2.2 Endocrinology of reproduction
The genital tract of non-pregnant cows normally lies in the pelvic cavity and consists of the vulva, vagina, cervix, uterus, Fallopian tubes (oviducts), ovaries and their supporting structures (Figure 2). Most of the reproductive structures can be palpated through the rectum; this is the basis of routine fertility work described in subsequent sections of this monograph. In general the reproductive tract of Bos indicus cattle is smaller than that of taurine cattle. The reproductive tract is supplied by blood from the utero-ovarian and uterine arteries, of which the middle uterine artery is the largest.
Figure 2. The reproductive tract of the cow (lateral view) showing its position inside the pelvic and abdominal cavities
Source: Peters and Lamming (1987).
The uterus is a muscular organ consisting of a body, about 4 to 5 cm long, and two uterine horns (cornua), each 15 to 25 cm in length and 1 to 3 cm in diameter. The uterus is suspended by the broad ligament in a coiled or curled manner. Its size varies with breed, age, parity, pregnancy and disease.
The cervix is a sphincter-like structure with a thick wall and a narrow lumen. This lumen is tightly closed, except during oestrus and at parturition, and the cervix forms a barrier between the uterus and the outside environment. The length of the cervix varies from 1.5 cm in heifers to 8 cm in multiparous cows of larger breeds. Interlocking ridges and complex folding of the lumen mucosa can hamper insertion of a pipette or tube for intra-uterine insemination or infusions.
The vagina extends backwards from the cervix and opens into the vulva. Its length varies with breed and stage of pregnancy. The vaginal epithelial cells near the cervix secrete mucus, especially around the time of oestrus.
The ovaries are oval-shaped structures, 1 to 4 cm long and 1 to 3 cm in diameter; their size depends on the stage of the reproductive cycle. They are linked to the uterus by the Fallopian tubes which open anteriorly into the fimbriae - funnel-shaped structures close to, but not attached to the ovaries. The fimbriae guide unfertilized eggs from the ovary into the Fallopian tubes.
Figure 3 is a schematic representation of the mammalian ovary. The ovary has two major functions: gametogenesis (the production of female gametes) and steroidogenesis (the production of steroid hormones, which play vital roles in the reproductive cycle).
Figure 3. The morphology and architecture of a mammalian ovary during the reproductive cycle
Source: Erickson et al (1985).
The ovary of a new-born heifer may contain up to 100,000 primordial follicles (Erickson, 1966). However, only a few of these mature and release an ovum. From birth to shortly before puberty the primordial follicles are in a state of arrested development (dictyotene; the resting stage). Shortly before puberty, many primordial follicles start to grow and develop in response to hormone (gonadotrophin) stimulation. The presence of developing follicles indicates active gametogenesis and steroidogenesis. During each oestrous cycle, several follicles may develop to the Graafian stage, but usually only one reaches full maturity and ruptures to release the ripe ovum (ovulation): the others become atretic (also known as degenerating, luteinising or anovulatory follicles) (McDonald, 1980).
Ovulation involves changes in steroid, gonadotrophin and prostaglandin secretions along with alterations in ovarian neuromusculature, in particular the breakdown of the follicular wall, escape of the ovum and release of follicular fluid. The ovulation fossa formed after ovulation fills with blood to become the corpus haemorrhagicum and eventually the corpus luteum (yellow body), which protrudes from the surface of the ovary. For details on the histology of the corpus luteum, see, for example, Corner (1915); McNutt (1924); Foley and Greenstein (1958); Gier and Marion (1961); Donaldson and Hansel (1965); Parry et al (1980); Alila and Hansel (1984); and Braden et al (1988). The corpus luteum is part of the endocrine system.
2.2.1 Endocrine changes in the prepubertal heifer
2.2.2 Endocrinology of the oestrous cycle
2.2.3 Endocrinology of pregnancy
2.2.4 Maternal recognition of pregnancy
2.2.5 Endocrinology of the postpartum period
The development of radio-immunoassay (RIA), competitive protein binding (CPB) and enzyme-linked immunosorbent assay (ELISA) methods since the 1960s has allowed rapid, accurate and sensitive measurement of the concentration of several pituitary, ovarian and adrenal hormones in blood, tissue, milk and urine. Ovarian tissue can now be studied in vitro. Cells can be broken down and the secretion and use of hormones by their organelles examined. The interrelations among hormones at various stages of the reproductive cycle in the female cow are therefore better understood, as are the physiological control mechanisms that govern reproductive function (Entwistle, 1983).
The endocrine system comprises a series of ductless glands, each of which secretes one or several hormones that integrate body functions. Hormones are secreted directly into the blood and act on tissues elsewhere in the body.
The reproductive cycle of the cow is mainly coordinated by hormones produced by the hypothalamus, pituitary and ovary. Gonadotrophic releasing hormone (GnRH), secreted by the hypothalamus, stimulates the anterior pituitary to secrete two gonadotrophic hormones - follicle stimulating hormone (FSH) and luteinising hormone (LH). Both of these hormones control ovarian function: FSH initiates maturation of follicles, and LH induces ovulation and luteinisation of granulosa and thecal cells. The major hormones produced by the ovaries are oestrogens (primarily oestradiol-17 b), which are produced by the follicles, and progesterone, secreted by the corpus luteum. Oestrogens play important roles in oestrus manifestation, and progesterone in maintenance of pregnancy. Both regulate the reproductive cycle through a series of feedback mechanisms acting on the hypothalamus and pituitary glands. In addition to these hormones, prostaglandins, which are produced by several tissues, including the uterus, also control the cow reproductive cycle in various ways. Details of the functions of these hormones at puberty, during normal oestrous cycles, during pregnancy, and in the postpartum period are given in the following sections. Other hormones, produced by the thyroid, parathyroid and adrenal glands, the placenta and the pancreas are also important in regulating reproduction.
For information on hormone action at the cellular level, see, for example, Eisenfeld (1972), Calandra et al (1974), Kato et al (1974), Catt et al (1975), Saxena (1976), Greenstein (1978), Cermak et al (1983), Spellberg et al (1983), Saxena et al (1984), McCracken and Okulicz (1984), Smith et al (1985) and Bergh et al (1985).
There are differences in the hypothalamic, pituitary and ovarian relationships in zebu and taurine cattle (Rollinson, 1955; Plasse et al, 1970; Griffin and Randel, 1978; Randel, 1976,1984; Rhodes et al, 1979). These differences probably account for differences in fertility between the two species even when similarly fed and managed (Rhodes et al, 1982).
This section highlights the endocrinology of reproduction in Bos indicus cattle. Data from taurine cattle and other species are used for emphasis and where information for zebu cattle is not readily available.
Little information is available on the hormonal control of puberty in the zebu. Early studies tended to compare the endocrine patterns in heifers with those of more mature animals. It was originally thought that the pituitary of prepubertal animals was incapable of elaborating sufficient gonadotrophic hormones to stimulate the ovaries. This was based on observations, such as those by Macfarlane and Worrall (1970) among Boran zebus, that administering gonadotrophins stimulated follicular growth and ovulation. However, follicle growth starts soon after birth (Desjardins and Hafs, 1968), as does the release of gonadotrophins, FSH and LH (Peters and Ball, 1987). The ovaries of 2-month old calves can respond to gonadotrophin therapy (Onuma et al, 1970) and calf follicles can secrete oestrogens. Prepubertal ovaries also respond when transplanted to mature animals (Russell and Douglas, 1945) and injecting oestradiol results in LH release in calves as young as 3 months old (Schillo et al, 1983). The possible causes of sexual maturation at puberty appear to be an increase in pituitary hormones output culminating in increased size and activity of the ovaries (Hunter, 1980), and maturation of the hypothalamo-pituitary axis, resulting in secretion of gonadotrophins (Ramirez, 1973).
126.96.36.199 Luteinising hormone
LH levels fluctuate before puberty (Desjardin and Hafs, 1968) but tend to increase as puberty approaches (Swanson et al, 1972). By taking samples more frequently from Angus heifers, Gonzalez-Padilla et al (1975a) confirmed that prepubertal heifers do not lack LH as such, but there is no cyclic pattern to its release. Two LH peaks were observed prior to puberty, the first (priming) peak at 9 to 11 days before first oestrus. This diphasic profile was also observed among Brown Swiss heifers by Schams et al (1981). In heifers attaining puberty at 10 months old, LH and FSH levels increased from birth to 3 months, declined to a nadir at 5-6 months and then increased to a second peak at about 9 months (Figure 4).
LH secretion in prepubertal heifers is probably suppressed through an inhibitory feedback (gonadostat) effect (Anderson et al, 1985). As noted above, components of the endocrine system can apparently function soon after birth (Ramirez and McCann, 1963). LH is secreted from the pituitary gland and stimulates ovarian follicles to produce oestradiol-17 b. However, the hypothalamus-pituitary axis is highly sensitive to the negative feedback effect of oestradiol, and further LH release is inhibited. Ovariectomy of immature rats significantly increases the concentration of plasma LH (Ramirez and McCann, 1963; Caligaris et al, 1972) and FSH (Kragt and Masken, 1972). The same is true in the calf (Odell et al, 1970). The sensitivity of the hypothalamus-pituitary axis to oestradiol must thus decrease prior to puberty in the heifer (Schillo et al, 1982). This allows LH to stimulate follicular growth and leads to increased oestrogen production (Foster and Ryan, 1981) and ovulation.
188.8.131.52 Oestradiol and progesterone
There are few reports on the plasma concentration of oestradiol-17 b in prepubertal heifers. Glencross (1984), using a sensitive and fully validated radio-immunoassay, found that the plasma oestradiol-17 b levels of four British-Friesian heifers varied randomly within the range 1 to 4 ng/litre between 59 and 15 days before puberty. About 8 days prior to puberty, oestradiol-17 b levels increased significantly (P<0.02) to a mean of 6.3 ±1.3 ng/litre, comparable to the normal preovulatory peak in post-pubertal heifers. It was not clear, however, if this induced an LH surge and ovulation. Progesterone levels subsequently rose (P<0.001) to a peak of 1.0 ±0.1 m g/litre on the fourth day, indicating some luteinisation. After the return of the progesterone to basal levels, oestradiol-17 b again rose significantly (P<0.001) to a second peak of 9.0 ±1.0 ng/litre on the day of first oestrus. Following this second peak, concentration of progesterone in the plasma remained high and pregnancy was confirmed in three of the heifers. The second peak in oestradiol-17 b concentration had therefore been followed by ovulation. A third oestradiol-17 b peak (P<0.02) of 4.3 ±0.8 ng/litre occurred 4 days later, when progesterone levels were rising sharply due to the formation of a corpus luteum. The changes in oestradiol-17 b and progesterone on or after the day of first oestrus were similar to those observed in post-pubertal heifers (Glencross and Pope, 1981; Glencross et al, 1981) and mature cows (Echternkamp and Hansel, 1973; Glencross et al, 1973; Smith et al, 1975).
Figure 4. Plasma LH and FSH concentrations (± SEM) of female calves from birth to 12 months of age
Source: Adapted from Schams et al (1981).
Studies among Bos taurus heifers showed that progesterone concentration is low through most of the prepubertal period with two rises before puberty (Gonzalez-Padilla et al, 1975a). The first occurred between 18 and 11 days before the LH peak and was thought to be of adrenal origin. The second, from 9 days before until the day of the LH peak, was assumed to be of ovarian origin. Schams et al (1981) also observed an increase in progesterone concentration for 8-12 days before first oestrus in four Brown Swiss heifers that attained puberty at about 10 months old (Figure 5). A fifth heifer, which showed first oestrus at 14 months, exhibited a progesterone secretion pattern resembling that of a normal corpus luteum during the 18 days before first oestrus. Prior to this rise, levels were elevated for 8 days, but only slightly (Figure 5). Similar elevations in progesterone concentration were reported by Berardinelli et al (1979) who attributed them to small luteal tissues, deeply embedded in the ovary, which could not be palpated. This agrees with Ojeda et al (1980), who stated that, in general, there is no compelling evidence for a role of adrenal sex steroids in the onset of puberty. These initial rises in progesterone may establish a phasic pattern to LH release and/or sensitise the ovaries to LH (Berardinelli et al, 1979) as in some postpartum cows.
Figure 5. Mean (± SEM) plasma progesterone levels before and after the first oestrus in heifers
a = 4 heifers attaining puberty at 303 + 11 days of age
b = 1 heifer attaining puberty at 414 days of ageThe arrows indicate the time of the preovulatory LH surges
· indicates the time of oestrus
Source: Schams et al (1981).
The above observations have led to attempts to stimulate puberty. Most efforts have tried to simulate the transient rise in progesterone prior to first oestrus using implants or daily injections of progesterone combined with oestrogen or pregnant mare serum gonadotrophin (Gonzalez-Padilla et al, 1975b; Rajamahendran et al, 1982). Generally the treatments have been more successful in animals approaching puberty.
Figure 6 illustrates how the concentrations of the main reproductive hormones in the plasma change during the cow oestrous cycle. Hypothalamic GnRH induces the release of both LH and FSH from the pituitary (Kaltenbach et al, 1974; Schams et al, 1974). LH is released in pulses (Rahe et al, 1980, 1982) (Figure 7). Each LH pulse appears to be in response to a release of GnRH from the hypothalamus and LH secretion can be stimulated by GnRH injections. LH induces ovulation and luteinisation of the granulosa and thecal cells. It also appears to be the principal luteotrophic factor in the cow (Peters and Lamming, 1983).
184.108.40.206 Luteinising hormone
LH concentration is low during most of the luteal phase of the oestrous cycle, with one pulse every 4 or more hours. It begins to rise a few days prior to oestrus. Pulse frequency increases to one or more per hour; pulse amplitude, however, falls. The large preovulatory LH peak or surge that occurs near the beginning of oestrus is preceded by a rise in the concentration of oestradiol one or 2 days before oestrus.
Figure 6. Schematic representation of the changes in plasma hormone concentrations during the bovine oestrous cycle
Day O = day cow first shows oestrus
Source. Peters and Lamming (1983).
Randel (1976) estimated the interval between oestrus onset and the LH surge to be 0.4 3.4 hours in Brahman cows, 6.8 ±2.1 hours in Brahman x Hereford cows and 5.3 ±1.3 hours in pure Herefords. Randel and Moseley (1977) recorded intervals of 2.0 ±1.3 in Brahman cows, 3.0 ±1.0 in Brahman x Hereford cows and 6.5 1.8 hours in Hereford cows. The preovulatory LH surge therefore seems to occur sooner after the onset of behavioural oestrus in zebu than taurine cows or their crosses. In addition, Randel (1976) estimated the interval between the LH surge and ovulation to be 18.5 ±3.1 hours in Brahman cows, 22.2 ±2.6 hours in Brahman x Hereford cows and 23.3 ±2.1 hours in Hereford cows. Zebu cows thus appear to ovulate sooner after the LH surge than Bos taurus cows
Oestradiol-17 b is the principal biologically active oestrogen. Randel (1980) measured total serum oestrogen (TSO) from 72 hours before oestrus until 24 hours after oestrus in Brahman, Brahman x Hereford and Hereford heifers. TSO did not differ significantly between breeds prior to oestrus. The highest pro-oestrous TSO level occurred 24 hours before oestrus in Brahmans, 8 hours before oestrus in Herefords and 16 hours before oestrus in the crossbreds. The pattern was similar for the lowest levels after oestrus: 24 hours after oestrus TSO levels were lower in Brahmans than the other two genotypes (P<0.05) and these lower values coincided with ovulation. This finding agreed with data showing that Brahman cows tend to ovulate within 24 hours of the onset of heat, earlier than taurine cattle (Plasse et al, 1970; Randel, 1976). Randel's (1980) data also indicate that the oestradiol surge has two peaks. Echternkamp and Hansel (1973) proposed that the pre-oestrus rise in oestrogen mediates the LH release from the bovine pituitary, which in turn might stimulate the second oestradiol rise.
Figure 7. Pattern of plasma LH concentration on day 3 and 11 and the preovulatory LH surge on day 19 in the cow
Glencross and Pope (1981), working with taurine cattle, found that oestradiol-17 b levels are low in peripheral plasma for most of the oestrous cycle and rise as the concentration of progesterone begins to fall, reaching a peak 3 to 4 days later. Probably the drop in progesterone concentration following luteal regression allows the preovulatory follicle to increase its secretion of oestradiol-17 b (Karsh et al, 1978). In the Holstein heifers used by Glencross and Pope (1981), plasma oestradiol-17 b concentration was low at the start of luteal regression (2.2 ±0.5 pg/ml), increased to 3.8 ±0.6 pg/ml the next day and reached 6.6 ±0.9 pg/ml when the concentration of progesterone in the blood had fallen to a minimum. The highest concentration of oestradiol-17 b (10.1 pg/ml) was recorded one or 2 days after complete luteolysis. A similar trend was reported by Wettemann et al (1972).
Glencross et al (1973), Smith et al (1975), Glencross and Pope (1981) and Hansel and Convey (1983) observed a second, postovulatory, peak in oestradiol 5 to 7 days after oestrus (Figure 8). The last authors observed this peak in non-pregnant cattle and those inseminated but failing to conceive. It appeared to be related to the presence of a large follicle. Its physiological significance is not clear but the follicle is not destined to ovulate (Peters and Ball, 1987).
Mukasa-Mugerwa et al (1989), using the ELISA method, found that the concentration of progesterone in the plasma of Ethiopian highland zebu cattle was less than 1.0 ng/ml from 2 days before oestrus to 3 days after oestrus. Llewelyn et al (1987) referred to this as the "basal progesterone" period. Progesterone concentration gradually increased from 4 days after oestrus (the "rising progesterone or early luteal" period), as the corpus luteum became functional. It reached a maximum of 8.0 to 10.0 ng/ml at 11 to 15 days after oestrus (the "plateau progesterone" period) and then declined (the "falling progesterone" period) to basal levels before the next oestrus and ovulation (Figure 9). A similar pattern in progesterone concentration was reported by Adeyemo and Heath (1980) in White Fulani cattle, Eduvie and Dawuda (1986) in Bunaji cattle, Coetzer et al (1978a) in Africander cattle, Vaca et al (1983) in InduBrazil cattle, Llewelyn et al (1987) in Boran cattle, Randel (1980) in Brahman cattle and their taurine crosses and Hansel (1981) in Holstein heifers.
Coetzer et al (1978a) observed a drop in the concentration of progesterone in the blood of zebu cows about 13 days after oestrus. Similar observations were made by Erb et al (1971) and Schams et al (1977) in taurine cows. Coetzer et al (1978a) associated the decrease with mid-cycle follicle growth and development. They also found a small, but consistent, peak in progesterone concentration, 18-25 hours after the preovulatory LH surge, the time of ovulation. If consistent, this could be used to determine more precisely the time of ovulation.
Figure 8. Follicular growth and endocrine changes from luteal regression to the resumption of luteal function
Source: Hansel and Convey (1983).
Figure 9. Mean (+ SD) plasma progesterone levels during the oestrous cycle of Ethiopian zebu cows
Source: Mukasa-Mugerwa et al (1989).
Randel (1980) noted that progesterone concentration was generally lower in Brahman and Brahman x Hereford crosses than in purebred Hereford heifers. Between-breed differences in progesterone concentration have sometimes been suspected to arise from differences in ovarian size (Adeyemo and Heath, 1980; Irvin et al, 1978; Segerson et al, 1984a). The exact relationship is, however, not clear. Randel (1984), for example, suggested that the lesser responsiveness of the ovaries of Brahman cows to gonadotrophic hormone during formation of a corpus luteum might also result in a smaller corpus luteum. Nevertheless, the corpora lutea of Brahman and Brahman x Hereford cows had similar total progesterone contents. The Brahman corpora lutea seemed to have compensated for the small size. In fact, the activity of 3 b -hydroxysteroid dehydrogenase, the enzyme responsible for converting pregnenolone to progesterone, was greater in corpora lutea from Brahman cows.
Oestrus usually occurs 1 to 5 days after the corpus luteum starts to regress. The regression of the bovine corpus luteum is brought about by the action of prostaglandin F2 a (PGF2 a) (Knickerbocker et al, 1988). Dobson and Kamonpatana (1986) suggested that the variation in this interval is due partly to differences in the time ovulatory follicles take to develop and mature. Views on the growth, distribution and selection of follicles in the ovary differ. Choudary et al (1968), Donaldson and Hansel (1965) and Marion and Gier (1971) suggested that follicular growth is continuous and independent of the phases of the oestrous cycle. Matton et al (1981) thought that numbers of follicles do not vary between stages of the cycle. In contrast, Rajakoski (1960) suggested that antral follicles grow and regress throughout the oestrous cycle in two waves: the first ends around day 12 and is followed by atresia; the second culminates in oestrus.
There is little information about prostaglandin concentration in zebu cows. Studies in the buffalo showed increases in prostaglandin concentration in blood plasma and milk from 250 to 900 pg/ml over the 2 or 3 days prior to oestrus (Batra and Pandey, 1983). Similar increases (150-750 pg/ml) were reported for the taurine cow by Kindhal et al (1976). Edquist and Kindhal (1981) give more information on the use of prostaglandins in animal reproduction.
When a cow conceives, plasma and milk progesterone levels rise as in a normal oestrous cycle but instead of declining at about 15 to 18 days after oestrus, remain high for the rest of the gestation period, preventing further ovarian cycles (Peters and Lamming, 1983). Working with Holstein heifers, Hansel (1981) noted that jugular plasma progesterone concentrations were higher (P<0.05) in pregnant than cyclic non-pregnant animals 10 days after oestrus (Figure 10), indicating that the bovine blastocyst is able to stimulate progesterone synthesis by as early as the 10th day after conception.
In a study of the blood progesterone levels during the gestation period of Ethiopian zebu cows, Mukasa-Mugerwa and Azage Tegegne (1989) (Figure 11) observed a trend similar to that found by Agarwal et al (1977, 1980) in Haryana Zebu and Coetzer et al (1978b) in Africander cows. Progesterone levels in Ethiopian zebu cows were high (over 5 ng/ml) until the last 12 to 18 days of pregnancy. This was followed by a decline to 3.7-8.2 ng/ml one to two days before parturition. Hashmat Shehata (1982) observed that progesterone levels declined to 1.2-2.0 ng/ml during the last 12-24 hours before calving and were less than 1 ng/ml 24-48 hours after delivery in local Egyptian cattle. The differences in progesterone concentration among the studies probably reflect breed and/or assay-technique variability.
Figure 10. Mean (± SEM) jugular plasma progesterone concentrations in pregnant, cyclic and inseminated non-pregnant heifers during an 18-day period
Source: Hansel (1981).
Although the corpus luteum remains active throughout pregnancy its weight and progesterone content do not perfectly reflect changes in jugular plasma progesterone, indicating an extra-ovarian source (Erb et al, 1971). Abortion can be induced using prostaglandin F2 a (PGF2 a) prior to 120 days of pregnancy (Dobson and Kamonpatana, 1986). After this time, both the placenta (Melampy et al, 1959) and adrenals (Wendorf et al, 1983) can produce progesterone. Progesterone from these sources probably maintains pregnancy after mid-term ovariectomy (Estergreen et al, 1967). These sources probably also account for the low abortion rates after administering prostaglandins at 120 to 250 days of gestation, and the administration of PGF2 a is not very successful until after 250 days of pregnancy Johnson, 1981).
Figure 11. Mean (± SEM) jugular plasma progesterone concentration in zebu cattle during pregnancy
Source: Mukasa-Mugerwa and Azage Tegegne (1989).
Coetzer et al (1978b) reported that the level of unconjugated oestrogens was low (193 to 267 pg/ml) and fairly constant during pregnancy in Africander cattle. Between 2 and 6 days prior to delivery the level increased sharply to 271 to 523 pg/ml. The upper limit is comparable to the 501 pg/ml obtained by Stellflug et al (1978) for Angus and Hereford beef cows. Two days after parturition the level stabilised at around 110 pg/ml, with little variation between individuals. Erb et al (1971) had noted the same trend in taurine cattle. Substantial amounts of oestrogens are also produced by the bovine placenta after 100 days of pregnancy (Veenhuizen et al, 1960; Ainsworth and Ryan, 1966; Robertson and King, 1979; Evans and Wagner, 1981; Shelton and Summers, 1983).
The exact mechanisms involved in maternal recognition of pregnancy are not fully understood. However, it appears necessary that (i) corpus luteum function is maintained and (ii) the cyclic release pattern of LH must be terminated, prostaglandins must be stopped from reaching the corpus luteum, or some substance must be secreted to check the cyclic action of prostaglandins. It is suspected that interactions between the developing conceptus and maternal system are involved. The mechanisms regulating the establishment and maintenance of pregnancy in cattle must be understood before techniques can be developed to reduce the incidence of early embryonic mortality (Segerson et al, 1984a).
Plasma progesterone levels are significantly (P<0.05) higher in pregnant than cyclic non-pregnant Holstein heifers by as early as 10 days after fertilisation (see Figure 10). Shemesh et al (1979) found that the bovine blastocyst can produce progesterone, some testosterone and limited amounts of oestradiol-17 b by day 13 to 16. Blastocysts 15-17 days old are also able to convert androstenedione to oestrogen in vitro (Eley et al, 1979). Oestrogens have a luteolytic action in the cow (Wiltbank et al, 1961; Eley et al, 1979).
Homogenates of sheep embryos (Martal et al, 1979) and sheep conceptus secretory proteins (Godkin et al, 1984) can extend corpus luteum function and cycle length when administered into the uterine lumen of cyclic ewes. Thus, in sheep the pre-implantation embryo appears to produce a luteotrophic substance that contributes to the maintenance of early pregnancy by directly stimulating progesterone secretion by the corpus luteum. In cattle too, Knickerbocker et al (1986) found that treating cycling Holstein cows with conceptus secretory proteins extended the life-span of corpora lutea and inter-oestrus interval. An evaluation of spontaneous prostaglandin response suggested that proteins synthesised and secreted by the bovine conceptus accommodate luteal maintenance during early gestation via an attenuation of prostaglandin production. In sheep and cows, oestrogen, which is luteolytic in the late luteal phase, may indirectly induce uterine prostaglandin synthesis. The role of the embryonic hormone, which the above substance appears to be, may be to counter the lytic action of the prostaglandins. Further details on the subject and the proteins secreted are presented in Roberts and Parker (1974, 1976), Laster (1977), Segerson et al (1984a, 1984b), Thatcher et al (1984) and Roberts et al (1985).
The interval between calving and conception depends on the reestablishment of normal ovarian cycles after calving, the occurrence of oestrous behaviour at the appropriate time in the cycle, and the pregnancy rate following service (Peters, 1984).
The interval between parturition and ovulation is characterised by sexual quiescence (postpartum anoestrus). The duration of this interval varies with breed, milk yield level, animal age, suckling or lactating status, nutritional level before and after calving, season and associated photo-periodism, climate, health status and calving difficulty. Of these factors, nutrition and suckling appear to be very important.
Schallenberger et al (1978) noted that pregnancy reduces the sensitivity of the pituitary to GnRH. Sensitivity of the pituitary to GnRH increases only gradually after calving. The resumption of ovarian cyclicity depends on the establishment of a pulsatile pattern of LH secretion (Peters and Lamming, 1983). The observed delay is due, probably, to their being insufficient oestradiol to induce the pre-ovulatory LH surge (Peters et al, 1981).
220.127.116.11 Luteinising hormone
Peters and Lamming (1983) estimated that a pulsatile pattern of LH secretion with a frequency of 0.25 to 1 per hour appears to be a prerequisite for the first ovulation postpartum. This results in gradually increasing LH concentration before the first LH surge. Hansel and Alila (1984) stated that the frequency of LH pulses is due to increased frequency of pulsatile releases of GnRH and that the factors affecting the duration of postpartum anoestrus also affect the time taken to establish the pulsatile pattern of LH release. Peters and Lamming (1986) thought that changes in gonadotrophin concentration may be brought about by changes in pituitary responsiveness to GnRH during the postpartum period. The time at which the pulsatile releases of LH appear and pituitary sensitivity increases varies among breeds and is also affected by suckling.
The exact mechanisms by which suckling interferes with the hypothalamus-hypophyseal axis are not well defined (Hanzen, 1986), but it is unlikely that teat manipulation alone can alter LH release patterns in the cow (Williams et al, 1984). Other factors, such as the presence of the calf or social interactions, might be necessary before teat stimulation has an effect (Hanzen, 1986). Suckling reduces the frequency and amplitude of LH release (Carruthers and Hafs, 1980), pituitary sensitivity to GnRH (Carruthers et al, 1978) and the pulsatile release of GnRH by the hypothalamus (Carruthers et al, 1980).
Suckling probably inhibits LH and GnRH release and their action rather than their synthesis (Hanzen, 1986). Both the hypothalamic concentration of GnRH and the pituitary concentration of LH are similar in milked and suckled cows (Carruthers et al, 1978; Saiduddin et al, 1967; Graves et al, 1968). Suckling can also inhibit the positive effect of endogenous or exogenous oestradiol on the release of pituitary LH (Short et al, 1979, Stevenson et al, 1983). Temporary weaning (48 hours) may, however, increase plasma LH concentration (Walters et al, 1982a), but LH concentration falls to previous levels within 4 hours of calf return (Walters et al, 1982b). Suckling delays LH release and reduces the amount of LH released in response to GnRH injection. Temporary calf removal can, however, enhance the total amount of LH released in response to GnRH injection (Dunn et al, 1985).
Using suckled beef cows, Rawlings et al (1980) noted that the maximum magnitude and frequency of LH peaks occurred 10 to 33 days before the initial increase of plasma progesterone, i.e. when there was a marked development of large follicles and a large variation in oestradiol-17 b. These and similar observations have led researchers to (i) suggest that there might be a deficiency of GnRH during the early postpartum period and (ii) attempt to stimulate ovulation and ovarian cycles by repeated injections of GnRH (Peters and Lamming, 1986) in order to simulate the events of the preovulatory period. Results from GnRH treatment have been inconsistent (Riley et al, 1981; Walters et al, 1982b; Edwards et al, 1983). The number of animals, whether milked or suckled, that respond positively to GnRH injection increases during the postpartum period. Suckled cows are, however, less likely to respond during the initial 15 days than milked cows. This difference diminishes during subsequent weeks (Hanzen, 1986). After this period the magnitude of GnRH-induced LH release appears to be directly proportional to follicular development (Smith et al, 1983).
It is difficult to monitor pulses of oestradiol-17 b in the jugular vein. Peters and Lamming (1986) reported unpublished work in which cannulae were inserted into the posterior vena cava, anterior to the junction of the ovarian veins, of recently calved cows. Oestradiol-17 b pulses, both naturally occurring and induced by three-hourly injections of GnRH, were detected: similar pulses could not be registered in the jugular vein, although increases in oestradiol concentration were measured. It was concluded that the early postpartum cow is sensitive to GnRH-induced gonadotrophin release and responds by secreting oestradiol.
Two types of luteal activity have been observed in the postpartum cow. Fifty to 80% of milked or suckled dairy cows exhibit an initial luteal phase in which increases in plasma progesterone concentration are of shorter duration and progesterone concentrations are lower than in the normal cycle (Donaldson et al, 1970; Schams et al, 1978; Odde et al, 1980; Peters and Riley, 1982). This is referred to as the short luteal phase and lasts 6 to 12 days. The second luteal phase lasts about 14 days and tends to have lower than normal progesterone levels. This dual progesterone pattern occurs even after GnRH challenge (Fonseca et al, 1980), early weaning (Odde et al, 1980) or limited suckling with (Dunn et al, 1985) or without GnRH treatment (Flood et al, 1979).
Tribble et al (1973) thought that progesterone can be released from follicles that fail to ovulate. Schams et al (1978) felt that short progesterone cycles are caused by shortage of LH or its receptor. However, no differences in plasma LH concentration are observed before or after an oestrus associated with a short cycle (Ramirez-Godinez et al, 1982a). Troxel et al (1980) suggested that the short life-span may be the result of short GnRH-induced LH surges. Alternatively, the amount of LH receptor and number of granulosa cells may not be sufficient to give optimum response to this luteotrophic stimulus (Channing et al, 1981).
Hanzen (1986) pointed out that the corpus luteum from a cow with regular cycles responds positively in vitro to LH addition but that a corpus luteum formed after GnRH injection does not. This may be due to luteal tissue of the latter being unable to recognise LH (Kesler et al, 1981). It has been shown that the in vitro response of the postpartum corpus luteum during the first three cycles is related to the integrity of luteal tissue at the time of removal (Duby et al, 1985). These observations suggest that there may be premature luteolysis due to PGF2 a synthesised after calving by caruncular uterine tissue (Troxel and Kesler, 1984). It has been suggested that oxytocin can increase production of PGFM (a metabolite of PGF2 a) and lead to earlier luteolysis (Troxel et al, 1984). Whatever the cause, the abnormal short cycles can result in increased early embryonic mortality (Ramirez-Godinez et al, 1982b; Troxel et al, 1983).
In a study of 20 postpartum pluriparous Brahman cattle, Rutter and Randel (1984) reported a higher incidence of abnormal cycles after the first heat than the second (35% vs 5%; P<0.05). It was suggested that the higher incidence of abnormal luteal function following the first and second postpartum cycles contributes to lower conception rates in animals bred during the first or second postpartum heat.
Adeyemo O and Heath E. 1980. Plasma progesterone concentration in Bos taurus and Bos indicus heifers. Theriogenology 14: 411-419.
Agarwal S P. Rahman S A, Laumas K R. Agarwal V K and Gautam O P. 1977. Studies on steroid hormones. Progesterone concentration in the blood serum of zebu cows. Indian Journal of Animal Sciences 47: 715-719.
Agarwal S P. Agarwal V K and Ahmad A. 1980. Studies on steroid hormones. 3. Serum progesterone concentration in pregnant zebu cows. Indian Journal of Animal Sciences 50: 706-709.
Ainsworth L and Ryan K J. 1966. Steroid hormone transformations by endocrine organs from pregnant mammals. I. Oestrogen biosynthesis by mammalian placental preparations in vitro. Endocrinology 79: 875-883.
Alila H W and Hansel W. 1984. Origin of different cell types in the ovine corpus luteum as characterised by specific monoclonal antibodies. Biology of Reproduction 31: 1015-1025.
Anderson W J, Forrest D W. Schulze A L, Kraemer D C, Bowen M J and Harms P G. 1985. Ovarian inhibition of pulsatile luteinizing hormone secretion in prepubertal Holstein heifers. Domestical Animal Endocrinology 2: 85-91.
Batra S K and Pandey R S. 1983. Relative concentration of 13,14-dihydro-15-keto prostaglandin F2 a in blood and milk of buffaloes during the estrous cycle and early pregnancy. Journal of Reproduction and Fertility 67: 191-195.
Berardinelli J G. Dailey R A, Butcher R L and Inskeep E K. 1979. Source of progesterone prior to puberty in beef heifers. Journal of Animal Science 49: 1276-1280.
Bergh A, Nikula H. Dember J E, Clayton R and Huhtaniemi I. 1985. Altered concentrations of gonadotrophin, prolactin and GnRH receptors, and endogenous steroids in the abdominal testes of adult unilaterally cryptorchid rats . Journal of Reproduction and Fertility 74: 279-286.
Braden T D, Gamboni F and Niswender G D. 1988. Effects of prostaglandin F2 alpha-induced luteolysis on the populations of cells in the ovine corpus luteum. Biology of Reproduction 39: 245-253.
Calandra R S. Podesta E J, Rivarola M A and Blanquier J A. 1974. Tissue androgens and androphillic proteins in rat epididymis during sexual development. Steroids 24: 507-518.
Caligaris L, Astrada J J and Taleisnik S. 1972. Influence of age on the release of luteinising hormone induced by oestrogen and progesterone in immature rats. Journal of Endocrinology 55: 97.
Carruthers T D and Hafs H D. 1980. Suckling and four-times daily milking: Influence on ovulation, estrus and serum luteinizing hormone, glucocorticoids and prolactin in postpartum Holsteins. Journal of Animal Science 50: 919-925.
Carruthers T D, Kesner J S. Convey E M and Hafs H D. 1978. Hypothalamic GnRH and pituitary function in suckled and non-suckled Holstein cows. Journal of Animal Science 47 (Supplement 1): 349.
Carruthers T D, Convey E M, Kesner J S. Hafs H D and Cheng K W. 1980. The hypothalamo-pituitary gonadotrophic axis of suckled and nonsuckled dairy cows postpartum. Journal of Animal Science 51: 949-957.
Catt K J, Dufau M L, Neaves W B. Walsh P C and Wilson J P. 1975. LH-HCG receptors and testosterone content during differentiation of the testis in the rabbit embryo. Endocrinology 97: 1157-1165.
Cermak D L, Braden T. Manns J. Niswender G D and Nett T M. 1983. Contents of hypothalamic GnRH, pituitary FSH and LH, and pituitary receptors for GnRH and estradiol in postpartum suckled beef cows. Journal of´ Animal Science 57 (Supplement 1): 322.
Channing C P. Anderson L D, Hoover D J. Gagliano P and Hodgen G. 1981. Inhibitory effects of porcine follicular fluid on monkey serum FSH levels and follicular maturation. Biology of Reproduction 25 885-903.
Choudary J B. Gier J T and Marion G B. 1968. Cyclic changes in bovine vesicular follicles. Journal of Animal Science 27: 468.
Coetzer W A, van Niekerk C H. Morgenthal J C and van der Westhuysen J M. 1978a. Hormone levels in peripheral plasma of the Afrikaner cow. 1. Progesterone and luteinising hormone levels during the oestrous cycle. South African Journal of Animal Science 8: 1-5
Coetzer W A, van Niekerk C H and Morgenthal J C. 1978b. Hormone levels in peripheral plasma of the Afrikaner cow. 2. Progesterone and total unconjugated oestrogen levels during late gestation and parturition. South African Journal of Animal Science 8: 7-10.
Corner G W. 1915. The corpus luteum of pregnancy as it is in swine. Publication No. 222, Carnegie Institution of Washington, Washington, DC, USA. 69 pp.
Desjardins C and Hafs H D. 1968. Levels of pituitary FSH and LH in heifers from birth through puberty. Journal of Animal Science 27: 472.
Dobson H and Kamonpatana M. 1986. A review of female cattle reproduction with special reference to a comparison between buffaloes, cows and zebu. Journal of Reproduction and Fertility 77: 1-36.
Donaldson L and Hansel W. 1965. Histological study of bovine corpora lutea. Journal of Dairy Science 48: 905-909.
Donaldson E L, Bassett J M and Thorburn G D. 1970. Peripheral plasma progesterone concentrations of cows during puberty, estrus cycles, pregnancy and lactation and the effects of undernutrition or exogenous oxytocin on progesterone concentration. Journal of Endocrinology 48: 599-614.
Duby R T. Browning T. Carey D and Block D L. 1985. Progesterone synthesis and histology of postpartum bovine corpora lutea. Theriogenology 23: 619-630.
Dunn R T, Smith M F, Garverick H A end Foley C W. 1985. Effects of 72 hour calf removal and/or gonadotropin-releasing hormone on luteinising hormone release and ovarian activity in postpartum beef cows. Theriogenology 23: 767-776.
Echternkamp S E and Hansel W. 1973. Concurrent changes in bovine plasma hormone levels prior to and during the first postpartum estrous cycle. Journal of Animal Science 37: 1362-1370.
Edquist L and Kindhal H (eds). 1981. Prostaglandins in animal reproduction. Acta Veterinaria Scandinavica, Supplementum 77: 390.
Eduvie L O and Dawuda P M. 1986. Effect of suckling on reproductive activities of Bunaji cows during the postpartum period. Journal of Agricultural Science (Cambridge) 107: 235-238.
Edwards S. Roche J F and Niswender G D. 1983. Responses of suckling beef cows to multiple, low-dose injections of GnRH with or without progesterone treatment. Journal of Reproduction and Fertility 69: 655-672.
Eisenfeld A J. 1972. Ontogeny of estrogen and androgen receptors. In: M M Grummbach, G D Grave and F E Mayer (eds), The control of the onset of puberty. John Wiley and Sons, New York, USA. pp. 271-306.
Eley R M, Thatcher W W and Bazer F W. 1979. Luteolytic effect of oesterone sulphate on cyclic beef heifers. Journal of Reproduction and Fertility 55: 191-193.
Entwistle K W. 1983. Factors affecting reproduction in beef cattle in Australia. AMRC Reviews No. 43. Australian Meat Research Committee, Sydney, NSW, Australia. 30 pp.
Erb R E, Randel R D and Callahan C J. 1971. Female sex steroid changes during the reproductive cycle. Journal of Animal Science (Supplement): 80-106.
Erickson B H. 1966. Development and senescence of the postnatal bovine ovary. Journal of Animal Science 25: 800-805.
Erickson G F. Magoffin D A, Dyer C A and Hofeditz C. 1985. The ovarian androgen producing cells: A review of structure/ function relationships. Endocrine Reviews 6(3): 371-399.
Estergreen V L, Frost O L, Gomes W R. Erb R E and Bullard J F. 1967. Effect of ovariectomy on pregnancy and parturition in dairy cows. Journal of Dairy Science 50: 1293.
Evans G and Wagner W C. 1981. In vitro oestrogen synthesis by bovine placenta during pregnancy and induced parturition. Acta Endocrinologica (Copenhagen) 98: 119-125.
Flood P F. Manns J G. Humphrey W D and Mapletoft R J. 1979. The first corpus luteum of the postpartum beef cow. Proceedings of the Society for the Study of Fertility and the Société Nationale pour l'Etude de la Stérilité et de la Fécondité, 14-16 December 1979, Gaillon, France. Journal of Reproduction and Fertility 59: 249-253.
Foley R C and Greenstein J S. 1958. Cytological changes in the bovine corpus luteum during early pregnancy. In: F X Gassner (ed.), Reproduction and infertility. Pergamon Press, New York, USA. pp. 88-96.
Fonseca F A, Britt J H. Kosugiyama H. Ritchie H D and Dillard E V. 1980. Ovulation, ovarian function and reproductive performance after treatment with GnRH in postpartum suckled cows. Theriogenology 13: 171-181.
Foster D L and Ryan K D. 1981. Mechanisms governing transition into adulthood. Journal of Reproduction and Fertility (Supplement 30): 75-90.
Gier H T and Marion G B. 1961. The formation of the bovine corpus luteum. Journal of Dairy Science 44: 1187.
Glencross R G. 1984. A note on the concentrations of plasma oestradiol-17 ß and progesterone and the time of puberty in heifers. Animal Production 39: 137-140.
Glencross R G and Pope G S. 1981. Concentrations of estradiol 17 ß and progesterone in the plasma of dairy heifers before and after cloprostenol-induced and natural luteolysis and during early pregnancy. Animal Reproduction Science 4: 93-106.
Glencross R G. Esslemont R J, Bryant M J and Pope G S. 1981. Relationships between the incidence of pre-ovulatory behaviour and the concentrations of Oestradiol-17 ß and progesterone in bovine plasma. Applied Animal Ethology 7: 141-148.
Glencross R G. Munro I B. Senior B E and Pope G S. 1973. Concentrations of oestradiol-17 ß, oesterone and progesterone in jugular venous plasma of cows during the oestrous cycle and in early pregnancy. Acta Endocrinologica (Copenhagen) 73: 374.
Godkin J D, Bazer F W. Thatcher W W and Roberts R M. 1984. Proteins released by cultured day 15-16 conceptuses prolong luteal maintenance when introduced into the lumen of cyclic ewes. Journal of´ Reproduction and Fertility 71: 57-64.
Gonzalez-Padilla E, Wiltbank J N and Niswender G D. 1975a. Puberty in beef heifers. 1. The interrelationships between pituitary, hypothalamic and ovarian hormones. Journal of Animal Science 40: 1091-1104.
Gonzalez-Padilla E, Ruiz R. Le Fever D, Denhan A and Wiltbank J N. 1975b. Puberty in beef heifers. 3. Induction of fertile estrus. Journal of Animal Science 40: 1110-1118.
Graves W E, Lauderdale J W. Hauser E R and Casida L E. 1968. Relation of postpartum interval to pituitary gonadotropins, ovarian follicular development and fertility in beef cows. University of Wisconsin Research Bulletin No. 270. 23 pp.
Greenstein B D. 1978. The role of hormone receptors in development and puberty. Journal of Reproduction and Fertility 52: 419-426.
Griffin J L and Randel R D. 1978. Reproductive studies of Brahman cattle. II. Luteinizing hormone pattern in ovariectomised Brahman and Hereford cows before and after injection of gonadotropin releasing hormone. Theriogenology 9: 437-445.
Hansel W. 1981. Plasma hormone concentrations associated with early embryo mortality in heifers. Journal of Reproduction and Fertility (Supplement 30): 231-239.
Hansel W and Alila H W. 1984. Causes of postpartum anoestrus in cattle in the tropics. In: Nuclear techniques in tropical animal diseases and nutritional disorders. Proceedings of the Consultative Meeting, 13-16 June 1983, Vienna. International Atomic Energy Agency, Vienna, Austria.
Hansel W and Convey E M. 1983. Physiology of the estrous cycle. Journal of Animal Science 57 (Supplement 2): 404-424.
Hanzen Ch. 1986. Endocrine regulation of postpartum ovarian activity in cattle. Reproduction, Nutrition and Development 26: 1219-1239.
Hashmat H A and Shehata Y M. 1982. Changes of progesterone, oestrogen and corticosteroid in bovine peripheral plasma before and after parturition. Indian Journal of Animal Sciences 52: 1173.
Hunter R H F. 1980. Physiology and technology of reproduction in female domestic animals. Academic Press, London, UK. 393 pp.
Johnson C T. 1981. Induction of parturition in cattle. Acta Veterinaria, Scandinavica, Supplementum 77: 311-319
Irvin H J. Randel R D, Haensly W E and Sorensen A M Jr. 1978. Reproductive studies of Brahman cattle. III. Comparison of weight, progesterone content, histological characteristics and 3 ß-hydrogenase activity in corpora lutea of Brahman, Hereford and Brahman x Hereford heifers. Theriogenology 10: 417-427.
Kaltenbach C C, Dunn T G. Kiser T E, Corah L R. Akbar A M and Niswender G D. 1974. Release of FSH amd LH in beef heifers by synthetic gonadotropin releasing hormone. Journal of Animal Science 38: 357.
Karsh F J. Legan S J. Ryan K D and Foster D L. 1978. The feedback effects of ovarian steroids on gonadotropin secretion. In: D B Crighton, G R Foxcroft, N B Hynes and G E Lamming (eds), Control of ovulation Butterworths, London. pp. 29-48.
Kato J. Atsumi Y and Inaba M. 1974. Estradiol receptors in female rat hypothalamus in the developmental stages and during pubescence. Endocrinology 94: 309-317.
Kesler D J. Weston P G. Pimental C A, Vincent D L and Hixon J E. 1981. Diminution of the in-vitro response to luteinising hormone by corpora lutea induced by gonadotropin-releasing hormone treatment of postpartum suckled beef cows. Journal of Animal Science 53: 749-754.
Kindahl H. Edquist L E, Bane L and Granstrom L. 1976. Blood levels of progesterone and 15-keto-13, 14-dihydro prostaglandin F2 a during normal oestrous cycles and early pregnancy. Acta Endocrinologica (Copenhagen) 82: 134-149.
Knickerbocker J J. Thatcher W W. Bazer F W. Drost M, Barron D H. Fincher K B and Roberts R M. 1986. Proteins secreted by Day-16 to -18 bovine conceptuses extend corpus luteum function in cows. Journal of Reproduction and Fertility 77: 381-391.
Knickerbocker J J, Wiltbank M C and Niswender G D. 1988. Mechanisms of luteolysis in domestic livestock. Domestic Animal Endocrinology 5: 91-107.
Kragt C L and Masken J R. 1972. Puberty - physiological mechanisms of control. Journal of Animal Science 34 (Supplement 1): 1.
Laster D B. 1977. A pregnancy specific protein in the bovine uterus. Biology of Reproduction 16: 682.
Llewelyn C A, Munro C D, Luckins A G. Jordt T. Murray M and Lorenzini E. 1987. Behavioural and ovarian changes during the oestrous cycle in the Boran (Bos indicus). British Veterinary Journal 143: 75-82.
Macfarlane J S and Worrall K. 1970. Observations on the occurrence of puberty in Bos indicus heifers. East African Agricultural and Forestry Journal 35: 409-410.
Marion G B and Gier H T. 1971. Ovarian and uterine embryogenesis and morphology of the non-pregnant female mammal. Journal of Animal Science 32 (Supplement 1): 24-47.
Martal J, Lacroix M, Loudes C, Saunier M and Winternberger-Torres S. 1979. Trophoplastin, an antiluteolytic protein present in early pregnancy in sheep. Journal of Reproduction and Fertility 56: 63-73.
Matton P. Adelakoun V, Couture Y and Dufour J J. 1981. Growth and replacement of the bovine ovarian follicles during the estrous cycle. Journal of Animal Science 52: 813.
McCracken J A and Okulicz W C. 1984. Hormone receptor control of pulsatile secretion of PGF2 a from the ovine uterus during luteolysis and its abrogation in early pregnancy. Animal Reproduction Science 7: 31-55.
McDonald L E. 1980. Veterinary endocrinology and reproduction. Lea and Febiger, Philadelphia, USA. 560 pp.
McNutt G W. 1924. The corpus luteum of the ox ovary in relation to the estrous cycle. Journal of the American Veterinary Medical Association 64: 556.
Melampy R M, Hearn W R and Rakes J M. 1959. Progesterone content of bovine reproductive organs and blood during pregnancy. Journal of Animal Science 18: 307-313.
Mukasa-Mugerwa E and Azage Tegegne. 1989. Peripheral plasma progesterone concentration in zebu (Bos indicus) cows during pregnancy and parturition. Reproduction, Nutrition and Development 29(3) (in press).
Mukasa-Mugerwa E, Azage Tegegne, Mattoni M and Cecchini G. 1989. Effect of oestrous synchronization with prostaglandin F2 a in Ethiopian Highland (Bos indicus) cows. Animal Production 48: 367-373.
Odde K G. Ward H S. Kirafoe G H. McKee R M and Kittok R J. 1980. Short oestrous cycles and associated progesterone levels in beef cows. Theriogenology 14: 105.
Odell W D, Hescox M A and Kiddy C A. 1970. Studies on hypothalamic pituitary-gonadal interrelations in prepubertal cattle. In: W R Butt, A C Croke and M E Ryle (eds), Gonadotropins and ovarian development. Longman, Edinburgh and London, UK.
Ojeda S R. Andrews W W. Advis J P and White S M. 1980. Recent advances in the endocrinology of puberty. Endocrine Reviews 1: 228-257.
Onuma H. Hahn J and Foote R H. 1970. Factors affecting superovulation, fertilisation and recovery of superovulated ova in prepubertal cattle. Journal of Reproduction and Fertility 21: 119-126.
Parry D M, Willcox D L and Thorburn G D. 1980. Ultrastructural and cytochemical study of the bovine corpus luteum. Journal of Reproduction and Fertility 60: 349-357
Peters A R. 1984. Reproductive activity of the cow in the postpartum period. 1. Factors affecting the length of the postpartum acyclic period. British Veterinary Journal 140: 76-83.
Peters A R and Ball P J H. 1987. Reproduction in cattle. Butterworths, London, UK. 51 pp.
Peters A R and Lamming G E. 1983. Hormone patterns and reproduction in cattle. In Practice 5: 153-157.
Peters A R and Lamming G E. 1984. Reproductive activity of the cow in the postpartum period. 2. Endocrine patterns and induced ovulation. British Veterinary Journal 140: 269-280.
Peters A R and Lamming G E. 1986. Regulation of ovarian function in the postpartum cow: An endocrine model. Veterinary Record 118: 236-239.
Peters A R and Riley G M. 1982. Milk progesterone profiles and factors affecting postpartum ovarian activity in beef cows. Animal Production 34: 145153.
Peters A R. Lamming G E and Fischer M W. 1981. A comparison of plasma LH concentration in milked and suckling postpartum cows. Journal of Reproduction and Fertility 62: 567-573.
Plasse D, Warnick A C and Koger M. 1970. Reproductive behavior of Bos indicus females in a subtropical environment. IV. Length of estrous cycle, duration of estrus, time of ovulation, fertilization and embryo survival in grade Brahman heifers . Journal of Animal Science 30: 63-72.
Rahe C H. Owens R E, Fleeger J L and Harms P G. 1980. Pattern of plasma luteinizing hormone in the cyclic cow: Dependence upon the period of the cycle. Endocrinology 107: 498.
Rahe C H. Owens R E, Fleeger J L and Harms P G. 1982. Pulsatile secretion of the luteinizing hormone during the estrous cycle of the cow. In: Beef cattle research in Texas. Texas Agricultural Experiment Station, The Texas AM University System, College Station, Texas, USA. pp. 26-27.
Rajakoski E. 1960. The ovarian follicular system in sexually mature heifers with special reference to seasonal, cyclical and left-right variation. Acta Endocrinologica (Copenhagen), Supplementum 52: 7-68.
Rajamahendran R. Lague P C and Baker R D. 1982. Serum progesterone and initiation of ovarian activity in prepubertal heifers treated with progesterone. Canadian Journal of Animal Science 62: 759-766.
Ramirez V D and McCann S M. 1963. Comparison of the regulation of luteinizing hormone (LH) secretion in immature and adult rats. Endocrinology 76: 452.
Ramirez-Godinez J A, Kiracofe G H. Schalles R R and Niswender G D. 1982a. Endocrine patterns in the postpartum beef cow associated with weaning: A comparison of the short and subsequent normal cycles. Journal of Animal Science 55: 153-158.
Ramirez-Godinez J A, Kiracofe G H. Carnahan D L, Spire M F. Beeman K B. Stevenson J S and Schalles R R. 1982b. Evidence for ovulation and fertilization in beef cows with short estrous cycles. Theriogenology 17: 409-414.
Randel R D. 1976. LH and ovulation in Brahman and Brahman x Hereford and Hereford heifers. Journal of Animal Science 43: 300 (Abstract).
Randel R D. 1980. Total serum estrogens before and after estrus in Brahman, Brahman x Hereford and Hereford heifers. Journal of Animal Science 51 (Supplement 1): 2.
Randel R D. 1984. Reproductive endocrinology of Brahman cattle. In: P Chemineau, D Gauthier and J Thimonier (eds), Reproduction des ruminants en zone tropicale. INRA (Institut national de la recherche agronomique) Publication No. 20. pp. 159-188.
Randel R D and Moseley W M. 1977. Serum luteinizing hormone surge and progesterone near estrus in Brahman compared to Brahman x Hereford and Hereford heifers. Journal of Animal Science 45(Supplement 1): 199.
Rawlings N C, Weir B and Todd J. 1980. Some endocrine changes associated with the postpartum period of the suckling beef cow. Journal of Reproduction and Fertility 60: 301.
Rhodes R C III, Randel R D and Harms P G. 1979. Prolactin levels in ovariectomized Brahman, Brahman x Hereford cows following a 20 mg dose of estradiol-17 ß. Theriogenology 12: 85-95.
Rhodes R C III, Randel R D and Long C R. 1982. Corpus luteum function in the bovine: In vivo and in vitro evidence for both a seasonal and breed type effect. Journal of Animal Science 55: 159-167.
Riley G M, Peters A R and Lamming E. 1981. Induction of pulsatile LH, FSH release and ovulation in postpartum acyclic beef cows by repeated small doses of GnRH. Journal of Reproduction and Fertility 63: 559-565.
Roberts R M, Godkin J D, Bazer F W. Fincher K B. Thatcher W W. Knickerbocker J J and Bartol F F. 1985. Antiluteolysins produced by mammalian conceptuses. In: R G Edwards (ed), Implantation of the human embryo. Academic Press, London, UK. pp. 253-282.
Robertson H A and King G J. 1979. Conjugated and unconjugated oestrogens in foetal and maternal fluids of the cow throughout pregnancy. Journal of Reproduction and Fertility 55: 463-470.
Rollinson D H L. 1955. Oestrus in zebu cattle in Uganda. Nature 176: 352.
Russell W L and Douglas P M. 1945. Offspring from unborn mothers. Proceedings of the National Academy of Sciences of the United States of America 31: 402.
Rutter L M and Randel R D. 1984. Luteal competency during the resumption of ovarian cyclicity in postpartum Brahman cows. Theriogenology 21: 713-725
Saiduddin S. Riesen J W. Graves W E, Tyler W J and Casida L E. 1967. Effect of suckling on the interval from parturition to first estrus in dairy cows. Journal of Animal Science 26: 950-951 (Abstract).
Saxena B B. 1976. Gonadotropin receptors. In: M Blecher (ed), Methods for receptor research. Becker, New York, USA. pp. 251-299.
Saxena B B. Rathman P and Dattatreyamurty B. 1984. Isolation and molecular assembly of the luteinizing hormone-human chorionic gonadotropin receptor of bovine corpora lutea. In: B B Saxena (ed), Hormone receptors in growth and reproduction. Raven Press, New York, USA. pp. 63-86.
Schallenberger E, Schams E D and Zottmeier K. 1978. Response of lutropin (LH) and follitropin (FSH) to the administration of gonadoliberin (GnRH) in pregnant and postpartum cattle including experiments with prolactin suppression. Theriogenology 10: 35-54.
Schams D F. Hofer E, Schallenberger E, Hartl M and Karg H. 1974. Pattern of luteinizing hormone (LH) and follicle stimulating hormone (FSH) in bovine blood plasma after injection of a synthetic gonadotropin-releasing hormone (GnRH). Theriogenology 1: 137.
Schams D, Schallenberger E, Hoffmann B and Karg H. 1977. The oestrus cycle of the cow: Hormonal parameters and time relationships concerning oestrus, ovulation and electrical resistance of the vaginal mucus. Acta Endocrinologica (Copenhagen) 86: 180-192.
Schams D, Schallenberger E, Menzer C, Stangl J. Zootmeier K, Hoffman B and Karg H. 1978. Profiles of LH, FSH and progesterone in postpartum dairy cows and their relationship to the commencement of cyclic functions. Theriogenology 10: 453-468.
Schams D, Schallenberger E, Gombe S and Karg H. 1981. Endocrine patterns associated with puberty in male and female cattle. Journal of Reproduction and Fertility (Supplement 30): 103-110.
Schillo K K, Dierschke D J and Hauser E R. 1982. Regulation of luteinizing hormone secretion in prepubertal heifers: Increased threshold to negative feedback of estradiol. Journal of Animal Science 54: 325-336.
Schillo K K, Dierschke D J and Hauser E R. 1983. Estrogen induced release of luteinizing hormone in prepubertal and postpubertal heifers. Theriogenology 19: 727.
Segerson E C, Hansen T R. Libby D W. Randel R D and Getz W R. 1984a. Ovarian and uterine morphology and function in Angus and Brahman cows. Journal of Animal Science 59: 1027-1046.
Segerson E C, Libby D W. Getz W R and Randel R D. 1984b. Immunosuppressive effect of uterine secretory protein from Angus and Brahman cows upon lymphocytes in vitro. Journal of Animal Science 59 1047-1059.
Shelton J N and Summers P M. 1983. Progesterone and estradiol-17 ß in peripheral plasma of Brahman cows with single and twin pregnancies. Theriogenology 20: 47-52.
Shemesh M, Ayalon N and Mazor T. 1979. Early pregnancy diagnosis in the ewe, based on milk progesterone levels. Journal of Reproduction and Fertility 56: 301-304.
Short R E, Randel R D, Staigmiller R B and Bellows R A. 1979. Factors affecting oestrogen-induced LH release in the cow. Biology of Reproduction 21: 683-689.
Smith J F. Fairclough R J. Payne E and Peterson A J. 1975. Plasma hormone levels in the cow. 1. Changes in progesterone and oestrogen during the normal oestrous cycle. New Zealand Journal of Agricultural Research 18: 123-129.
Smith M F. Nix K J. Kramer D C, Amoss M S. Herron M A and Wiltbank J N. 1982. Fertilization rate and early embryonic loss in Brahman crossbred heifers. Journal of Animal Science 54: 1005-1011.
Smith M F. Lihman A W. Lewis G S. Harms P G. Ellersieck M R. Inskeep E K, Wiltbank J N and Amoss M S. 1983. Pituitary and ovarian responses to gonadotropin-releasing hormone, calf removal and progestagen in anestrous beef cows. Journal of Animal Science 57: 418-424.
Smith R A, Branca A A and Reichert L E Jr. 1985. The subunit structure of the follitropin (FSH) receptor. Photoactivity labelling of the membrane-bound receptor follitropin complex in situ (cattle). Journal of Biological Chemistry 260: 14297-14303.
Spellberg T C, Littlefield B A, Seelke R. Dhni G M, Toyodo H. Boyd-Leinen P. Thrall C and Kon O L. 1983. Role of specific chromosomal proteins and DNA sequences in the nuclear binding sites for receptors. Recent Advances in Hormone Research 39: 463-514.
Stellflug J N. Randel R D and Moody E L. 1978. Plasma estrogens in periparturient beef cows. Journal of Animal Science 37: 330 (Abstract).
Stevenson J S. Spine M F and Britt J H. 1983. Influence of the ovary on estradiol-induced luteinizing hormone release in postpartum milked and suckled Holstein cows. Journal of Animal Science 57: 692-698.
Swanson L V, Hafs H D and Morrow D A. 1972. Ovarian characteristics and serum LH, prolactin, progesterone and glucocorticoid from first estrus to breeding size in Holstein heifers. Journal of Animal Science 34: 284-293.
Thatcher W W. Bartol F F. Knickerbocker J J. Curl J S. Wolfenson D, Bazer F W and Roberts R M. 1984. Maternal recognition of pregnancy in cattle. Journal of Dairy Science 67: 2797-2811.
Tribble R L, Sorensen A M, Woodward T L, Connor J S. Beverly J R and Fleeger J L. 1973. Serum progestins and luteinising hormone levels in non-suckled primiparous heifers. Nature 246: 495.
Troxel T R and Kesler D J. 1984. Effect of progestin and GnRH treatments on ovarian and reproductive hormone secretions of anestrous postpartum suckled beef cows. Theriogenology 21: 699-711.
Troxel T R. Kesler D J. Noble R C and Carlin S E. 1980. Ovulation and reproductive hormones following steroid pretreatment, calf removal and GnRH in post-partum beef cows. Journal of Animal Science 51: 652-659.
Troxel T R. Conarik G F. Ott R S. Lock T F and Kesler D J. 1983. The effect of method of GnRH administration and short-term calf removal on ovarian function and reproductive performance in postpartum suckled beef cows administered PGF2 a for estrus synchronization. Theriogenology 20: 417 433.
Troxel T R. Opsomer M J and Kesler D J. 1984. The effect of days postpartum, indomethacin and oxytocin on prostaglandin metabolite concentration in postpartum suckled beef cows. Theriogenology 22: 187196.
Vaca L A, Galina C, Fernandez Baca S. Escobar J and Ramirez B. 1983. Progesterone levels and relationships with the diagnosis of a corpus luteum by rectal palpation during the estrous cycles in zebu cows. Theriogenology 20: 67-76.
Veenhuizen E L, Erb R E and Gorski J. 1960. Quantitative determination of free oestrone, oestradiol-17 ß, and oestradiol-17 a in bovine fecal cotyledons. Journal of Dairy Science 43: 270.
Walters D L, Short R E, Convey E M, Staigmiller R B. Dunn T C and Kaltenbach C C. 1982a. Pituitary and ovarian function in postpartum beef cows. II. Endocrine changes prior to ovulation in suckled and non-suckled postpartum cows compared to cycling cows. Biology of Reproduction 26: 647-654.
Walters D L, Short R E, Convey E M, Staigmiller R B. Dunn T C and Kaltenbach C C. 1982b. Pituitary and ovarian function in postpartum beef cows. III. Induction of oestrus, ovulation and luteal function with intermittent small-dose injections of GnRH. Biology of Reproduction 26: 655-662.
Wendorf G L, Lawyer M S and First N L. 1983. Role of the adrenals in the maintenance of pregnancy in cows. Journal of Reproduction and Fertility 68: 281-287.
Wettemann R P, Hafs H D, Edgerton L A and Swanson L V. 1972. Estradiol and progesterone in blood serum during the bovine estrous cycle. Journal of Animal Science 34: 1020-1024.
Williams G L, Kirsh J D, Post G R. Tilton J E and Slanger W D. 1984. Evidence against chronic teat stimulation as an autonomous effector of diminished gonadotropin release in beef cows. Journal of Animal Science 59: 1060-1069.
Wiltbank J N, Ingalls J E and Rowden W W. 1961. Effects of various forms and levels of estrogen alone or in combinations with gonadotrophins on the estrous cycle of beef heifers. Journal of Animal Science 20: 341.