3. MILK PRODUCTION

Growth of lambs depends essentially on the milk production of the ewes and their maternal qualities, especially during the first four weeks of age, when the lamb's nourishment is exclusively milk. It is therefore essential to estimate the quantity of milk furnished by the ewe and consumed by the lamb until weaning to perfect new management techniques leading to the improvement of flock productivity.

Milk production was studied in housed (Bonsma, 1939; Wallace, 1948; Hugo, 1952; Thomson and Thomson, 1953; Gyer and Dyer, 1954) and in grazing ewes (Barnicoat et al . 1949; Owen, 1957). Moreover, it has been demonstrated that ewe milk yield can be affected by many factors such as breed, age, sex, birth weight and number of suckled lambs (Bonsma, 1939; Wallace, 1948; Owen, 1953; Munro, 1955; Rlcordeau et al. 1960; Folman et al. 1966; Peart et al. 1972). On the other hand, high correlations between dam milk production and lamb growth rate were found by Wallace (1948), Ricordeau and Boccard (1961), Doney and Munro (1962) and Poujardieu (1969).

Two experiments on the Barbary breed at the Experimental Station of Ousseltia studied the effects of age of ewes, sex of lambs and milk yield under grazing conditions (Khaldi, 1979) and the influence of nutrition in late pregnancy and in lactation on milk production of ewes and growth rate of their progeny (Khaldi, 1983).

3.1 Grazing ewes

The aim of this experiment was to study the effect of the age of ewes and the sex of lambs on the milk yield of the former and the growth rate of the latter. Fourty red-race females were used and divided into 4 groups:

Adult and young females (yearlings) were 5 and 2 years old respectively. The animals were grazed on natural pastures of mainly couch-grass and did not receive any supplementation, the year having been particularly favourable. Ewes were weighed monthly and lambs weighed at birth just after parturition and then weekly. The ewe milk yield was estimated indirectly by weighing lambs before and after suckling (Ricordeau et al. 1960) during the first 13 weeks of lactation.

3.1.1 Changes in ewe weight

The change in ewe weight (Figure 8) showed clearly that they were undernourished during both pregnancy and the suckling period. This is the case of all flocks managed under extensive management conditions. In fact, adult ewes were heavier than yearlings at mating in May (55 vs 45 g). Figure 8 shows that all the females kept a constant liveweight during the first 3 months of pregnancy and then gained weight until lambing. Nevertheless, this weight gain in late pregnancy resulted exclusively from the growth of the pregnant uterus since all females lost about 4.2 g between mating and parturition (liveweight after lambing - liveweight at mating). Furthermore, the liveweight of all the suckling females decreased during lactation but the weight loss was significantly higher (P < 0.05) in adult than in young ewes (11.8 vs 8.7 g).

3.1.2 Milk production

The sex of lambs had no significant effect on ewe milk yield. However, total milk production was significantly affected (P < 0.05) by the age of the females (Figure 9; Table 2). Milk production was estimated at about 91 and 80 g in adult and young ewes respectively.

The relationship between the liveweight of the females at mating and their milk production was significant (P < 0.05) only during the last 4 weeks of lactation (r = 0.50).

Table 2: Comulative milk production (g)

Source: Khaldi (1979).

3.1.3 Growth of lambs

The mean growth rate of lambs was not significantly affected either by their sex or the age of their ram. Nevertheless, male lambs issued from adult ewes tended to have a higher growth rate than the other lamb categories during the first 9 weeks of age (Table 3).

Table 3: Growth rate of Barbary lambs (g/day)

Source: Khaldi (1979).

Lamb growth rates were relatively high (about 230 g/day) during the first month after birth but they decreased dramatically to 180 g/day during the following two weeks and to only 95 g/day between the 8th and the 13th week. These data reflect the insufficiency of natural pastures available to the lactating females and the necessity of supplementation by concentrates, at least after lambing.

Contrary to growth rate, lamb liveweight was significantly (P < 0.05) affected by both their sex and age of the dam. Male lambs were heavier at birth and subsequent ages than ewe-lambs with both adult and young mothers (Figure 10). The effect of sex became more pronounced with age only in lambs produced by adult ewes. On the other hand, lambs issued from ewes were significantly (P < 0.05) heavier at birth than those issued from yearlings. However, the maternal influence, of great importance during the first month of life, decreased in relative value with age of lambs.

Phenotypic correlations between lamb birth weight and liveweight at different ages were high and significant (P < 0.01), but the higher correlation coefficients were obtained during the first 4 weeks of age (r = 0.80 to 0.90). This result demonstrates the close relationship between pre-natal and post-natal lamb growth when extensively managed.

3.1.4 Relationship between milk yield and lamb growth

The correlation coefficient between dam milk production and lamb weight gain during the first 6 weeks of age was 0.722. It increased slightly to 0.732 when weight gain was substituted by the corresponding liveweight. These two similar values are statistically significant (P < 0.01). Thus, the liveweight of lambs at this age is a good means to estimate milk production of ewes as well as their weight gain since birth. Linear regressions connecting these different variables are:

Y = 5.406 X₁ - 2.944

and Y = 4.498 X₂ - 12.173

with:

Y = ewe milk production during the first 6 weeks of lactation (g)

X₁ = lamb weight gain between birth and 6 weeks of age (g)

X₂ = lamb liveweight at 6 weeks of age (g).

The sex of lambs had no significant effect on their milk-to-weight-gain ratio, but males tended to convert better maternal milk to meat than females (Table 4). This ratio decreased slightly during the first 2 weeks and then increased until the 6th week. This increase was certainly related to the increase of lamb liveweight and nutritional requirements. It decreased again after six weeks probably because the lamb started to graze and was no longer dependent on the dam.

Table 4: Milk-to-weight-gain ratio of Barbary lambs

Source: Khaldi (1979).

3.2 Effect of nutrition in late pregnancy and in lactation on ewe milk yield and lamb growth

Ewe nutritional requirements increase rapidly in late pregnancy and in lactation and it is often difficult to cover them. Generally, these two physiological stages occur in late summer or in autumn, which frequently are the dry seasons in North Africa when sheep are undernourished.

An experiment was peformed at the Experimental Station of Ousseltia on 80 red-head females to study the influence of nutritional level in late pregnancy and in lactation on the performances of Barbary ewes (Khaldi, 1983). Two nutritional levels, high (H) and low (L), were used during the last 6 weeks of pregnancy and the first 13 weeks of the suckling period on 4 groups of 20 adult ewes: LL, LH, HL and HH. During all the experimental period, the females were grazed on dry natural pastures and only the well-fed animals received lucerne hay ad libitum supplemented by 400 g of concentrate/ewe/day. Ewes were weighed weekly, just before lambing and one day after. Lambs were weighed at birth and then every week. Ewe milk production was estimated weekly by weighing lambs before and after suckling according to the method of Ricordeau et al. (1960).

3.2.1 Supplementary feed consumption of ewes

In late pregnancy, the quantity of hay consumed by the well-fed ewes (HH and HL) decreased progressively until parturition (Table 5). During the first 13 weeks of lactation, and in addition to pastures, the females of the HH and HL supplemented groups consumed 110.13 and 114.90 hg of dry matter of hay/ewe. In both periods, the supplemented animals consumed all the concentrate they received (400 g/ewe/day).

Table 5: Quantities of supplementary feed intake consumed by groups HH and HL in late pregnancy (kg/ewes/day)

Source: Khaldi (1983).

3.2.2 Changes in ewe weight

Ewe weight gains during the last 6 weeks of pregnancy were higher in well-fed (HH and HL) than in under-fed groups (LL and LH) , but the difference between the values of these weight gains was not significant. Nevertheless, the effect of feed intake level in late pregnancy became more important (P < 0.05) when the proper weight change of ewes was considered (weight 24 hours after parturition - weight 6 weeks before parturition). In fact, a weight gain of 3.6 and 2.1 g was recorded in supplemented ewes (HH and HL) with 1 and 2 foetuses respectively. In contrast, all the under-fed females lost between 2.0 and 5.3 g of their body mass during the same period according to litter size.

The average weight loss of ewes during the first 13 weeks of the suckling period was 4.05, 8.01, 9.90 and 14.10 in groups LH, HH, LL and HL respectively. This weight loss was not significantly affected by the number of suckled lambs. The differences were only significant (P < 0.05) between groups LH and HL on the one hand and beween groups HH and HL on the other.

3.2.3 Milk production

Whatever the nutritional level and the number of suckled lambs, the milk production peak was recorded during the first week of lactation (Figures 11and 12). The nutritional level in late pregnancy had no significant effect on ewe milk yield. In contrast, this milk yield was significantly (P < 0.05) affected by the post- partum feed level. In effect, total milk production was similar in groups HH and LH on the one hand and in groups HL and LL on the other (Table 6). Milk production was higher, but not significantly, in ewes suckling twins than in those suckling single lambs, especially in well-nourished lactating females.

Table 6: Total milk production (g)

Source: Khaldi (1983).

3.2.4 Lamb growth

The birth weight of lambs issued from well-fed ewes in late pregnancy was higher than the birth weight of those produced by under- fed ewes during the same period (Table 7). However, the effect of the pre-partum nutritional level of dams on the birth weight of their progeny was only significant (P < 0.05) with twins.

Table 7: Effect of pre-partum nutritional level of Barbary ewes on lamb birth weight (g)

Source: Khaldi (1983).

In both single and twin lambs, the growth rate was significantly (P < 0.05) affected by post-partum nutritional level of ewes. This growth rate was higher in groups HH and LH than in groups LL and HL (Table 8).

At 10 days of age, the liveweight of single lambs was statistically similar in the 4 groups. At this age, only twins of group HH were significantly (P < 0.05) heavier than those of group LL (Figures 13 and 14). When they were 35 days old, single lambs of both groups HH and LH became significantly (P < 0.05) heavier than those of groups LL and HL. In twins, the sole significant difference (P < 0.05) was recorded between groups HH and LL. When lambs reached the age of 90 days, their liveweight was higher in groups HH and LH than in groups LL and HL, but the differences were significant (P < 0.05) only in the case of single lambs.

Table 8.: Growth rate of lambs (g/day)

Source: Khaldi (1983).

3.2.5 Relationship between milk production of ewes and growth of lambs

Lamb birth weight has very little influence on dam milk production. In fact, the correlation coefficient between these two parameters was at its maximum during the first week (r = 0.28). It then decreased rapidly to reach 0.03 only at the end of the 4th week of lactation. This can probably be related to the high vigour of the Barbary lamb at birth which allows it to suck all the milk present in the dam's mammary gland. On the other hand, there was a high correlation between lamb weight at 35 days of age and ewe milk production during the first 5 weeks of lactation (r = 0.90). Linear regression is:

Y = 4.49 X - 9.17 with:

Y = milk production during the first 5 weeks (g)

X = liveweight of lambs at 35 days of age (g)

The highest correlation coefficient between daily milk production of the ewe and lamb growth rate was observed at the 5th week for singles (r = 0.80) and at the 3rd week for twins (r = 0.59).

The pattern of milk-to-weight-gain ratio is given in Table 9 which shows that the utilization of milk is more efficient in twins than in single lambs on the one hand and in males than in females on the other. Meanwhile, the differences between these three categories of young animals are not significant.

Table 9: Milk-to-weight-gain ratio of Barbary lambs (g milk/1 g gain)

Source: Khaldi (1983).