Most of the improvement work on sheep in Europe and other temperate regions of the world to which European sheep have been exported, has concentrated on meat conformation and growth or on fleece weight and wool quality. Recently it has been pointed out that this may no longer be the optimum method for increasing lifetime production. Selection of the largest individuals for breeding, without reference to type of birth, may mean selection of singles rather than twins and therefore may in the long run be counterproductive. Spedding et al. (1976) show that reproductive rate is one of the major factors influencing feed conversion efficiency in meat-producing animals. Reproductive rate can be increased in sheep by reducing the age at first lambing, reducing the proportion of barren ewes, increasing the number of lambs per birth, reducing lamb mortality and reducing the interval between successive parturitions. In regard to number of lambs per birth (litter size), Large (1966) investigated in England the relationship between the amount of food consumed by a ewe and her lambs over the entire year and the amount of carcass produced in the same period. Efficiency of food conversion was 35–50 percent higher for ewes with twins than for ewes with singles; ewes with triplets were 20 percent more efficient than those with twins. Turner (1978) has repeated this with Merinos in Australia and also found a 40 percent greater efficiency for ewes with twins.
Unfortunately the heritability of litter size, like that for other aspects of reproduction, is usually found to be low. This means that progress from selection is likely to be slow. Nevertheless if observations are restricted to adult and to fertile ewes heritabilities of litter size of up to 40 percent have been calculated and indeed considerable progress has been made in selection for this character (Turner and Young, 1969).
There is an alternative to selection, namely to cross with a known prolific breed. There are breeds in Europe which have a high prolificacy, presumably as a result of earlier selection. The “discovery” of the Finnish Landrace and its export, for the first time, to Edinburgh, Scotland, in 1962 was one of the first indications of the rising interest in high fertility (Maijala and Österberg, 1977). In the following year the similar Romanov brought from Yaroslavl, U.S.S.R., was used experimentally in France (Ricordeau et al., 1978).
On account of its high prolificacy Finnish sheep have been imported to over 30 countries all over the world, but chiefly Europe and North America, either for experimental purposes or to increase the fertility of local breeds by crossbreeding. First crosses are intermediate in litter size between Finnish and local breeds with little or no heterosis (Jakubec, 1977).
In several countries (Ireland, France, U.K., U.S.A.) new breeds are being formed or have been formed combining the prolificacy of the Finnish (or Romanov) with the desirable characters (usually meat or milk production) of the local breeds.
To a lesser extent the Finnish and Romanov breeds have been tried experimentally in some tropical and sub-tropical countries with the aim of improving litter size. However, there are no reports of favourable results from these experiments. There is reason to belive that the lack of adaption of these north European breeds to the high temperatures, irregular feed supplies, local diseases and less satisfactory management conditions in the receiving countries, has prevented the prolific breeds from exhibiting their potential. Certainly there is plenty of evidence that rams of thin-tailed breeds are incapable of mating with fat-tailed ewes, that the libido and fertility of rams from temperate countries are adversely affected by heat and that European breeds are more sensitive to trypanosomiasis and helminthiasis than tropical breeds.
Therefore, if genetic improvement in prolificacy is desirable - and this is a point to which we must return later - it would appear more sensible either to achieve it by selection, or to import a prolific breed from an environment as similar as possible to that of the breed to be improved. The present review has shown that there are such breeds in the tropics and subtropics but, like the Finnish and the Romanov, they have been unknown or unappreciated outside their own country, and sometimes within it.
The reproductive characteristics of the prolific tropical and subtropical breeds reviewed herein, together with those of the temperate breeds, are summarized in Table 24. It is clear that, in addition to their high litter size, these breeds are similar in having a low age at first lambing and a short interval between lambs. Even the temperate breeds can lamb every seven months although this is unusual. In addition, less complete figures show that these breeds have a high conception rate e.g. 98.5 percent for 3184 adult Finnish Landrace ewes (Maijala and Österberg, 1977). The males have a large testis size, high libido and early sexual maturity.
These similarities in reproductive performance do not reflect genetic similarity in other characteristics. The breeds listed in Table 24 are of every possible type as regards appearance and production. The two West Indian breeds are hair sheep; the others are coarse-wooled sheep. The East Javanese, Hu-yang, Svanka and Chios are fat-tailed and the rest are thin-tailed. Within these groups there are differences in the size of the fat tail and the length of the thin tail. There are also differences, both within and between breeds, in colour, size, type of fleece and presence or absence of horns.
Nor can the prolificacy be related to a similar physical environment. The breeds listed come from eight countries in three continents and the climates in which they are found range from cool temperate (Finland and Russia) through subtropical (Mediterranean and China) to humid tropical (West Indies and Java).
Furthermore each of these groups of breeds forms an island in the middle of populations either of different breeds, or breeds which are phenotypically similar but do not show the prolific characteristic. The prolific West Indian breeds are of the same general type as the hair sheep of Cuba, Mexico, Colombia and Brazil but none of these has a twinning rate of more than 30–40 percent at the most. The prolific Indonesian sheep have as neighbours the wooled sheep of Malaysia and, further away, the hair sheep of Sri Lanka and southern India.
|Breed||Country||Age at first oestrus (months)||Age at first lambing (months)||Lambing interval (months)||Litter size|
|First lambing||Later lambings|
|White Virgin Island||13–14||6–8||1–2|
|East Javanese Fat-tailed||Indonesia||6–12||6–9||1.6|
Sources: (1) Maijala and Österberg, 1977
(2) CBABG (n.d.)
(3) Ricordeau et al., 1978
These breeds normally produce only single lambs. The D'man of Morocco is quite different in both appearance and fertility from all the other breeds of Morocco and Algeria. All the sheep of China (excluding Tibet) are coarse-wooled and fat-tailed but only the Han-yang and Hu-yang are prolific. The Svanka and Imeritian of Georgia resemble the other Caucasus breeds in fleece and tail type but differ in their high prolificacy. The Chios is quite special among the coarse-wooled fat-tailed breeds of Greece and Turkey. The other (thin-tailed) prolific Greek breeds show no similarity in fleece, tail, horns or conformation. The Finnish Landrace and Romanov belong to the North European Short-tailed group but the other breeds in this group (Iceland, Shetland, Old Norwegian, Swedish Landrace, Russia, Northern Short-tailed) do not show exceptional prolificacy.
For a common thread linking the prolific breeds we must look to the way they are kept. All these breeds are kept in small flocks in agricultural areas; none is a range sheep. In Barbados flock size is 5–10 sheep; they are usually tethered. In Java each farmer owns 3–5 sheep; they graze by day and are brought back at night into pens near the houses where they are fed. In Morocco the D'man sheep are found in the pre-Saharan oases where they are kept in small groups of 1–3 ewes which are almost permanently housed; in the rest of the country sheep are herded in large flocks. The two Chinese breeds are in the agricultural areas of the east and well away from the range areas of the north and west. The Hu-yang are kept throughout the year in small sheds attached to the homesteads. In Greece the prolific breeds all originated on islands where they are kept on a family scale. Most of the Skopelos and Kymi sheep are kept in flocks of 2–6; they are grazed by day and brought home at night. Sakiz sheep in Turkey are kept in groups of 2–4 animals. During most of the year they are fed in fruit and vegetable gardens; in winter they are kept and fed in simple stalls. In Finland the pasture season is only 4–5 months and for the rest of the year sheep are kept and fed indoors; 85 percent of flocks have only 1–4 adult sheep (Goot, 1973).
It would appear that keeping these sheep in close association with the household has made possible the careful observation which is needed to make selection effective, especially if it is done by eye and memory rather than on the basis of written records. The various aspects of fertility in sheep have a reputation for low heritability. However, Turner and Young (1969) have shown that if the selection is on number of lambs per ewe lambing (rather than overall lambing rate including barren ewes) and if first lambings are considered separately from later lambings, heritability can be considerably increased.
Furthermore high prolificacy seems to be one of a group of related reproductive characters which are shared by the prolific breeds, namely early sexual maturity, absence of lactation anoestrus, high libido in males, high conception rate. The common factor linking them would appear to be high level of gonadotropic hormone (or high sensitivity to the hormone). Thus selection may have been on any or all of these characters, not on litter size alone.
Using this technique the litter size of one Australian Merino flock has been increased from about 1.1 to 2.1 in about 25 years of selection (Turner, 1978).
It is therefore tentatively concluded that almost any breed of sheep could be selected for prolificacy and that the principal requirement is that the owners should conceive the idea. To carry it to a successful conclusion they must then have the will, the right management system, the time and the patience.
Owing to the small flock size and limited distribution the numbers of some of the prolific breeds described here are very low and in many cases they are declining. For the following breeds conservation programmes are urgently needed.
White Virgin Island - There are only a few thousand sheep on the U.S. Virgin Islands and not many more on the U.K. Virgin Islands. Many of these are of other breeds or are crossbreds. The breeders need government encouragement which should be conditional on the abandonment of crossbreeding and the initiation of a selection programme for prolificacy and meat production and against wool and horns which are indications of European blood. The scourage of stray dogs should be tackled systematically and above all a marketing system should be developed which will enable the local producers to compete with organized imports.
Bahama native - This interesting breed is receiving attention at the station of the Bahamas Agricultural Research, Training and Development Project on North Andros Island with gratifying results. It is to be hoped that the new project on Long Island will continue to explore its potential while at the same time implementing a programme for selection and development.
Greek breeds - There are about 10,000 sheep on the island of Zakynthos but many are crossed with East Friesian or Greek Zackels from the mainland. The total number of sheep on the island of Skopelos is 850 of which 250 are of the Skopelos breed or its crosses; possibly purebred Skopelos sheep on the island number only 100. However some large flocks have now been formed on the mainland. The Kymi breed numbers only 1,200 and is declining. It is confined to about a dozen village south of Kymi town on the island of Euboea. It is very similar to the Skopelos breed from which it may have been derived.
These Greek breeds form a unique genetic resource of thin-tailed prolific dairy sheep adapted to a Mediterranean environment. Their breeding should be subsidized, especially on the islands themselves, so that a source of purebred stock is available. At the same time for each breed a recording system should be established or supported - particularly for milk yield and prolificacy - so that there is a sound basis for continued selection. The Skopelos and Kymi breeds are so similar that they should be treated as the same breed, thus increasing the numbers available for recording and selection. They should be selected for milk yield, prolificacy and lamb survival, as well as for wool production. The Zakynthos breed should be selected for mutton production as well as for milk and lamb production. Crossbreeding on the islands should be discouraged - it may possibly increase milk yield but it will certainly reduce prolificacy and adaptability.
Experiments should be started or continued on the mainland to explore the use of these breeds in crossbreeding. All three should be useful in increasing milk yield and fertility in local mainland breeds. They might also be useful as a first cross in a stratified system of crossbreeding for meat production (like the Border Leicester in U.K.) and the Zakynthos might also be a useful terminal crossing breed.
For other prolific tropical breeds numbers remain reasonably high and comparatively stable. This refers to the Barbados Blackbelly, the Priangan and East Javanese Fat-tailed, and the D'man. For these breeds the important thing is to monitor their progress to make sure that they are not contaminated by ill-conceived crossbreeding policies introduced in the name of progress. Before any crossbreeding with temperate breeds is started in village herds it must be tested experimentally under ordinary feeding and management conditions and the results must be analysed on an economic basis so as to include reproductive rate and viability as well as growth rate.
There are four ways in which prolific breeds can be exploited:
They can be bred pure in the environment in which they are found.
The females can be used as foundation stock for crossing with males of improved meat breeds in a commercial crossing system.
The males can be used in crossbreeding to increase the prolificacy of breeds elsewhere.
They can be used to form a new breed by crossbreeding followed by selection.
The tropical breeds reviewed herein form an important genetic resource because of their adaptation to an environment which is supposed to be unsuitable for sheep - namely the humid or subhumid tropics. This section thus has relevance not only to prolific breeds but to all the sheep in the climates designated Aw (tropical savanna), Am and Af (tropical rainforest) in Köppen's classification (Trewartha, 1957). This includes the hair sheep of tropical America, the hair sheep of West and west equatorial Africa, the hair sheep of south India and Sri Lanka, and the wool sheep of southeast Asia.
That sheep can thrive in these climates is shown by the 3 million sheep in Java. Their adaptation appears to be physiological rather tha morphological. A wooled fleece is present in southeast Asia but not elsewhere. Fat tails occur only in East Java. Although there has been little experimental work to prove it, circumstantial evidence would indicate that this adaptation involves heat tolerance, disease resistance, particularly to gastro-intestinal parasites, and tolerance of a low and irregular nutritional level. It is an adaptation which has been acquired by natural selection over a long period of time and may turn out to be the single most important economic characteristic of sheep in these environments kept under current management systems. Therefore it should not be lightly ignored. In particular it should not be jeopardized by crossing with so-called improved breeds which exhibit the more showy traits of large size, good mutton conformation or heavy fleece.
Therefore the primary breeding programme for the local breeds in these regions should be selection of the purebred for an accentuation of its characters of fitness (fertility and viability) combined with an improved meat production (chiefly growth rate). -The need for such a policy has been appreciated in some countries - notably in Mexico where the Tabasco sheep are bred pure and there is not even any experimental crossing.
In other countries programmes for the study and improvement of the local breeds have barely started. They are particularly needed in Brazil, Cuba and Colombia. In each country a census is needed of the number, types and distribution of the sheep and their breeding structure. Their performance should be studied both in the field and in government stations under “reasonably acceptable” management levels, i.e. improved only to the extent that may be practicable on farms within the foreseeable future. At the same time a selection programme for fertility, viability and growth rate should be undertaken, preferably in the field, but if this is not possible, then in a series of government stations.
Lamb mortality is the major problem in all these regions. Research should be continued or initiated to discover its causes and explore simple husbandry methods to reduce it e.g. later weaning, later age at first lambing, longer lambing interval etc.
It must always be borne in mind that these tropical breeds are an important resource not only in their original habitat but also in countries with a similar climate which lack sheep or lack prolific sheep. Thus the Pelibüey from Cuba has spread most successfully in the tropical areas of Mexico. The Barbados Blackbelly has been imported into many Caribbean islands and mainland American countries. In the same way the Priangan and East Javanese Fat-tailed could be used with advantage in other countries of southeast Asia.
It it is desired to obtain the advantages of the improved breeds it is probably best to do so by a system of commercial crossing. In this system the local breed- again either prolific or not - is partly bred pure to supply replacements, and partly bred to rams of exotic mutton or mutton/wool breeds. Experimental work on these lines is being carried out in Barbados, and Rastogi (Section 2.1) describes a possible stratified breeding scheme using Dorset and Suffolk rams for crossbreeding.
The crossbred animals must be given special treatment and will all be slaughtered for meat. They will need a high level of feeding, good housing, and regular deworming, dipping and foot-bathing.
If conditions are favourable the F1 ewes may be crossed again with a second mutton breed in order to produce three-quarter-bred European lambs for slaughter. It is important that the animals with the European blood should not be kept for further breeding since they lack the adaptation of the local breed.
In Jamaica (Mason, 1978) such a crossbreeding programme is being explored experimentally using several imported breeds: Suffolk, Polled Dorset, Rambouillet, Columbia, and various crossbreds involving Columbia and Finnish Landrace. The basic local breed is the St Elizabeth, a wooled Criollo type. The present scheme is to mate the St Elizabeth sheep to Rambouillet, Dorset or Columbia rams. The Rambouillet is used because of its subtropical origin, the Dorset because of its extended breeding season. These F1 ewes will be distributed to farmers. They can use a Suffolk ram if a high standard of feeding and management is available; otherwise they are recommended to backcross to the St Elizabeth. Wool is an important product as well as meat - hence the crossing with improved wool-meat breeds is doubly important.
For such a commercial crossing system it is essential to use as the female line the local adapted breed. It is not essential that this be a prolific breed but the advantage of such a breed is that fewer ewes are needed to breed purebred replacements and more can be used for breeding the more profitable crossbreds.
This is one of the ways in which the Finnish Landrace breed is used in temperate countries. For tropical and sub-tropical countries one of the breeds described in Sections 2.1, 2.2, 3.1 and 4 would be more appropriate. Indeed it is partly with the aim of using these breeds in just this way that the conservation and improvement programmes have been recommended for the White Virgin Island and the Greek breeds (see Section 5.3). The Barbados Blackbelly is already being used in this way in the Caribbean. The Priangan and East Javanese Fat-tailed could be so used in southeast Asia.
However, before such a programme is instituted, it must be established that an increase in prolificacy is in fact desirable. In steppe and semi-desert country a ewe cannot be expected to rear more than one lamb per year. Nor must it be assumed that it is necessarily the genetic potential which is the limiting factor - Dr. Allenby has shown in the FAO Sheep and Goat Project in Kenya that frequent drenching against worms can increase the lambing rate of the local Masai sheep by over 50 percent.
This is also a system which can apply to both prolific and non-prolific adapted breeds. Its aim is to combine the adaptive characters fitness of the local breed with the improved meat characters of an imported exotic breed. Its justifications, as compared with commercial crossbreeding, are: 1. It does not involve the keeping of purebred exotic sheep to supply males for crossbreeding. 2. It is easier for small farmers to operate a one-breed system than to keep part of their flock for breeding purebred replacements and for producing commercial crosses. (But there could be division of labour within the industry with some flocks breeding purebred replacements and others producing crossbreds).
Genetically this breeding system implicitly assumes that additive gene action is more important than hybrid vigour.
This is the plan adopted by Cuba where a new breed is planned with Suffolk and Pelibüey blood (see Section 2.4). There is also some crossbreeding in Java but it is not clear whether the aim here is an infusion of exotic blood (Suffolk, Dorset and Merino) or a commercial crossing scheme. It will be interesting to see the results of the Dorset x Fat-tailed crossing experiment in Yogyakarta. The preliminary results from new breed formation in Sri Lanka (Buvanendran, 1978) are not encouraging and may lead to a review of the policy of introducing European breeds; certainly the present emphasis is on Indian breeds.
When the results of these crossbreeding experiments are available the pros and cons must still be carefully weighed. Can the ordinary subsistence farmer give the imported breeds and their crosses the extra attention they need? Can he afford the medicaments necessary for regular dipping against ticks and lice, drenching against worms, and treatment against foot-rot? Can he supply the extra food and better housing needed by a more exigent animal? Or would he be better to rely on the natural resistance of his local breed and the wide ranging grazing which minimizes the work build-up?
About three subtropical breeds mentioned above, namely the Svanka and Imeritian of U.S.S.R. and the Hu-yang of China, no new or first-hand information has been obtained. When an opportunity arises it should be grasped in order to make a thorough study of these breeds and assess whether they have a role to play in improving sheep production either within or outside their countries of origin. The Chinese study should also include the Han-yang. The Omani breed also needs further study.
The information on most of the other breeds is sufficient to make clear their value in their present habitat. However, we know little about their comparative performance or their behaviour on being transferred to a new environment. These things must be known before advice based on firm facts can be given to those wanting to use these breeds for increasing the prolificacy of their own sheep.
It is therefore suggested that a project be established where several of these breeds can be bred under the same conditions and compared with each other and with the Finnish Landrace (or Romanov). The country chosen should be one with its own prolific breed. It would have to be intermediate in climate between equatorial and cool temperate and it would have to have liberal veterinary regulations about the importation of sheep from other countries. The institution at which this experiment was conducted would have to be suitably equipped with an experimental farm and laboratories. It should have well qualified staff who already have worked on the production, reproduction or genetics of prolific sheep.
Either Greece or Morocco would be a suitable country. In Morocco a suitable institution would be the Institut National Agronomique et Vétérinaire Hassan II at Rabat. They are currently working on the D'man sheep and would be prepared to establish a Centre for the Study of Prolific Sheep. The Institute has good laboratory facilities in Rabat (supported by an FAO project) and an active team working on the reproductive physiology of sheep. There is a large experimental farm some distance out of Rabat. Veterinary regulations should not cause any difficulties.
The experiment would by preference by undertaken in both Morocco and Greece in order to detect any genotype environment interaction. Such a development would involve a two-way exchange of breeding material on an international level and Morocco would have to relax its present export ban on D'man sheep in order to make this possible.
The first aim would be to import sheep of one or two other prolific breeds to compare them with the present flock of D'man. The breeds to start with would be the Finnish (or Romanov) and the Chios since they have been most studied and should be the easiest to obtain. Flocks of at least 50 ewes and 10 rams of each breed would be kept under the same conditions and their reproductive and productive performance and their reproductive physiology would be studied. Later their reaction to different treatments (nutritional level, regular deworming etc.) could be compared.
If the first two or three years of the experiment are a success steps should be taken to introduce other breeds, i.e. the Barbados Blackbelly, White Virgin Island, and Javanese. If the Chinese could be interested to send some of their Hu-yang and Han-yang sheep this would indeed be most desirable. The extension to further centres would be foreseen.
The Barbados Blackbelly should be the subject of a thorough-going improvement programme based on recording throughout the island flock.
The White Virgin Island should be conserved and selected for prolificacy and meat production.
The Bahama native should be selected for prolificacy and growth rate.
The Zakynthos, Skopelos and Kymi breeds of Greece should be conserved and selected.
Adapted local breeds with large populations (e.g. the Colombian Africana, Morada Nova of Brazil, Pelibüey of Cuba) should be further studied and selected.
The Svanka and Imeritian breeds of the U.S.S.R. and the Han-yang and Hu-yang breeds of China should be studied.
A Centre (or Centres) for the Study of Prolific Sheep should be established in order to compare the characteristics of different prolific breeds - temperate, subtropical and tropical.