There are very many situations in which the ideal producing animal is some intermediate between a tropical adapted and an improved temperate breed. This applies particularly to dairy cattle kept under intensive conditions in the humid tropics and in many parts of the wet-dry tropics and to beef cattle under range conditions in the wet-dry tropics and the subtropics. This intermediate can most easily be formed by crossbreeding. Some examples to bear out these statements are given in Sections 6.3.1 and 2.
When it has been established that in a given environment (including climate, disease situation, feeding and management systems) the most productive animal is one containing a proportion of local blood and a proportion of imported genes, then an appropriate breeding system must be chosen which will maintain this intermediate type after the initial crossing. There are several possibilities.
This is also called the formation of a gene pool. After the first cross, F1 animals are mated together to form an F2 followed by an F3, and so on. If the desired proportion of European blood is 75 percent then a backcross is made before the inter se mating starts. The advantage of the gene-pool approach is that the population is self-replacing and after the initial cross no further outside blood is needed.
However, because of the former emphasis in Europe and America on the importance of purebreeding and on uniformity within a breed, there has in the past been reluctance to take the step of attempting to form a new breed especially in a species with a long generation interval. People are afraid that breeding from crossbreds will lead to excessive variation, and therefore that formation of a new breed is only possible if immense resources of animals are available. In fact, the increase in variation on breeding an F2 generation has been much exaggerated. To begin with, it is only the genetic variation which is increased, and with characters of low heritability this is only a small part of the total variation. Secondly, the obvious effects are due to segregation of colour, colour pattern and morphological characters (e.g. horns) which are controlled by single genes. The increase in variation between F1 and F2 for such quantitative characters as growth rate and milk yield will be very small and, in view of other sources of variation, may not be detectable.
Certainly there is a danger that if the initial crossing is not done on a large enough scale then problems of inbreeding may be encountered. This is particularly the case if the breed is based on a single herd or on only one or two imported sires. Inbreeding leads to decline in fertility, vaibility and growth rate to a greater or lesser extent. This has often not been realized and inbreeding has been deliberately employed in order to concentrate the blood of outstanding sires or to produce uniformity in a new breed. But uniformity is an elusive goal, a will-o'-the-wisp. It must not be sought at the expense of productivity. Indeed it is essential at the outset to have as much variation as possible in order to allow scope for selection. The aim of crossbreeding is to combine the high yield of the European breed with the resistance of the local breed, and intense selection is needed to find the few animals combining both characters. If a breed is to be formed in a single experimental herd it is therefore essential to use a large number of unrelated sires (at least 10–15) in the formative stages. As early as possible the programme should be extended to other herds and sires should be exchanged between herds. In the case of dairy cattle a large cow population will be needed for progeny testing the bulls. Above all, animals should not be selected according to colour, conformation or other fancy points but for important economic characters. A programme for the selection of a new dairy breed, the AMZ, has already been outlined in Section 4.2.4.
Where resources for intense selection are not available it would be easier to import a new intermediate breed from elsewhere and grade the local population to this imported breed. Unfortunately such new breeds are rare. Some have been mentioned in Section 5.3.
The advantage of the single breed approach is that in an unsophisticated social environment it is easier for the farmer to deal with one type of animal. He can either keep his own sire or obtain semen from an AI service if available. There is no question of different sires for different females which is the essence of a crossbreeding system.
The major disadvantage of new breed formation is that it is a long-term programme and by the time it yields results the environment or market demand may have changed so that a different type of animal is needed. A systematic crossbreeding system is much more flexible.
The essence of a systematic crossing system is that breeding animals of both pure breeds involved (local adapted and temperate improved) are used in each generation. There are two main categories - terminal crossing and rotational crossing.
1. Terminal crossing
In this type of cross the progeny are not used for breeding. It is therefore not suitable for dairy animals but is important for meat animals. There are several varieties:
Re-creation of the F1 in each generation. Since it is the commercial F1 animals which are produced every generation this is also called “commercial crossing”. Both sexes are slaughtered (for meat) without being used for breeding.
Three-way crossing
Commercial crossing exploits only the hybrid vigour in the growing animal. In order to cash in also on that in maternal ability a second stage of crossing is needed so as to breed from a crossbred dam.
This is very important because a major proportion of the total advantage may come from the use of crossbred dams. The second cross may be a backcross to the sire breed or a cross to a third breed.
This system has its maximum advantage if the crossing is stratified according to the environment. In Britain, for instance, local breeds of sheep are maintained on the mountains; they are crossed with a hardy improver breed and the crossbreds are kept in the hills; the crossbred ewes are crossed with a second improver breed and the second-cross lambs fattened in the lowlands.
A similar scheme could be applicable in the tropics with the environment being superior in each phase in terms of rainfall, temperature or feeding level rather than altitude.
2. Rotational crossing
In crisscrossing (or reciprocal backcrossing) sires of the temperate and tropical breeds are used in alternate generations (see Figure 6.1).
Figure 6.1 Crisscrossing (reciprocal backcrossing) between temperate and tropical breeds
At equilibrium two-thirds of the hybrid vigour of the F1 is retained. In alternate generations progeny will be two-thirds tropical or two-thirds temperate in their genotype. This system is very flexible; if more than an average of 50 percent temperate blood is needed then there can be two generations of crossing with the temperate breed alternating with one cross with the tropical breed. In this case at equilibrium succeeding generations will have 3/7, 5/7 and 6/7 of the temperate blood (see Figure 6.2). Or the tropical breed can be replaced by a halfbred - either a new intermediate breed or a half-bred sire.
Figure 6.2 Rotational crossing between temperate and tropical breeds
A similar result can be achieved by using three different breeds in the crossing. It then becomes rotational crossing and 87 percent of the maximum heterosis is retained.
This system is also flexible in the face of changing market demand - the type of crossing breed can be changed immediately according to the product needed.
Systematic crossbreeding systems need a source of crossing sires. The tropical sires should come from an improved local breed under selection. The temperate sires should also come from nucleus herds selected in the local environment rather than from selection programmes overseas in view of the phenomenon of genotype x environment interaction (see Section 5.5).
3. Combination of terminal crossing and rotational crossing
A meat sire can be used on the crossbred females which emerge in a crisscrossing or rotational crossing system. It is terminal because the offspring of this sire are not used for breeding but are all slaughtered for meat.
In this scheme halfbred sires are produced in each generation and used to grade up the local females. It is thus suitable for situations in which it is desired to stabilize a 50:50 mixture of local and exotic strains. The halfbred sires would have to be bred in a special herd and supplied to the local farmer either on a loan system (so that each was not used too long in one herd) or by means of AI. Lindhé (1978) describes a breeding plan which starts with the pure local breed and uses F1 sires on these females. The female population will in a few generations approach 50 percent European blood. Hickman (1979) has a variant of this scheme which he calls the “repeated hybrid male cross”. The first cross on to the local females is by a pure European sire and the specially bred F1 bulls are then used on these F1 cows. The population is then 50 percent European from the first generation.
The use of crossbred sires (or a new intermediate breed) is the only system possible in a herd too small to maintain two bulls for crisscrossing or with no access to AI.
This scheme has the flexibility of the systematic crossing programmes and the simplicity (one type of sire only) of the gene-pool concept. Indeed it can easily lead to the formation of a new breed if at any time it is decided to use sires bred within the population instead of using the specially bred F1 bulls.
It must be emphasized once more that all these schemes require a source of temperate sires which should preferably be selected in an environment similar to that in which they are to be used. They also require a selection programme for the indigenous breed which is used in the crossing programme.
The various options available in choosing a breeding system are shown diagrammatically in Figure 6.3. This includes the questions we have already discussed, i.e. whether to select in the local population or to cross with an exotic breed, which exotic to use, whether to grade up completely to the exotic or whether to maintain a crossbred population. However, one point is not made clear in this diagram. In conditions too adverse to maintain any proportion of European blood, the best breeding system may be to cross two tropically adapted breeds. Examples of this are given in Section 6.3.
Figure 6.3 Strategic options involving crossbreeding and selection (Cunningham, 1981)
The gains from crossbreeding arise from:
complementarity, i.e. the combination of the adaptation of the tropical breed with the productivity of the improved temperate breed, and
heterosis or hybrid vigour which, whatever its genetic explanation, demands a heterozygous genotype.
The first effect is additively genetic; the second is non-additive.
From a genetic viewpoint the optimal method to maintain a crossbred population depends on the amount of heterosis exhibited in the cross. If heterosis is important then one of the systematic crossing systems should be used. The commercial crossing system exploits 100 percent of the heterosis, the 3-breed rotational cross 87 percent and the criss-cross 67 percent. A new breed, on the other hand, retains only half the heterosis of the first cross and this is the genetically preferable system if complementarity is important rather than heterosis.
In theory heterosis should be greater for those characters which are related to “fitness” and have therefore been under the influence of natural selection for a long time and now have a low heritability. It is also expected to be higher in a stressful environment. In practice it is necessary to test each cross in each situation.
Cunningham (1981) has presented a model which would explain the common experience that heterosis is greater in a stressful than in a favourable environment. His model is shown in Figure 6.4.
Figure 6.4 A model for differential heterosis and additive effects of two breeds in good and poor environments. (Cunningham, 1981)
HET = Heterosis
ADD = Additive effect
In a very unfavourable environment the local breed and the improved exotic breed may not differ greatly in production. The F1 often shows a high performance, indicating heterosis. In a favourable environment, on the other hand, the improved breed will yield nearer to its genetic potential but the local breed does not significantly improve in performance. The F1 is improved by an intermediate amount which may put it only slightly above the mid-parent indicating very modest heterosis. On this model the difference between the F1 and the local strain is largely due to heterosis in the poor environment and to the additive genetic difference between the breeds in the favourable environment. This might be explained by the physiological limitations of stress in the poor environment and of genetic potential in the good one.
With dairy cattle crossbreeding has the longest history and has been most extensive in India. Katpatal (1977) reviewed its results and showed that the highest milk yields were usually reached when the proportion of European blood was between one-half and three-quarters. Figure 6.5 shows this clearly from the results of the crossbreeding between Friesian and Indian dairy zebus (chiefly Sahiwal, Red Sindhi and Hariana) on the Military Dairy Farms. The curves for body weight at one and at two years closely followed that for milk yield, indicating a lack of adaptation in animals of more than ¾ Friesian breeding. At younger ages body weight increased with increasing Friesian blood.
Figure 6.5 Regression of least squares breed group constants for estimates of most probable milk producing ability on fraction of Friesian breeding (Data from Military Dairy Farms in India) ( from Katpatal, 1977)
Fewer results are reported on reproduction and viability. However on Military Dairy Farms, age at first calving, calving interval and service period were generally lowest in the half breds and 5/8 breds. Striking results are quoted on mortality to first calving: it was 28 percent in purebred Sahiwals, and 47 percent in 31/32 bred Friesians; in half-breds and 5/8 breds on the other hand it was only 3–4 percent.
These results are quoted at some length because they refer to several breeds, to large numbers of animals, to a variety of environments many of which are north of the tropic of Cancer, and to well managed government farms. They should therefore have some general application.
Indeed in the semi-arid and coastal areas of Kenya and mainland Tanzania it is now clearly established that the Sahiwal x European dairy cross outyields both purebreds and has a better reproductive record and lower mortality. The optimum proportion of European blood varies according to management between onehalf and three-quarters (Kimenye, 1979; Trail and Gregory, 1981). Even in the high potential areas of Kenya under conditions of poor feeding and management the crossbred often has an advantage in reproductive rate (including fertility, abortions and calf mortality) and may even outyield the purebred European (Meyn and Wilkins, 1974).
In the humid tropics where management is optimal, the yield of the purebred may be better than that of the crossbred but the problems of poor health, low fertility and high calf mortality remain. For instance, at the Thai-Danish farm in Thailand, where the cattle were fed and managed according to Danish standards, the Danish Red purebreds yielded more than the Indian zebu but their fertility and viability were unacceptably low (see Table 6.1). The results of European x zebu crossbreeding in the coastal region of Tanzania also favoured the halfbreds; they were significantly better than both ¼ breds and ¾ breds when evaluated on the basis of the average daily yield of milk between calvings (Mahadevan and Hutchison, 1964).
Table 6.1
PERFORMANCE OF DANISH RED, INDIAN ZEBU (SAHIWAL
AND RED SINDHI) AND THEIR CROSSBREDS IN THAILAND
| Purebred Sahiwal and Red Sindhi | Halfbred | Purebred Danish Red | |
| First lactation yield (kg) | 1 000 | 2 000 | 2 300 |
| Percent abortions | 4.6 | 1.8 | 21.5 |
| Percent stillbirths | 6.6 | 4.4 | 5.1 |
| Mortality to 6 months (%) | 15.5 | 5.9 | 7.3 |
| Mortality from 6 months to | |||
| calving (%) | 5.4 | 0 | 23.7 |
| Calving interval (days) | 467 | 443 | 525 |
Source: Madsen and Vinther (1975)
There is now good evidence for hybrid vigour when zebu and temperate cattle are crossed in the humid tropics. It is most often revealed by the decline in yield and reproductive rate as the crossing proceeds from F1 to F2, from F1 to backcross or from the ¾ bred to the 7/8 bred. If the improvement in yield were entirely due to additive effect of genes the yield should increase as the proportion of European blood increased and the F2 should have the same yield as the F1. This decline from F1 to F2 is shown clearly in the figures that Katpatal (1977) quotes from India. It is particularly striking in Sri Lanka - See Table 6.2.
Table 6.2
HYBRID VIGOUR IN DAIRY CATTLE CROSSES IN
SRI LANKA AS SHOWN BY DECLINE FROM F1 TO F2
| Milk yield (kg) | Calving interval (days) | |
| Friesian x Sinhala | ||
| F1 | 1 573 | 393 |
| F2 | 987 | 448 |
| Jersey x Sinhala | ||
| F1 | 1 215 | 370 |
| F2 | 809 | 412 |
| Jersey x Sindhi | ||
| F1 | 1 929 | 368 |
| F2 | 1 115 | 430 |
Source: Buvanendran and Mahadevan (1975)
This same decline will occur as soon as the first crosses are interbred in the second stage of forming a new breed. It has not prevented work on the formation of new breeds (see Section 4.2.4), but it would suggest that in the humid tropics a systematic crossbreeding system would be genetically more efficient. A reformation of the F1 every generation is not applicable to dairy cattle; nor is a two-stage crossing system. The generation interval is too long and it is the breeding animal which is important, not its progeny for slaughter.
In spite of these shortcomings Madalena (1981b) reports that in the north of Minais Gerais, Brazil, halfbred zebu heifers are produced for sale to more specialized dairying regions. Because of the short productive life of the zebu cows and the high mortality of their calves it is calculated that only 30 percent of the zebu herd could be used to produce F1 calves. It would probably be better to use these herds for crossing with European beef bulls.
On the other hand a crisscrossing or rotational crossing system would be ideal and easy to apply, especially in a zebu x European breed cross. The semen from the two breeds could be distributed by an AI system; the hump of the ⅔ zebu cows would indicate the need for European semen and the zebu semen would be used on the humpless cows (⅔ European). If AI is not available it would be best to separate the two types of cows into different herds and to run a zebu bull with one and a European bull with the other.
Apart from its genetic merits this breeding scheme has several advantages. The total resources of the farm can be used for keeping cows and breeding their replacements. Bulls are not reared; all male calves are slaughtered and bulls (or semen) are brought in as necessary. The continuing genetic improvement stems from these bulls - no recording is needed in the herd except to breed replacement cows from the highest yielders and best breeders. They must be chosen equally from the two types, even though the ⅔ exotic cows may give more milk. Otherwise the proportion of the two types in the herd will become unbalanced. It would be desirable to start the crisscrossing by backcrossing half the F1 cows to the zebu bull and half to the exotic bull. This would enable the two types to be kept in equilibrium from the start.
There may initially be some resistance to such a scheme. After the boost obtained by one generation of crossing with the dairy breed, farmers may be reluctant to return to the zebu bull. However, in some places this crossing system has been gradually developed by the farmers' own experience - for instance in Mexico and in Brazil.
Meyn and Wilkins (1974) recommend a crisscross between Sahiwal and European dairy breeds (Friesian, Simmental, Brown Swiss or Red Poll) for dairy ranching in the semi-arid and coastal regions of Kenya. Likewise Mahadevan (1970) presents evidence of the advantages of a rotational crossbreeding system for the lowlands of Sri Lanka.
If more European blood is needed then the zebu bull is replaced by a halfbred or by a bull from a new halfbred breed. Madalena (1981b) suggests that in Brazil the new Pitangueiras breed (Red Poll x zebu) could be tried experimentally in this role. The extra European blood could also be introduced by using two European breeds to one zebu in a rotational cross. These might well be the Friesian and the Jersey in order to exploit the features of both the two outstanding dairy breeds. In alternate generations the cows would then have the following composition:
4/7 Friesian 2/7 Jersey 1/7 zebu x zebu bull
2/7 Friesian 1/7 Jersey 4/7 zebu x Jersey bull
1/7 Friesian 4/7 Jersey 2/7 zebu x Friesian bull
It will be seen that the bull is used of the breed with the lowest proportion of blood in the cows. Since cows of all three types will in fact be present in the herd at any one time some care will have to be taken that accurate breeding records are kept and the right breed of bull is used for each cow. This demands either AI or segregation of the cows into three separate herds. Each cow should be marked with its breed type.
In the most adverse conditions the local breed must be retained but it might be worth trying a cross with another tropical dairy breed. In Latin America the cross might be between a dairy zebu and a dairy Criollo, in West Africa between a dairy zebu and the N'Dama, in the eastern Mediterranean between a dairy zebu and the Shami breed. In Kenya the initial improvement of the local zebu was with Sahiwal bulls.
All these crossing systems are fairly sophisticated and some are highly so. When the first cross has been successful and the question arises, what to do next, the simplest solution is to use crossbred bulls and it is also satisfactory genetically. Many development projects which started with purebred dairy bulls are now using halfbreds on the crossbred cows e.g. in India, Ethiopia.
For beef cattle it has been clearly demonstrated that there are many pastoral areas in Africa, Australia and tropical America, where the European crossbred performs better than the local adapted cattle or the purebred European beef breed. For instance, Sacker et al. (1971b) showed that Red Poll x Boran crossbred calves were 26–30 percent heavier than the mean of their purebred parents at 3–12 months of age (see Figure 6.6).
Figure 6.6 Body weights of Boran, Red Poll and their reciprocal crosses in Uganda (from Sacker et al. 1971b)
The most striking results have come from Queensland, Australia, where the zebu x European crossbred has excelled the purebred European breed in growth rate, fertility and viability. In these experiments the zebus were Africanders and Brahmans and the European breed was a Shorthorn x Hereford cross. Most of the experimental work compared the crossbreds only with the European parent. Smaller trials showed them to be superior also to the zebu parental breeds except in postweaning and adult mortality (Turner, 1975).
Since beef cattle are more exposed to the environment than are dairy cattle, crossbreeding for beef is advantageous even in climates where the best dairy breed would be a pure European. For instance in the southern United States cows which are crosses between the American Brahman and European beef breeds exceed the average of purebred controls by 27–46 percent when productivity is measured as weight of weaned calf per cow mated (Koger et al., 1973).
All these examples show the high degree of heterosis exhibited when European beef breeds and zebus are crossed in the tropics and subtropics. Madalena (1977) gives examples from Latin America of the improvement in growth rate and reproductive rate when the Criollo is crossed with the zebu or the zebu with a European beef breed. However, in few cases have both parental breeds and the cross (or better the two reciprocal crosses) all been included in the trial so that heterosis is not proved. This is an important point. If only the local breed and its cross with a temperate breed are available, the superiority of the cross should not be interpreted as a reason for continuing to grade to the imported breed. This point was overlooked in South America. The initial improvement from crossing zebus on to the Criollo in the tropical areas has led to grading up to the zebu so that the pure Criollo is now in danger of extinction. But Plasse (1981) has shown that the zebu x Criollo cross is superior to the pure parental breeds in age at first calving, conception rate, birth weight and growth to weaning. For later ages hybrid vigour is not consistently exhibited and at slaughter age the pure zebu is the heaviest of the three types.
These examples would suggest that systematic crossbreeding is the best breeding system for beef cattle in the tropics (and indeed probably also in temperate regions). This is now generally accepted. Nevertheless some new breeds have been formed in the past (see Section 5.3.3) but they have been subjected to several generations of selection to reach the performance level of the original F1.
The simplest crossing system is a straight cross between zebu and European breeds with the slaughter of all the F1 progeny. This might be a Hereford x Boran cross in East Africa, a Hereford or Simmental x Sanga cross in southern Africa, a Chianina or Marchigiana x meat zebu cross in Brazil, and so on.
However, the genetically most efficient system is a terminal cross; this exploits the hybrid vigour in the cows in such characters as fertility and mothering ability. It is a common system in Britain where the Galloway (a hill breed) is first crossed with a white Shorthorn to give the Blue Grey cow and the terminal cross is with a Hereford (or one of the large European breeds). The first-cross steers and all the terminal cross - steers and heifers - are reared for beef. The first cross should be with a small breed (for low maintenance costs) with high fertility. In Latin America the first cross might be Criollo x zebu and the terminal cross with a large European beef breed. In Africa the first cross on to the local zebu or Sanga breed might be an improved zebu (Boran or Africander or American Brahman) or an intermediate breed (Santa Gertrudis) or a small European breed (Jersey or Aberdeen-Angus) according to the environment. The terminal cross would be with a Hereford or one of the large European beef breeds (Charolais or Simmental) according to the feeding level which can be provided for the progeny.
In many situations crisscrossing or rotational crossing is to be preferred to terminal crossing. It has the great advantage that young females are not slaughtered for meat. It is thus appropriate in countries where there is a legal ban on the slaughter of young females, and also in circumstances where the aim is to increase herd numbers. A crisscrossing system has been in operation among beef cattle breeders in southern Africa for many years on an ad hoc basis. It usually involved crosses between South Devon or Hereford and Mashona in Zimbabwe and between Hereford and Africander in South Africa. Because it was based on the farmers' experience rather than on genetic theory they tend to describe it in a defensive, almost guilty, way which is entirely unnecessary since it is genetically sound. Based on breed availability one might suggest a Hereford x Boran crisscross in East Africa and a zebu x European beef cross in tropical America.
For harsher conditions a cross between two unrelated tropical breeds would probably give the best results, for instance, Brahman x Tswana in Botswana, zebu x Criollo in Latin America. If three tropical breeds are available a rotational cross can be made. In Costa Rica the advantage of crossing Santa Gertrudis, Brahman and Criollo in all possible ways has been demonstrated (see Madalena, 1977) but this experimental result has not yet developed into a commercial breeding system.
The terminal crossing and crisscrossing systems can be combined by using a crisscross between two tropical breeds to produce the cow herd. They will combine tropical adaptation and hybrid vigour and so should be hardy, fertile, long lived and good mothers. Onto a proportion of these crossbred cows which are not needed for breeding replacements is put a bull of a European beef breed. The progeny should be good beef animals but they will need better conditions than their mothers and adequate feeding to develop their potential. Both males and females will be reared for beef.
All these crossbreeding systems need careful identification of cows so that the right type of bull can be used on each type of cow. Some are too sophisticated for general use. The important thing is to use a crossbred cow. The additional advantage from more refined systems are not often worth the extra trouble. In reviewing the results of much crossbreeding work Koger et al. (1973) emphasize that the major portion of the total advantage from crossbreeding comes from the use of crossbred dams. Progeny from F1 dams mated to a third breed were only slightly superior to backcross progeny, and progeny from an established crisscross or rotational crossing system were only slightly inferior. As expected on theoretical grounds inter se mating of F1 bulls and cows leads to a considerable decrease in hybrid vigour. They advocate the crisscross breeding system-especially for zebu x European breeding systems in environments comparable to the Gulf Coast of America. It is simple in having only two breeding groups; all the females are crossbreds; there is scope for selection among the females; and heterosis is almost as high as in the system of breeding from F1 females (terminal crossing).
Most of the sheep crossbreeding in the tropics has been in the form of gradingup or the formation of new breeds. There have been few successful systematic crossing programmes. This is not because heterosis has been shown to be absentin fact there have been few experiments designed to detect it and little data on which it could be studied. Sheep in the dry tropics are kept extensively on the range and under these conditions it is not possible to cross with an unadapted breed however productive it may be in its home environment. In the wet tropics sheep are kept on a subsistence basis by smallholders and in small numbers. They could not easily support the sophistication of even the simplest crossing scheme.
1. Wool
Merinos have been successfully introduced into many subtropical areas (Australia, South Africa, Uruguay, Mexico) but the only areas within the tropics where they flourish are Queensland and the north of Western Australia on the one hand, and the highlands of Kenya on the other. Little crossbreeding was involved in these introductions.
In Egypt (which is not strictly tropical) there have been two attempts with Merinos. The first was in the middle of the 19th century but failed owing to the poor care the sheep received. The second was in the 1950s and 1960s. Various breeds of European Merino were imported but they suffered very much from pneumonia due to dust. Crosses onto the local coarse-wooled breeds were intermediate in fleece weight and wool quality but had a highly variable fibre diameter and coloured patches on the body. However, they showed hybrid vigour in lamb viability and growth rate.
In West Africa Merino crossing has not been a success. Grading of local hair breeds in Chad during the 1930s was discontinued owing to a lack of market for wool. Undertaking a similar cross in northern Nigeria, Burns (1967) found that the F fleece was excessively kempy. She argued that another breed with “central checking” might be more successful than the Merino in reducing kemp and introduced a Wensleydale cross for this purpose. Incidentally little trace now remains of the crossbred animals produced in Katsina.
All the above trials have been rather ad hoc and experimental. None led to any permanent results. It is only in India that there has been a large-scale development programme for sheep which has included crossbreeding to improve wool production (Turner, 1978b). In Kashmir and other Himalayan regions the local breeds have been successfully graded to American Rambouillet and various Russian finewool breeds to produce new breeds. Elsewhere the plan was to keep Corriedales in state farms and use them to cross on to farm flocks. Tremendous difficulties were encountered in the early years in keeping the Corriedales alive and reproducing. There has thus been a delay in starting the comparison of crossbreds with indigenous sheep and there has not been any evaluation of the crossbred wool and its uses. These steps are an essential preliminary to any large-scale crossbreeding project.
In the same article Turner (1978b) compares the gains from selection with those from crossbreeding. She bases her calculations on the Indian project and the vital statistics of Corriedales and their crosses. For the crossbreeding an initial 5 years is allowed for the testing of the various possible crossbreds. The rate of spread of exotic genes is then calculated based on the distribution of crossbred rams which at first are 50 percent rising to 75 percent exotic blood. In 10 years, even starting with purebred rams, the level of exotic genes in the general population will be only 36 percent. Under these conditions it is calculated that the exotic breed must be 60 percent superior to the indigenous for crossing to produce the same genetic improvement in the population as selection in a period of 10 years.
Two other points should be made. Selection can be done at several centres simultaneously whereas crossing is limited by the number of exotic males. The second point is that, if a finewool is wanted but sheep with over 50 percent exotic genes will not thrive, what use can be made of the crossbred wool?
The several difficulties encountered in the trials described above, and especially the questions of adaptation and markets, suggest that the comparative merits of selection and crossbreeding should be carefully evaluated before crossbreeding to improve wool production is attempted in the tropics or subtropics.
2. Meat
One would expect that meat production of tropical sheep could be improved by some sort of systematic crossing system, as with beef cattle, based on the local breed and on an improved mutton breed. No such programmes have developed. There have been sporadic crossbreeding trials but none has left permanent results. Even in subtropical countries such as Iran and Afghanistan, Yalçin (1979) concluded that the results of experimental crossbreeding had not been sufficiently encouraging for it to be extended to the much poorer environmental conditions in the field. In addition to the prevailing unfavourable nutritional and managemental conditions there is the difficulty of natural mating between thin-tailed foreign rams and fattailed native ewes.
In the tropics results have been equally discouraging. Wool and meat-wool breeds have been imported into Java off and on since 1864 but no trace of these early imports remains (Mason, 1980a). More recent trials with Dormers (Dorset x Merino) and Suffmers (Suffolk x Merino) from Australia have not proceeded far but they are confined to well managed experimental herds in the highlands.
European sheep were also imported at various times into the Caribbean islands but were not used systematically. They account for the partially woolled animals which appear among the generally woolless population. The Wiltshire Horn was the commonest import (see Mason, 1980a).
In Barbados the Ministry of Agriculture is undertaking some crossbreeding trials with Suffolk and Dorset Horn to produce market lambs. But to quote Rastogi et al. (1980) “It is hoped that crossbreeding will occupy a minor place in the programmes ..... it is very tempting for a farmer to be misled by the hybrid vigour in the cross and to continue to use the crossbred females for breeding in the hope that their superiority will be maintained. Genetical theory and past experience with woolled sheep in Barbados indicate that this is a forlorn hope”.
European breeds and Indian woolled breeds did not thrive in Sri Lanka. The best results were obtained with hair breeds from south India (especially the Bannur) under extensive conditions and with Dorset x local hair sheep for intensive husbandry (Buvanendran, 1978).
In other places haphazard crossing has led to the formation of new intermediate populations which are now recognized as breeds. For instance the sheep of the Nilgiri hills in southern India, which have a short fine fleece, are derived from a cross of the local coarse-wooled or hair sheep with Merino and Southdown in the early 19th century. A more recent new breed is the Santa Inês of north-east Brazil. This developed since the 1940s by crossing the Bergamasca breed of Italy with the local hair sheep. It gave them greater size without significantly reducing their adaptation or clothing them with wool. It was formed by chance but is now accepted and is being selected as a new breed (see Mason, 1980b).
In South Africa many crosses were made between various European wool breeds and local hair sheep in order to combine their mutual advantages. The one which was most successful was the Dorset Horn x Blackhead Persian. The crossing programme was begun in the 1930s and a breed society for Dorper sheep was formed in 1950. Its breeding is 50:50 but there have been many generations of selection to fix its characteristics. The Dorper inherits the black head, hornlessness, and adaptation of the Blackhead Persian but it lacks its fat deposits on rump, tail and brisket. It has the mutton conformation and growth rate of the Dorset. The coat is a mixture of wool and hair but selection is against excess of wool. This breed has been used as an improver in other parts of southern and eastern Africa, especially in Kenya.
Similar efforts are being made elsewhere. In Cuba, the government policy is to form a new breed by crossing the local hair sheep with the Suffolk and trials are in progress to determine the optimum proportion of Suffolk blood. This policy may be satisfactory for State farms but probably under ordinary levels of feeding and management it is the pure hair breed which will perform best (Mason, 1980a).
Too little work has been done on the crossing of two adapted breeds to exploit either hybrid vigour or complentarity. There are some indications that in the dry subtropics the local coarse wool breed can be improved by crossing with a similar breed from elsewhere. Thus in Libya the Barbary has been improved in size, weight and fleece weight, by crossing with the White Karaman from Turkey. In Egypt hybrid vigour is exhibited when the Ausimi and Rahmani are crossed. In Iran the Shal breed has been improved by crossing with the Israeli Awassi: heterosis was exhibited in lambing rate, litter size, milk yield and growth rate (see Yalçin, 1979).
There has also been some crossbreeding among hair sheep breeds in the tropics. The chief improver breed used has been the Blackhead Persian which has left extensive traces in southern and eastern Africa and in the Caribbean. In Brazil a new breed has been developed - the Brazilian Somali - which retains the black head but has almost lost the fat rump. Crossing with the woolled Criollo has introduced a certain amount of wool.
That these crossbreds have persisted in the absence of any special breeding programmes indicates their thriftiness and suggests that more systematic effort should be put into mutual improvement of tropical breeds - and especially hair breeds - with the eventual aim of forming new breeds based on crossbred foundations. In this connection the prolific tropical breeds have a special role to play since, to improve meat production, increase in number of lambs may be more important than increase in growth rate. The two most useful improver breeds would be the Barbados Blackbelly and the White Virgin Island sheep. They may also be subject to improvement themselves by crossing with European mutton breeds. Indeed the high litter size of the Barbados Blackbelly may itself be the result of early crossing between the hair sheep introduced from Africa and a prolific breed from the Netherlands.
The White Virgin Island sheep has been used (in Maine, USA) to form an improved prolific hair breed by crossing with the Suffolk and Wiltshire Horn - namely the Katahdin. They are larger than the Virgin Island sheep but just as prolific (lambing rate up to 200 percent) (see Mason, 1980a).
Similar initiatives should be undertaken in the tropics using local sheep.
In conclusion it may be suggested that there are three ways of improving meat sheep in the tropics, depending on the severity of the environment and the standard of management.
In the hot wet tropics selection of the local hair sheep or crossing with another tropical hair sheep, particularly a prolific one, to produce a new breed.
In the hot dry tropics selection of the local coarse wool breeds or crossing with another coarse wool breed from a similar environment.
In the wet dry tropics or subtropics grading the local sheep to crossbred rams (European x local) or to a new breed of this genetic composition.
In the tropics the goat is the producer par excellence of milk and meat for home consumption. The small size of the flocks of subsistence farmers precludes a sophisticated crossing programme and the poor feeding and management are not suitable for the introduction of European blood.
The most successful results have been obtained by the infusion of blood of improved tropical breeds. Above all the Jamnapari from India has been used to improve milk yield and size (and hence meat production) in several countries of southeast Asia. In Malaysia and Indonesia the majority of goats now show evidence of Jamnapari blood in their size and lop ears as well, of course, as their milk production. The Jamnapari has also been used in the West Indies. The Anglo-Nubian has been used for upgrading indigenous stock in the West Indies, Mauritius, Malaysia and the Philippines (see Devendra and Burns, 1982).
Although these two breeds have also been used in Brazil the breed which has had the most influence in the northeast of that country is the Bhuj (from Gujarat, India). It has introduced lop ears and larger size into the original Crioulo goats. However it has not been proved that its economic return is better considering the reduced hardiness which accompanies the crossing (see Mason, 1980b).
Crossing with Swiss breeds to improve milk production has been attempted in the Caribbean, East Africa, Malaysia, Fiji, Mauritius and Queensland. In West Malaysia the Toggenburg was the least satisfactory. Saanen and British Alpine crosses on to the local goats were superior to the local goats in milk production but for all round performance the Anglo-Nubian was the most successful improver. Among meat breeds only the Boer (of South Africa) has a reputation but it has in fact not been very successful in East Africa (see Devendra and Burns, 1982).
It can be concluded that with goats even more than with sheep crossing with European breeds must be undertaken only with the greatest caution. For both milk and meat production the need is for selection programmes to produce improved tropical breeds which can be used for grading up populations in which such a selection programme is not possible.
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THE FAO TECHNICAL PAPERS
FAO ANIMAL PRODUCTION AND HEALTH PAPERS
1. Animal breeding selected articles from World Animal Review, 1977 (C* E* F* S*)
2. Eradication of hog cholera and African swine fever, 1976 (E* F* S*)
3. Insecticides and application equipment for tsetse control, 1977 (E* F*)
4. New feed resources, 1977 (E/F/S*)
5. Bibliography of the criollo cattle of the Americas, 1977 (E/S*)
6. Mediterranean cattle and sheep in crossbreeding, 1977 (E* F*)
7. Environmental impact of tsetse chemical control, 1977 (E* F*)
7 Rev. Environmental impact of tsetse chemical control, 1980 (E* F*)
8. Declining breeds of Mediterranean sheep, 1978 (E* F*)
9. Slaughterhouse and slaughterslab design and construction, 1978 (E* F* S*)
10. Treating straw for animal feeding, 1978 (C* E* F* S*)
11. Packaging, storage and distribution of processed milk, 1978 (E*)
12. Ruminant nutrition: selected articles from World Animal Review, 1978 (C* E* F* S*)
13. Buffalo reproduction and artificial insemination, 1979 (E***)
14. The African trypanosomiases, 1979 (E* F*)
15. Establishment of dairy training centres, 1979 (E*)
16. Open yard housing for young cattle, 1981 (E* F* S*)
17. Prolific tropical sheep, 1980 (E*)
18. Feed from animal wastes: state of knowledge, 1980 (E*)
19. East Coast fever and related tick-borne diseases, 1980 (E*)
20/1. Trypanotolerant livestock in West and Central Africa, 1980
Vol. 1 - General study (E* F*)
20/2. Trypanotolerant livestock in West and Central Africa, 1980
Vol. 2 - Country studies (E* F*)
21. Guideline for dairy accounting, 1980 (E*)
22. Recursos geneticos animales en America Latina, 1981 (S*)
23. Disease control in semen and embryos (E* F* S*)
24. Animal genetic resources - conservation and management, 1981 (E*)
25. Reproductive efficiency in cattle, 1982 (E*)
26. Camels and camel milk, 1982 (E*)
28. Feed from animal wastes: feeding manual, 1982 (E*)
29. Echinococcosis/hydatidosis surveillance, prevention and control: FAO/UNEP/WHO guidelines, 1982 (E*)
30. Sheep and goat breeds of India, 1982 (E*)
31. Hormones in animal production, 1982 (E*)
32. Crop residues and agro-industrial by-products in animal feeding, 1982 (E/F*)
33. Haemorrhagic septicemia, 1982 (E*)
34. Breeding plans for ruminant livestock in the tropics, 1982 (E*)
FAO PLANT PRODUCTION AND PROTECTION PAPERS: 39 titles published
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FAO FORESTRY PAPERS: 34 titles published
FAO FOOD AND NUTRITION PAPERS: 25 titles published
FAO AGRICULTURAL SERVICES BULLETINS: 45 titles published
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FAO SOILS BULLETINS: 46 titles published
Availability September 1982
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The FAO Technical Papers are available through the authorized FAO Sales Agents or directly from Distribution and Sales Section, FAO, Via delle Terme di Caracalla, 00100 Rome, Italy.
