Genetic improvement programs for livestock in the developed countries have advanced to the present stage, mainly because the infrastructural elements necessary for planned breeding programs such as thorough performance recording and AI (Artificial Insemination) are available. In addition, advances have also been made in population genetics and statistics which have made it possible to develop genetic improvement schemes applicable on a national scale (buvanendran, 1982).
SELECTION PROGRAMS IN DAIRY CATTLE
The importance of selection programs in tropical breeds
No serious attempt has previously been made with national selection programs to improve the dairy quality of indigenous cattle. This is partly due to the belief that the limiting factor to increased production is not the genetic value of the animals but rather the opposing environment, and those improvements in husbandry and veterinary care should precede the setting up of breeding schemes. But when one considers the performance of most indigenous breeds, a notable similarity is noticed in their late age at maturity, short lactation, long dry period, low average daily milk yield and lack of dairy temperament. Most of these characters are conveyed even under conditions of improved husbandry, therefore indicating that poor productivity is attributable to low genetic quality. Breeding schemes to raise their genetic potential are therefore fully justified (buvanendran, 1982).
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The determined rate of genetic gain in milk yield reachable by selection even in temperate breeds is of the order of 2.0 percent per annum. This low rate of improvement has discouraged the application of selection schemes for indigenous breeds since the overall increase in production possible by selection in cows yielding 500–1 000 kg is not of any great scale. It should, however, be remembered that even European breeds were as unproductive as the tropical breeds before the application of selection programs and it is the application of planned selection programs that have brought the advance that we see today (Cunningham, 1979). Since the methods for genetic improvement are even more established today than when they were first used on European cattle, it is not unimaginable that similar accomplishments could be made in tropical dairy breeds (buvanendran, 1982).
Where progressive selection efforts have been made to improve tropical breeds, the results have been promising. Annual genetic gains of approximately 2.5 percent of the herd mean yield have been reported by (J.L.Lush, 1968) in Haryana (zebu) cattle in India and by (I.R.Franklin, 1976) in the Australian Milking Zebu (Jersey x zebu cross). The high rate of genetic gain in both populations was partly attributed to the high coefficient of variation for milk yield in the populations, which is typical of most dairy cattle in the tropics (Mahadevan, 1966). It is also clear that, in populations that failed to show important gains, it was the inefficient selection procedures that were responsible, e.g. East African zebu (Kimenye, 1979).
In formulating a breeding program for dairy cattle in the tropics, the final objectives of the program and the environment and production system under which these objectives are to be realized should be defined. Therefore, the objectives should explain whether it is milk alone or milk and beef or milk and draught or all three that are to be improved. Further, depending on market demands, the importance to be attached to compositional quality should also be explained. Similarly, the production system, which can range from nomadism through small sedentary herds to large commercial farms, will regulate the importance given to adaptation traits, mainly those concerned with disease resistance and reproduction (buvanendran, 1982).
Selection in institutional herds in the tropics
Established herds have an important role in the tropics since these may be the only herds where genetic improvement of indigenous stock can be carried out. Established facilities are also necessary for selection programs for exotic breeds which are required in pure or crossbreeding programs (buvanendran, 1982).
Artificial insemination should be active for breeding even in established herds so that large sire progeny groups are obtained for accurate progeny tests. Where the established herd is too small, it is necessary to recruit the support of the progressive farmers in the area to contribute to the progeny testing scheme. In situations where only natural mating can be active, progeny testing loses much of its efficiency since the number of bulls required is so large that the selection concentration, as well as the accuracy of the test, is much reduced. Selection of bulls on their dams' records will then be similarly efficient provided that bulls are replaced annually to minimize the generation interval (buvanendran, 1982).
An example of a successful breeding program involving an established herd is that used in the Sahiwal herd in Naivasha, Kenya. The breeding plan given below is based on the explanation by (J.V.Wilkins, 1974) whose schematic diagram is also reproduced in Figure 1.1.
The herd consists of about 500 cows. Breeding is by Artificial Insemination with semen from the best two progeny tested bulls. Female selection is almost completely on milk yield during the first two lactations. About 50 percent of the heifers that enter the milking herd annually are culled at the end of the first lactation and a further 50 percent after the second lactation. Therefore only 25 percent of the heifers freshening annually are kept to the third and higher parities. These animals create the best herd consisting of about 180 animals. They are bred only by progeny tested bulls while the balance of the herd (320 animals) consisting of heifers and first calves is used for progeny testing young bulls.
Bulls are selected from among those born in the best herd. They are first performance tested for growth rate on pasture to 2 years of age and 15 bulls (out of the 70 born annually) are chosen on a catalog that combines growth rate with dam's milk yield. A further five animals are removed for poor physical conformation and semen quality and the remaining 10 are progeny tested. They are bred to females outside the best group (about 320 animals) and each bull is evaluated on the first lactation yields of about 14 daughters. The best two bulls are chosen for use in the best herd and national Artificial Insemination scheme. The genetic progress with this type of program is estimated at 3–4 percent per year (buvanendran, 1982).
Selection in new breed formation
In programs involving a crossing of exotic and indigenous stocks for developing a new breed, selection has an important role to play after the original crossings are made to create the desired gene pool. Selection can be not only for the improvement of production traits but for adaptability traits as well:
Selection for milk yield
The procedures used in selection for milk yield involve progeny testing. Testing of bulls is, however, not possible in the early generations of new breed formation (F1 or backcross depending on the composition of the new breed, since young bulls are required to mate with heifers of their own age and generation. Further, even from a genetic viewpoint, selection should start only after segregation and recombination of the gene combinations of the parental breeds have occurred. The chance for this to happen will arise after inter se mating starts. Progeny testing of bulls should, therefore, start with bulls born after this stage.
Private Farmer contribution is important in sire assessment for reasons given earlier but problems may arise if the dams mated to the sires differ in their genetic make-up. This is usually the case in situations where crossbreeding programs in private herds have been underway for a number of years before selection schemes start. The crossbred dams may then differ in respect of both breed and grade of exotic blood and bias the progeny test due to their different contributions to the progeny.
Adaptability traits
Selection for adaptability traits should wherever possible continue simultaneously with improvement for milk yield. Measurements for these traits may, however, have to be limited to trial herds because of the facilities required.
The heat tolerance test was aimed to select bulls that upheld their body temperature under heat stress while simultaneously having a high energy conversion rate (S.M.Donegan, 1974). Animals were tested in climatic chambers and measurements recorded of physiological changes (rectal temperature, sweating rate, and respiration rate) and feed and water intake when they were moved from low to high temperatures (from the dry and wet bulb of 21° and 18°C to 38° and 24°C respectively). Body temperature and respiration rate were given a negative score and sweating rate and feed conversion given a positive score. The best third of the animals chosen on this test were put through tick challenges. This was done by artificially infesting each animal with 40 000 larvae of Boophilus microplus and counting the number surviving to the adult stage from the 18th to the 22nd day after infection. The best 50 percent of the animals that had the least number of engorged ticks were finally chosen for progeny testing after examination of semen quality.
Selection programs in the absence of either Artificial Insemination or milk recording or both
In the absence of Artificial Insemination and milk recording programs, established herds offer the only chance to carries out recording and selection. (Cunningham, 1979) has proposed a kind of open nucleus breeding scheme (Figure 1.2) that may be appropriate for this situation. The plan works around a central herd which should preferably be under government control and carry out proper recording and breeding practices. The supporting base population is the village herds which provide cows to replace about 10 percent of the cows in the central herd annually. Selection from village herds will be by simple procedures involving judgment by eye, milking ability, size, conformation, and condition. Where feasible, test milking’s may also be done. The selection of these animals will be done by officers who would tour many villages to select the best animals from as wide a population as possible.
Bulls are bred from the best cows and selected bulls in the central herd. They are then assessed on their own growth rates and dams' and milk yields and the best are chosen as sires. The lowest yielding 10 percent of the cows are replaced with new animals drafted from village herds. The last animals are recorded during the following year with the remainder of the central herd and re-evaluated.
The benefits to the cooperating farmers from this scheme will accumulate from the sale of selected bulls from the central herd. Assuming that the central herd has 200 cows and that the best third of the bulls born annually are chosen, about 20–25 bulls will be available for sale to farmers after meeting the central herd's requirements. This may satisfy the requirements of the cooperating farmers but will have hardly any impact among farmers outside the scheme. This is a serious shortcoming of programs involving central herds where supporting Artificial Insemination services are inadequate (buvanendran, 1982).
References
- buvanendran, i. m. (1982). breeding plans for ruminant livestock in the tropics. In SELECTION PROGRAMS IN DAIRY CATTLE. Rome: FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS.
- Cunningham. (1979). The importance of continuous genetic progress in adapted breeds. Report of the FAO Expert Consultation on Dairy Cattle Breeding in the Humid Tropics, 35-41.
- I.R.Franklin, R. a. (1976). Progeny testing and expected rate of genetic improvement . Bos indicus and Bos taurus crossbred dairy cattle in Austrailia, 309-321.
- J.L.Lush, R. A. (1968). Genetic progress through selection in a closed herd of Indian cattle. Dairy Science , 1059-1064.
- J.V.Wilkins, K. a. (1974). Breeding for milk in Kenya, with particular reference to the Sagiwal stud. World Animal Review (FAO), 24-30.
- Kimenye. (1979). Review od breeding programs and genetic change of dairy cattle in East Africa. Paper for FAO Expert Consultation on Dairy Cattle Breeding in the Humid Tropics.
- P.Mahadevan. (1966). Breeding for Milk Production in Tropical Cattle. Common-wealth Agricutural Bureaux.
- S.M.Donegan, T. a. (1974). A climate room test of heat tolerance used in the selection of young sires for progeny testing. Bos indicus and Bos taurus crossbred dairy cattle in Australia, 1023-1035.
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