Productivity data are presented and evaluated for
- purebred Sahiwal (Bos indicus) cattle,- Sahiwals crossed in different combinations with cattle of the Ayrshire, Friesian and Brown Swiss (Bos taurus breeds,
- purebred Ayrshire (Bos taurus cattle,
- cattle of two indigenous Bos indicus breeds, Boran and East African Zebu
reared under different environmental conditions in Kenya, varying from sea level to 2200 m and from 610 to 1043 mm mean annual rainfall. These environments include the subhumid ecological zone, the lower- to middle-potential areas of the temperate highlands and the higher-potential areas of the semi-arid highlands of sub-Saharan Africa. The data derive from five ranches whose production objectives varied from primarily beef to primarily milk production, with different levels of emphasis in between.
A primary objective of this study was to evaluate the potential for both milk and beef production of the Sahiwal breed in various African environments. The focus was on Sahiwal cattle as contributors of Bos indicus germ-plasm to populations also based on Bos taurus breeds, as the Sahiwal breed is known to have the greatest potential for milk production among the Bos indicus breeds generally available in Africa. Bos indicus cattle generally have a low response capability for milk and beef production characters as straightbreds, hut they are well adapted to the African climatic and nutritive environments represented by the five locations which contributed data for this study. These environments are not adequate in terms of climate or feed resources to support the high milk and beef production response capability of Bos taurus breeds. It is also not economically or technically feasible to modify these environments sufficiently to meet the requirements of the high-performing Bos taurus breeds. Thus, this study included an examination of concepts and approaches for combining the adaptability characteristics of the Sahiwals with the high-performance characteristics of Bos taurus breeds to achieve and maintain the most favourable additive genetic composition for production in the given environments.
In two of the higher-potential locations (Deloraine Estates and Cedarvale Farms) Sahiwal crosses with Bos taurus breeds were markedly superior to purebred Sahiwals in terms of characters relating to milk production and an index of cow productivity. In what should be the highest-potential site (Deloraine Estates) situated in the temperate highlands at an elevation of 2100 m with 1007 mm annual rainfall, the Ayrshire-Sahiwal cross cows exceeded the straightbred Ayrshire cows by 3.8% in terms of a productivity index per cow, in spite of the preferential feeding provided to the straightbred Ayrshires. The straightbred Ayrshire cows produced 185 kg more milk per lactation than the Ayrshire-Sahiwal crosses, but the higher calving percentage and higher calf survival rate of the crosses resulted in a higher index of cow productivity.
For the three highest-potential units (Kilifi Plantations, Deloraine Estates and Cedarvale Farms) where the primary objective was milk production, both milk production and the cow productivity index were reduced when the Sahiwal breed contributed more than one-half of the germ-plasm to a population where the remainder was contributed by the Ayrshire breed. In the herd that provided the highest nutritive environment and had the highest level of milk production (Kilifi Plantations), the 2/3 Ayrshire - 1/3 Sahiwal crosses were superior in most characters related to milk production, including the cow productivity index, over the 1/3 Ayrshire - 2/3 Sahiwal crosses, even though the herd is located in a relatively stressful climatic environment at sea level, 3.5º latitude south. Thus, it is concluded that the Ayrshire breed excels the Sahiwal breed in transmitted effects for milk production. On the basis of results from a relatively wet area in the subhumid ecological zone, a lower to middle-potential area in the temperate highlands, and a lower-potential area in the temperate highlands, the optimum contribution by the Sahiwal breed appears to he in the range of one-fourth to one-half, with the remainder contributed by Bos taurus breeds.
When straightbred Sahiwals were compared with indigenous Boran cattle in a beef production environment representative of the higher-potential areas of the semi-arid highlands, the differences between the two breeds were small for most characters evaluated. In terms of fitness-related characters, the Sahiwal breed proved well adapted to the environment. The Sahiwals excelled the Boran in maternal ability, while the Boran excelled in transmitted effects for growth. When credited with milk extracted for domestic use, the Sahiwals proved superior to the Boran in terms of a cow productivity index. It is concluded that the Sahiwal breed can make a useful contribution to beef production programmes in the higher-potential areas of the arid to semi-arid ecological zone. Their greatest potential contribution in this zone is either through organized crossbreeding systems to use heterosis or as a contributor to composite breeds. Results from Deloraine Estates and Cedarvale Farms show the Sahiwal breed superior to the Ayrshire in transmitted effects for growth rate, indicating that Sahiwals can usefully he included in programmes geared towards beef production.
At the Ilkerin Project in a higher-potential area of the semi-arid highlands, the Sahiwal breed greatly excelled the indigenous Small East African Zebu in characters related to milk production and growth. Further, the Sahiwals averaged a 38-day shorter calving interval, which indicates a high level of adaptability to a fairly harsh nutritional environment.
Although available data were limited for obtaining reliable estimates of heterosis, the estimates made are in general agreement with prior reports. The importance of heterosis for both milk and beef production characters is well documented, particularly in the case of crosses of Bos taurus and Bos indicus breeds.
The optimum contribution of each foundation breed cannot he achieved and maintained by a two-breed rotation crossbreeding system because of the fluctuation between generations of one-third to two-thirds in genes contributed by the breed of the sire and of the maternal grandsire. Furthermore, substantial numbers of cattle throughout the world are kept in herds which are too small to use organized cross-breeding systems on a self-contained basis. In these situations, the formation of composite breeds is a logical alternative, based on comprehensive characterization of foundation breeds in the relevant climatic and nutritive environments.
In addition, composite breed formation has the advantage of utilizing the effects of heterosis. Retention of initial heterozygosity after crossing and subsequent random meting with the crosses (inter se) is proportional to 1 -S in Pi2 where Pi is the fraction of each of n breeds contributing to the composite breed. Thus, retention of heterozygosity favours including as many breeds as possible in a composite breed as long as each makes a positive contribution to average additive genetic merit. A further important consideration in the formation of composite breeds is to maintain a sufficiently large population so that the initial advantage of increased heterozygosity is not dissipated by early re-inbreeding.
Loss of heterosis has been shown to be proportional to loss of heterozygosity in breed rotation crossbreeding systems. The importance of the contribution to composite breeds of favourable epistatic genetic combinations that either have become fixed or are maintained by selection in parental breeds has not been determined in cattle. To the extent that such favourable epistatic genetic combinations contribute to increased performance in parental purebreds, the loss in performance in composite breeds would be greater than the loss of heterozygosity relative to initial heterozygosity reflected by the mean for the F1 crosses of the parental breeds. Results from rotation crossbreeding systems show loss of heterosis to be linearly associated with loss of heterozygosity in cattle, but also indicate that heterosis may be primarily the result of the dominance effects of genes. If this is so, composite breeds may have performance advantages from retained heterosis approaching those obtainable from rotational crossbreeding. The linear association between loss of heterosis and loss of heterozygosity in composite breeds still needs to be demonstrated, along with the additive genetic variation of composite breeds relative to their parental breeds, particularly for characters related to fitness. Research is currently in progress, at the U S Meat Animal Research Center in Clay Center, Nebraska, which addresses these two relevant issues.
Suggestions are presented for specific composite breeds in each of the environmental situations for which data were analyzed. The primary objective is to synchronize the formation of a composite breed with the natural environment in which the herd is to be maintained, including climatic, nutritive, disease and parasite conditions, with the level of management which is technically and economically feasible and with specific production goals. The Sahiwal breed has been selected to contribute from one-fourth to three-fourths of the germ plasm for each of the composite breeds suggested. There are strong indications that the optimum contribution by a Bos indicus breed is at least one-fourth in any of the environmental situations described in this study.
