Evaluation of genetic diversity of Sahiwal cattle in Kenya

Abstract

The Sahiwal cattle breed in Kenya is a product of a long-term upgrading program involving Sahiwal bulls and the Small East African Zebu (SEAZ) dams. It is an important animal genetic resource that play dual purpose role but information about its population structure, demographic trends and genetic diversity is lacking. The breed faces several challenges that include high risks of inbreeding, drought related challenges, competition from exotic breeds and indiscriminate crossbreeding. Therefore, the breed could be at a risk of losing genetic diversity and also increased vulnerability to extinction. The objectives of this study were to explore geographical distribution of the breed, characterize its population structure, assess its status and trends of genetic diversity and lastly to conduct a population viability analysis (PVA) of the main herd of the breed at the National Sahiwal Stud (NSS). Geographical distribution was analysed using ArcView GIS 3.2. Population structure and genetic diversity were analyzed using POPREP software system while population viability was evaluated using VORTEX version 9.98. The number of registered Sahiwal cows and bulls used for breeding annually indicated an unstable situation and even a downward trend of the breed population growth though the geographical distribution was sparse. Males stayed in the breeding herd much longer than females. Generation Interval (GI) was longer by about 3 years in males than in females. Family sizes varied widely in male pathways than in female pathways. The mean number of discrete generation equivalents traced was 2.46. The mean level of inbreeding was 0.58% for all animals and 2.23% for inbred animals while the mean additive genetic relationships (AGR) in the whole population was 0.87%. The average inbreeding for inbred animals decreased slowly at the rate of -0.0012% per annum while for the entire breed increased by 0.025% per annum. The overall population size of the NSS herd was predicted to increase at a deterministic rate of 12.5% and stochastic rate of 3.2% before any truncation due to limited carrying capacity. Female mortality prolonged generation interval for both males and females whereas male mortality had no effect on generation interval. The proportion of breeding males had no effect on deterministic growth rate but it improved genetic diversity of the herd. An increase in adult breeding females increased the viability of the NSS herd. The study revealed that the Sahiwal population is not going extinct but it is below the level required to maintain genetic diversity in the long term.