Genetic diversity, sperm quality and cryo-tolerance in Zimbabwean Sanga cattle breeds
Sanga-type cattle breeds dominate smallholder farm systems and constitute the bulk of the cattle genetic resources of Zimbabwe. These breeds are under-utilized in commercial production systems, yet valuable due to their diversified production ability and adaptive traits. The present research sought to address the key gaps that exist in Zimbabwean indigenous cattle genetics and reproduction research. This was done in four successive studies designed to understand the technical and social context of smallholder farm systems and their potential for adopting novel assisted reproduction technologies; characterise the within and between breed diversity that native cattle breeds possess; evaluate the extent to which semen from native cattle breeds could be frozen for use in conservation or artificial reproduction practice, in order to ensure cheaper and wide scale access to improved genotypes by farmers; and quantify the association between quantitative variation in semen traits and neutral genetic variation in native Sanga cattle breeds. A survey based study of 261 smallholder farmers in Mashonaland West Province of Zimbabwe revealed that these farms are diversified, are resource driven, have multiple production objectives, and keep multi-purpose cattle breeds of predominantly indigenous blood. Farmers face problems related to poor reproduction and access to affordable good quality breeding stock. Traits related to adaptation (hardness), growth potential, fertility and draught power are highly valued. It was concluded that current and future interventions in cattle production should emphasise local breeding of Mashona, Tuli, Brahman and related composite breeds, as well as application of novel reproductive techniques for improved access to breeding stock and distribution of genetic gain in these preferred breeds. Conservation populations of Mashona, Tuli and Nkone breeds were then genetically characterized using a panel of sixteen microsatellite markers to estimate the within and between breed genetic diversity. The markers used displayed sufficient polymorphism for elucidation of genetic diversity and uniqueness in these breeds. Results showed that Sanga cattle in Zimbabwe fall into distinct breed groups with moderate allelic diversity and moderate genetic differentiation. In addition, breed private alleles and rare alleles were observed, indicating presence of unique genetic traits in the breeds studied. The Mashona was observed to have closer genetic similarities with Tuli compared to the Nkone. It is therefore of utmost interest to the country to maintain these breeds as viable targets for utilization or conservation, and to safeguard the genetic variability that each breed possesses. A phenotypic performance experiment to evaluate breed and individual bull variation in semen production, quality and freezability traits in Mashona and Tuli bulls was then conducted. Thirty-five ejaculates from ten bulls were diluted in two experimental Tris-egg yolk extenders that differed in the type of penetrating cryoprotectant (ethylene xiii glycol, glycerol) and a commercial extender (Triladyl®), and evaluated at three time periods: fresh, pre-freeze and post-thaw. Results showed significant breed and bull variations in semen production, sperm quality and cryo-survival, and those differences were not modified by the type of cryoprotectant used. Tuli bull semen had poor cryo-survival compared to semen from Mashona bulls. Ethylene glycol produced comparable results in cryoprotection as glycerol in both breeds, and for good or poor freezers. The last study quantified the association between observed quantitative variation in semen traits and microsatellite genetic variation between pairs of Sanga bulls. Pairwise tri-distance matrices based on Nei’s genetic distance (DA) and phenotypic differences in semen traits were generated and Pearson’s correlation coefficient (Rxy) was computed using the Mantel test in GenAlEx 6.5. Rxy was positive for all semen traits. It was weak, non-significant (P >0.05) and triangular shaped for ejaculate volume, concentration and sperm morphology. Rxy was significant and moderate for sperm motility variables (0.40-42, P < 0.05) and high for sperm viability (0.61, P < 0.001). Given the weak to moderate and triangular-shaped relationship of microsatellite genetic diversity with most of the traits, use of neutral genetic diversity as a proxy for quantitative variation in semen traits should be approached with caution. The entire study was successful in answering the objectives set out at the beginning.