Application of Genomics in Tropical Dairy Systems: Matching Appropriate Dairy Breeds to Smallholder Production Systems in Tanzania

Anecdotal evidence and previous research suggests that the optimal level of Taurine inheritance in crossbred animals lies between 50% and 75% when considering total productivity in tropical systems. While increasing the exotic percentage of cattle results in more milk, the cumulative benefits relative to farmer socio-economic status, input level and production environment are not clear. We set out to assess insitu the relationship between breed composition and productivity for various smallholder production systems in Tanzania.

We surveyed 654 smallholder dairy households over a one year period and grouped them into production clusters. Based on supplementary feeding, milk productivity and sale as well as household wealth status four clusters were described: low-feed-low-output subsistence, medium-feed-low-output subsistence, maize germ intensive semi-commercial and feed intensive commercial systems. About 800 crossbred cows were genotyped at approximately 150,000 SNP loci and their breed composition determined. Percentage dairyness (proportion of genes from international dairy breeds) was estimated using admixture analysis using Holstein, Friesian, Norwegian Red, Jersey, Guernsey, Ndama, Gir and Zebu as references. A fixed regression model using the G matrix was used to analyze milk test day yield accounting for year-month-test-date, parity, age, breed type and the production clusters as fixed effects in the model with random effects of animals and permanent environment effects.

Results indicated that farmers practicing in the low-feed-low-output subsistence systems had the lowest milk yields and some kept animals with dairyness below 34%. These farmers would be best served with cows having no more than 65% Norwegian Red - Friesian genes. In all management groups, except in the feed intensive commercially oriented systems, the best performing animals had higher proportion of Norwegian Red-Friesian genes. These results also seemed to suggest that dairyness between 75 to 85% is the most appropriate for these systems. For farmers in the feed intensive system, animals with a Holstein genetic background with at least 75% dairy composition were the best performing.

These results indicate that matching breed type to production system is central to maximizing productivity. The outcome observed will serve as a basis to inform the development of the dairy sector in Eastern Africa.