This thesis is a contribution to improvement of efficiency of dairy cattle breeding programmes in the presence of between and within trait genotype by environment
interactions in Kenya. The thesis aimed at
1) analyzing the breeding structure and estimating current and future trends in genetic variability of the Holstein-Friesian cattle population in Kenya to identify genetically important herds which contribute to genetic improvement of the population,
2) evaluating nature and extent of genotype by environment interactions for milk yield and fertility traits between nucleus and commercial herds, and the expected correlated response in the traits to sire selection in the nucleus,
3) assessing nature and extent of genotype by environment interaction for milk yield across alternatively defined herd environments and expected correlated response in milk yield and fertility traits in the low opportunity herd environments to sire selection for increased milk yield in the average and high opportunity herds environments,
4) describing the lactation curve of cows in the nucleus and commercial herd environments to determine the appropriate polynomial order for describing the additive genetic and permanent environmental curves and to determine whether variance components differ between the two herd environments,
5) estimating genetic parameters of variously defined measures of genetic persistency of lactation and their genetic relationship with milk yield and assessing the efficiency various selection criteria in genetic improvement of lactation milk yield and genetic persistency of lactation.