Genetic evaluation of the dairy cattle herd at the University of Nairobi veterinary farm
Muasya, T K
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A total of 963 performance records were collected from 250 daughters of 71 sires, covering a period of 15 years (1989-2003), were used to genetically evaluate the performance of a dairy herd at the UoN veterinary farm (Vet. Farm) located in the sub humid zone of the Kenyan highlands. Analyses for the effect of fixed factors and estimation of genetic and phenotypic parameters were carried out using computer packages of Harvey (1987) and Derivative Free Restricted Maximum Likelihood (DFREML) (Meyer, 2000), respectively. During the period of study, the herd performance was rainfall-driven. The respective breed means for the Kenya Holstein-Friesian (HF), Ayrshire (Ayr), Guernsey (GNS) and Jersey (JRS) were 2953,2856.2,2352.5,2334.2 kg for 305-day milk yield (305-MY), 449.5, 460, 465.8and 444.5 days of calving interval (CI), 1.6, 1.7, 1.9 and 1.8 services per conception (SPC) and 36.2, 36.7, 33.4, and 33.6 months of age at first calving (AFC), respectively. The respective length of productive life (LPL) were 3.7, 4.2, 3.9 and 3.6 parities while mean Annual milk yields (AMY) and productivity index (PI) were 2466, 2409.7, 1966.3, 1986.6 kg and 5.9, 5.8,4.8 and 4.7 respectively. Year of calving and year of birth and parity affected 305-MY and AMY in the large breeds (HF and AYR) only, while season effects were not important. The large breeds performed better than the small breeds (Guernsey and Jersey) in terms of 305-day milk yield, age at first calving and the two indices (annual milk yield and production per day of calving interval). The farm had high pre-productive losses of heifers (40%) and high replacement rates (191 and 130% respectively) for the large breeds. Heritability and repeatability estimates of 305-day, annual milk yield, PI, calving interval, and age at first calving for the large breeds were low. The respective estimates of annual genetic and environmental trends of Holstein-Friesian and Ayrshire for milk yield (0.08, - 0.32 and -0.3, 0.006 kg) and calving interval (-0.32, -0.05 and 0.34, 0.25 days) indicated negligible change in the genetic merit of the cows and the management standards in this herd. The respective genetic correlations for Holstein-Friesian and Ayrshire between 305-day milk yield and calving interval of 0.2 and 0.4 were low and positive. The respective genetic correlations for Holstein-Friesian and Ayrshire of productivity index with 305-day milk yield of 0.171 and 0.21 were also low but positive, while those of the index with calving interval of 0.5 and 0.6 and annual milk yield of 0.54 and 0.72 were medium and positive in both breeds. Improvement in dairy productivity in this herd would be achievable if there was improvement in management and efficient use of sires.