Reproductive traits and milk yield relationship of the Kenya sahiwal cattle
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Records of the Kenya Sahiwal Stud were analysed with regard to factors affecting reproductive traits and milk yield in temales born in the period 1963 through 1983. Genetic parameters and the relationship among the traits were studied. Two data sets were analysed: one with 2238 heifer records, and the other with 3518 heifer and cow records. The overall means and standard deviations were 881.1~107.7 days for age at first service, 930.3~130.5 days for age at conception, 1218.0~130.7 days tor age at first calving, 2.7~1.7 number of services per conception, 49.6~85.0 days for insemination period, 287.6~7.5 days for gestation length, 1145~523 kg for 305-day milk yield. For the combined data set, corresponding values were 2.1~1.6 for number of services per conception, 50.2~89.7 days for insemination period 287.7~7.7 days for gestation length, and 1287.8~565.9 kg for 305-day milk yield. Heritability estimates were O.25~O.06 for age at tirst service, O.15~O.05 for age at conception and O.29~O.06 for age at calving, O.08~O.05 tor number of services per conception, O.12~O.05 for insemination period, O.08~O.05 for gestation length, and O.35~O.07 tor milk yield in the heiter data set. The corresponding values for the combined data set were O.02~O.03 for number of services per conception, O.05~O.03 for insemination period, O.07~O.03 for gestation length and 0.36~0.06 for 305-day milk yield. Genetic correlations among the reproductive characters examined ranged from -0.1 to 0.9, with the high values among the ages at first service, at conception and at calving. Corresponding phenotypic and environmental correlations were also high, ranging from 0.7 to 0.9 among the ages at first service, at conception and at calving, but relatively low (within a range of -0.03 to 0.6) among the other traits. Favourable, negative genetic correlations were reported between 305-day milk yield and reproductive performance characters (-0.12 to -0.02) except gestation length which had positive genetic correlation with milk yield. The phenotypic and environmental relationship between milk production and reproductive traits were low but positive in the two data sets. It was concluded from this study that the reproductive characters could be improved by environmental improvement as selection for such traits would be unrealistic due to their low heritability estimates. However, selection for milk yield should be continued at the stud since no antagonism was observed in the relationship between milk yield and the reproductive traits.