Genetic and environmental trends in a large commercial Jersey herd in the Central Rift Valley, Kenya.

Records from a large commercial Jersey herd covering the period 1980 to 1993 were used to estimate genetic and environmental trends in production traits, fertility traits and age at first calving. The farm is located in the upper midland agroecological zone of the Central Rift Valley in Kenya. The estimated annual genetic trends were 0.8 kg (milk yield), 0.003% (butterfat per cent), 0.09 kg (butter fat yield), 0 d (calving interval), -0.001 (number of services per conception), 0.22% (conception rate) and -0.5 d (age at first calving). All the traits studied showed an environmental trend in the desired direction: 14.6 kg (milk yield), 0.029% (butterfat per cent), 1.31 kg (butterfat yield), -2.52 d (calving interval), -0.62 (number of services per conception), 0.89% (conception rate), and -17 d (age at first calving). Despite great genetic variation in the production traits, the genetic trend in all traits was very small reflecting ineffective selection, whereas the results for the environmental trends indicate a substantial improvement in the management.