Crossbreeding of Ayrshire, brown Swiss, and Sahiwal cattle for annual and lifetime milk yield in the lowland tropics of Kenya.

Records from 1577 cows born from 1973 to 1983 on a ranch in lowland Kenya were analyzed for age at first calving, lactation length, lactational milk yield, calving interval, annual milk yield, and, for cows born from 1973 to 1979, number of calvings, productive lifetime in the herd, and lifetime milk yield. Partial lifetime yield (< or = 4 lactations) was studied for all cows. Rotational crosses of Ayrshire and Sahiwal, the crosses between them (interbreds), and the crosses sired by Brown Swiss from the rotational crosses were compared. The Ayrshire-sired rotational crosses yielded 4.8% more milk during a 19-d longer lactation and with a calving interval that was 14 d longer than those sired by Sahiwal. Milk yield for completed lactations of interbred cows was 4.5% less, and milk yield of Brown Swiss crosses was 9.5% more, than the average of the rotational crosses. Lifetime milk yields were 48% greater for the rotational crosses sired by Ayrshire than for those sired by Sahiwal. Interbreds yielded 34% less than the average of the rotational crosses. Over partial lifetimes, the rotational crosses sired by Ayrshire yielded 45% more milk than those sired by Sahiwal; the interbreds and the crosses sired by Brown Swiss yielded 34% less and 62% more milk, respectively, than the mean of the rotational crosses.