In a long-term experiment, the performance of British Friesian, Ayrshire and Jersey cattle and their first and second crosses was evaluated over 526 first and 477 second lactations. Live weight at 18 mo of age and the ratios of lactation yield to live weight were examined as well as milk yield and composition of milk.
Percentage heterosis in each combination of two pure breeds was estimated. For the three first crosses combined, heterosis for 305-day milk yield averaged 6·4% in first and 3·7% in second lactations. Heterosis in live weight at 18 mo averaged 2·9%; none was found in the Ayrshire × British Friesian crosses, but the others averaged 4·7%. Second crosses overall had an average live weight near to that of first crosses, but their milk yields were lower and, in the combined lactations, approximately equalled the purebreds' total.
Components of variance due to breed and environmental factors were calculated. Breed accounted for 50% of variance in live weight at 18 mo but less than 10% of variance in milk yield.
The influence of fertility on heterosis is discussed with reference to earlier findings, and economic implications are considered.