Mixed model estimation of age and month of calving adjustment factors for milk and butterfat yields of New South Wales dairy cattle

Abstract

Month-age adjustment factors for milk and butterfat yields were estimated from 504063 lactations on 239 518 cows located in 6 145 herds in New South Wales by mixed model methods, which eliminate biases due to genetic trends, environmental trends and culling. June, July and August are the most favourable months for calving, and December and January are the least favourable, resulting in 8 to 22% less production. Friesian cows are less affected by calendar month of calving than the other breeds. Results indicate the existence of large differences between breeds in rate of maturity. Ayrshire and Jersey cows attain mature yields at a younger age than AIS, Guernsey, Friesian and AMZ cows.