Effects of Level of Herd Environment. I. Relationship between Yield and Age

Milk records in herds producing at high, medium, and low levels were analyzed for biases in adjustments for age at freshening. A highly significant relationship between age of cow and yield remained in the age-corrected records at all levels of enviromnent. The bias appeared to be less marked in the highenvironment herds for milk production but not for fat production. However, this apparent change in the yield-age relationship with increasing herd environment was not statistically significant. A method for predicting first-lactation from herd yield at older ages to minimize this bias and, at the same time, to provide a more efficient use of first-lactation records for sire evaluation in small herds is described.