Abstract
Monthly data on completed lactations were employed to estimate a three-stage least squares lactation curve model for milk production, milk fat content, milk protein content, and body weight change in lactating Holstein cattle. In comparison with previous work on the lactation curve, our study employed an augmented incomplete gamma model of the lactation curve, a simultaneous rather than single equation estimation technique, monthly rather than daily or weekly observations, and a pragmatic treatment of the genetic background of individual cows using sire proof data. In addition to considering genetic and dietary effects on the lactation curve, the model isolates the seasonal effect of calving date and current production month as well as the age of the cow. By allowing for the simultaneous explanation of various measures of cow performance, the model accommodates formulation of diets tailored for individual cows or groups of cows and can be used in profit-maximizing mathematical programming models. Diet, production, and body weight changes are determined simultaneously and are not independent of one another.
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