Lactation Curves of Purebred and Crossbred Dairy Cattle

Abstract

First lactations of 397 cows with varying percentages of Holstein and Guernsey breeding provided a means of studying genetic and environmental effects on coefficients of the lactation curve y = anbe-cn (1 + u sin(x)+v cos(x)), where a, b, c, u, and v are coefficients to be estimated; e is the base of natural logarithms (in); n is the day of lactation; and x is the day of year. The logarithmic form of the equation ln (y) = ln(a) + b ln(n) + cn + u sin(x) + v cos(x) was fit for each cow. Analysis of variance on each estimated coefficient used a fixed model with genetic and environmental factors. There was no evidence of additive genetic variation associated with coefficients of the lactation curve except for ln(a), the initial yield or general scaling of the curve. There was some evidence of nonadditive genetic variation with significant interaction of breed of sire with breed of dam. There was no effect for age fresh or days open on any coefficient; however, effects of year fresh and month fresh on most coefficients were highly significant. To change the shape of the lactation curve through selection probably would not be successful; however, initial production or scaling of the curve probably can be changed. It may be possible to change the shape of the lactation curve by exploiting nonadditive genetic variance.