Using Spline Test-Day Model for Estimating the Genetic Parameters for Cow’s Milk Yield

Genetic parameters of Montbeliarde cows were estimated for test-day milk yield with a random regression spline model. The spline model has been considered as a good alternative to Legendre polynomials to direct interpretation of parameters. With this model the lactation curve is divided into sections by knots. The milk yield between any two knots is assumed to be changing linearly. The random regression was fitted with linear splines with five knots: 7, 54, 111, 246, 302. The herd-test-day is the fixed effect of the model. The heritability estimates for test-day milk yield ranged from 0.346 at 65th day in milk, to 0.422 at 35th day in milk. Genetic correlations between individual test days were positive and tended to decrease with an increase in days between when the yields were measured. Random regression models using linear splines were proved to be useful for genetic evaluations of cows.