MODELS FOR ESTIMATING TYPICAL LACTATION CURVES IN DAIRY-CATTLE

Milk yields from monthly tests of 59474 Holsteins cows were used to es timate model parameters of the incomplete Gamma, Y-(t) = at(b)e(-ct), Compartmental, Y-(t) = me(-nt) (1-e(-p(t-q))), and Modified Compartmen tal, Y-(t) = ge(-ht)/1+e(-s(t-r))), models to describe the shape of ty pical first lactations, where Y-(t) is daily milk production on day t. Gamma parameters were estimated by both linear (L) and non-linear (NL procedures. Models were evaluated on the basis of average bias (B) of the estimated 305-day (305-d) milk yield compared with that reported under the Agriculture Canada official milk recording program (ROP). Ga mma (L) underestimated milk yield more than Gamma (NL) (p < 0.05), and used in fitting all three models, the Compartmental model had least b ias (p < 0.05). Heritabilities for parameters oi the Compartmental mod el were small (from 0.02 to 0.18). Genetic and phenotypic correlations of parameters from this model with 305-d milk yield ranged from -0.11 to 0.17. Parameters associated with the beginning of the lactation we re positively correlated with 305-d yield, while the parameter represe nting decrement of production had a negative correlation.