USE OF LACTATION CURVES FOR ANALYSIS OF MILK-PRODUCTION DATA

Nonlinear equations were compared with categorical analysis to account for DIM effects on milk production. Five different models for lactati on curves were evaluated. Derived from a multiphasic lactation curve, the selected lactation curve appeared to result in random residuals an d performed more consistently than the multiphasic curve. Residuals fr om the fitting of lactation curves were then used for split-plot analy sis (continuous model) to estimate treatment effects. Statistical perf ormance of this model was compared with split-plot analysis based on a discrete model with regularly spaced intervals to account for DIM eff ects (discrete model). The fitting of lactation curves accounted for h erd, lactation number, and interaction effects of herd and lactation n umber and accounted for 34.1 and 44.3% of variance among cows for prim iparous and multiparous cows, respectively. The continuous model detec ted interactions of genetic and management factors with treatment of m ultiparous cows that were not detected by the discrete model. No stati stically significant differences were detected between the two modelin g approaches. The continuous model appeared to violate fewer assumptio ns regarding data distribution than did the discrete model, which redu ced the risk of introducing bias during the estimation of treatment ef fects. The continuous model seemed to be more sensitive to subtle inte ractions of treatment and other factors.