COMPARISON OF THREE METHODS OF PREDICTING 305-DAY MILK AND FAT PRODUCTION IN DAIRY COWS

Abstract

The data used in this study consisted of 119 075 complete lactation records accumulated by record of performance (ROP) from 1976 to 1981. Each individual record consisted of 7, 8 or 9 monthly test day milk weights and corresponding fat percentage. The data for each breed (Ayrshire, Guernsey, Holstein, Jersey) were grouped into two ages at freshening — less than 36 mo and greater than or equal to 36 mo. Months of calving were grouped into four seasons: March to May, June to August, September to November, and December to February. For both milk and fat yield, separate sets of extension factors to predict 305-day yield were estimated for each breed, group, and season of calving, using three methods (multiplicative, regression, nonlinear model). A new set of data collected by ROP during 1982 was used to compare three sets of extension factors. The nonlinear model method was better than either the multiplicative or regression methods because of smaller bias and better precision, and because it can be easily implemented in Canadian milk recording programs. Separate extension factors are not required for different geographic zones allocated by ROP. Key words: Extension factors, cattle (dairy)