Analysis of Persistency of Lactation Calculated from a Random Regression Test Day Model

Persistency of lactation is the ability of a cow to continue producing milk at a high level after the peak of her lactation. Differences in persistency, if not properly accounted for, reduce accuracy of genetic evaluation when incomplete records are used. Persistency has also direct economic value. Reduction of feed, health and reproductive costs are the major factors which favour more persistent cows (Dekkers et al., 1996; Sölkner and Fuchs, 1987). Persistency can be measured in many different ways (Gengler, 1996). The major concepts are based on ratios of partial and total yields, variation of yields during the lactation or shape of the lactation curve. Random regression models for test day (TD) yields (Jamrozik et al., 1997a) allow curves to be estimated for each lactation of every cow. Therefore, a measure of persistency based on the shape of the lactation curve after peak seems to be a natural way of describing potential to maintain the level of production. Milk, fat and protein TD yields have slightly different lactation curves. Curves for different lactations also differ in shape. Therefore, persistency for milk, fat and protein yields in consecutive lactations might be considered as different traits. The objective of this study was to estimate genetic and phenotypic parameters for milk, fat and protein persistencies, defined as the slope of the lactation curve after the peak, in the first three lactations of Canadian Holsteins. Possible expressions of estimated breeding values for first lactation milk persistency are also discussed. Materials and Methods