Calculation of a cow culling merit index including specific heterosis in a multibreed dairy population

Abstract. The objective of this study was to compare two models for the estimation of producing values (EPV) for lactation yields of milk, fat and protein, and calving interval (CI), which were combined in an index called the Cow Culling Merit Index (CMI), in Irish dairy cattle. Data comprised 188 927 records for production and 157 117 records for CI, collected on North American Holstein Friesian (HO), Friesian (FR), Jersey (JE), and Montbéliarde (MO) pure breeds, and some of their crosses. Cows calved from 2002 to 2006 and were from parities 1 to 5. Coefficients of specific heterosis for HO×FR, HO×JE, and HO×MO were calculated for each cow from parental breed information. The coefficient of general heterosis (GH) for each cow was obtained as the sum of the specific coefficients previously estimated. Model 1 included fixed effects of contemporary group, age at calving within parity, linear regression on gene proportions for FR, JE, and MO, and linear regression on the coefficient of expected GH. Additive genetic, permanent environmental, and error were random effects. Model 2 was based on Model 1 but GH was replaced by linear regressions on coefficients of expected specific heterosis for HO×FR, HO×JE, and HO×MO. Estimated producing values were calculated as the sum of estimated breeding value, permanent environmental and heterosis effects. The inclusion of coefficients of specific heterosis in the model did not produce re-ranking of animals but important differences in EPVs were observed in crossbred cows. These changes are important if EPVs are used to develop a culling merit index.