INVESTIGATION OF PREFERENTIAL TREATMENT FOR MILK-YIELD IN CANADIAN HOLSTEINS

Milk production data collected from 1980 to 1986 for Holsteins in Canada were examined for evidence of preferential treatment of cows. Herds were split by milk-recording organization and within organization into Embryo Transfer herds, Bulldam herds, and other herds. Cows were classified as registered or grade within herds. Within-herd coefficients of variation were calculated for all herd-years and were generally higher for Bulldam herds. Cow estimated transmitting abilities (ETA) for milk yield were regressed on dam ETA and sire ETA to compare the regression coefficients with the theoretical regression coefficients and to examine the residuals. Residual variances increased with predicted ETA in all the sets of other herds and in ET herds located in Ontario. There was little evidence of effects due to preferential treatment. Frequency distributions of deviations from herd-year-season mean were calculated separately for grade and purebred cattle within lactation number and herd type. All distributions except for grades in Bulldam herds were significantly lepto-kurtotic (P < 0.01). There was little difference in distributions of grades and purebreds. No evidence of effects which could be caused by preferential treatment was obtained by these methods.