Possible association between the breeding value of dairy bulls and milk yield of their mates

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Abstract
British Friesian and Jersey cows were mated either to high contemporary comparison (CC) bulls for their first pregnancy and to low bulls for their second pregnancy, or vice versa. Sixteen proven bulls were used, four ‘high’ and four ‘low’ in each breed. Least squares analysis of data on 175 lactations gave estimates of each bull's effect on his mates' milk yield in the ensuing lactation, but there were no significant differences between bulls for this ‘mate effect’. The high bulls tended to depress their mates' milk yield in the subsequent lactation and the low bulls tended to increase it. Weighted regressions of the ‘mate effect’ on breeding value had negative slopes for both breeds and for first and second lactations, separately or combined. The high CC bulls appeared to depress their mates' milk yield by at least 10% of the amount by which their daughters' yield was expected to increase.