The effect of cow genetic merit on the performance of spring calving Holstein Friesian dairy cows in first, second and third lactation was investigated. The study contained 96 first lactation animals in 1995, 96 second lactation animals in 1996, and 72 third lactation animals in 1997. Half of the animals were of high genetic merit (HG) and half of medium genetic merit (MG) for milk production. Genetic effects for the traits of interest were estimated as the contrast between the two genetic groups. The HG cows produced significantly higher yields of milk, fat, protein and lactose when compared to the MG cows. During the grazing season the HG cows had significantly (P < 0.001) higher grass DM intake (GDMI). In very early lactation when cows were indoors, offered grass silage ad libitum plus 7.9 kg of concentrate DM daily, there was no difference in DM intake. During the non-lactating period the HG cows had significantly (P < 0.01) higher silage DM intake (SDMI). Cow genetic merit had no significant effect on live weight with the exception of pre-calving weight at the beginning of second lactation when the HG cows had significantly (P < 0.05) higher live weight. At all stages of lactation the MG cows had significantly (P < 0.001) higher condition score. In early lactation the HG cows had greater (not significant) live weight loss and significantly (P < 0.05) greater condition score loss (indicating greater negative energy balance). In the dry period the HG cows had significantly (P < 0.01) greater live weight gain. The results of this study suggest that present day HG cows will produce high milk yields on a grass-based feeding system where an adequate quantity of high quality grass is available. (C) 2000 Elsevier Science B.V.
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