Genetic merit for milk production and reproductive success in dairy cows

The effect of genetic merit for milk production traits - fat, protein and m ilk yield - in dairy cows on milk production, body condition, blood metabol ites, reproductive hormones, feed intake and reproductive performance was s tudied over a period of 2 years. Cows were grouped into two categories, bas ed on calculated pedigree indices using multiple-trait across country evalu ation (MACE). Cows of high genetic merit (HGM, n = 48 in year 1 and n = 46 in year 2) had a mean predicted difference +/-standard deviation for milk p roduction of 475 +/- 76 kg. The cows of medium genetic merit (MGM, n = 48 i n both years) had a mean predicted difference for milk production of 140 +/ - 68 kg. The cows calved between January and April, and were offered grass silage ad libitum plus 9 kg concentrates per cow per day, irrespective genetic merit , from calving to turnout in March, when they were subjected to one of thre e grazing systems. Cows were available for rebreeding from late April until late July of each year. High genetic merit cows had higher milk production, incurred greater body c ondition loss between calving and first service and had lower plasma glucos e and insulin-like growth factor-1 (IGF-1) concentrations than medium genet ic merit cows. Furthermore, HGM cows had lower first and second service and overall conception rates, and required more services per conception than t he MGM cows. Cows that did not conceive to first service were retrospectively compared t o those that conceived to first service within each genetic merit group. Th ere were no significant differences between the HGM cows that did not conce ive to first service and those that conceived to this service in terms of m ilk production, body condition score change between calving and first servi ce, feed intake at first service. or in plasma concentrations of glucose, n on-esterified fatty acids (NEFA) or IGF-I. Medium genetic merit cows that d id not conceive to first service lost more body condition between calving a nd first service than did those that conceived to this service. In the present study, HGM cows had higher milk production and reduced repro ductive performance in comparison with MGM cows. However, reproductive perf ormance was not associated with milk production, feed intake or plasma conc entrations of glucose, NEFA or IGF-I between calving and first service, sin ce there were no significant differences in these variates between high or medium genetic merit cows that did not conceive to first service and those that conceived to this service. Therefore. these variates are unlikely to h e useful predictors of reproductive performance, under the conditions of th e present study. (C) 2001 Elsevier Science B.V. All rights reserved.