Effect of genetic merit and concentrate feeding on reproduction of grazingdairy cows in a subtropical environment

This study was conducted to determine the effects of genetic merit for milk production and concentrate feeding on the intervals to the onset of postpa rtum cyclic ovarian activity, first insemination, and establishment of preg nancy. Other factors considered included biochemical measures of energy bal ance, liveweight, and measures of production. A farmlet study was conducted with two lines of cattle that differed in Australian Breeding Value by 735 L milk and 47 kg of fat plus protein that were fed 300 or 2000 kg of conce ntrate per cow per year in a subtropical grazing system based on ryegrass, clover, and kikuyu pasture. Milk yield was higher in cows fed 2000 kg of co ncentrate per cow per year (23.6 L/d) than those fed 300 kg/cow per year (1 8.2 L/d), and cows of medium genetic merit (19.6 L/d) produced less milk. t han those of high genetic merit (22.2 L/d). Plasma glucose concentration wa s lower in the cows of high genetic merit (3.11 mmol/L), than in the cows o f medium genetic merit (3.39 mmol/L). The cumulative probabilities of ovula tion, insemination, and establishment of pregnancy by any day postpartum we re determined by a Weibull distribution. Medium genetic merit cows in winte r were more likely to ovulate by any day postpartum than in summer and more likely than high genetic merit cows in either summer or winter. The cumula tive probability of insemination was lower in primipara than multipara and in summer than in winter. Primiparous cattle had a lower cumulative probabi lity of pregnancy by any day postpartum.