Optimum liveweight, feed intake, reproduction, and calf output in beef cows on North Island hill country, New Zealand

Abstract

Hereford × Friesian beef cows were managed on one of five lifetime liveweight profiles with the relative differences maintained throughout the 4‐year period of the project. The objective was to determine optimum beef‐cow liveweight profiles for use by farmers. In the first year, 115 in‐calf heifers entered the trial in January 1991 at age 18 months. This process was repeated annually to replace culled and empty cows so that in the second, third, and fourth years there were 139, 152, and 144 cows, respectively, on the trial with 4 age classes on the trial in the fourth year. Four times each year (pre‐calving, early lactation, mid lactation, and post weaning) dry matter intake by the cows was estimated using slow release chromic oxide capsules, faecal grab sampling, and estimates of in vitro digestibility of pasture samples. The cows and their calves were weighed every 4 to 6 weeks, and the calves were weaned at about 180 days when they left the trial. During most of the trial, mean cow liveweight spanned a range from 350 to 500 kg. The profiles were significantly different, as were their effects on cow condition score, cow pasture intake, calf liveweights, and, sometimes, cow reproduction. For every kg extra that the cows were heavier, dry matter intake increased by 15 to 60 g d ‘ depending on physiological status. When cow mating weight was fitted in regressions against cow reproductive items, optimum cow liveweight was estimated to be 440 kg with very little change in output over the range 420 to 450 kg liveweight. Calf growth rate showed a linear response of 1.2 g additional daily gain per kg increase in cow mating weight. Over 180 days from birth to weaning, this amounted to 0.22 kg extra calf weaning weight per kg of cow mating weight.