Effects of weight and pelvic area at mating on dystokia in beef heifers

Abstract

Over six years, lighter heifers mated in spring at 15 months of age conceived later in the joining period and, especially if mated after the first three weeks, had a higher (P < 0.001) incidence of dystokia than did heavier animals. Low nutrition in late pregnancy reduced calf birth weights and reduced heifer weights before calving, but there was no reduction in difficult births. Angus x (Angus x Friesian) heifers were lighter at joining and had higher dystokia rates than Angus x Friesian heifers. They also had higher dystokia rates than Angus heifers, which had lower calf birth weights than either of the cross-breeds. Heifers with dystokia had calves on average 2.3 kg heavier and pelvic areas 14 cm2 smaller than those calving normally. Twice as many heifers with bull calves had dystokia than did those with heifer calves. Our best prediction of dystokia was obtained from data on pelvic area at the start of joining. We suggest that heifers that are light at 15 months of age and have small pelvic areas should not be joined until 19 to 24 months of age.