Heifer development systems for March-born heifers and improving pregnancy rates in June-calving cows

Typical heifer development programs are designed to feed heifers to reach 65 percent of mature body weight prior to the first breeding season, however, recent research from the Gudmundsen Sandhills Laboratory reported no differences in performance in heifers reared to 55 or 60 percent of mature weight before breeding. A three year follow-up trial was conducted to compare developing heifers to 55 percent of mature weight with a 45-d breeding season versus 50 percent of mature weight followed by a 60-d breeding season. There was no difference in first- or second calf conception rates between the two systems; however, development to only 50 percent of mature resulted in a twelve dollar per head reduction in the cost to place one bred two-year old into the cow herd. The extended breeding season in the 50 percent system did result in lighter weaning weight of calves, but the reduction in returns from lower weight calves is more than offset by the reduced feed costs during the development period associated with that system. Low second-calf conception rates in June-calving cows create a challenge if replacement cows are reared from heifers within that system. Two experiments were conducted in attempt to increase the rebreeding success rate of lactating, primiparous June-calving cows. In both experiments, cows were supplemented for 45-d prior to the breeding season. Supplements were designed to meet net energy and metabolizable protein requirements. Forage quality, and supplement composition, resulted in marked differences in response to supplementation between the two experiments. During periods of severe nutrient deficiencies, supplementation improved cow weight gains and reproductive performance, while supplementation was less effective during periods of increased forage quality. Pregnancy rates in mature June-calving cows have remained numerically lower compared to the overall pregnancy rates in the March-calving herd. Therefore, a 3-year experiment was conducted to determine the effects of plane of nutrition (pasture type) and bull exposure during the pre-breeding period and plane of nutrition during the breeding season on pregnancy rates in lactating, multiparous June-calving cows. Pre-breeding pasture type had no effect on subsequent reproductive success, but pre-breeding bull exposure did interact with breeding pasture type when influencing pregnancy rates. Previous exposure to bulls is effective in increasing pregnancy rates in June-calving cows only if cows are bred on meadow pastures