Producers are often told that supplementing bred heifers with protein prior to calving increases calf birth weight and leads to greater calving difficulty. Nutrition of bred heifers during gestation does have lasting consequences for both the calf and the future productivity of the cow. Decades of research clearly demonstrate that maternal undernutrition during pregnancy negatively influences not only the cow’s reproductive performance, but also immune transfer, calf survival, weaning weight and post-weaning performance.
Research conducted in the 1970s at Oklahoma State University (OSU) compared productivity of heifers managed to achieve either “adequate” or “obese” body condition from 12 months of age through 5 years of age. At first calving, 58% of the obese heifers required calving assistance, compared with only 8% of heifers that managed to maintain adequate body condition. These data are the basis for current recommendations that heifers calve at a body condition score (BCS) of approximately 6, but not reach “fleshy” (BCS 7) or “obese” (BCS 8) conditions.
Energy and protein supply during late gestation can influence calf birth weight. Classic research published in 1975 demonstrated that heifers restricted to 65% of energy requirements during the final 100 days of gestation produced calves that were approximately 4.4 pounds lighter at birth. However, these calves experienced greater neonatal mortality and reduced weaning rates compared with calves from adequately fed dams. Importantly, lighter birth weight was not associated with reduced calving difficulty, illustrating that undernutrition compromises calf viability rather than preventing dystocia. Restricting nutrients prior to calving weakens both the cow and the calf, increasing calving difficulty and reducing calf survival.
Birth weight responses to pre-calving supplementation are highly variable. The changes in calf birth weight reported from 24 studies looking at late-gestation supplementation were evaluated. The average increase in calf birth weight was only 3 lbs., with responses ranging from a 3-lb. decrease to a 10-lb. increase. The largest increases occurred when high levels (approximately 5 lbs. per day) of energy-dense supplements were fed. Fifteen of these studies also reported subsequent pregnancy rates. Although variable, the average pregnancy rate of unsupplemented cows was 86%, compared with 92% for cows supplemented during late gestation, with the greatest response observed in first-calf heifers.
Excessive energy intake during late gestation partitions more nutrients toward fetal growth, resulting in larger calves. When coupled with excess fat deposition in the pelvic region, this increases the risk of dystocia. Thus, excessive energy, rather than protein supplementation alone, is the primary contributor to increased calving difficulty in many heifer programs.
Ensuring bred heifers meet—rather than greatly exceed—energy and protein requirements, promoting moderate BCS gain during mid- and late gestation, and strategically supplementing key nutrients improves calf survival, pre-weaning growth, immune function, and long-term reproductive performance without increasing calving difficulty.
Avoiding maternal undernutrition remains one of the most consistent strategies for improving whole-herd productivity and profitability. — Paul Beck, OSU Extension Beef cattle nutrition specialist
