47. Effects of Mild Nutritional Restrictions During Mid-gestation on Neonatal Calf Performance and Placental Function in Extended-grazed Beef Cows.

Abstract

Extended grazing (EG) is a cost-effective and sustainable winter-feeding strategy for the management of pregnant beef cows. In spring calving herds, EG often overlaps with mid-gestation, exposing cows to mild protein restrictions, potentially affecting placental development and fetal growth. This study investigated whether protein supplementation during EG enhances placental morphology and health, and how these changes relate to calf vigor, birth weight, and growth. Angus cows (n = 32) were assigned to two groups during a simulated EG to either receive an individual protein supplement (S; n = 16) or remain unsupplemented (NS; n = 16). Treatments were applied during mid-gestation (86.0 ± 3.5 to 176.0 ± 3.5 days of gestation), during which S and NS cows were provided either 100% or 80% of their crude protein (CP) requirements, respectively. During early (<86 days) and late (>176 days) gestation, all cows were managed on pasture under similar conditions. Cows were sampled from the start of supplementation until calving to measure serum bovine pregnancy-associated glycoprotein (bPAG) concentrations as an indicator of placental health, and placental cotyledon tissues were collected at calving for histological assessment of vascularization and collagen distribution within the cotyledon tissue. Additionally, calf birth weight, vigor, and weaning weight were recorded. Repeated measures data were analyzed using a generalized linear mixed model approach, while variables measured at a single time point were analyzed using either Student's t-tests or Wilcoxon rank sum tests in R. Mild protein restriction (20% CP decrease) during mid-gestation did not affect calf birth weight, vigor, weaning weight, ADG, or maternal serum bPAG concentrations during gestation (p > 0.05). However, cotyledon vascular area density (NS: 14.01 ± 1.88% vs. S: 9.60 ± 1.16%; p = 0.04) and collagen distributed area fraction (NS: 24.78 ± 3.99% vs. S: 13.35 ± 2.62%; p = 0.01) were significantly higher in NS than S. These findings suggest a compensatory increase in placental vascularization along with an increase in extracellular connective tissue surrounding blood vessels, under protein-restricted conditions. Overall, mild protein restriction during mid-gestation did not affect calf outcomes, possibly due to compensatory placental adaptations. Because these mild restrictions do not produce immediate effects that producers can detect in calves, it remains important to determine whether these compensatory mechanisms occur at the expense of long-term calf health. Therefore, ongoing investigations will evaluate the effects of mild nutrient restrictions during EG on calf immune priming.