Abstract Despite growing evidence linking maternal nutrition to placental development, the specific effects of mid-gestation supplementation remain unclear, even though this period marks the peak of placental growth and vascular expansion essential for fetal development. The current study evaluated the effects of mid-gestation protein and energy supplementation on placental development in beef heifers. We hypothesized that maternal supplementation would enhance placental growth and vascularization, potentially supporting fetal development and long-term offspring productivity. Angus based heifers (n = 44; initial body weight (BW) 339.7 ± 33.0 kg.), were bred via artificial insemination using male-sexed semen from a single sire, and at d 90 of gestation were randomly assigned to receive either: 1) a forage-based control diet fed for target gains of 0.28 kg/d (CON; n = 22); or 2) the forage-based diet with the addition of a protein and energy supplement to target 0.79 kg/d (SUPP; n = 22) through d 186 of gestation. All heifers were then managed as a single group after d 186 through calving. Immediately upon expulsion of the placenta, samples of cotyledon (CON = 18; SUPP = 18) were collected and preserved in neutral buffered formalin. Tissues were subsequently stained using immunohistochemistry for key indicators of vascularization (using CD 31/34) and cell proliferation (using Ki-67), with data used to calculate capillary area density (CAD), capillary number density (CND), Ki-67 positivity ratio (PR), and spatial cell density (SCD). All statistical analyses were performed using the General Linear Model (GLM) procedure of SAS 9.4 with individual animal as the experimental unit. Data are reported as least squares mean ± standard error of the mean. Significance was considered at P ≤ 0.05, and tendencies were recognized at 0.05 P ≤ 0.10. Results showed that CAD tended to be greater (P = 0.07) in SUPP compared with CON, indicating enhanced placental vascular development and potential for improved nutrient transfer to the fetus. Conversely, CND was reduced (P = 0.02) in the SUPP, suggesting a more efficient network of fewer, but larger and more functional, capillaries. No significant differences were present between treatments for PR or SCD (P ≥ 0.51), indicating that overall cellular proliferation and tissue cellularity were not affected by maternal supplementation. These findings suggest that mid-gestation protein and energy supplementation can enhance placental vascularity, which is critical for fetal development, particularly by promoting vascular efficiency rather than increasing cellular proliferation. The development of a more advanced and efficient vascular network may improve nutrient and oxygen delivery to the fetus, potentially supporting greater calf growth and birth weights during the neonatal period.
Sullivan et al. (Wed,) studied this question.