Effects of heat stress during late gestation on placental morphometry at term in primiparous and multiparous dairy cows

Abstract

Heat stress (HS) during final third gestation (FTG) can impair placental development in dairy cows. Placental development and function in primiparous (P), which are still growing, may respond differently to HS compared to fully grown multiparous (M) dairy cows. The aim of this study was to evaluate the impact of voluntary access to Natural shade on term placental morphometry in P and M dairy cows exposed to HS during FTG. The experiment was carried out at the Salto Experimental Station (Faculty of Agronomy, Udelar, Latitude: 31º 23´ S and Longitude: 57º 57´ W) in summer 2024 with a ITH index >72 (approved ethical protocol No. 1389). Holstein Friesian primiparous (P; n = 10) and multiparous (M; n=12) pregnant dairy cows (BW = 566 ± 55 kg; BCS = 2,9 ± 0,27) managed in a grazing system were dried off 72 ± 17.1 days before calving and randomly assigned to two treatments: with (ANS) and without (NNS) voluntary access to NS. Placentas were collected at birth (4.6 ± 2.4 h) and analyzed for placental weight (PW), intercotyledonary membrane weight (ICW), and cotyledon number (CN), weight (CW), and surface area (CSA). Cotyledons were classified into six size categories based on their major and minor diameters (1 = smallest, 6 = largest). The cotyledon surface area was calculated based on an elliptical shape, and the mean values were subsequently analyzed. P showed higher PW (6.0 vs 5.0 kg; p = 0.044), ICW (4.3 vs 3.1 kg; p = 0.007), CN (94 vs 72; p = 0.025), MCW (19 vs 28 g; p = 0.008), and MCSA (0.040 vs 0.059 m²). Additionally, higher CW3, higher CN1, and larger SA1, SA2, and SA3 were observed compared to M, with tendencies for higher CW2, lower CW5, and higher CN2. P exhibited a compensatory placental development, probably due to the overlapping growth and gestation nutrient requirements.