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Heat stress reduces fetal growth in late gestation in cattle, driven by shifts in placental form and function. Sheep are short-day breeders, but they are also bred off-season, allowing late summer lambing, which is associated with heat stress exposure in late pregnancy. Citrulline is known to induce nitric oxide release and vasodilatation and may not be degraded by the rumen, but its impacts on placental function are unknown. This study aimed to: 1) evaluate the effects of late gestation heat stress on placental/fetal development in ewes; and 2) evaluate if citrulline supplementation mitigates heat stress impacts. To that end, 28 pregnant ewes were randomly assigned to each treatment in environmental chambers: control (CN; n = 14, 18 °C) or heat stress (HT; n = 14: 28 °C daytime and 25 °C nighttime) during their last month of gestation. Within temperatures, animals received citrulline (0.5 % dry matter intake (DMI), CT) or not (NO), resulting in a 2 × 2 factorial design: CNCT, CNNO, HTCT, and HTNO (n = 7/trt). Respiration rates (RR) and rectal temperature (RT) were measured once weekly and placental perfusion was estimated by quantitative power Doppler ultrasonography at the beginning and 15 days after the start of the experiment. Gestation length (GL) and lamb birth weight (BW) were recorded. Placentas were collected at spontaneous delivery (3 ± 0.66 h postpartum). Data analysis used the GLIMMIX procedure in SAS with fixed effects of ewe, lamb sex, category, treatments, and their interaction. In ewes, HT increased RR, whereas CT decreased RT in both treatments. HT decreased GL and lamb BW tended to be lower in the HTCT group. Placental morphology did not differ among treatments, but female twins had greater cotyledon number and lower placental efficiency. In conclusion, exposure of ewes to HT during late gestation reduced GL, confirming observations in cattle, and CT tended to reduce BW in heat stress conditions. Further investigations are ongoing in placental function and transcriptomics are currently being evaluated.
Casarotto et al. (Thu,) studied this question.