Study reveals potential link between limited cold stress during pregnancy and birth weight

Research out of South Dakota State University has highlighted a potential connection between limited exposure to cold stress during pregnancy and birth weight. The work showed that that shearing ewes at day 50 of gestation, rather than after 100 days, resulted in larger and heavier lambs at birth.

"It's always been known that heat stress, in any pregnant animal, is going to cause problems for the developing fetus, so I wondered what cold stress does," said Alex Vasquez-Hidalgo, Ph.D. an assistant professor in the SDSU Department of Veterinary and Biomedical Sciences. "It appears that some cold stress, not extreme of course, but some cold stress is beneficial for the physiology of the pregnant ewe."

In the United States, ewes that will birth in the spring are usually sheared at around 120 days of pregnancy to improve cleanliness during lambing. Vasquez-Hidalgo found that in other countries with similar climates, researchers had shorn ewes earlier in pregnancy to simulate cold stress and had seen an increase in birth weights. He wondered if the same results could be replicated in the U.S. and was the first to conduct a study of this type. His findings were recently published in the Journal of Animal Science.

"The idea is that while the ewe's body works to keep warm, it passes physiological information to the growing lamb, preparing the baby to be adaptable to a colder climate," Vasquez-Hidalgo said. "Our research showed that twin and triplet lambs born to ewes shorn at day 50 had higher birth weights. This is good for producers because there is a positive correlation between birth weight and survivability."

Choosing to shear the ewes at day 50 wasn't a random interval. Vasquez-Hidalgo and graduate student Mara Johnson chose that day because it aligns with when the placenta, the organ that passes nutrients from mother to fetus, is growing. From days 50 to 110 of pregnancy, they used ultrasounds to measure umbilical blood flow and fetal and placental development in ewes that had been sheared at 50 days and ewes that hadn't been sheared at all. When the lambs were born, at around 147 days, they recorded the weight of the lambs and the placentas and also collected placental tissue to assess how gene expression in the organ varied between the groups.

In addition to higher birth weights, twin and triplet lambs whose mothers had experienced limited cold stress were also larger in size, had increased umbilical blood flow and months later were heavier at weaning. All good signs for long-term health.

"We can tell a producer it is likely if you shear ewes on day 50 of pregnancy, specifically if they're carrying twins or triplets, you're probably going to improve lamb birth weights and survivability," Vasquez-Hidalgo said. "This is a managerial tool that producers don't have to spend more on."

Vasquez-Hidalgo is currently building on this work and investigating if the lambs of early-shorn ewes are in fact perceiving temperatures differently than their counterparts and if that gives them a survival and growth advantage. This research project is looking at if limited cold stress is the only factor that is contributing to the increased birth weight or if the act of shearing itself has the same results.