Early shearing of pregnant ewes linked to heavier, hardier lambs in first U.S. trial

Shearing ewes at day 50 of pregnancy, rather than at the customary later stage, produced larger and heavier lambs at birth in a South Dakota State University study, the first of its kind conducted in the United States. The work, led by Alex Vasquez-Hidalgo, an assistant professor in the SDSU Department of Veterinary and Biomedical Sciences, was recently published in the Journal of Animal Science. The findings point to a potential connection between limited exposure to cold stress during pregnancy and higher lamb birth weight, a factor directly tied to lamb survivability.

Vasquez-Hidalgo’s curiosity began with a basic physiological question. “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,” he said. “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 give birth in the spring are usually sheared at around 120 days of pregnancy, mainly 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 observed an increase in birth weights. He wondered whether the same results could be replicated in the U.S., and he became the first to conduct a study of this type there.

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

The choice of day 50 was not arbitrary. Vasquez-Hidalgo and graduate student Mara Johnson selected that interval because it aligns with the period when the placenta, the organ that passes nutrients from mother to fetus, is growing. From days 50 to 110 of pregnancy, the team used ultrasounds to measure umbilical blood flow and fetal and placental development in ewes that had been sheared at 50 days and in ewes that had not been sheared at all. When the lambs were born, at around 147 days, the researchers 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.

The measurements revealed a consistent pattern. 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. According to the researchers, all of these are good signs for long-term health.

The practical implications for sheep producers could be significant. “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.”

Because the study focused on ewes carrying multiples, the findings speak most directly to flocks where twin and triplet births are common, a situation in which low birth weight and reduced survivability are persistent challenges. The article reporting the work, titled “Shearing during early mid-gestation improves umbilical blood flow and fetal size in ewes carrying multiples,” was published on 10 September 2026, with the authors declaring no conflicts of interest.

Important questions remain before the practice can be fully explained. Vasquez-Hidalgo is currently building on this work and investigating whether the lambs of early-shorn ewes are in fact perceiving temperatures differently than their counterparts, and whether that difference gives them a survival and growth advantage. A separate line of inquiry is examining whether limited cold stress is the only factor contributing to the increased birth weight, or whether the act of shearing itself produces the same results.

For now, the study offers U.S. producers a low-cost option worth considering: moving the shearing calendar earlier for pregnant ewes carrying multiples. If the early results hold, a routine husbandry decision, timed to coincide with a critical window of placental growth, could translate into heavier lambs at birth, better blood flow in utero, improved weights at weaning and, ultimately, more lambs surviving to adulthood.

Subject of Research: Effects of limited cold stress from early-gestation shearing on lamb birth weight and survivability

Article Title: Limited cold stress during pregnancy could lead to increased lamb birth weight, survivability

Article References: Limited cold stress during pregnancy could lead to increased lamb birth weight, survivability. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: sheep, ewes, pregnancy, cold stress, lamb birth weight, survivability, placenta, umbilical blood flow, shearing, Journal of Animal Science, South Dakota State University, twin and triplet lambs