Supplementing late-gestation mares with omega-3 fatty acids appears to help shrink the uterus faster after foaling, which might make them more likely to conceive on their foal heat, researchers have reported.

“A rapid uterine involution could improve fertility in foal heat, which occurs around five to 12 days after foaling, because the uterine involution process is still incomplete at this stage,” said Julia Rizzo de Medeiros Ferreira, PhD, of the Department of Animal Nutrition and Animal Production at the University of São Paulo, in Pirassununga, Brazil.

Rizzo de Medeiros Ferreira and her fellow researchers worked with 18 embryo recipient mares of mixed breeds at a Brazilian stud farm. All the mares were pregnant with Paint Horse foals and maintained on pasture with a standard ration of commercial concentrate feed.

The scientists provided half the mares with a daily supplement of about 25 grams (less than an ounce) of microalgae rich in docosahexaenoic acid (DHA), an omega-3 fatty acid naturally present in algae oil. They began supplementation 90 days before each mare’s expected foaling date and ended it seven days after her first ovulation post-foaling.

They assessed each mare’s uterus and ovaries via ultrasound six times: Days 3, 7, 11, and 15 after foaling, and Days 4 and 7 after ovulation. After the mares foaled, the researchers took endometrial (uterine lining) samples after seven and 11 days, as well as four days post-ovulation.

Rizzo de Medeiros Ferreira said each study mare’s uterus became smaller over time after foaling. However, the diameters of the uterine horns in the supplemented mares were consistently smaller than those of the nonsupplemented mares.

“The uterine involution process begins with a drastic decrease in diameter of uterine horns during the first week after parturition (foaling),” she said. “Our study demonstrated that dietary supplementation with microalgae rich in DHA resulted in a lower horn diameter, such that the involution process required less effort to return to normal size prior to cycling and ovulation.”

The researchers also found that the uteri of supplemented mares appeared lighter in color on ultrasound—meaning they bounced back stronger sound waves (known as echogenicity)—than in nonsupplemented mares. This might suggest the mares had less circulating estradiol—a hormone associated with uterine inflammation and heat behavior, said Rizzo de Medeiros Ferreira.

“Although we were not able to conclude the exact reason for this greater uterine echogenicity, one of the possibilities would be the reduction of the inflammatory process during the postpartum period,” she said. “Mares from the control group could have a more inflamed uterus postpartum, leading to less echogenicity.”

The scientists detected no notable differences in other reproductive factors, including body condition score, time to first foal heat, endometrial inflammatory gene expression, or the presence of intrauterine fluid after foaling, said Rizzo de Medeiros Ferreira.

“Increasingly, studies are showing that diet has a direct influence on progeny and fertility,” Rizzo de Medeiros Ferreira said. “I believe that polyunsaturated fatty acids from (the) omega-3 family have great potential for use in mare diets; however, more studies evaluating pregnancy outcomes should be performed.”

The study, ”Uterine Involution of Mares Supplemented with Dietary Algae-Derived Omega-3 Fatty Acids During the Peripartum Period,” was published in the Journal of Equine Veterinary Science in November 2021.