Starch Source's Effect on Broodmares' Fecal Bacteria

Most easy-keeping horses can thrive on forages alone, but others, such as broodmares, need additional feedstuffs to meet their daily nutrient requirements. Highly palatable and digestible grain-based concentrates can help supply calories as nonstructural carbohydrates (NSC), most being starch. Because too much starch can upset horses’ gut microbial balance, a research team at the University of Kentucky (UK) sought to determine if starch source affects fecal levels of these microbes in broodmares.  

Normally, enzymes in the foregut (everything ahead of the large intestine) act on NSCs to aid absorption. If all the NSC is not digested, it spills over into the hindgut, negatively affecting the microbial environment and leading to hindgut upsets and potentially colic. As part of her graduate research in the Department of Animal and Food Sciences, Morgan Pyles and a UK team compared the effects of oat-based (OB) or corn- and wheat-middlings-based (CWB) pelleted concentrates on fecal amylolytic (capable of breaking down starch), cellulolytic (capable of breaking down cellulose), and Lactobacillus spp bacteria in mares prior to foaling through post-foaling.

Eighteen Thoroughbred mares took part in the study from 310 days gestation through four weeks post-foaling and were randomly assigned to either the OB or  CWB concentrate. Mares received 3.2 kilograms (7.05 pounds) of their respective concentrate per day prior to foaling and 4.8 kilograms (10.58 pounds) per day after foaling. Researchers collected fecal samples from mares two weeks after starting their assigned diet prior to foaling and at Day 1, 14, and 28 post-foaling to determine the number of cellulolytic, Lactobacillus spp, and amylolytic bacteria present.

Starch intake averaged 1.05 grams per kilogram of body weight per meal prior to foaling and 1.32 grams per kilogram of body weight per meal after foaling. Surprisingly, said the researchers, the starch source in the concentrate did not affect fecal amylolytic, lactobacilli, or cellulolytic bacteria in the current study.

Previous research in horses on a forage-only diet found alterations in fecal bacteria when researchers introduced a diet with similar starch levels to the ones in the current study. One reason for this could be that the mares in this study consumed a pelleted concentrate, whereas horses in the former study consumed minimally processed grains. Pelleting allows for opening up of the starch molecules in the feed, making way for enzymatic breakdown in the horse’s stomach and small intestine, reducing starch spillover into the hindgut.

Researchers then combined all the data from the two treatments to evaluate the fecal bacterial changes over time in the mares. Fecal amylolytic bacteria did not change significantly prior to foaling and post-foaling. However, fecal lactobacilli and cellulolytic bacteria were altered significantly. Lactobacilli numbers decreased at one day post-foaling, then returned to prepartum values by two weeks post-foaling. Cellulolytic bacteria also decreased one day after foaling and returned to prepartum numbers by four weeks post-foaling. Changes in hormones, stalling prior to foaling, and decreased hay intake rates might have affected fecal bacteria in these mares.

It’s possible the lack of differences in fecal bacteria between treatment groups was due to the processing of the cereal grains or because the mares had been fed concentrates for several months before the study began, Pyles explained. Therefore, the gut bacteria might have already adapted to a diet consisting of concentrates with forage.

“There are many changes happening around parturition in mares,” said Pyles, so “it is not surprising that we found the gut bacteria are also affected by this major event. Having a better understanding of the changes occurring around parturition may provide insight on the best management practices to prevent gastrointestinal upsets.”

Take-Home Message

Although the researchers saw no effect of starch source on fecal bacteria in broodmares, parturition did appear to alter the hindgut microbiota. Further studies are needed to address whether these changes during foaling can put mares at risk for hindgut upset.

Kristen Janicki, MS, PAS, is an equine nutritionist and freelance writer living in Nicholasville, Kentucky.

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