Rapid Starch Increases, Decreases in Horse Diets Upset Gut Microbiome
It’s one of the golden rules of horse feeding: Make all grain and concentrate changes gradually. Veterinarians have long known the equine digestive system can be sensitive to dietary changes and prone to upset, but, to date, there’s still little research in live horses focusing on how dietary changes can affect the hindgut’s microbial community.

So, a group of researchers at Texas A&M University, in College Station, led by Christine Warzecha, MS, set out to discover how short-term starch inclusion in the diet—essentially, a rapid increase and then decrease in the amount of starch a horse consumes—affects the equine cecal microbiome.

The team fed seven cecally cannulated Quarter Horse geldings (meaning they had tubes permanently placed through their sides and into the cecum so the researchers could collect intestinal contents) a treatment diet for two 28-day periods with a 28-day washout in between. The diets consisted of a commercial concentrate with 30% nonstructural carbohydrates fed at 0.6% body weight (the low-starch diet) or 1.2% body weight (the high-starch diet), along with free-choice coastal bermudagrass hay.

On the first day of each test period, the researchers collected cecal samples prior to the morning meal and again at three-hour intervals for 12 hours to observe the diet’s immediate effects. On Days 2, 3, and 7, they gathered cecal samples six hours after the morning meal. The team analyzed the samples’ pH levels immediately and then froze them for later volatile fatty acid (VFA) content analysis and microbial DNA extraction.

The researchers found that cecal pH tended to decrease (become more acidic) during the 12 hours after the Day 1 meal and remained lower for the seven subsequent days when horses consumed the high-starch diet. The average cecal pH did not reach levels indicative of hindgut acidosis (a pH of less than 6; hindgut acidosis can cause problems including anorexia, colic, and laminitis) during this period; however, two individual horses on the high-starch diet did have a cecal pH of less than 6 at one time point during the first three days of consuming the diet.

Previous study results have shown that feeding a diet containing highly fermentable feeds (such as the high-starch diet provided in this study) can alter VFA proportions—specifically, increasing propionate and decreasing acetate production. So it wasn’t surprising that horses consuming the high-starch diet in this study produced more total VFAs at all periods measured. In the Day 1 samples for each test period, the acetate-to-propionate decreased from 3.54 to 3.07 in low-starch horses and from 3.54 to 2.60 in high-starch horses.

Finally, the researchers used core microbiome analysis to identify which microorganisms were present in each sample. They identified a core microbiome consisting of only 10 operational taxonomic units (OTU), a term used to classify groups of closely related organisms present in all samples. Adding starch to the diet adversely affected the cecum’s microfloral diversity, and the core microbiome only represented 0.18% of the total OTUs present in the cecal samples.

The researchers also noted that, in response to increased dietary starch concentrations:

  • Proteobacteria increased at all sampling times and to the greatest degree with the high-starch diet, indicating it might play a role in fermenting protein and starch in the cecum;
  • Fibrobacter, a group of fiber-fermenting bacteria, decreased at all sampling times when concentrate was added to the diet;
  •  YRC22 concentrations increased when horses consumed the high-starch diet;
  • Paludibacter and S24-7 levels increased after Day 1 when horses consumed the low-starch diet;
  • Prevotella increased only in the first day after concentrate introduction;
  • Anaerovibrio, which produces the VFAs propionate and acetate, and Oscillospira, which breaks down lipids (fats), increased immediately after researchers added concentrates to the diet;
  • Streptococcus, Anaerostipes, and Phascolarctobacterium concentrations increased over the sampling times with both diets; and
  • Ruminococcaceae, Clostridiaceae, Coriobacteriaceae, and Clostridiales decreased over all sampling times after concentrates were added, indicating they play a role in fermenting fiber.

What Does it All Mean?

Feeding grain can help our horses receive balanced nutrition and meet increased energy and nutrient demands of growth, training, pregnancy, and lactation. But, as this study revealed, adding concentrates to horses’ diets resulted in immediate and short-term effects on the cecal microbiome, pH, and VFA production, thus reducing the cecum’s microbial diversity.

Further research using sudden changes in diet and starch levels will be key to unveiling the specific role of these microbes, but this study yielded some practical information owners can use immediately.

“From this set of data, my recommendation would be to transition horses to new concentrates over seven to 10 days and never short them on forage, which will promote diversity of the microbial community,” Warzecha said.

The next steps will be to observe microbiome changes with these concentrate rations over longer periods.

The study, “Influence of short-term dietary starch inclusion on the equine cecal microbiome,” was published in the Journal of Animal Science.