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Sudden Dietary Changes in Horses: Yeast ResearchBy Kentucky Equine Research Staff · February 25, 2015

Among the risk factors for colic in horses, an abrupt change in feed or hay ranks high. Sudden variations in diet disrupt the balance of microbes in the horse’s hindgut, and the result of this disruption can be acidosis, colic, or laminitis.

If such a change can’t be avoided, horse owners sometimes use hindgut buffers, probiotics, or other dietary supplements to moderate any adverse effects. French researchers* evaluated the effect of yeast supplementation on the microbial balance in the hindguts of horses that were subjected to a sudden change in hay.

Researchers used six mature geldings in the study. The horses were fitted with cannulas in the cecum and right ventral colon. Horses consumed hay and grain twice a day for seven weeks, with three horses getting Saccharomyces cerevisiae yeast with the morning meal, and the others acting as a nontreated control group. At the start of the fifth week, all horses were switched from an early-cut meadow hay to a later-cut hay of the same type. The later-cut hay contained a large proportion of Rumex foliage (herbs also known as sorrel or dock). All horses ate the later-cut hay for the final three weeks of the period. The horses were then fed the early-cut hay without yeast supplementation for a six-week washout period, after which the experimental treatment period of seven weeks was repeated with the groups switched.    

The researchers collected samples of each horse’s cecum and colon contents four hours after the morning meal at intervals both before and after the abrupt change in hay types. Researchers recorded total anaerobic bacteria, fibrolytic bacteria, volatile fatty acid (VFA) concentrations, and pH levels for each sample.

Results did not show a relationship between yeast supplementation status and changes in the cecum and colon. In all horses, pectin-digesting bacteria increased in the cecum after the change in hay, and a significant increase in the total VFA concentration in the colon documented.

The researchers commented that although the two batches of hay were very similar in composition, significant changes were seen after the abrupt change, indicating that the microbial balance is sensitive to even minor dietary variations. With a greater difference in hay types, a larger imbalance could be expected and yeast supplementation could have led to a more significant difference between horses in the treatment groups.

*Grimm, P., V. Julliand, C. Philippeau, and S. Sadet-Bourgeteau. 2014. Effect of yeast supplementation on hindgut microbiota of horses subjected to an abrupt change of hays. In: Proc. 7th European Workshop on Equine Nutrition, p. 53-55.