Effect of dietary starch source and concentration on equine fecal microbiota

Authors: HARLOW, BRITTANY - University Of Kentucky; LAWRENCE, LAURIE - University Of Kentucky; HAYES, SUSAN - University Of Kentucky; CRUM, ANDREA - University Of Kentucky; Flythe, Michael

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2016
Publication Date: 4/29/2016
Citation: Harlow, B.E., Lawrence, L.M., Hayes, S.H., Crum, A., Flythe, M.D. 2016. Effect of dietary starch source and concentration on equine fecal microbiota. PLoS One. 11(4): e0154037. doi:10.1371/journal.

Interpretive Summary: Horses evolved as forage-utilizing animals, but cereal grains are included in equine diets to increase energy intake. Starch from cereal grains may be digested in the small intestine or it may bypass small intestinal digestion and be fermented by the microbial community of the hindgut. When the bacteria in the hindgut ferment the starch, the hindgut can become more acidic and cause a number of health problems for the horse. Corn, oat, and wheat starches have similarities and differences, and this study was conducted to determine how each of these grains impacts specific groups of bacteria in the equine hindgut. Control horses received a hay-only diet. Experimental horses received the same hay, but were also gradually adapted to consume either 1 or 2 grams of starch per kilogram body weight. The form of the starch was either corn, oats or wheat middlings. Fecal samples were collected before the starch was added, during the adaptation and while the horses received the full amount of starch. The fecal samples from wheat- and corn-fed horses were more acidic than samples from hay-only or oat-fed horses. Several types of bacteria were counted in the fecal samples. The groups of bacteria were consistent for the hay-only horses, but the addition of the cereal grains changed the number of bacteria. Starch (2 grams per kilogram) caused the number of starch-utilizing bacteria to increase, and the cellulose (a fiber)-utilizing bacteria to decrease. These effects were more pronounced in the horses that received corn or wheat than oats. The lower level of starch (1 gram per kilogram) caused similar, but smaller changes in the bacteria. The low level of oats was very similar to hay-only. Another difference between corn and oats was that oats appeared to promote lactobacilli, a group of related bacteria that are generally considered beneficial. There were fewer lactobacilli in samples from corn-fed horses. The predominant starch-utilizing bacteria in the wheat- and corn-fed horses were enterococci, primarily Enterococcus faecalis. The conclusion was that different grains promote the growth of different groups of bacteria in the equine hindgut, even when the grains contain equal amounts of starch. The type of cereal grain, not just the amount of starch should be considered in the diets of horses, but further research is required before specific recommendations should be made. There are three major impacts of this finding. First, different starch sources not only promote different bacterial groups, but also affect which bacteria dominate those groups. Second, E. faecalis appears to be a major starch-utilizing bacterium in horses, which has not previously been described. Third, the predominant enterococci, including E. faecalis, were not detected with a method that is normally used to count these bacteria; therefore, their prevalence in horses might be underestimated.

Technical Abstract: Starch from corn is less susceptible to equine small intestinal digestion than starch from oats, and starch that reaches the hindgut can be utilized by the microbiota. The objective of the current study was to examine the effects of starch source on equine fecal microbiota. Thirty horses were assigned to treatments: control (hay only), HC (high corn), HO (high oats), LC (low corn), LO (low oats), and LW (low pelleted wheat middlings). Horses received an all-forage diet (2 wk; d -14 to d -1) before the treatment diets (2 wk; d 1 to 14). Starch was introduced gradually so that horses received 50% of the assigned starch amount (high = 2 g starch/kg BW; low = 1 g starch/kg BW) by d 4 and 100% by d 11. Fecal samples were obtained at the end of the forage-only period (S0; d -2), and on d 6 (S1) and d 13 (S2) of the treatment period. Cellulolytics, lactobacilli, Group D Gram-positive cocci (GPC), lactate-utilizers and amylolytics were enumerated. Enumeration data were log transformed and analyzed by repeated measures ANOVA. There were sample day × treatment interactions (P < 0.0001) for all bacteria enumerated. Enumerations from control horses did not change during the sampling period (P > 0.05). All treatments except LO resulted in increased amylolytics and decreased cellulolytics, but the changes were larger in horses fed corn and wheat middlings (P < 0.05). Feeding oats resulted in increased lactobacilli and decreased GPC (P < 0.05), while corn had the opposite effects. LW had increased lactobacilli and GPC (P < 0.05). The predominant amylolytic isolates from HC, LC and LW on S2 were identified by 16S RNA gene sequencing as Enterococcus faecalis, but other species were found in oat fed horses. These results demonstrate that starch source can have a differential effect on the equine fecal microbiota.