Not all cereal sources of starch are created equal when it comes to equine digestion, a study reveals.
Researchers from the Department of Animal and Food Sciences at the University of Kentucky set about examining the effects of dietary starch sources on the microbial communities that inhabit horse feces.
Cereal grains are often included in equine diets to increase energy. Corn, oat, wheat, and barley starches are similar in that they consist of amylose and amylopectin, but differ in the proportion of these carbohydrate molecules, and also in the form and structure of the starch granule.
Horses get a significant amount of their energy from fermentation of plant materials, particularly cellulose, in their massive hindgut.
With large carbohydrate-rich meals, some carbohydrates can reach the hindgut, where its fermentation can trigger a change in the balance of microbes, which can spark rising lactic acid levels and an increase in toxins. Such changes have been implicated in colic and laminitis.
Researchers Brittany Harlow, Laurie Lawrence, Susan Hayes, Andrea Crum and Michael Flythe set out to examine the effects of cereal-based starch sources on the microbial community found in horse dung – what is known as the equine fecal microbiota.
Thirty healthy horses were used in the research, all of whom first spent 14 days on a forage-only diet, near the end of which fecal samples were collected to assess the makeup of their microbial communities.
The horses were then randomly assigned to one of six treatments: hay only (the control group), high-corn, high-oats, low-corn, low-oats, and low-wheat middlings.
The researchers used coarsely cracked corn, whole but slightly rolled cleaned oats, and pelleted wheat middlings to make up the feeds. All horses continued to receive a measured hay allotment to maintain forage in their diet.
The oats, cracked corn and pelleted wheat middlings used in the study were analysed to obtain the sum of starch and ethanol-soluble carbohydrates in each feed. The amount given to the horses was adjusted to provide 1 gram of starch per kilogram of bodyweight in the low-treatment regimes, and double that in the high-treatment options.
In all cases, the starch was introduced gradually so that the horses received half the assigned starch amount by the fourth day on the new diets, and 100 percent by day 11. Fecal samples were obtained for analysis on the sixth day and on the 13th day of the feed period.
All samples – including those collected when all the horses were still on the forage-only diet – were analyzed to get counts for cellulolytic bacteria, lactobacilli, Group D gram-positive cocci, lactate-utilizing bacteria and amylolytic bacteria.
The researchers, writing in the open-access peer-viewed journal PLOS ONE, found no significant changes in the microbial counts in the control horses.
They found that all other treatments except the low-oats diet resulted in increases in amylolytic bacteria and decreases in cellulolytic bacteria, but the changes were larger in horses fed corn and wheat middlings. Feeding oats resulted in increased lactobacilli and decreased Group D gram-positive cocci, while corn had the opposite effects.
The low-wheat diet triggered increased lactobacilli and Group D gram-positive cocci.
The most common amylolytic bacteria from both corn diets and the low-wheat diet was Enterococcus faecalis, but other species were found in the oat-fed horses.
“A variety of predominant amylolytic bacteria were observed in horses fed hay and oats. However, the predominant amylolytic bacterium in corn and wheat fed horses was E. faecalis, which was present at much greater number (10,000-fold) in the horses on these diets.”
They noted that, at equal starch intakes, corn produced more marked changes in the fecal microbial ecosystem than oats, the researchers reported.
“Therefore, differences in composition and morphology [structure] of the starch granule, or in other components of the grains, could also affect amylolytic proliferation.
“These results demonstrate that starch source can have a differential effect on the equine fecal microbiota,” the study team wrote.
Starch from corn was less susceptible to digestion in the small intestine than starch from oats – and any starch undigested in the small intestine was available for fermentation in the hindgut.
The addition of starch to horse diets has often been linked to decreased digestion of fiber, the study team noted. The pointed to a 1984 study that found fiber digestibility decreased in horses when 60% or more of dietary forage was replaced with oats.
Discussing their findings, the researchers said it was important to consider that the starch concentrations used in their study were typical of feeding rates used for horses in many practical situations.
“Additionally, no horses displayed any outward symptoms of acidosis or related conditions over the course of the study.”
They noted 1992 research that suggested the the upper limit of starch digestion in the equine small intestine was between 3.5 and 4 grams of starch per kilogram of bodyweight per feeding.
“However, others have suggested that to limit starch bypass to the large intestine, starch intake should not exceed 2 grams per kilogram of bodyweight per meal.
“In this study, changes to the fecal bacterial community occurred when horses were fed low-wheat and low-corn.
“This latter result suggests that starch bypass to the large intestine occurs even at starch intakes below 2 grams of starch per kilogram of bodyweight per meal, at least when corn and wheat middlings were fed.
“The extent of starch digestion in the small intestine can be affected by processing, so the amount of starch that will result in bypass to the large intestine is likely to vary with both starch source and processing.
“In this study, the wheat middlings were pelleted, the corn was coarsely cracked and oats had been cleaned but not otherwise processed. Grinding and pelleting the oats and the corn may have resulted in less bypass of starch and fewer effects on the microbial ecosystem.”
They said their study employed a gradual introduction of starch to the feeding program, as was commonly recommended.
“More marked changes might have been noted if the addition of a high level of starch to the diet had been more abrupt,” they noted.
“Thus, the observed changes represent what can occur during the initial period when cereal grain starch is gradually introduced to horses consuming a forage-based diet. This study did not examine changes to the fecal bacteria that might occur after long-term adaptation of horses to a starch-containing diet.”
The suggested additional research was warranted to evaluate potential changes with long-term adaptation.
“Overall,” they said, “this study revealed that starch source can affect the microbial ecology of the equine digestive tract as measured in freshly voided feces.”
The use of feces as a proxy for microbial changes in the equine large intestine had become common, they said, and was minimally invasive.
“Conclusions drawn from changes observed in feces should be viewed cautiously as they may not precisely reflect either the magnitude or the time-course of changes in the cecum or colon. However, it seems likely that if differences due to diet are observed in feces at the end of the digestive process, that those differences would have been present in the earlier sites of digestion as well.”
They suggested further studies were needed to develop a better understanding of the development of several detrimental health conditions, such as colic and laminitis, that can be triggered by carbohydrate overload. Such research also had the potential to aid in the development of effective treatment strategies.
Harlow BE, Lawrence LM, Hayes SH, Crum A, Flythe MD (2016) Effect of Dietary Starch Source and Concentration on Equine Fecal Microbiota. PLoS ONE 11(4): e0154037. doi:10.1371/journal.pone.0154037