Acidosis and Starch Fermentation Studied

Horses in race training usually need more energy than they can derive from all-forage diets, so most racehorses are fed large amounts of grain in addition to hay. The horse’s gastrointestinal system is not set up to process large grain meals, a fact that may lead to digestive and metabolic problems when undigested starch impacts the hindgut.

This study was designed to provide information on grain feeding practices in Australia and to determine their impact on the hindgut environment. Digestibility of various grains was quantified and fecal analyses were conducted to establish which grains held the greatest risk of hindgut starch fermentation and acidosis.

How was the study performed?

Trainers in New South Wales were contacted by postal survey to invite them to participate in the study and to ask about their feeding practices. Information was collected about feeds and feed management, and site visits were made to collect samples of grain, chaff, and feces.

Grain samples were analyzed for total starch, enzymatic starch digestibility, dry matter, and nitrogen content. Fecal samples were analyzed for pH, dry matter, starch, nitrogen, volatile fatty acids, and lactic acid.

What results were found?

For the 72 trainers involved in the study, each trained between 1 and 35 horses (average 9.6). In all, information was collected on 690 horses. A small number were colts or stallions, with the remainder split between geldings (48%) and fillies/mares (43%).

Oats, corn, commercial premixed feeds, and barley were fed most commonly. About 25% of trainers fed sunflower seeds, and smaller numbers added faba beans, soybeans, lupins, or chick peas. A majority of trainers based feed amounts on how much work a particular horse was performing, while about a quarter of the trainers chose quantities based on the appetite of the horse. Small numbers of trainers said that temperament and body weight were the most important factors to consider when determining how much to feed each horse. Amounts fed ranged from 3.8 kg to 13.2 kg (8.4 lb to 29 lb) of grain per day, with an average of 7.3 kg (16 lb).

Horses were fed grain twice a day by 82% of trainers; 15% of trainers fed three times a day; and a few fed four times a day. Hay was supplied at the same time as grain by just under half the trainers. Others provided hay a few hours before to a few hours after grain meals. Hay was available from no time (one trainer fed no hay at all) to constantly, varying by trainer. Commercial feeds most commonly consisted of mixed grains and contained whole, cracked, or other processed grains. Outside of commercial feeds, of the total corn, faba beans, and chickpeas, 40% were cracked before feeding. About 27% of grains were fed whole with no processing. Oats, sunflower seeds, and some corn fell into this category. Other forms of processing included grinding, micronizing, steam flaking, extruding, and boiling.

About three-quarters of the trainers formulated a specific diet for each horse, while the others fed a standard diet to all horses at a particular level of training. The trainer-formulated diets typically contained three or four types of grain, and most included at least one commercial premixed feed that comprised about half of the ration. Virtually all trainers included chaff in the feed.

Analysis of grains showed oats had the lowest and corn had the highest average starch content.

However, enzymatic starch digestibility was highest in oats and lowest in corn. For commercial mixed feeds, starch content was lower than in oats or corn, while starch digestibility was higher than that of corn. The in vitro digestibility of corn was approximately doubled by micronization or extrusion. Fecal pH ranged from 5.5 to 7.9, with an average of 6.5. Average fecal starch content was 1.2% DM, and fecal volatile fatty acid concentration averaged 30.9 mmol/L. Acetate to propionate ratio was 3.7, and fecal lactate concentrations ranged from 0.05 mmol/L to more than 3 mmol/L. A strong negative relationship was found between fecal propionate concentration and fecal pH, with high propionate concentrations found at low fecal pH. Quantity of indigestible starch fed per horse per day was positively related to the concentration of total fecal acid.

The authors of this study commented, “Approximately 27% of fecal samples collected had a pH lower than 6.2, which is below that considered optimal for cellulolytic and lactate utilizing bacterium. Fecal pH was negatively related to fecal propionate, indicating that these declines in fecal pH are caused by the fermentation of starch by amylolytic bacteria in the hindgut, which characteristically produce more propionate during carbohydrate fermentation than cellulolytic bacteria.” Fecal starch concentrations were also negatively related to fecal pH.

What does this tell us about feeding horses to prevent hindgut acidosis?

Grain is not a natural part of the equine diet, but is often required for the energy demands of high performance. Consumption of large amounts of grain may lead to hindgut starch fermentation and acidosis. Studies have shown that as meal size and starch intake increase, there will be a rise in the amount of undigested starch reaching the hindgut. In this study, however, the type of starch fed was found to be more significant than the amount. The authors stated, “In contrast to the total quantity of grain fed/horse/day, it appeared that the quantity of indigestible starch fed/horse/day had an effect on total fecal acid concentrations and thus on the incidence of hindgut starch fermentation. More specifically, the quantity of indigestible corn starch fed in the diet appeared to be the major factor affecting the incidence of hindgut starch fermentation and thus total fecal acid concentrations.”

This study indicated that feeding practices among the participating trainers resulted in some degree of hindgut starch fermentation and acidosis in at least one-quarter of the horses studied. Hindgut acidosis can impair performance, and could be career-threatening if it leads to laminitis. The authors suggest the following management steps to decrease the occurrence and impact of hindgut acidosis: Feeding mixed grains or commercial horse feeds rather than straight whole corn. Feeding processed (micronized, steam-flaked, extruded or otherwise heat-processed) grains and/or selecting grains for ease of small-intestine digestibility.

The full text of this article was published in the Australian Veterinary Journal, Vol. 84, No. 11 (November 2006), pp. 402-407.

The study “The effect of current grain feeding practices on hindgut starch fermentation and acidosis in the Australian racing Thoroughbred,” was conducted by N Richards, GN Hinch, JB Rowe.