Effects of direct-fed microbial supplementation on digestibility and fermentation end-products in horses fed low- and high-starch concentrates

¹ University of Maryland, College Park, MD 20742
² Cargill Animal Nutrition, Innovation Center, Elk River, MN 55330

^* To whom correspondence should be addressed. E-mail: amyburk{at}umd.edu.

	Abstract

A study was conducted to determine whether direct-fed microbials (DFM) could be used to increase digestibility and minimize the risk of acidosis associated with feeding an increase in the amount of starch fed to horses. Fifteen mature Thoroughbred geldings were randomly assigned to 1 of 3 treatments in a 3 x 3 Latin square design balanced for carry-over effects. Within each 26-d period, horses were offered grass hay + low-starch concentrate (LS; 1.2 g starch·kg BW^-1·meal^-1) from d 1 to 13, and then abruptly changed to hay + high-starch concentrate (HS; 2.4 g starch·kg BW^-1·meal^-1) on d 14 continuing through d 26. The DFM treatments were offered in concentrate pellets at target dosage of 10⁸ cfu/(50 kg BW·d) as follows: no DFM (CON; control), Lactobacillus acidophilus (LAC1; single species DFM), or a mixture of L. acidophilus, L. casei, Bifidobacterium bifidium, and Enterococcus faecium (LAC4; multiple species DFM). Total feces were collected over 72 h from d 11 to 13 (LS; low dietary starch collection), from d 15 to 17 (AC; abrupt change in dietary starch collection), and at the end of each experimental period, from d 24 to 26 (HS; high dietary starch collection). Data collected consisted of total DM intake and fecal output, fecal pH, fecal acetate and propionate concentrations, and viable numbers of DFM in the feed. With the exception of Fe digestibility, there were no starch by DFM interactions. There was an effect of starch level (P 0.02) on most nutrient digestibilities, except for Mg (P = 0.08) and CP, K, and Zn (P > 0.10). Horses supplemented with LAC4 had increased ether extract (P < 0.05) and a tendency for decreased Na (P < 0.10) digestibilities compared to CON horses. All DFM supplemented horses had increased Cu (P < 0.05) and Fe, and numerically increased Zn digestibilities compared to CON horses. Fecal pH decreased (P < 0.05) and fecal propionate concentration increased (P < 0.05) as dietary starch content changed from LS to HS. There was a tendency for elevated fecal pH (P < 0.10) in LAC1 horses compared to CON horses. These results confirm that increasing starch in the equine diet can enhance nutrient digestibility of the diet. Supplementing equine diets with either a single or mixed strain direct-fed lactic acid bacteria had limited effects on nutrient digestibility or on reducing the risk of acidosis associated with feeding high-starch concentrates to horses. The potential response of DFM supplementation should be evaluated when a more acute acidotic state is induced in horses than in the current study.

Key Words: digestibility, direct-fed microbials, horses, Lactobacillus acidophilus, probiotics, starch