Effects of phytase supplementation in mature horses fed alfalfa hay and pelleted concentrate diets.

An experiment was conducted to study P digestibility in mature horses because of the growing environmental concerns regarding P runoff and previous equine research focused mostly on young and growing animals or used ponies as a model. Phytase supplementation of swine and poultry diets can result in greater phytate-P digestibility, leading to a decreased need for inorganic P supplementation and a decrease in P excreted to the environment; this, however, has not been demonstrated in the horse. Six mature Arabian geldings were fed 6 diets consisting of pelleted concentrate and alfalfa hay. The concentrates consisted mainly of soybean hulls, ground corn, wheat midds, broken rice, and beet pulp, and phytase was added to the concentrates accordingly before pelleting. There were 3 diet types: control (concentrate and hay), high P (greater P concentrate and hay), and forage only, and each diet type included 1 phytase-supplemented diet and 1 non-phytase-supplemented diet, resulting in 6 total diets. Phytase supplementation for the forage only diet was accomplished by feeding a nominal amount of concentrate formulated solely as a vehicle for the phytase. Horses had unrestricted access to water throughout the experiment. Using a Latin square design, all horses received all diets over a period of 12 wk. In each week, the new diet was fed for 11 d of diet acclimation, which was followed by a 3-d total collection of feces and urine for each horse. There was no effect (P < 0.05) of phytase supplementation on P output in the urine or feces, resulting in no differences in P apparent digestibility. Analysis of the feed and feces for phytate revealed a 93% average disappearance rate of phytate, indicating that horses are highly capable of degrading phytate and that phytase supplementation was not beneficial. Thus, the results indicate that mature horses are able to maintain a near 0 P balance, with adequate P provided in the diet even as phytate, and increased P intakes above requirement may result in increased potentially detrimental outputs to the environment.