Background Federal regulations to address nutrient losses from concentrated animal feeding operations now explicitly include horse farms. These regulations increase the amount of land needed for manure application. While many horse farms have limited land base for manure application, nutrient losses from horse farms have not been an area of focus. Similarly, little is known about factors affecting P excretion. Our research focuses on digestion of phytic acid. Phytic acid P is unavailable for digestion and use by simple stomached animals, because they do not possess the enzyme phytase. Ruminants and hind-gut fermentors are thought to be able to digest phytic acid as a result of the microbes within their GI tract that secrete phytase. Unknown as are the factors which affect phytic acid digestion. Reducing the environmental impact of horse farms through dietary nutrient management Sarah Marpet and Dr. Burt Staniar, Dept of Animal and Poultry Science; Dr. Katharine Knowlton, Dept of Dairy Science Methods In spring of 2006, three groups of open mares were offered pasture and hay, a high fat/fiber feed to supplement pasture and hay, or a traditional high starch feed with pasture and hay. Feed and feces were collected. In summer of 2006 two different groups were sampled, open mares and mares with standing foals. The open mares were fed pasture only while the mares with foals were fed the high fat/fiber feed and pasture. Feed, pasture, feces, and urine were collected. Total P (nitric/perchloric digestion with Pi release measured colorimetrically with molybdovanadate) was measured in samples from both studies, while phytate P (low pressure ion chromatography/molbydate blue) was measured only in the spring 2006 samples. Fractionation was confirmed on a subset of samples using NMR spectroscopy. Acknowledgements The Middleburg Agriculture Research & Extension Center Cathy Parsons, Tzu-Hsuan Yang, and Karen Hall, Dept of Dairy Science; Dr. Chao Shang, Dept of Crop and Soil Environmental Sciences The John Lee Pratt Animal Nutrition Foundation Figure 1. Digestibility of phytic acid was increased in non-lactating, non-pregnant mares fed a sugar-starch grain compared to those fed a fat-fiber diet or pasture only. Figure 2. In lactating mares, feeding a fat-fiber diet decreased digestibility of phosphorus. This is contrary to our hypothesis. Summary Non-lactating, non-pregnant mares with low nutrient needs excreted all consumed P. In non-pregnant, non-lactating mares, the sugar-starch diet increased digestibility of phytic acid. This suggests increased microbial phytase activity in the hindgut and may be due to: Increased supply of desired substrate. Starch-fermenting bacteria have greater phytase activity than fiber-digesting bacteria. Decreased pH in the hindgut, giving these starch fermenters with high phytase capacity a competitive advantage over fiber digesting bacteria. In both studies, our hypothesis that the increased retention time observed with the fat-fiber diet would increase digestion of phytate P was not supported. Urinary P was measured and is negligible (1 to 5 ppm, < 1 g/day) Dr. Katharine Knowlton Department of Dairy Science To quantify excretion of total and phytate P by grazing mares (open or with foal at side). We evaluated the effect of high fat/fiber feeds formulated to increase growth and performance while optimizing health as compared to high starch feeds or traditional pasture-only diets. Objectives This work will help identify dietary strategies to reduce potential nutrient losses from land-limited horse farms. Hypothesis We hypothesize that efficiency of phytic acid digestion will be greater in the horses fed the high fat/high fiber feed because of increased retention time of digesta in the hindgut as well as diet-induced differences in gut microbial population Results Implications Phytic acid analysis Nitric/perchloric acid digestion Phytic acid