Effect of total phosphorus and phytate-phosphorus intake on phosphorus digestibility in horses

Abstract

s / Journal of Equine Veterinary Science 33 (2013) 321-399 352 than in both LP and LP+Phy (P<0.0001) but did not differ between LP and LP+Phy (P1⁄40.66). Likewise, fecal phytate P (both in g and in mg/kg BW) was higher in C than in both of the low P groups (P<0.0001), though it did not differ in the two low P groups, regardless of whether phytase was incorporated or not (P1⁄40.99). Phytate P disappearance was high for all treatments (C 1⁄4 82.9%, LP 1⁄4 91.3%, LP+Phy 1⁄4 89.7%) and did not differ between either of the low P groups, regardless of whether phytase was incorporated into the diet or not (P1⁄40.70). These results suggest the microbial population present in horses allows sufficient release and absorption of phytate P, similar as in ruminants, despite the population being present primarily in the hindgut of the horse. Incorporation of phytase into equine diets is not warranted. Effect of total phosphorus and phytate-phosphorus intake on phosphorus digestibility in horses L.K. Warren , J.M. Weir , P.A. Harris , and J. Kivipelto 1 1 Department of Animal Sciences, University of Florida, Gainesville, Florida, USA, WALTHAM Centre for Pet Nutrition, Melton Mowbray, Leicestershire, United Kingdom Over two-thirds of the phosphorus (P) in grains, grain byproducts and oilseeds is organically bound in the form of phytate-P (inositol hexakisphosphate). Mammals do not produce phytase; thus, P from phytate-P is limited to that made available via microbial degradation in the gastrointestinal tract. Phytate-P is highly unavailable to swine and poultry, necessitating the addition of inorganic P to the diet, which ultimately results in large quantities of P excreted in manure. Although horses are thought to have an advantage over other monogastrics due to the fermentative capacity of their hindgut, evidence is conflicting on the availability of phytate-P. To determine how total P and phytate-P intake affects P digestibility, P balance, and the excretion of calcium (Ca), 8 mature (mean SE, 522 17 kg) geldings were used in a 4 X 4 Latin square design study. The 4 diets contained either 100% or 300% of NRC (2007) maintenance requirements for P, with either a low (< 2%) or high ( 25%) proportion of the P provided as phytate-P. A 2:1 calcium to P ratio was maintained in all diets. Each study period consisted of an 11-d diet adaptation phase followed by a 3-d collection of all feces and urine. Data were analyzed using ANOVA as part of the MIXED procedure of SAS (v9.3) with horse, period, P level, phyate-P level, and P*phytate-P interaction as fixed effects. Apparent P digestibility was affected by dietary phyate-P level (P< 0.0001) but not P level (P 1⁄4 0.08) nor the interaction of P*phytate-P (P 1⁄4 0.99). Mean apparent P digestibility was higher when horses were fed low phytate-P diets (54.5 1.4%) compared to high phytate-P diets (43.0 1.4%). Estimated true P digestibility was affected by dietary phytate-P level (P< 0.0001), P level (P< 0.0001), but not P*phytate-P (P1⁄4 0.90). Mean true P digestibility was higher in diets containing 100% NRC P (75.6 1.4%) compared to 300% NRC P (57.3 1.4%), and higher in low phytate-P diets (73.1 1.4%) compared to high phytate-P diets (59.9 1.4%). All horses were in positive P balance, with greater P retention in diets containing 300% NRC P compared to 100% NRC P (P< 0.0001). Calcium excretion in feces was affected by dietary phytate-P level (P< 0.0001) and P level (which paralleled dietary Ca level; P< 0.0001), as it was greater in diets containing 300% NRC P (and Ca) compared to 100% NRC, and greater in high phytate-P diets compared to low phytate-P diets. All horses on all diets were in apparent negative Ca balance. Results indicate phytate-P can limit Ca and P availability in the horse when it makes up 25% of P intake. Effect of ideal protein on muscle mass development P.M. Graham-Thiers, and L.K. Bowen Department of Equine Studies, Virginia Intermont College,