Phosphorus digestibility and phytate degradation by yearlings and mature horses1,2

Abstract

Inorganic P is often added to growing horse diets because organic P, or phytate-P, is believed to have lower digestibility. If horses can efficiently digest organic P, then the need for inorganic P may be reduced. Much of the P in grain-based concentrates fed to growing horses is in the form of phytate-P. Little is known about the ability of growing horses to degrade phytate-P or whether horse age affects mineral digestion in horses. The objective of this study was to examine the effect of age on P, Ca, and Mg digestibility as well as phytate-P degradation. Four yearling geldings and 4 mature geldings were fed a diet of alfalfa cubes, timothy cubes, and a pelleted concentrate. The diet contained 0.28% total P and 17.4% of that P was in the phytate form. There was a 14-d diet adaptation period followed by a 4-d fecal collection period. Apparent total tract P digestibility was higher for yearlings than mature geldings ( = 0.036; 7.7 and -6.6% for yearlings and mature geldings, respectively). Phytate-P disappearance was 94.8% and did not differ between ages ( = 0.190). Apparent Ca digestibility was lower in mature geldings ( = 0.043), but apparent Mg digestibility did not differ between ages ( = 0.414). Phytate is broken down in the gastrointestinal tract, but the low P digestibilities suggest that either degradation occurs after the site of P absorption or liberated P is recycled back into the gastrointestinal tract. Yearlings can utilize organic P as well as mature horses; therefore, diets without inorganic P are acceptable for growing horses.

The Effect of Diet Composition on the Digestibility and Fecal Excretion of Phosphorus in Horses: A Potential Risk of P Leaching?

“…The excretion of phosphorus observed here was within the ranges presented in the literature [7,12,18,33,34]. P excretion is linearly related to its intake, and the intake increases with the increasing concentrate ingestion [10,12,33].…”

“…There may be some improvement in phosphorus absorption as the need for phosphorus increases, for example, through exercise or when the phosphorus content of the diet increases [6]. Fowler et al [7] concluded that yearlings can utilize organic P as well as mature horses.…”

“…The main site of the gastrointestinal tract of phosphorus absorption in horses is the dorsal colon, but some phosphorus is also absorbed from the small intestine [8,9]. Fowler et al [7] suggested that degradation either occurs after the site of P absorption, or liberated P is recycled back into the gastrointestinal tract.…”

“…Cereals are good sources of phosphorus [3], but a significant proportion of the phosphorus in cereals is bound to phytic acid, which is poorly digestible in monogastric animals [16]. However, the horse is able to digest phytate phosphorus [7,17,18]. The content of phosphorus in grasses approximately equates to that in cereals (about 3 g/kg DM), but the phosphorus content of grasses is significantly influenced by the age of the crop at the time of harvest [19,20].…”

Recycling nutrients from horse manure: effects of bedding type and its compostability

“…The amount of nutrients excreted by the horse in dung and urine depend highly on the nutrition and feed quality (Graham-Thiers and Bowen 2011, Ögren 2013, Ögren et al 2014, Fowler et al 2015, Trottier et al 2016, optimization of which is an important tool in reducing excess nutrient load from horse operations. Averages of around 500 horse manure samples analyzed in Finland in a commercial laboratory, Eurofins Viljavuuspalvelu Oy, between 2005 and 2009 were 15 g kg -1 dw for total N, 3 g kg -1…”