Phosphorus availability of oilseed meals determined by the apparent faecal digestibility technique

Abstract

This study aimed to determine the phosphorus availability of five oilseed meals and the effect of formaldehyde treatment with or without an exogenous phytase supply. Eleven diets were optimised so that 70% of the phosphorus supply was provided by one of the following oilseed meals: untreated (UT) linseed, groundnut, soybean, rapeseed, sunflower meals, formaldehyde-treated (FT) soybean, rapeseed, sunflower meals, the FT oilseed diets being with or without phytase supplementation (FTP). Eleven mature sheep were restrictively fed on five of these diets successively according to a Youden square experimental design so that each diet was fed successively to five different sheep. After 3 weeks adaptation to the diet, the sheep were placed in digestibility crates. Dry matter (DM) intake, and faecal and urinary output were weighed and phosphorus contents determined. Phosphorus intake was 2.85 g per day for each sheep. Apparent phosphorus digestibility was calculated and true phosphorus digestibility was estimated according to three endogenous faecal phosphorus prediction models from the literature. Apparent phosphorus absorption was for soybean meal 0.230 (UT), 0.249 (FT) and 0.235 (FTP), for rapeseed meal 0.126 (UT), 0.193 (FT) and 0.143 (FTP), for sunflower meal 0.041 (UT), 0.182 (FT) and 0.241 (FTP), for groundnut meal 0.127 (UT) and for linseed meal 0.149 (UT). Apparent phosphorus absorption from UT soybean meal was greater than that from other UT meals ( P<0.01). Formaldehyde treatment effect was significant for rapeseed and sunflower meals ( P<0.01). Phytase supplementation significantly increased apparent phosphorus digestibility in FT sunflower meal but decreased it in FT rapeseed meal ( P<0.01). Urinary phosphorus output averaged 0.012 g per day (<1.2% of total phosphorus output). The faecal endogenous phosphorus prediction models lead to different phosphorus true digestibility values, the lowest with the ARFC model (from 0.493 to 0.621) and the highest with model 1 (from 0.597 to 0.697). Regardless of the model used, more phosphorus was digested in UT soybean than from other UT oilseed meals. Formaldehyde treatment increased phosphorus true absorption from rapeseed and sunflower meals ( P<0.01). Phytase supplementation decreased phosphorus digestibility in FT rapeseed meal but increased it in FT soybean meal ( P<0.01). After discussing the choice of method used to determine phosphorus true absorption and the possible effects of formaldehyde treatment and phytase in the digestive tract, the authors conclude that it is relevant to determine specific phosphorus absorption rates for each group of feedstuffs.