Cereal grain fiber composition modifies phosphorus digestibility in grower pigs

Lay Summary Increased fermentable carbohydrates (e.g., beta-glucan, amylose) may increase intestinal endogenous phosphorus (P) losses and thereby reduce P digestibility. The study assessed effects of cereal grains varying in fermentable carbohydrates on non-phytate-P and phytate-P. Phytate is the major binding form of P in plant seed and is incompletely degraded. Seven barrows cannulated at the terminal ileum were fed diets containing 80% of one of five cereal grains: 1) high-fermentable, high-beta-glucan, hull-less barley; 2) high-fermentable, high-amylose, hull-less barley; 3) moderate-fermentable, hull-less barley; 4) low-fermentable, hulled barley; and 5) low-fermentable wheat. Diet ileal digestibility of P was greater for low- and moderate-fermentable grain than high-fermentable grain, and diet total tract digestibility of P was greater for moderate-fermentable barley than high beta-glucan barley. Total tract digestibility of phytate-P was greater for high beta-glucan than low-fermentable barley. Total tract P release was greater for high-fermentable barley, and wheat than moderate- and low-fermentable barley. In conclusion, cereal grains high in fermentable fiber had lower diet ileal and total tract digestibility of P resulting in greater excretion of P, but greater total tract digestibility of phytate-P. Carbohydrate fermentation, thus, increases total tract P release from phytate-P degradation. Increased fermentable carbohydrates (e.g., beta-glucan, amylose) may increase endogenous losses including for P, and thereby reduce apparent total tract digestibility (ATTD) of P. The present study assessed effects of barley cultivars varying in fermentable starch and fiber on apparent ileal digestibility (AID) and ATTD of P, myo-inositol 1,2,3,4,5,6-hexakis (dihydrogen phosphate; InsP(6)) and Ca, and standardized total tract digestibility (STTD) of P and the presence of lower inositol phosphates (InsP) compared to wheat. In a 6 (period) x 5 (diet) Youden square, seven ileal-cannulated barrows (initial BW, 27.7 kg) were fed diets containing 80% of one of five cereal grains differing in amylose, beta-glucan, and fiber content: 1) high-fermentable, high-beta-glucan, hull-less barley (HFB); 2) high-fermentable, high-amylose, hull-less barley (HFA); 3) moderate-fermentable, hull-less barley (MFB); 4) low-fermentable, hulled barley (LFB); and 5) low-fermentable, Canadian Western Red Spring wheat (LFW). On dry matter (DM) basis, cereal grains contained between 0.32% to 0.53% total P and 0.24% to 0.50% InsP(6)-P. The InsP(6-2)-P was calculated as the sum of all detected InsP-P (InsP(6)-P to InsP(2)-P) in the sample. The P release of degraded InsP-P was calculated by using the following equation: sum InsP(6-2)-P-diet (g/kg DM) x (AID or ATTD sum InsP(6-2)-P (%)/ 100). Data were analyzed using a mixed model with diet as fixed effect, and pig and period as random effects. On DM basis, diets contained 41.4% to 50.6% starch, 0.88% to 8.54% beta-glucan, 0.81% to 0.89% total P, and 0.19% to 0.35% InsP(6)-P. The MFB, LFB, and LFW had greater (P < 0.05) diet AID of P than HFB and HFA, and MFB had greater (P < 0.05) diet ATTD and STTD of P than HFB. The ATTD of InsP(6)-P was greater (P < 0.05) for HFB than LFB and the ATTD of the sum InsP(6-2)-P was greater (P < 0.05) for HFB and HFA than LFB. Total tract P release was greater (P < 0.001) for HFB, HFA, and LFW than MFB and LFB. The LFW had greater (P < 0.05) ATTD of Ca than LFB. Diet beta-glucan content was not correlated with STTD of P (R-2 = 0.03) or ATTD of InsP(6) (R-2 = 0.05). In conclusion, cereal grains high in fermentable fiber, e.g., amylose and beta-glucans included in specific hull-less barley cultivars, had lower diet AID, ATTD, and STTD of P, but greater ATTD of InsP(6)-P and sum InsP(6-2)-P. Carbohydrate fermentation, thus, results in greater total tract P release from InsP-P hydrolysis. Cereal grains high in fermentable fiber, e.g., amylose and beta-glucans, had lower diet phosphorus digestibility, but greater apparent total tract digestibility of myo-inositol 1,2,3,4,5,6-hexakis (dihydrogen phosphate)-P. Carbohydrate fermentation, thus, increases total tract P release from inositol phosphate-P hydrolysis.