Partial and total tract nitrogen digestion in ponies fed forages, cereal grains and protein supplements

Abstract

Mature, ileal cannulated ponies were used to quantify partial and total tract nitrogen digestibility of forages, cereal grains and protein supplements. Feed, ileal, fecal and urinary nitrogen was determined and nutrient:chromic oxide ratios facilitated calculation of digestibility. Blood was analyzed for total plasma free amino acids. Analyses of variance appropriate to the Latin square (SAS, 1979) were conducted and means differences tested using Tukey's procedure (Steel and Torrie, 1960). Digestibility of individual feedstuffs was determined by-difference (Schneider and Flatt, 1975). True nitrogen digestibility was estimated by linear regression of digested nitrogen on nitrogen intake. In trial 1, bermudagrass hay containing 11.7% crude protein and two alfalfas containing 15.0% and 18.1% crude protein were fed at 2.0% of body weight daily. Apparent total trace and prececal nitrogen digestibility and nitrogen retention was higher for high-alfalfa than low-alfalfa or bermudagrass (P < .05). Total tract nitrogen digestibility of Coastal bermudagrass was lower than low-alfalfa (P < .05). Apparent postileal nitrogen digestibilities were similar (P > .05). The large intestine was major site for nitrogen digestion, but 28.45% of high-alfalfa nitrogen digestion occurred in the small intestine. In trial 2, rations containing rolled corn, oats or sorghum were fed in a 3:1 ratio with bermudagrass hay at 1.28% of body weight daily. There were no significant differences (P > .05) in partial or total tract nitrogen digestibility across treatments. The small intestine was site for 54.92% of total ration nitrogen disappearance and 61.64% of concentrate nitrogen disappearance. In trial 3, soybean or cottonseed meal provided over 50% of dietary nitrogen when added to a basal corn and bermudagrass ration fed at 1.24% of body weight daily. Prececal nitrogen digestibility, plasma free amino acid concentrations and retained nitrogen were affected by supplementation. By-difference calculations indicated prececal cottonseed meal nitrogen digestibility was greater (P < .05) than soybean meal. The reverse was apparent postileally. The small intestine was site for 95.56% and 56.96% of total cottonseed and soybean meal digestion, respectively. True disgestibility estimates indicated endogenous or metabolic fecal nitrogen was significant and may be influenced by diets fed to horses.