Urinary excretion of purine derivatives, microbial protein synthesis, nitrogen use, and ruminal fermentation in sheep and goats fed diets of different quality1

Abstract

The objective of this study was to compare N balance, microbial N flow (MNF) estimated from purine derivatives (PD) urinary excretion, and its variation when estimated using purine bases:N ratios in liquid associated bacteria (LAB) from models reported in the literature (MNF - response models) or measured ratios in liquid and solid-associated bacterial (SAB) pellets (MNF-LAB+SAB), diet digestibility, and rumen fermentation variables in sheep and goats fed 3 different practical, quality diets to study interspecies differences concerning N use as accurately as possible. Four mature female Merino sheep and 4 mature female Granadina goats, each fitted with a ruminal cannula, were used in 3 × 3 Latin square design with an extra animal. Two experimental diets had a forage-to-concentrate ratio of 70:30 (DM basis) with alfalfa hay (ALC) or grass hay (GRC) as forage, and the third diet contained 70% concentrate and 30% alfalfa hay (CAL). All animals were fed the diets at a daily rate of 56 g/kg BW(0.75) to minimize feed selection. Digestibility of nutrients was similar (P = 0.16 to 0.88) in the 2 species, but some animal species × diet interactions (P = 0.01 to 0.04) were detected. There were small differences between the fermentation patterns of both animal species. Goats showed decreased VFA concentrations (P = 0.005) and butyrate proportions (P = 0.04), and greater acetate proportions (P = 0.02) compared with sheep, whereas N intake and percentage of N intake excreted in feces were similar in both species (P = 0.58 and 0.15, respectively), the percentage excreted via the urine was greater in goats compared with sheep (P < 0.001). As a consequence, sheep had greater (P < 0.001) N retention than goats (averaged across diets, 32.6% and 16.1% of N intake, respectively). There were no differences (P = 0.95) between animal species in total PD excretion, but goats showed a greater excretion of allantoin (P = 0.01) and decreased excretion of xanthine (P = 0.008) and hypoxanthine (P = 0.007) compared with sheep. In general, differences between sheep and goats were more pronounced for the medium-quality diet (GRC) compared with those of high-quality diet (ALC and CAL). The greater urinary losses in goats would indicate a greater contribution of goats to N environmental contamination compared with sheep.