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

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anglais. The objective of this study was to compareN balance, microbial N fl ow (MNF) estimatedfrom purine derivatives (PD) urinary excretion, and itsvariation when estimated using purine bases:N ratios inliquid associated bacteria (LAB) from models reportedin the literature (MNF – response models) or measuredratios in liquid and solid-associated bacterial (SAB)pellets (MNF-LAB+SAB), diet digestibility, and rumenfermentation variables in sheep and goats fed 3 differentpractical, quality diets to study interspecies differencesconcerning N use as accurately as possible. Four maturefemale Merino sheep and 4 mature female Granadinagoats, each fi tted with a ruminal cannula, were used in3 × 3 Latin square design with an extra animal. Twoexperimental diets had a forage-to-concentrate ratioof 70:30 (DM basis) with alfalfa hay (ALC) or grasshay (GRC) as forage, and the third diet contained 70%concentrate and 30% alfalfa hay (CAL). All animalswere fed the diets at a daily rate of 56 g/kg BW0.75 tominimize feed selection. Digestibility of nutrients wassimilar (P = 0.16 to 0.88) in the 2 species, but someanimal species × diet interactions (P = 0.01 to 0.04)were detected. There were small differences betweenthe fermentation patterns of both animal species. Goatsshowed decreased VFA concentrations (P = 0.005) andbutyrate proportions (P = 0.04), and greater acetate proportions(P = 0.02) compared with sheep, whereas Nintake and percentage of N intake excreted in feces weresimilar in both species (P = 0.58 and 0.15, respectively),the percentage excreted via the urine was greaterin goats compared with sheep (P < 0.001). As a consequence,sheep had greater (P < 0.001) N retentionthan goats (averaged across diets, 32.6% and 16.1% ofN 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 weremore pronounced for the medium-quality diet (GRC)compared with those of high-quality diet (ALC andCAL). The greater urinary losses in goats would indicatea greater contribution of goats to N environmentalcontamination compared with sheep.