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The assessment of microbial protein synthesis (MPS) from the urinary excretion of purine derivatives (PD) and creatinine (C) was studied in double-muscled Belgian Blue bulls (BBb) receiving silage-based diets. The first step of the work consisted in establishing an index relating the duodenal flow of purine bases (PB) to urinary PD and C (PD = 0.752 (+/- 0.128) x PB + 0.4 (0.052) x C ; R2 = 0.997 ; P <0.001). The urinary excretion of endogenous PD, estimated by the second term of the index, was high in BBb (0.66 mmol/kg P0.75) and largely varied according to the diets. The second step consisted in converting duodenal PB into microbial nitrogen (MN) using the purine-N/total N ratio determined in bacteria extracted from the solid phase of the rumen. This ratio varied according to the nature of the forage in the diets. The plasma concentrations of PD were poorly correlated to the urinary PD since their tubular reabsorptions were high and variable according to the diet. Hence, this parameter cannot be used to estimate precisely the MPS in BBb. The PMPS was high (31.2 g MN/kg of organic matter apparently digested in the rumen (OMADR)) in BBb receiving maize silage alone as a result of an important recycling of endogenous urea in the rumen. Consequently, the nitrogen supplementation of maize silage had no significant effect on the PMPS (28.1 g MN/kg OMADR). The energy supplementation of grass silage did not increase the MPS in comparison with grass silage alone (31.1 vs 29.9 g MN/kg OMADR) since the OM digestion of grass silage was high in the rumen. Finally, the MPS was extremely low (11.9 g MN/kg OMADR) in rams of the Black Thibar breed receiving oat silage alone, presumably as a result of an energy deficit for the microorganisms in the rumen. The supplementation of the silage with a broiler litter-based concentrate increased the MPS (18.3 g MN/kg OMADR), but not with ones based on soyabean meal and urea.

ruminants --- ruminants --- protein synthesis --- protein synthesis --- Digestive system --- Digestive system --- microbial flora --- microbial flora --- feeds --- feeds --- rations --- rations --- Purines --- Purines --- Creatinine. --- Creatinine --- Urine --- Urine --- Quantitative analysis --- Quantitative analysis --- Plasma --- Plasma

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Sheep --- Sheep --- Goats --- Goats --- animal husbandry --- animal husbandry --- reproduction. --- reproduction --- Breeds (animals) --- Breeds (animals) --- Semiarid zones --- Semiarid zones --- Tunisia --- Tunisia