Urea recycling in the rumen of dairy goats: Effects of physiological stage and composition of intake

Rumen Recycled Urea (RRU) was measured on dairy goats at different physiological stages: drying off, pregnancy (weeks 7, 15 and 20) and lactation (weeks 4, 8 and 20). In three experiments 45 measurements were obtained from animals fed diets varying in crude protein contents (10 and 14% of dry matter). Average RRU (6.58 g day(-1)) represented 15% of N intake but with high variations (sigma = 5.73 g day(-1), cv = 87%). Rumen Recycled Urea varied with physiological stage being higher for lactation and lower during drying period. For lactating goats, there was a significant correlation between RRU and milk protein yield. Food composition, nitrogen utilization, blood urea and rumen ammonia content explained 60% of RRU variation. There was an effect of physiological stage on RRU residues (measured RRU - estimated RRU): residues being higher at the end of pregnancy than during lactation or drying period. For the same nutritional status, rumen recycled urea is increased at the end of pregnancy and the beginning of lactation.