Aspects of urea metabolism in ruminants with reference to the goat.

In goats and other ruminants, urea functions as a source of nitrogen for protein biosynthesis in the digestive tract. Ammonia can be absorbed in the digestive system when formed in excessive quantitites and enhance formation of urea, or it can be derived from urea of blood plasma when its formation from feed sources is small. Entry rates of urea into plasma may vary from 4 to 80 mumol/min per kg.75 body weight depending on dietary conditions. Urea formation is related to nitrogen intake of which approximately 70% passes into the urea pool of plasma. Irreversible losses of urea of plasma into the digestive tract vary between 10 and 90% depending on the protein to energy ratios of the diet. Entry of urea from plasma into the rumen appears to be a passive process which is sensitive to short-term changes of urea concentrations in plasma. Permeability of ruminal epithelium to urea may be altered by fermentation products of rumen (ammonia, carbon dioxide, volatile fatty acids). The influx of nitrogen into the rumen is related to needs for nitrogen of microbial populations and is associated with changes of renal excretion and tubular reabsorption of urea. Combined gastrointestinal and renal responses exert a synergistic effect on improved utilization of urea of plasma when uptake of dietary nitrogen is limited in goats and other ruminants.