Aspects of Urea Metabolism in Ruminants with Reference to the Goat

Abstract

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.

The effect of CP concentration in the diet on urea kinetics and microbial usage of recycled urea in cattle: a meta-analysis

“…However, the intercept of the linear regression observed by these authors was significant, whereas in this current meta-analytical approach it was not significant. According to Harmeyer and Martens (1980), N intake is the major factor influencing UER; they observed a correlation of 0.88 between these variables, which was similar to the current study (r = 0.91). These authors found that the feeding schedule (twice daily or feeding hourly), site of N supplied to the animal (oral or abomasal), and type of N fed (non-protein N v. true protein) did not affect significantly the amount of urea synthesized in the liver.…”

“…The UUE is one of the fates of the urea synthesized in the liver and released into blood, and factors such as N intake relative to nutritional requirements affect it (Huntington and Archibeque, 1999). According to Harmeyer and Martens (1980), the kidney has specific mechanisms that modify urinary urea N excretion (or reabsorption). The amount of urea excreted by the kidneys may be influenced by three main factors: changes of PUN and corresponding changes in filtered urea loads, variations of the glomerular filtration rates, and modifications of tubular reabsorption of urea (Harmeyer and Martens, 1980).…”

“…According to Harmeyer and Martens (1980), the kidney has specific mechanisms that modify urinary urea N excretion (or reabsorption). The amount of urea excreted by the kidneys may be influenced by three main factors: changes of PUN and corresponding changes in filtered urea loads, variations of the glomerular filtration rates, and modifications of tubular reabsorption of urea (Harmeyer and Martens, 1980). According to these authors, the quantity of urea reabsorbed in the tubules increases with increased PUN and that the percentages of filtered loads of urea reabsorbed remain constant.…”

“…In addition, specific mechanisms of the kidneys for saving urea (e.g. glomerular filtration rate, tubular reabsorption) from being excreted with urine may be triggered when N intake is lowered further and when N supply to the animal remains inadequate for several months (Harmeyer and Martens, 1980). The UUE:TUN may indicate the priority for excretion of excess N in relation to muscle mass, muscle growth or perhaps acid-base regulation (Huntington and Archibeque, 1999).…”

“…Urea N recycling and assimilation by ruminal microbes are influenced by dietary and ruminal factors, and intakes of CP and digestible organic matter (OM) are major factors (Harmeyer and Martens, 1980;Kennedy and Milligan, 1980). A better understanding of the relationships between dietary factors and urea kinetics and microbial usage of recycled urea would allow for a more accurate estimation of N supply and N requirements by current feeding models.…”