Estimating ruminal nitrogen-to-energy balance with in situ disappearance data

Abstract

Microbial growth in the rumen is a pivotal part of any ruminant protein system, and there is an optimal balance between available nitrogen (N) and energy in the rumen. When the nitrogen-to-energy balance in the rumen is optimal, apparent ruminal N digestion (percentage of intake) equals 0. In situ digestion can be used to estimate the ruminally degraded N:ruminally degraded organic matter (OM;g/kg) ratio. The relationship between in vivo apparent ruminal N digestion and dietary N concentration (percentage of OM), dietary N concentration relative to in vitro digestible OM (IVDOM; percentage of IVDOM), and the ruminally degraded N:ruminally degraded OM ratio were evaluated with data from 10 studies in which cattle consumed forage diets. A moderate relationship (r2 = 0.49) was noted between apparent ruminal N digestion (Y) and dietary N (X; % of OM; Y = 42.94X -110.54); this equation predicted that apparent ruminal N digestion would equal 0 at a N concentration of 2.57 +/-0.95% of OM. There was a weak relationship (r2 = 0.14) between apparent ruminal N digestion (Y) and the N:IVDOM ratio (X; Y= 21.64X -97.77); this equation predicted that apparent ruminal N digestion would equal 0 at a N concentration of 4.57% of IVDOM. A strong relationship (r2 = 0.67) was noted between apparent ruminal N digestion (Y) and ruminally degraded N:ruminally degraded OM (X; Y = 4.327X -117.04); this equation predicted that apparent ruminal N digestion would equal 0 at a ruminally degraded N:ruminally degraded OM ratio of 27.03 +/- 0.71 g/kg. The ruminally degraded N:ruminally degraded OM ratio was a better predictor of apparent ruminal N digestion than dietary N concentration expressed relative to either OM or IVDOM. The ruminally degraded N:ruminally degraded OM ratio seems to be a useful tool for predicting apparent ruminal N digestion and managing the nutrition of forage-fed cattle.