ESTIMATING RUMINAL NITROGEN-TO-ENERGY BALANCE WITH IN-SITU DISAPPEARANCE DATA

Microbial growth in the rumen is a pivotal part of any ruminant protei n 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 deg raded N:ruminally degraded organic matter (OM; g/kg) ratio, The relati onship between in vivo apparent ruminal N digestion and dietary N conc entration (percentage of OM), dietary N concentration relative to in v itro digestible OM (IVDOM; percentage of IVDOM), and the ruminally deg raded N:ruminally degraded OM ratio were evaluated with data from 10 s tudies in which cattle consumed forage diets, A moderate relationship (r(2) = 0.49) was noted between apparent ruminal N digestion (Y) and d ietary N (X; % of OM; Y = 42.94X -110.54); this equation predicted tha t apparent ruminal N digestion would equal 0 at a N concentration of 2 .57 +/- 0.95% of OM. There was a weak relationship (r(2) = 0.14) betwe en apparent ruminal N digestion (Y) and the N:IVDOM ratio (X; Y = 21.6 4X -97.77); this equation predicted that apparent ruminal N digestion would equal 0 at a N concentration of 4.57% of IVDOM. A strong relatio nship (r(2) = 0.67) was noted between apparent ruminal N digestion (Y) and ruminally degraded N:ruminally degraded OM (X; Y = 4.327X - 117.0 4); this equation predicted that apparent ruminal N digestion would eq ual 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 con centration expressed relative to either OM or IVDOM, The ruminally deg raded N:ruminally degraded OM ratio seems to be a useful tool for pred icting apparent ruminal N digestion and managing the nutrition of fora ge-fed cattle.