Accounting for the effects of a ruminal nitrogen deficiency within the structure of the Cornell Net Carbohydrate and Protein System

The Cornell Net Carbohydrate and Protein System (CNCPS) prediction of fiber digestion and microbial mass production from ruminally degraded carbohydra te has been adjusted to accommodate a ruminal N deficiency. The steps for t he adjustment are as follows: 1) the ruminal available peptide and ammonia pools are used to determine the N allowable microbial growth; 2) this value is subtracted from the energy allowable microbial growth to obtain the red uction in microbial mass; 3) this mass reduction is allocated between pools of bacteria digesting fiber (FC) and nonfiber (NFC) carbohydrate according to their original proportions in the energy allowable microbial growth; 4) the reduction in fermented FC is computed as the FC bacterial mass reducti on divided by its yield (g bacteria/g FC digested); and 5) this reduction i s added to the FC fraction escaping the rumen. Five published studies inclu ded information that allowed us to evaluate the response of animals to adde d dietary N, These evaluations compared observed and CNCPS-predicted ADG wi th and without this adjustment. The adjustment decreased the CNCPS overpred iction of ADG from 19.2 to 4.7%, mean bias declined from .16 to .04 kg/d, a nd the r(2) of the regression between observed and metabolizable energy (ME ) or metabolizable protein allowable ADC was increased from .83 to .88 with the adjustment. When the observed dry matter intake was regressed against CNCPS-predicted DMI with an adjustment for reduction in cell wall digestibi lity, the r(2) was increased from .71 to .88. These results indicated the a djustment for ruminal nitrogen deficiency increased the accuracy of the CNC PS model in evaluating diets of growing animals when ruminally degraded N i s deficient.