PROTEIN REQUIREMENT AND NON PROTEIN NITROGEN UTILIZATION1

Factors governing nitrogen utilization by dairy cattle include the following: (a) Maintenance of ruminal ammonia nitrogen in excess of 5 mg/100 ml rumen fluid has no effect on microbial protein production; (b) Supplemental NPN is not utilized in typical dairy and feedlot beef rations containing, more than 12 to 13 % CP (DM basis); © NPN is approximately equal to true protein as a nitrogen source in typical dairy and feedlot rations containing not more than 12 to 13 % CP; (d) A scheme based upon metabolizable protein (absorbable protein) for calculating requirements and comparing protein sources in superior to the crude or digestible protein designations. Ultimate expression of the requirement may be in terms of CP for the sake of simplicity; (e) One kg of CP regardless of nitrogen source, equals about.75 kg metabolizable protein in typical dairy and feedlot beef rations containing not more than 12 to 13 % CP. One kg of plant protein (true protein) fed in excess of an amount equivalent to 12 to 13 % dietary protein equals about 0.3 kg metabolizable protein; (f) Protein supplementation of lactating cows might better be related to stage of lactation than level of milk production; (g) Lactating cows having above-average lactation Al ability may benefit from dietary protein levels reaching as high as 16 to 18 % (DM basis) during the first third of lactation; (h) Cows in the latter two-thirds of lactation appear to require 12.5% dietary protein or less; (I) Plant protein (true protein) should be the supplemental source of nitrogen during the first third of lactation, with NPN providing most, if not all, the supplemental nitrogen during the last two-thirds of lactation