Nitrogen Requirement and Utilization in Dairy Cattle

Formulation of dairy cow rations should consider the following points regarding nitrogen utilization by lactating cows. (a) Maintenance of ruminal ammonia nitrogen in excess of 5 mg/100 ml rumen fluid has no effect on microbial protein production. (b)Supplemental nonprotein nitrogen is not utilized in typical dairy and feedlot beef rations containing more than 12 to 13% crude protein(dry matter basis). (c)Nonprotein nitrogen is approximately equal to true protein as a source of nitrogen in typical dairy and feedlot rations containing not more than 12 to 13% crude protein. (d)A scheme based upon metabolizable protein (absorbable protein) for calculating requirements and comparing protein sources is superior to crude or digestible protein designations. Ultimate expression of the requirement may be in terms of crude protein for the sake of simplicity. (e)One kilogram of crude protein, regardless of nitrogen source, equals about .75kg metabolizable protein in typical dairy and feedlot beef rations containing not more than 12 to 13% crude protein. One kilogram of plant protein (true protein) fed in excess of an amount equivalent to 12 to 13% dietary protein equals about .3 kg metabolizable protein. (f)Protein supplementation of lactating cows might be related more to stage of lactation than to milk production. (g)Lactating cows having above average lactational ability may benefit from dietary protein as high as 16 to 17% (dry matter 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 sources 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.