Evaluation of protein nutrition by metabolizable protein and urea fermentation potential.

Metabolizable protein and selected metabolizable amino acid requirements for lactating cows were described, and tentative values were established for differnt yields of milk.A new expression"urea fermentation potential of feeds," describes urea use in lactation rations for partial satisfaction of protein and amino acid requirements. Tentative urea fermentation values were established for the more common cattle feedstuffs. Four lactation rations were formulated with different fermentation values to illustrate the variable feeding value of urea in satisfying amino acid requirements at different yields of lactation. Urea had the highest feeding value in lower-protein rations when fed to cows with medium to low yields of milk.Conversely, urea, by the metabolizable protein system, had little or no feeding value in lactation rations having more than about12% protein on a dry matter basis or in rations supporting lactations in excess of 25 to 30 kg of milk per cow per day.