Updating protein requirements in ruminants and determination of the responses of lactating females to metabolisable protein supply

Abstract

As part of the revision of ruminant feed unit systems (INRA “Systali” project) the protein requirements as well as responses to changes in protein supply were updated. The maintenance requirements take into consideration the various mandatory nitrogen losses which lead to non-productive protein requirements: metabolic fecal protein loss, urinary endogenous nitrogen loss and loss of protein from scurf. These updates have been made by factorial approaches and from meta-analyzes of large databases of literature. These new values of “non-productive” metabolisable protein supply (PDI) requirements were applied to numerous experiments performed on cows or goats with the aim to study the variation of milk responses to protein supply. For estimation of the response of the efficiency of PDI to milk protein (EffPDI in %) it was decided to apply the same EffPDI value for milk protein synthesis and for the other body proteins as well as to the accretion or mobilization of body proteins which was related to the calculated energy balance. EffPDI% is a nonlinear decreasing function of the PDI concentration, similar between cows and goats. It was also proposed to predict the response of milk protein production to the level of supply of available PDI by using, as a pivot situation, either the production for EffPDI = 67% or the potential of production. The collected data were also used to calculate, with a factorial manner, the urinary nitrogen losses. Their two major causes are the inefficiency of PDI and the ruminal nitrogen nutritional status calculated through the rumen protein balance.