MILK-PRODUCTION AND COMPOSITION AS A FUNCTION OF POSTRUMINAL LYSINE AND METHIONINE SUPPLY - A NUTRIENT-RESPONSE APPROACH

A total of 57 trials using 164 different diets in which measurements were made of lactational responses of dairy cows to postruminal supplements of lysine and methionine, were analysed. Duodenal flow of digestible lysine (LysDI) and methionine (MetDI) was estimated and expressed in relative terms (%PDI). To allow inter-trial comparison which would have been invalidated by large differences between control treatments in LysDI and MetDI, milk production responses in terms of total yield, protein yield and content, fat yield and content, were expressed relative to new reference values calculated for the LysDI and MetDI concentrations (7.0 and 2.1% PDI, respectively), common for all analysed trials. Milk protein yield responses (dPY, g/d) were described by the following equation derived for lysine (LysDI): dPY=21.0{1-exp[-1.15(LysDI-7.0)]} R(2)=0.87, RSD=21.0, n=63 The equation describing responses to methionine (MetDI) was calculated for concurrent lysine concentrations greater than 6.5%PDI: dPY=34.1{1-exp[-1.90(MetDI-2.1)]} R(2)=0.81, RSD=11.7, n=37 The relationships obtained for protein yield could be used consequently to determine lysine (7.3 LysDI) and methionine (2.5 MetDI) allowances (%PDI), which optimise milk protein secretion in dairy cows and which conform to standard production conditions. The validity of the present approach as well as its strengths and weaknesses are discussed. The present values were compared with those obtained by either direct physiological or indirect factorial methods.