Influence of dietary methionine to methionine plus cysteine ratios on nitrogen retention in gilts fed purified diets between 40 and 80 kg live body weight

Abstract

The relationship between the ratio of available methionine (MET) to methionine plus cysteine (TSAA) intake and wholebody protein deposition (PD) was established using the nitrogen (N) balance technique in gilts between 40 and 80 kg live body weight (BW), according to a repeated 5 × 5 Latin square design. Pigs were fed casein- and cornstarch-based diets that supplied equal moles of TSAA supporting a PD of approximately 80% of the gilts’ PD potential. On a weight basis, the target ratios of MET to TSAA were 42, 47, 52, 57 and 67% for the five experimental diets, respectively. This calculated to 37, 42, 47, 52 and 62.5% on a molar basis. Total N excretion (urine plus feces) was reduced (linear; P < 0.001) and PD was increased (linear; P < 0.001) when the available MET to TSAA ratio was increased to 52%; these values did not change (P > 0.05) when the MET to TSAA ratio was further increased. A number of statistical models were fitted to the data to establish the best fit of the model parameters. The greatest proportion of the variation (R2 = 0.990) was explained with an asymptotic model; based on this model the optimum available MET to TSAA ratio (supporting 90% of the asymptotic value for PD) was 55% on a weight basis or 50.5% on a molar basis. The results indicate that the minimum contribution of available MET to TSAA requirements of growing pigs is higher than the value currently suggested by the National Research Council (46 to 48% on a weight basis). Key words: Methionine, cysteine, pigs, nitrogen balance, protein, deposition