Screening tests of the protein quality of grain legumes for poultry production

Three screening tests for protein quality, modified limiting amino acid score (MLAAS), net weight gain (NWG) and net protein ratio (NPR), were compared. Two experiments using young broiler chickens were conducted in a temperature-controlled room at 28.5 +/- 0.5 degrees with no adaptation to cages and diets, or at 31 +/- 0.5 degrees with 2 d adaptation to cages and diets. Nine isoenergetic diets containing nominally 100 g crude protein/kg supplied by legume meals and one isoenergetic N-free diet were randomly allocated to chicks in single cages in each side of a four-tier battery brooder. Each dietary treatment had eight replicates. The chickens had access ad lib. to diet and drinking water throughout a 14 d observation period. Body weight and feed were measured at the start on day 7 and at the end on day 21. The results indicated that keeping the chickens at 31 +/- 0.5 degrees and giving them a 2 d adaptation period decreased the variability of chickens' responses to each treatment. MLAAS, NWG and NPR methods could distinguish legume proteins of high, medium and low feed values. MLAAS correlated well with NWG (r 0.90; P < 0.001) and NPR (r 0.78; P < 0.01) in evaluating the protein quality of grain legumes used as sole sources of protein for meat chickens. However, MLAAS did not predict the exact order of NWG and NPR. Growth was limited because dietary methionine, the first limiting amino acid, provided only 27.6-55.2% of the recommended proportion in the protein. Although the results should be interpreted cautiously since a small sample size was used, it was concluded that the MLAAS calculation could be used as a reasonable estimate of the relative protein quality of most grain legumes, but that NWG and NPR were better methods as they detected limiting factors other than limiting amino acids in raw and processed legumes.