Determination of the fourth and fifth limiting amino acids in broilers fed on diets containing maize, soybean meal and poultry by-product meal from 28 to 42 d of age.

1. Valine (Val) is considered the 4th limiting amino acid for broilers fed on diets containing ingredients from vegetable origin. However, Val and Isoleucine (Ile) may be co-limiting for broilers fed on diets containing animal protein meals. An experiment was conducted to examine growth responses and meat yield of broilers provided diets varying in digestible Val (dVal) and digestible Ile (dIle) concentrations from 28 to 42 d of age. 2. Eight experimental diets varying in dVal (dVal to dLys ratios from 0·66 to 0·76) and dIle (dIle to dLys ratios from 0·57 to 0·67) were studied; digestible Lysine (Lys) was formulated to 9·9 g/kg in all diets. 3. Broilers fed on a negative control (NC) diet supplemented with crystalline Val (L-Val) and crystalline Ile (L-Ile), dVal to dLys = 0·76 and dIle to dLys = 0·67, grew faster and had higher breast meat yield than birds fed on NC + L-Ile (dVal to dLys = 0·66 and dIle to dLys = 0·67), NC + L-Val (dVal to dLys = 0·76 and dIle to dLys = 0·57), and NC + reduced L-Val and L-Ile (dVal to dLys = 0·71 and dIle to dLys = 0·62). 4. Feeding broilers on the NC + L-Val and L-Ile (dVal to dLys = 0·76 and dIle to dLys = 0·67) diets gave similar BW gain, carcase weight and yield and total breast meat weight and yield to birds fed on the positive control-fed broilers with no added L-Val and L-Ile (dVal to dLys = 0·76 and dIle to dLys = 0·67). 5. These results indicate that Val and Ile are co-limiting in diets containing poultry by-product meal.