An experiment was carried out with the objective of evaluating the effects of graded increases in the ratios between digestible threonine (Thr) and lysine (Lys) in the diet on broilers from 15 to 37 days old. The increases on Thr levels were obtained with higher amounts of soybean meal (SBM) or L-threonine (L-Thr) in the diets. One thousand and two hundred Ross x Ross 308 male chicks were placed in a factorial arrangement with the three ratios between Thr and Lys (63.5, 66.0 and 68.5%) and the two sources of Thr previously cited. Each of the six treatments had eight replicates of twenty five birds. The diets were formulated to attain minimum requirement levels of other nutrients and energy. There were no effects of the Thr level on live performance responses, water intake, litter dry matter, foot pad dermatitis, carcass and processing yields. In addition, there were no effects of the source of Thr on body weight, feed intake, litter dry matter, foot pad dermatitis, carcass, breast tenders, thighs and drumsticks yields. However, feed conversion was better (P=0.009) and water intake was higher (P=0.011) when the increase on Thr was originated by SBM, whereas breast meat yield was higher (P=0.035) with diets supplemented with L-Thr. An interaction between level and source of Thr was identified for weight gain and feed conversion demonstrating improvement when birds were fed 68.5% Thr to Lys ratio originated from SBM. Amounts of essential amino acid (AA) quantities after the third limiting AA were greater in the diets with increased SBM. Supposely, differences obtained in broiler responses when fed diets with Thr increased from SBM or L-Thr were related to different amounts or balances in the intake of essential AA. A Thr to Lys ratio of 63.5% was considered sufficient to maximize broiler performance, whereas increases in Thr obtained exclusively from SBM influence the results probably because of higher amounts of AA supplied after the fourth limiting one.
amino acids; broilers; ideal protein; threonine
