Dietary threonine requirement of Pekin ducks from 15 to 35 days of age based on performance, yield, serum natural antibodies, and intestinal mucin secretion.

A study was conducted to establish the dietary Thr requirement of Pekin ducks from 15 to 35 d of age. Experimental diets were formulated to contain 0.55, 0.60, 0.65, 0.75, and 0.85% Thr (0.57, 0.60, 0.64, 0.72, and 0.80% on an analyzed basis) and were studied in 2 experiments. In experiment 1, each diet was fed to 10 pens of 52 drakes per pen. Samples were collected at d 35 for determinations of carcass yields, serum immune parameters, and intestinal characteristics. Experiment 2 was a digestibility study, wherein 0.5% chromic oxide was mixed into the experimental diets and fed from 15 to 19 d. Ileal digesta were collected at d 19 to analyze mucin secretions and apparent ileal Thr digestibility. The results showed that feeding 0.72% versus 0.64% Thr improved 15 to 35 d BW gain by 55 g (P < 0.05), reduced feed-to-gain by 0.04 (P < 0.05), as well as increased carcass and breast meat yields by 22 and 24 g, respectively. Also, 0.72% Thr had the highest crude mucin secretion on a DM intake (DMI) basis (P < 0.05), although Thr had no effect on villus height, crypt depth, goblet cells, and MUC2 gene expression in the jejunum and ileum. In addition, serum natural IgY linearly increased (P < 0.0001) with dietary Thr increase. Using nonlinear regressions, Thr requirement was estimated to range from a low of 0.70% to maximize dry crude mucin secretion on a DMI basis to a high of 0.80% to maximize carcass weight and serum IgY production by the linear or quadratic regression. Equivalently, Thr requirement varied between a low of 0.62% to minimize mortality and a high of 0.73% to maximize dry crude mucin secretion expressed as DMI using the quadratic broken-line model. Correspondingly, the apparent ileal digestible Thr requirements were estimated to be 0.52 to 0.66% (0.70 to 0.80% dietary Thr) by quadratic and 0.47 to 0.56% (0.62 to 0.73% dietary Thr) by quadratic broken-line model.