Digestible sulphur amino acid to digestible lysine ratios in relation to performance, feather traits and skin properties in broiler chickens

1. This study evaluated the effects of digestible sulphur amino acids to digestible lysine ratios (dSAA:dLys), applied in increasing and decreasing series, on growth performance, gene expression, plasma homocysteine and taurine and feather quality in broiler chicken.2. Two trials were conducted using 720 one-day-old broiler chicks (Ross 308) from 1 to 24 d of age in a completely randomised experiment using four treatments with nine replicates of 20 birds. Treatments consisted of dSAA:dLys ratios applied across the pre-starter (1-10 d) and starter (11-24 d) phases in a decreasing (Trial 1) or an increasing manner (Trial 2).3. In Trial 1, dSAA:dLys ratios (pre-starter/starter) were: TRT1 (84/82%), TRT2 (82/80%), TRT3 (80/78%) and TRT4 (78/76%). In Trial 2, dSAA:dLys ratios (pre-starter/starter) were: TRT1 (78/80%), TRT2 (76/78%), TRT3 (74/76%) and TRT4 (72/74%).4. In the decreasing series of the dSAA:dLys ratio, higher levels improved feed conversion ratio (FCR) at 24 d of age. The increase favoured feather structural development, providing a linear increase in primary follicle diameter, while feather weight and calamus thickness showed quadratic responses. The maxima was estimated at approximately 78% of the dSAA:dLys ratio, which indicated greater structural development in feathers near this level.5. For the increasing treatments, a linear response was observed in the length of vane 1 of the eighth primary feather and greater skin hardness was estimated at approximately 78% of the dSAA:dLys ratio. This suggested enhanced feather development and greater mechanical resistance of the skin.6. In conclusion, dSAA:dLys ratios at 78% improved feather structure and FCR in the decreasing series and maximised skin hardness in broiler chickens in the increasing series.