Interrelationships between dietary lysine, sex, and porcine somatotropin administration on growth performance and protein deposition in pigs between 80 and 120 kg live weight

Sixty male and 60 female crossbred pigs were allocated to an experiment to investigate the effects of porcine somatotropin (pST) administration (0 or 6 mg/d) and dietary lysine content on growth performance, tissue deposition , and carcass characteristics over the live weight range of 80 to 120 kg. P igs receiving PST were given diets containing 6.9, 7.8, 8.8, 9.7, 10.6, or 11.5 g lysine/kg, whereas control pigs received diets containing 4.8, 5.8, 6.9, 7.8, 8.8 or 9.7 g lysine/kg. These dietary levels ranged from 0.40 to 0.70 g available lysine/MJ of DE for pST-treated pigs and from 0.28 to 0.58 g available lysine/MJ of DE for control pigs. Pigs were individually house d in pens, and there were five replicates of each treatment. All diets cont ained 14.5 MJ of DE/kg and were offered for ad libitum consumption to pigs between 80 and 120 kg live weight. Growth rate increased exponentially and food conversion ratio (FCR) decreased exponentially with increasing levels of lysine. In addition, there was a significant sex x pST interaction such that PST reduced the sex difference in FCR. Growth rate was faster in boars than in gilts and was increased by pST at the higher levels of dietary lys ine. Similarly, FCR was lower for boars than for gilts and was decreased by PST at the higher dietary lysine levels. The Optimum growth rate and FCR w ere defined as the lysine level at which growth rate and FCR were 95% and 1 05%, respectively, of the lysine plateau. The optimum growth rate and FCR w ere achieved at similar dietary lysine contents and were approximately 0.35 and 0.52 g available lysine/MJ of DE for control and pST-treated pigs, res pectively. Protein deposition in the carcass increased exponentially with i ncreasing dietary lysine level, was higher in boars than in gilts, and was increased by pST at the higher dietary lysine contents. Sex had no effect o n dietary lysine required to maximize protein deposition. The dietary lysin e contents required to ensure 95% of plateau protein deposition of 104 and 153 g/d were 0.39 and 0.55 g available lysine/MJ of DE for control and pST- treated pigs, respectively. The increase in lysine requirement with pST see ms to be commensurate with the increase in protein deposition.