The responses of growing pigs to dietary lysine, as free lysine hydrochloride or in soya-bean meal, and the influence of food intake

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Abstract
One hundred and ninety-two individually fed pigs, half of them castrated males and half females, were used to examine the effects of reducing the concentration of dietary lysine below the value of 0·84 g/MJ digestible energy suggested by the Agricultural Research Council (1981). Lysine was varied by altering the inclusion of either free lysine hydrochloride or soya-bean meal. Over the whole experiment from 18 to 65 kg live weight, there were significant reductions in live-weight gain and increases in food: gain ratio in response to reducing lysine concentration; these changes were greater with soya-bean meal than with free lysine. During later growth (45 to 65 kg) alterations in lysine concentration had no significant effect.For pigs given soya-bean supplements, both the live weights after fasting and the carcass weights were significantly lower than for those given free lysine hydrochloride, suggesting that at least part of the difference in growth between pigs given free lysine hydrochloride and soya-bean was attributable to this difference. The food required per kg carcass weight gain, or per kg lean tissue gain, was not affected by the source of lysine.In a subsidiary experiment using nitrogen balance, supplements of five amino acids in addition to lysine did not increase nitrogen retention, providing further evidence that lysine was the only amino acid responsible for the response seen with soya-bean meal. It was concluded that for growing pigs with daily food intakes of 1·5 to 1·8 kg, a lysine concentration of at least 0·81 g/MJ digestible energy is required to maximize performance.