Abstract
A total of 120 pigs were used to investigate the effects of yucca extracts on the growth performance, nutrient digestibility and excretion of growing pigs fed different levels of dietary protein. Pigs were allotted into $2{\times}3$ factorial design by the supplementation with yucca extract (YE, 0 and 120 mg/kg) and 3 levels of dietary protein (16, 18, 20%). During the whole experimental period (18 to 52 kg body weight), there were no significant differences in ADG, ADFI or F/G by YE addition or different protein levels among treatments (p>0.05). Overall, although addition of YE to the diet and elevation of protein level showed better ADG, there were no significant differences in growth performance among treatments. Pigs fed diets with YE showed significantly (p<0.05) higher dry matter (DM), crude ash (CA) and crude protein (CP) digestibility than did the others during the growing period. Concerning the levels of dietary protein, only the CP digestibility was significantly higher in pigs fed high protein diet. Pig fed the low protein diet without YE showed a significantly low CP digestibility (p<0.05). No significant differences were found in crude fat (CF), calcium (Ca) and phosphorus (P) digestibilities regardless of YE supplementation or dietary protein levels. Pigs fed YE supplemented diets showed significantly (p<0.05) higher amino acid digestibility. Also, high CP level diets showed a higher amino acid digestibility than low CP diets (p<0.05). DM and N excretion did not show any significant differences among treatments, there was a slightly lower excretion with increase in dietary protein level. Supplementation with YE significantly decreased the DM and N excretion. Interaction (YE$\times$protein) was found in P excretion. Pigs fed a medium protein diet without YE showed the lowest P excretion during the growing period. The NH3-N content in the feces tended to be increased by the increased dietary protein levels and with YE supplementation. During the whole experimental period, the cost for YE supplementation was similar to value of the improvements of performance obtained. The cost of feeding high level protein was significantly higher than that of medium level protein by 10% and low level protein by 9% (p<0.05). It could be concluded that the effects of dietary protein level and yucca extract on growth performance, nutrient digestibility and excretion might play a role to some extent in growing pigs from the aspect of pollution control.