GROWTH-PERFORMANCE OF PIGS GIVEN A CHOICE OF NATURAL INGREDIENT DIETSCONTAINING DIFFERENT LEVELS OF PROTEIN AND AMINO ACID(S)

Seventy-two crossbred pigs with an initial bodyweight (BW) of 24.6 kg were assigned to 24 groups of three pigs to determine whether pigs hav e the ability to select a lysine intake and/or a protein intake to mee t their requirements and to determine the effects of supplementation o f a combination of lysine (Lys), methionine (Met), and threonine (Thr) on diet selection when pigs were given a choice of a pair of isoenerg etic diets from 25 to 70 kg BW. Dietary treatments consisted of contro l, C, (conventional two phase feeding 16-14% crude protein), and three different choice feeding regimens: 1) protein choice, PC, (18 vs. 10% crude protein CP), 2) lysine choice, LC, (choice between a Lys-supple mented and -unsupplemented diet 0.90 vs. 0.53% lysine with 14% CP diet s), 3) amino acid choice, AC, (choice between a diet supplemented with a combination of Lys, Met, and Thr on an ideal protein basis) and uns upplemented diet Lys (0.90 vs. 0.53%), Met (0.50 vs. 0.45%), and Thr ( 0.59 vs. 0.40%) with 14% CP diets). The diets based on ideal protein w ere formulated by supplementing synthetic Lys, Met, and Thr to meet a ratio relative to lysine of 65% for Thr, 55% for Met + cystine, and 18 % for tryptophan. The two growth phases for the C treatment were 25 to 50, and 50 to 70 kg BW. All diets contained the same energy level (13 .7 MJ DE kg(-1)). At any weight interval and the overall period, daily feed intake was not affected (P > 0.05) by different feeding regimens and pigs in C and PC grew faster (P < 0.05) and had superior (P < 0.0 5) feed:gain ratio than LC or AC pigs. Supplementation of Lys or a com bination of Lys, Met, and Thr did not improve pig performance when cho ice-fed with the unsupplemented diets. Protein choice pigs showed an e xtremely strong preference for the high protein diet and consumed 89.8 % of the high protein diet and 10.2% of the low protein diet for the o verall period. Supplementation of Lys alone or a combination with Met, and Thr did not increase feed consumption. Selection of the Lys suppl emented diet and the Lys, Mel, and Thr supplemented diet accounted for 35.7 and 31.8% of total feed intake for the overall period. During th e overall period, dietary protein intake increased (P < 0.05) in PC pi gs compared with control pigs and there was no significant difference in dietary lysine intake between C and LC pigs and C and AC pigs, resp ectively. Protein choice and LC pigs required (P > 0.05) a similar amo unt of dietary protein and lysine per unit gain of BW to that of contr ol pigs. These results suggest that pigs are unable to control their p rotein and lysine intake to meet their requirement for growth when giv en a choice of two isoenergetic diets differing in protein and lysine content. Supplementation of Lys alone, or in combination with Met, and Thr did not affect the pattern of diet selection.