Responses of pigs fed corn-soybean meal or low-protein amino acid-supplemented diets at different feeding levels

Two experiments with growing(Exp. 1) and finishing (Exp. 2) gilts were conducted comparing one vs four gilts/pen to identify factors involved in the growth retardation of pigs housed in groups. Pens were provided with one feeder, one waterer, and a space allowance of 1.5 m$\sp2$/pig, regardless of the number of pigs per pen. Pigs fed individually had greater final BW and ADG (Exp. 1, P $\u3c$.10; Exp. 2, P $\u3c$.05), backfat thickness (Exp. 1, P $\u3c$.10), DM, CP, and energy apparent digestibilities (Exp. 1, P $\u3c$.01), and loin weight and primal cut percentage (Exp. 2, P $\u3c$.10), but had lower (Exp. 1, P $\u3c$.05) plasma NEFA concentrations. Two experiments, each with 39 barrows with high lean-gain potential, were conducted to evaluate the responses of pigs fed corn-soybean meal diets (CONTROL) and low-CP diets supplemented with crystalline Lys, Thr, Trp, and Met either on an ideal protein basis (IDEAL) or to a pattern similar to the control diet (AACON). In Exp. 3 (growing-finishing pigs), the CONTROL and IDEAL diets were fed on an ad libitum basis, or at 90 or 80% of ad libitum. In Exp. 4 (growing pigs), the CONTROL, IDEAL, and AACON diets were fed on an ad libitum basis or at 80% of ad libitum. Pigs fed the CONTROL diet had greater ADG and ADG/ADFI (P $\u3c$.05) than pigs fed the IDEAL and AACON diets. As the level of feed intake decreased, ADG decreased (P $\u3c$.05), but ADG/ADFI tended to improve (P $\u3c$.10) for pigs fed 90% of ad libitum (Exp. 1) and for pigs fed 80% of ad libitum (Exp. 2). Plasma urea concentrations were lower in pigs fed the IDEAL and AACON diets than in pigs fed the CONTROL diet, regardless of feeding level. For pigs fed the CONTROL diet, urea concentrations were lower when feed intake was 80% of ad libitum (diet x level, P $\u3c$.01). On a whole body basis, the protein concentration (g/kg), and the accretion rates of protein (g/d) were greater (P $\u3c$.05) for pigs fed the CONTROL than for pigs fed the IDEAL and AACON diets. In Exp. 1, pigs fed the CONTROL diet had a trend (P =.10) for greater water and lower fat concentration and had greater (P $\u3c$.05) water and ash accretion rates. Whole body protein concentration was greatest (P $\u3c$.05) for pigs fed at 80% of ad libitum, but protein, water, and ash accretion rates were greatest (P $\u3c$.05) for pigs allowed ad libitum access to feed. The data suggest that a deficiency of one or more essential amino acids or a reduction of the Lys:energy ratio may have limited the growth potential of pigs fed the IDEAL and AACON diets