LIMITING AMINO ACIDS IN AN 11% CRUDE PROTEIN CORN-SOYBEAN MEAL DIET FOR GROWING PIGS Louis E. Russell 1 , Brian J. Kerr and Robert A. Easter 2 University of Illinois a~ Urbana, 61801 ABSTRACT Three experiments were conducted to test the hypothesis that methionine, isoleucine, valine or nitro- gen either singly or in combination are limiting in an 11% crude protein, lysine-tryptophan-threonine- supplemented, corn-soybean meal diet for growing pigs. A 16% crude protein diet was used as a positive control in each experiment and all amino acid additions were made, at a minimum, to equal requirements. Average initial weights were 21.3 kg, 23.0 kg and 19.1 kg in Exp. 1, 2 and 3, respectively. The experi- ments averaged 4 wk in duration. In Exp. 1 and 2, neither the addition of glutamic acid as a source of nitrogen, nor methionine to the 11% diet resulted in any improvement (P>.20) in rate or efficiency of growth. Addition of the combination of isoteucine and vatine to the 11% diet resulted in an increase (P<.05) in both growth rate and feed efficiency to a level not different (P>.20) from that of the pigs consuming the positive control diet. The addition of valine to the 11% crude protein diet with supple- mental lysine, tryptophan and threonine (Exp. 3) caused an improvement in daily gain (P<.05) but not feed efficiency (P>.10). Isoleucine addition tended to reduce pig performance. The results of these ex- periments suggest that an 11% crude protein, corn-soybean meal diet fortified with lysine, tryptophan and threonine is not limiting in sulfur amino acids or nitrogen. Valine may be the only limiting amino acid. (Key Words: Pigs, Amino Acids, Methionine, Isoleucine, Valine, Nitrogen.) Introduction If the protein level of corn-soybean meal (CS) diet fed to 20- to 40-kg pigs is reduced by two percentage units from recommended levels (16% crude protein; NRC, 1979) growth rate and efficiency of feed utilization can be re- stored to normal by the addition of lysine (Katz et al., 1973; Baker et al., 1975; Easter and Baker, 1980). Pigs fed similar diet reduced in crude protein content by four percentage units, i.e., 12% crude protein, clearly respond to lysine, tryptophan and threonine supplemen- tation (Corley and Easter, 1983; Russell et al., 1983, 1986). 1Current address: Feed Specialties Co., 1877 NE 58th Ave., Des Moines, IA 50313. 2Reprint requests should be addresses to this at 318 Mumford Hall, 1301 W. Gregory Drive, Urbana, IL 61801. s Dept. of Anita. Sci. 4 The assistance of Scott Williamson, Howard Cook, Dale Alexander, Ron Brewer and Steve Heffernan with diet preparation and animal care and that of Miss Julie Thode with manuscript typing is gratefully acknowl- edged. Received October 15, 1986. Accepted June 3, 1987. An unexpected, and sometimes contradic- tory, response to methionine addition has been observed in some experiments. Sh~rda et al. (1976) were unable to demonstrate a response in growing pigs to either methionine or thre- onine addition to a 12% crude protein, lysine- supplemented CS diet. However, Gomez-Rojas et al. (1982) reported improvements in feed efficiency but not daily gain when methionine was added to a 12% crude protein, CS diet with added lysine, tryptophan and threonine. In con- trast, Russell et al. (1983), in a similar experi- ment, found an improvement in growth rate (P<.05) but not in feed efficiency. Cromwell et al. (1983) reported numerical, but not signifi- cant, improvements in rate and efficiency of growth by finishing pigs when methionine was added to a 10% CS diet supplemented with ly- sine, tryptophan and threonine. The present experiments were conducted to determine if methionine, another amino acid, or nitrogen is fourth limiting in reduced-protein, CS diets fed to pigs weighing between 20 and 40 kg. Experimental Procedure Three growth experiments involving 474 Yorkshire-Hampshire-Landrace pigs derived 1266 J. Anim. Sci. 1987.65:1266-1272