The effects of diets formulated on an ideal protein basis on growth performance, carcass characteristics, and thermal balance of finishing gilts housed in a hot, diurnal environment.

Forty-eight finishing gilts (initial BW = 70.6 +/- .95 kg) were randomly assigned to one of eight experimental treatments in a 2 x 2 x 2 factorial arrangement with main effects including dietary lysine (.60 vs 1.00%), source of amino acid fortification (intact protein vs synthetic amino acids formulated on an ideal protein basis), and environmental temperature (thermoneutral [TN]: 20 degrees C vs hot, diurnal [HD]: 27.7 to 35 degrees C). The ideal protein diets were formulated by using corn and soybean meal to meet the fifth-limiting amino acid; synthetic lysine, threonine, tryptophan, methionine, or isoleucine were added to meet the gilts' estimated requirements. The ratios of other total amino acids relative to lysine were as follows: threonine, 66%; tryptophan, 17%; methionine and cystine, 56%; and isoleucine, 63%. Average daily gain, ADFI, and feed efficiency (G/F) were similar for gilts fed the intact and those fed the ideal proteins diets (P > .10). Increasing dietary lysine improved d 0 to 14 ADG (P < .01), but no differences were observed for the overall experiment. Gilts in the HD environment ate less feed and had lower ADG than gilts in the TN environment (P < .01). A temperature x lysine interaction was observed (P < .02) for G/F. Increasing dietary lysine had no effect on G/F of gilts in the TN environment but improved G/F of gilts in the HD environment. Gilts fed the intact protein diets had higher (P < .01) N intake and plasma urea concentrations. Gilts fed the ideal protein diets had lower (P < .05) plasma essential amino acids, with the exception of lysine. Carcass protein and lipid contents were improved (P < .01) for gilts in the HD environment and for those fed 1.00% lysine. Backfat thickness and longissimus muscle area (P < .01) were improved and lipid accretion rate tended to decrease (P < .08) in gilts fed 1.00% lysine. The source of amino acid fortification did not influence carcass characteristics (P > .10). Rectal, skin, and ear temperatures were higher for gilts in the HD environment (P < .05). Metabolic heat production was elevated by feeding gilts the ideal protein diets (P < .03). In conclusion, increased dietary lysine improved G/F and carcass leanness in gilts to a greater extent in HD than in TN environments. However, no improvements in growth performance or carcass traits resulted from feeding ideal protein diets.