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To find out why low-crude protein, amino acid-supplemented diets often reduce growing pig performance, we conducted two experiments. In the first experiment, a nitrogen balance trial, three standard corn-soybean meal diets and three corresponding low-crude protein, amino acid-supplemented diets were used. The diets were: 14% CP and 10% CP + AA, 16% CP and 12% CP + AA, and 18% CP and 14% CP + AA, fed to 12 (90 lb) gilts in three periods of 7 d each. The amino acids lysine, tryptophan, threonine and methionine were added to low-crude protein diets to reach the same total amount as that in their respective standard diet. All nitrogen balance variables studied were affected by the reduction of crude protein in the diets. For energy balance, only energy excreted in feces and the apparent digestibility of energy were affected by the concentration of crude protein in the diet. The second experiment was a growth performance trial, in which a standard corn-soybean meal, 16% crude protein, and five low-crude protein, amino acid-supplemented diets were fed to 36 (43 lb) gilts. The low-crude protein diets had 15, 14, 13, 12, and 11% crude protein, and were supplemented with crystalline lysine, tryptophan, threonine, and methionine to contain the same total concentration as that in the standard diet. There was no difference in average daily gain, average daily feed intake, feed/gain, longissimus muscle area, or average daily lean gain in gilts fed 12 to 16% crude protein diets. However, the 11% crude protein diet had negative effects on these variables. Plasma urea nitrogen increased as the crude protein increased from 11 to 16%. Backfat thickness was not affected by the crude protein concentration in the diet, but varied among the different diets. These results suggest that dietary crude protein can be reduced from 16 to 12% if amino acids are added, without affecting pig performance, and that this crude protein reduction can help reduce nitrogen excretion in the urine and feces.