Effects of diet acidity and protein level or source of calcium on the performance, gastrointestinal content measurements, bone measurements, and carcass composition of gilt and barrow weanling pigs

A total of 228 crossbred weanling pigs (average age of 25 d and BW of 6.44 kg) were used in two trials to evaluate the responses to sex, diet acidity, protein level, and source of calcium on the performance, gastrointestinal digesta measurements, bone measurements, and carcass composition. Diet acidity was manipulated by varying the sources of supplemental phosphorus in the diets. Trial 1 (5 wk) was conducted as a 2 x 3 x 2 factorial to evaluate sex (gilts and barrows), diet acidity (pH 5.9 and .90% P, pH 6.1 and .63% P, and pH 6.9 and .63% P), and level of protein (16 and 22% CP). In Trial 2 (6 wk), diet acidity (pH 5.5, 5.9, and 6.8, all with .7% P) and Ca sources (CaCO3 and CaSO4) were used with gilts and barrows. The sex X diet acidity interactions were significant for ADG in both trials. Barrows seemed to respond to both the more acidic diets and the buffered phosphate diets even though the pH was less acidic than that of the unbuffered diets. Gilts responded only to the more acidic diets. In trial 1, gilts ate more and grew faster (P < .05) than barrows, but no sex effects on performance were observed in Trial 2. Pigs fed 22% CP diets grew faster (P < .001) and more efficiently (P < .001) than did pigs fed 16% CP diets, but protein level x diet acidity and protein level x sex interactions were not significant. Stomach digesta DM, pH, and titration value were not consistently influenced by sex and diet acidity in Trials 1 and 2, by protein level in Trial 1, and by calcium source in Trial 2. Only the sex x diet acidity interaction for stomach DM tended to be significant in both trials; gilts fed the less acidic diets had the lowest DM, whereas barrows fed the more acidic diets had the lowest DM values. Although not significant in every case in both trials, bone (average of metacarpal and metatarsal) volume was lower and specific gravity and shear stress values were higher for gilts than for barrows. Pigs fed 16% CP diets had higher specific gravity (P < .05) and stress (P < .06) values than pigs fed 22% CP diets. A protein level x diet acidity interaction (P < .03) for stress suggested that pigs fed 22% CP diets were unaffected by diet acidity, whereas pigs fed 16% CP had the highest stress values when fed the more acidic diet and the lower P level. Pigs fed CaCO3 diets had higher (P < .08) stress than those fed CaSO4 diets. Longissimus muscle area and backfat thickness were slightly influenced by sex, diet acidity, protein level, or calcium sources. In summary, manipulation of diet acidity had minimal effects on gastrointestinal digesta and bone measurements and carcass lean/fat composition, but body weight gain favored the more acidic diets; gilts had a greater response to diet acidity than did barrows.