Effects of dietary carbohydrates and buffering capacity on nutrient digestibility and manure characteristics in finishing pigs.

A 2 x 3 factorial experiment was conducted with 24 finishing pigs (Yorkshire x [Finnish Landrace x Dutch Landrace]) to determine the effects of dietary buffering capacity (BC) and carbohydrate sources on apparent total tract digestibility (TD), N retention, and manure characteristics. Twelve of these pigs were fitted with steered ileo-cecal value cannulas to measure the apparent ileal digestibility (ID) of N. Experimental variables were two levels of BC (High = 600 mEq/kg and Low = 530 mEq/kg) and three dietary carbohydrates (tapioca [28%], soybean hulls [25%], and sugar beet pulp [25%]). The two levels of BC were achieved by adjusting the amounts of alkalogenic limestone and acidogenic Ca sulfate in the diet. Pigs were fed twice daily at 2.4 x maintenance requirement for metabolizable energy (418 kJ ME/BW(0.75)). Chromic oxide was used as an indigestible marker. Feces and urine were collected over 5 d for nutrient balance and in vitro measurements of pH and ammonia emission over 7 d. No significant interactions between BC and dietary carbohydrates on the ID, TD, N retention, or manure characteristics were observed. Lowering BC with Ca sulfate did not affect digestibility of most nutrients, except for a negative impact on the TD of Ca (P = 0.015) and Mg (P = 0.003). Although all pigs receiving Ca sulfate had more acidic urine (by 0.49 pH units; P = 0.001), ammonia emission from manure was not lowered, irrespective of the carbohydrate source. Carbohydrates affected significantly the ID and TD of most nutrients, whereas N retention remained similar (P > 0.10). Urinary N:fecal N ratios in manure of pigs fed diets with tapioca, soybean hulls, and beet pulp were 2.09, 1.35, and 1.67, respectively. These ratios corresponded with in vitro ammonia emission (169, 125, and 148 mmol/7 d; P = 0.023). In conclusion, these results indicate that the acidification of urine in pigs fed Ca sulfate in the presence of NSP-rich carbohydrates was achieved. However, acidity of manure and ammonia emission were not affected. The addition of NSP from soybean hulls and(or) sugar beet pulp reduced ratios of urinary:fecal N, and thereby ammonia emission.