Influence of dietary undetermined anion on acid-base status and performance in pigs.

Dietary undetermined anion (dUA), calculated as (sodium + potassium + calcium + magnesium) - (chloride + phosphate + sulfate) and expressed in milliequivalents per kilogram, is a major determinant of the acid or alkaline contribution of a diet. Three experiments involving 218 7-wk-old pigs were conducted to investigate the effect of chloride-mediated alterations of dUA. The first two experiments evaluated the effect on pigs of decreasing dUA in either a corn-soybean meal type of diet (expt. 1) or one with a lower potassium content (expt. 2). As dietary chloride increased, and thus as dUA decreased, the rate of growth declined, mediated by changes in feed intake; the effect became more noticeable at extreme dUA levels. The increasing acidogenicity of the diets resulted in a lowering of blood pH, bicarbonate and base excess. The third experiment compared a corn-soybean meal control diet with diets containing two levels of added chloride (7.7 and 9.8 g/kg) at lowered dUA (388 vs. 172 vs. 98 mEq/kg) or at constant dUA (388 vs. 412 vs. 431 mEq/kg). Elevated chloride at constant dUA had no effect on pig performance or acid-base status (p greater than 0.05); however, chloride-mediated decreases in dUA depressed all growth parameters and induced an apparent acidemia. It was concluded that dUA is an important dietary component with the potential to alter pig performance and health. It was also clear that dUA is independent of specific mineral (chloride) effects.