Urine acidification and mineral metabolism in growing pigs fed diets supplemented with dietary methionine and benzoic acid

Abstract

Benzoic acid (BA) reduces pH of urine and thereby reduces the emission of ammonia and possibly also odorous sulphur-compounds from slurry. The effect of BA on mineral metabolism in growing pigs is not clear. The objective was therefore to study the effect of BA and methionine (Met) as a sulphur (S) source in diets for pigs on urine acidification and mineral metabolism. Twenty-four 45 kg pigs in a 2 × 2 factorial design were fed one of 4 diets, containing 0 or 2% BA and a low or high dietary S level provided through diet supplementation of 0 or 1% Met. The pigs were placed in metabolic cages for a 5 d adaptation period and a 7 d period with collection of faeces and urine. Benzoic acid was metabolized into hippuric acid which reduced urinary pH by 0.8 pH units ( P < 0.001), and dietary supplementation with 1% Met reduced urinary pH by 1.0 unit ( P < 0.001). The group of pigs receiving the unsupplemented diet showed the lowest apparent digestibility of phosphorus (P) ( P < 0.001) and calcium (Ca) ( P = 0.04). The urinary concentrations of Ca were increased ( P = 0.02) by BA supplementation, but the retained proportion of absorbed P and Ca was affected by neither BA nor Met supplementation. Sulphur apparent digestibility was not ( P = 0.58) affected by BA, but on the 1% Met diets, S excretion into urine was reduced ( P = 0.006) by BA. In conclusion, both BA and Met supplementation increased P and Ca apparent digestibility and decreased urinary pH. The increased acid load provided through dietary BA did not significantly affect the intermediary metabolism of P and Ca.