A method for determination of the acidifying value of organic acids used in pigs diets in the acid binding capacity at pH 4 (ABC-4) system

The objective of this study was to measure the ABC-4 values for organic acids using their compensating effects on various feedstuffs ABC-4 values and to propose a model predicting organic acids ABC-4 values using acid characteristics such as pKa and molecular weight. Measurements of the amount of HCl required to lower the pH of 1 kg of sample to pH 4.0 were determined as the ABC-4 on a 0.5 g sample of feed ingredient (wheat, soybean meal and dicalcium phosphate). A substitution of HCl 0.1 N by an organic acid: dl-2-hydroxy-4-methylthiobutyric acid (HMTBA), formic acid, butyric acid or malic acid was realized with three different proportions (11.7, 23.5 or 35.2 mEq H+). The amount of HCl needed to lower the pH at 4 was recorded and the new ABC'-4 determined. The acidifying effect of organic acids was calculated for wheat and soybean as the difference between ABC'-4 and ABC-4. Results indicated that ABC-4 determined for wheat was lower than ABC-4 of soybean meal and dicalcium phosphate. A mixture of these ingredients resulted in the addition of their ABC-4 values proportionally to their level of inclusion. The ΔABC-4 obtained for organic acids showed a significant difference between these acids with HMTBA and formic acid having the highest acidifying effect on equivalent dose base. The slopes obtained from the linear regression of ΔABC-4 in function of the amount of organic acid added, were not significantly different between wheat and soybean meal for all selected organic acids. Based on these observations and knowing that purity, pKa and molar mass of the acid itself can affect the amount of H+ released, an equation was proposed to predict the ABC-4 of organic acids. HMTBA is as acidifying as formic acid on equimolar basis. Inclusion of 1 kg of HMTBA per ton of diet would supply −4876 mEq H+. This study allowed feed formulation using ABC-4 of ingredients and acids for optimal weaned pig performance.