Development of a simple in vitro assay for estimating net rumen acid load from diet ingredients

The objective of these experiments was to develop a simple in vitro techniq ue for evaluating the production and neutralization of acid as feeds fermen t in the rumen. An in vitro approach was adopted to eliminate animal factor s. The procedure was based on the method of Tilley and Terry, with some mod ifications developed in this project. Residual acidity (acidogenicity value ) was determined by the dissolution of Ca from CaCO3 powder added to the me dia at the end of 24-h incubations. Acidogenicity values (AV) were higher w hen 20% strength buffer was used, while lowering buffer pH increased values , equally across all feeds. There was no effect of donor animal diet, but c onsiderable day-to-day variation in the fermentation activity of rumen flui d. This variation likely reflected the substrate preferences of differing m icrobial populations, so that several standard feeds may be required to acc ount for this effect. A series of 28 diverse feed ingredients was evaluated for AV using a mixture design, with 85 combinations of ingredients: 100% o f each ingredient (n = 28); 50% of each ingredient and an equal mixture of all others (n = 28); equal mixture of all ingredients, excluding one (n = 2 8); and an equal mixture of all ingredients (n = 1). The effects of most in gredients on AV were essentially linear, though some extreme ingredients sh owed nonlinear effects. Protein sources had low AV, forages intermediate AV and starchy feeds high AV. Calcium contained within feeds contributed to A V, particularly for legumes, sugar beet pulp, and citrus pulp, and must be accounted for.