The objective of these experiments was to develop a simple in vitro technique for evaluating the production and neutralization of acid as feeds ferment in the rumen. An in vitro approach was adopted to eliminate animal factors. The procedure was based on the method of Tilley and Terry, with some modifications developed in this project. Residual acidity (acidogenicity value) was determined by the dissolution of Ca from CaCO3 powder added to the media at the end of 24-h incubations. Acidogenicity values (AV) were higher when 20% strength buffer was used, while lowering buffer pH increased values, equally across all feeds. There was no effect of donor animal diet, but considerable day-to-day variation in the fermentation activity of rumen fluid. This variation likely reflected the substrate preferences of differing microbial populations, so that several standard feeds may be required to account for this effect. A series of 28 diverse feed ingredients was evaluated for AV using a mixture design, with 85 combinations of ingredients: 100% of 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 = 28); and an equal mixture of all ingredients (n = 1). The effects of most ingredients on AV were essentially linear, though some extreme ingredients showed nonlinear effects. Protein sources had low AV, forages intermediate AV and starchy feeds high AV. Calcium contained within feeds contributed to AV, particularly for legumes, sugar beet pulp, and citrus pulp, and must be accounted for.

Earlier studies developed a new approach to feed evaluation, measuring the net acid load that develops during rumen fermentation. Two concentrates were formulated to be isoenergetic and isonitrogenous, with extremes of rumen acid load. A third treatment comprised a 50:50 mixture of these concentrates. These concentrates were evaluated along with ryegrass silage and corn silage. The feeds were evaluated in a continuous culture system adapted to deliver and record the quantities of acid or alkali needed to maintain a constant pH (6.2 to 6.3). This study confirmed the anticipated ranking of concentrates for rumen acid load, as well as the highly acidogenic nature of corn silage. The concentrates were formulated to balance corn silage and were offered to early-lactation Holstein-Friesian cows at 50% of dry matter intake, with either ryegrass silage or corn silage. Feed intake was lower for animals offered corn silage-based diets (17.4 vs. 22.2 kg of dry matter/d). Increasing concentrate acid load led to a large decline in dry matter intake for corn silage, although not for grass silage. Feed intake effects were reflected in significant effects on yield of milk (31.0, 29.9, and 26.9 kg/d for low-, medium-, and high-acid load concentrates, respectively) and milk solids. Milk protein concentration was unaffected by concentrate type with corn silage diets but tended to be higher when high acid load concentrates were fed with grass silage. This may reflect the effect of the high starch concentrate rectifying a shortage of glucogenic precursors or microbial protein with the grass silage-based diet.

A new approach to evaluating feeds has been developed based on in vitro estimates of rumen acid load (acidogenicity value). The present work was conducted to establish effects of rumen acid load on dry matter intake and milk production of early-lactation dairy cows fed corn silage-based diets. The effects of diet acidogenicity were investigated independently of ingredient composition using a series of isoenergetic and isonitrogenous concentrates that used different raw materials to achieve the same acidogenicity value. Six concentrates were formulated to be fed at 7 kg/d, and two at 10 kg/d. Continuous culture equipment, which had been modified to control and record infusions of acid and alkali to maintain pH in the range 6.2 to 6.3, confirmed the ranking of concentrates based on the in vitro technique. The concentrates were offered to dairy cows in early lactation with corn silage available ad libitum. Forage intake declined with increasing concentrate acidogenicity, particularly at the higher level of concentrate feeding. However, concentrate acidogenicity had a significant negative effect on dry matter intake even at 7 kg/d. Feed intakes were similar for concentrates that were formulated to be isoacidogenic, but using different raw materials. There were only small and inconsistent effects of concentrate acidogenicity value on milk yield and milk fat percentage, while milk protein percentage was higher with the high AV concentrates. Effects on milk composition likely reflect differences in nutrient supply and there was no evidence for effects of rumen acid load per se on milk composition.

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Group z : control diet : cereals-! soyabean meal. Group 2 : replacement of So p. 100 soyabean meal by ! p. ioo untreated « keratin ». Group 3 : replacement of 5 o p. Too soyabean meal by ! p. 100 ultraground « keratin n. For the experiments on «gelatin», only 2 treatments were applied, i.e. 2 groups of 2 8 Large White pigs (iq castrated males, 14 females) for the lot trials between 30 and 100 kg live weight and 2 groups of 4 growing male pigs for the digestibility study. The diets used were the following : Group r : Control diet : cereals i soyabean meal. Group 2 : Replacement of So p. too soyabean meal by l p. ioo gelatin. For the lot trials, the pigs were kept in collective pens including animals of the same sex fed according to the following feeding schedule : a y p. roo protein diet during the growing period (30-6 0 kg live weight) and a r 5 p. too protein diet during the finishing period (6o-ioo kg