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The reticulorumen is now recognised to be an important site of net absorption of phosphate ions from ruminal fluid containing phosphate concentrations appropriate to those found in normal farming practice. These rates of absorption were measured in vivo from solutions placed in the washed reticulorumen, isolated in situ, in conscious, trained sheep. Reducing the ruminal sodium concentration led to reduced absorption of phosphate, suggestive that phosphate and sodium fluxes across the apical wall of the ruminal epithelial cell are linked, as they are in the kidney. Increased absorption of short chain fatty acids led to enhanced absorption of phosphate ions. Conversely, inhibition of carbonic anhydrase activity, by the addition of 1 mM acetazolamide to the ruminal fluid, led to a reduction in phosphate absorption. An increase in the acidity of the ruminal fluid also increased the absorption of phosphate, as did an increase in the ruminal Ca(2+) concentration over the range 1-4 mmol per litre. It is suggested that these effects can be accounted for by a Na(+)/H(+) antiporter coupled with a phosphate/proton symporter in the apical membrane of the ruminal epithelial cell., (Copyright 2002 Elsevier Science Ltd. All rights reserved.)

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.

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.

Net Ca2+ and Mg2+ absorption rates were measured in vivo from buffer solutions placed in the washed reticulo-rumen, isolated in situ in 30 conscious, trained sheep. An increase in concentration of short chain fatty acids (SCFA) in the buffer, over the range 0-50 mM, was shown to stimulate the net rates of absorption of Ca2+ and Mg2+ ions from the rumen. Similarly, the results of in vitro experiments, carried out with ovine rumen epithelium mounted in short-circuited Ussing chambers, showed that the absence of SCFA from the chamber fluid resulted in a reduction in Jnet Ca2+ caused by reduced flux of Ca2+ ions in the mucosal to serosal direction (Jms Ca2+). The addition of 1 mM acetazolamide, an inhibitor of carbonic anhydrase, to the ruminal buffer used in the in vivo experiments led to significant reductions in the net absorption rates of Ca2+ and Mg2+ ions in the presence of SCFA (50 mmol x l(-1)) but not in the absence of SCFA. However, in the in vitro experiments, the addition of 60 microM ethoxyzolamide had no significant effect on Jnet Ca2+. A reduction in pH of the intraruminal buffer in vivo from 6.8 to 5.4 led to significant increases in the net absorption rates of Ca2+ and Mg2+ ions, an effect which was duplicated for Ca2+ in preliminary in vitro experiments in which the pH of the mucosal buffer was reduced from 7.4 to 5.4. This stimulatory effect was confined to Jms Ca2+ and Jnet Ca2+. Ussing chambers were also used to demonstrate that Jnet Ca2+ was reduced by a high transmural potential difference (PD), caused by voltage clamping, independently of the mucosal K+ concentration. Both unidirectional Ca2+ fluxes consisted of a PD-dependent and a K+-insensitive PD-independent component. The latter may be represented by a Ca2+/ 2H+ antiporter. It is postulated that SCFA, and to a lesser extent H2CO3, can stimulate Jms Ca2+ by activation of an apical Ca2+/2H+ antiporter through the provision of protons within the ruminal epithelial cell. A mild reduction in ruminal pH may also lead to a similar stimulation of this putative electroneutral exchange.

The net absorption rates of strontium ions from the ovine reticulo-rumen, isolated in situ in trained conscious animals, were measured under controlled conditions. A linear positive response was obtained from the addition of Sr2+ ions to the artificial rumen fluid. This increase in the absorption of Sr was reflected in an increase in the plasma Sr concentration. In contrast to the discrimination observed elsewhere in favour of the absorption of Ca relative to Sr, the absorption rate of Sr from the reticulo-rumen was significantly greater than that of Ca, from solutions containing the same molar concentration. A graded increase in the Sr concentration in the ruminal fluid from 1 mmol/l to 4 mmol/l led to a corresponding reduction in the absorption rate of Ca but an increase in that of phosphate. The latter result is similar to that observed when the intra-ruminal concentration of Ca2+ ions is increased. It is suggested that Ca and Sr share a common pathway for absorption from the reticulo-rumen and that this may involve coupling with the absorption of phosphate ions.