Since Megasphaera elsdenii ferments a variable part of DL-lactate to butyrate, measurement of the percentage of DL-lactate fermented to propionate via the acrylate pathway in rumen contents will underestimate the participation of M. elsdenii in the DL-lactate fermentation. The percentage of DL-[2-13C]lactate fermented via the acrylate pathway and the percentage of DL-lactate fermented to butyrate can be measured with 13C-FT (Fourier transform)-nuclear magnetic resonance. On the average, the contribution of M. elsdenii to DL-lactate fermentation in the rumen of dairy cattle was found to be 74% (standard deviation, 13%), but differed with animal or diet. After feeding a cow readily fermentable carbohydrates, the contribution of M. elsdenii to the fermentation of DL-lactate increased as a consequence of catabolite repression in other DL-lactate-fermenting bacteria.High-concentrate diets induce a rapid microbial fermentation in the rumen and the accumulation of intracellular (H) within the microbes leads to an increased formation of reduced end products and of lactic acid (4). This in itself should not lead to DL-lactate accumulation in the rumen, provided that the capacity to remove DL-lactate was high enough. DL-Lactate formed in the rumen may be removed in three ways: (i) by passage to the lower gastrointestinal tract, (ii) by absorption directly from the rumen, and (iii) by microbial fermentation.Megasphaera elsdenii, a predominant DL-lactate-fermenting organism, shows no catabolite repression by carbohydrates such as glucose and maltose, whereas Selenomonas ruminantium subsp. lactilytica, another predominant DL-lactate-fermenting organism, ferments glucose, sucrose, and xylose first before fermenting DL-lactate (7, 11).M. elsdenii is commonly considered to be associated with the fermentation of readily fermentable carbohydrates (9,14). Therefore, it is important to know the participation of M. elsdenii in the fermentation of DL-lactate under several feeding conditions, in different animals, and especially after feeding readily fermentable
The regulation of lactic acid production, the regulation of lactate fermentation and the role of lactate as intermediate in the rumen metabolism was studied. The pH had a pronounced effect on all three processes and therefore buffer capacity of the rumen contents is also described. Starch gave much less rise to lactic acidosis than soluble sugars, as glucose and fructose. Most bacteria grow faster and therefore produce more lactic acid when amino acids and/or soluble proteins are present in the diet. Activity of LDH (lactate dehydrogenase) of mixed rumen microorganisms is regulated by the NADH/NAD(H) balance and the ATP concentration. About 60% of the LDH in mixed rumen microorganisms is fructose-1, 6-diphosphate independent. Megasphaera elsdenii ferments 60 to 80% of the lactate fermented in the rumen of dairy cattle. Lactate accumulates only when the glycolytic flux (hexose units fermented per unit time per microorganism) is high. During adaptation, the glycolytic flux is increased and lactate may accumulate. After adaptation to a certain diet, the number of microorganisms is changed and the glycolytic flux again is normal and lactate is only a minor intermediate in rumen metabolism.
The role of DL-lactic acid as an intermediate in the rumen of a Friesian X Holstein dairy cow adapted to a diet of hay ad libitum plus 12 kg of a concentrate mixture was studied in vitro and in vivo. Concentrations of soluble sugars in the rumen fluid became maximal at 30 min postfeeding, but at 90 min no sugars were detectable. The DL-lactate concentration increased very rapidly to about 30 mm at 30 min after feeding, whereas the maximum total VFA concentration was reached 15 min later. More than 80% of the DL-lactate fermented to VFA was converted by Megasphaera elsdenii. Whereas only 16% of L-lactate was fermented to propionate, 75% of the D-lactate was converted to propionic acid. When all soluble sugars had been fermented, the participation of M. elsdenii to lactate fermentation declined and fermentation patterns for D- and L-lactate became similar yielding mostly acetate. Except for a brief period immediately after feeding, DL-lactate did not appear to be an important precursor of VFA in the rumen of a cow adapted to concentrate feeding. DL-lactate may become a more important intermediate in rumen fermentation temporarily when dairy cows are gradually changed from a hay diet to a diet including concentrates. The first 30 d after parturition, when the changeover takes place, is an unstable period, during which the microbial population is changing to fit the new environment.
A method is presented for the analysis of buffer systems in the rumen using the first derivation of titration curves. Bicarbonate and volatile fatty acids (VFA) are the main components of the buffering system in the rumen fluid of dairy cattle under widely different feeding conditions. Phosphate from saliva is of little importance as a buffer, but neutralizes acids produced in the rumen. After studying five cows during the peripartal period a spontaneous and transient increase in the concentrations of VFA and a soluble marker (PEG) as well as a drop in pH and in the bicarbonate concentrations not related to feeding was observed in two animals that were sampled several hours before parturition. The potential risk of provoking rumen disturbances upon feeding animals close to the time of parturition, when buffering capacity may be minimal, is stressed.
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