Six lactating Holstein cows fitted with rumen and T-type duodenal cannulas were used in a crossover design to examine effects of yeast culture supplement on production parameters, rumen fermentation, and flow of N to the duodenum. Treatments were control and control plus 10 g/d of yeast culture. Dry matter intake was greater, and milk production tended to be higher, for cows supplemented with yeast culture, but milk composition was not affected. Rumen pH was not affected by yeast culture, but peak lactic acid concentration decreased from 1.93 to 1.73 mM. Rumen fluid acetate:propionate ratio, dilution rate (percentage per hour), and ammonia N concentration (milligrams per deciliter) were 2.28, .12, and 10.7 and 2.04, .13, and 9.6 for control cows and for cows supplemented with yeast culture, respectively. Although numbers of fiber-digesting bacteria were not affected by yeast culture, DM disappearance of wheat straw tended to be higher at 12 and 24 h, and CP and ADF digestibilities were greater. Duodenal NAN flow tended to be higher in cows supplemented with yeast culture because of higher bacterial N flow. Duodenal AA profile and flow of Met were significantly affected by yeast culture supplementation. The results suggest that yeast culture may alter the AA profile of bacterial protein.

1. The nature of the digestion-inhibiting substance in Kikuyu grass (Pennisetum clandestinum, Hochst), containing high levels of nitrate, was investigated using in vitro digestibility techniques.2. Nitrite, which accumulated during the reduction of nitrate to ammonia, seemed to be the primary factor reducing digestibility. Nitrate and ammonia did not affect digestion in vitro.3. Nitrite caused a reduction in the cellulolytic, xylanolytic and total microbial population, with a concomitant reduction in cellulase and xylanase activity of the digesta.4. The mode of action of nitrite on rumen microbial growth was investigated.5. The possibility that the growth of cellulolytic rumen microbes was depressed by a reduction in concentration of essential branched-chain volatile fatty acids by nitrite was discounted.6. Although nitrite caused a marked increase in the redox potential, due to its oxidizing properties, the more-positive redox potential did not reduce the digestibility of the grass.7. The growth of three of the four major cellulolytic bacteria commonly found in the rumen was severely depressed by nitrite, while some rumen bacteria were relatively insensitive to nitrite.8. Growth inhibition seemed to depend primarily on the extent to which these microbes derive their energy from electron-transport-mediated processes.9. It was suggested that, due to the sensitivity of some rumen bacteria to nitrite, digestibility and therefore animal performance could be affected long before clinical symptoms of nitrite toxicity become apparent.

SUMMARYFrom a large number of cultures of cellulolytic bacteria isolated from rumen contents of sheep conditioned to low-protein teff hay through the use of both a selective medium containing finely ground cellulose as energy source and a less selective medium containing cellobiose, xylan and starch, thirty isolates, representing three morphological types, were selected for detailed characterization. Nineteen isolates of Gram-negative curved rods belonged to the genus Butyrivibrio. Of these one was identical with and ten closely related to B. Jibrisolvens. The remaining eight Butyrivi brio cultures differed in several respects from the only other defined species within this genus, B. alactacidigens. Four coccal isolates were identified as Rminococcus albus and five as R.j ¶avefaciens. Two strains of sporeformers belonged to the genus Clostridium but could not be identified with any of the cellulolytic species of this genus listed by Bergey's Manual.

The relationship between the pH of the medium and specific growth rates, in well-buffered media at 38.5 degrees C, was determined for three strains of Butyrivibrio fibrisolvens and for one strain each of Streptococcus bovis, Selenomonas ruminantium subsp. lactilytica. Megasphaera elsdenii, Veillonella alcalescens, and Propionibacterium acnes. The pH optima for growth were between 6.1 and 6.6 for all six species, and the upper pH limits were between 7.3 and 7.8. The lower limit pH values for growth on glucose were 5.4 for B. fibrisolvens, near 5.0 for V. alcalescens, and between 4.4 and 4.8 for the other four species. These values fall within the minimum pH ranges found when these species are grown in poorly buffered medium with nonlimiting glucose concentrations. Acid sensitivity per se could cause the washout of B. fibrisolvens, but not of the other five species, from the rumens of animals on high-starch diets.

SUMMARYA method is described which is suitable for routine viable counts of rumen bacteria capable of fermenting various carbohydrates. The contribution of different steps in the preparation of the medium towards the final redox potential was determined and the choice of a suitable reducing agent, redox indicator and source of rumen fluid supplement is discussed. In a sheep on a constant practical ration, the variation in numbers of cellulolytic bacteria was greater than the differences between five successive samples withdrawn at one time, and much greater than the variation due to the method.