The effects of Saccharomyces cerevisiae strains on the rumen fermentation in sheep fed with diets of different forage to concentrate ratios

The objective of this study was to compare the effects of two different Saccharomyces cerevisiae yeast strains on the rumen fermentation of sheep fed with two different total mixed rations. Three rumen-cannulated Merino wethers were used in a pilot study in which a phase of feeding a higher proportion of concentrate in the diet (HC; forage to concentrate ratio 40: 60, as fed) was followed by a phase of lower proportion of concentrate in the diet (LC; forage to concentrate ratio 49: 51, as fed). Both phases consisted of three 3-week periods as follows: no yeast supplementation, trehalose non-producing and trehalose-producing S. cerevisiae supplementations, respectively. Rumen fluid samples taken in the last days of each period were measured for pH, short chain fatty acid (SCFA) concentrations and ammonia content. The degradability of neutral detergent fibre (NDF) and starch was estimated by the in sacco method. Supplementation with the trehalose non-producing strain did not alter any variables tested in the HC phase. In the LC phase, though degradability was not altered, total SCFA concentration increased, resulting in a decrease of rumen fluid pH. The trehalose-producing strain increased total SCFA content and effectively prevented the consequent decrease in pH in both phases. These effects were more pronounced in the high-concentrate phase, as shown by higher molar proportion of propionate, increased level of ammonia content, and higher ruminal degradability of NDF and starch compared to control and the trehalose non-producing yeast strain.