Sensitivity and improvement of in vitro methods to assess the effects of Saccharomyces cerevisiae on rumen fibre degradation
Thesis (MScAgric)--Stellenbosch University, 2017.
ENGLISH ABSTRACT: In this study, the effects of a yeast supplement, Saccharomyces cerevisiae CNCM I1077, on in vitro neutral detergent fibre digestibility (NDFd) and rate of digestion of NDF(kd), organic matter digestibility (OMd), dry matter digestibility (DMd), pH and volatile fatty acids (VFA) concentration were investigated when different parameters were changed, within the in vitro system, in order to challenge the yeast’s effectiveness. In the first experiment, the Kansas State, McDougall and Goering-Van Soest media where tested with 3 different yeast doses against a control (0, 1x105 , 1x106 and 1x107 cfu/ml) using oat hay and wheat straw. The maximum in vitro NDFd, DMd, OMd as well as rate of digestion of NDFwere obtained with dose 1x106 cfu/ml and with the McDougall or Goering-Van Soest media, although differences between doses were not always significant. The Kansas State medium resulted in the lowest values compared to the other two media. There was a decrease in pH estimates as yeast dose increased which corresponded with an increase in VFA concentrations. However, although the differences in pH were not all significant, dose 1x107 cfu/ml related to lower estimates compared to the control and 1x105 (6.53 vs. 6.61 and 6.61, respectively). In the second experiment, the effect of yeast (one-dose or two-doses) was tested while oxygen and distilled water were injected into the flasks every 3 hours using lucerne and maize silage. Yeast did not have any significant effect on NDFd, kd, OMd and DMd, nor did it interacted with forage, oxygen or time. Oxygen stress, however, reduced NDFd, VFA concentrations as well as rate of digestion of NDFby about 5% while there was no effect seen on the pH estimates. Two-doses of yeast caused a more stable and controlled pH environment over time as there was no significant interaction between pH and two-doses at 12, 24 or 48 h (6.80 vs. 6.82 vs. 7.02, respectively). Lastly, in the third experiment, the effect of yeast (one-dose or two-doses) was tested while having a starch-stressed environment with lucerne and maize silage with certain samples receiving additional wheat-starch. There were no significant interactions between dose and forage, starch or time for NDFd, kd, DMd, OMd or VFA. Forage had a positive interaction with yeast, showing increased pH estimates for yeast compared to the control, although differences were small. Yeast doses 1x106 and 1x107 cfu/ml showed increased digestibility as well as VFA concentrations demonstrating the yeast’s ability to facilitate fibre digestion in vitro and by supplying the microorganisms with a better micro-environment, as also seen with the MD & GV media. The two media provided a better environment for fermentation than the KS medium, resulting in higher NDFd, rate of digestion, DMd, OMd, pH estimates as well as VFA concentrations. The pH estimates did not always result in significant differences when different stressors were placed on the in vitro environment which may be as a result of the excellent buffering capacity of the medium. The differences that were seen were very small (< 0.1) which may not be biologically important as pH remained within normal ranges for fermentation to occur. The decreases in pH were often accompanied by increases in VFA concentrations which were expected. This study confirms the negative effect oxygen can have on the anaerobic rumen environment, while also showing how high levels of starch can cause significant decreases in rumen pH values. It also confirms the positive effects yeast has on pH by stabilizing the rumen environment; however, this may also be due to the medium used. More research is needed to study the mode of action of yeast supplements and to determine the optimal oxygen and starch concentrations that yeast can withstand. It is also important that care be taken when evaluating different fermentation parameters when yeast supplements are used, especially when comparing different variables such as media and forages.
AFRIKAANSE OPSOMMING: Geen opsomming