By: Dr. Melissa Williams, University of Guelph
Introduction
In the feedlot, cattle are fed high-grain diets to help them reach market weight quickly; however, the large amount of grain fed in the diet does have its downsides. One downside is the increased risk of ruminal acidosis caused by a reduced ruminal pH, which in turn can damage the gut wall. With increased damage to the gut wall bacteria make their way to the liver and colonize to form an abscess, reducing carcass value at slaughter. Since damage to the gut wall increases the wall permeability, harmful molecules can enter the bloodstream. This invasion can activate the animal’s inflammatory systems, which can draw energy away from growth. Therefore, reducing gut wall damage by a high-grain diet is an opportunity to improve overall animal health and productivity in the feedlot.
Yeast, similar to that which is used to make bread at home, has been heavily researched for many decades as a feed additive in the dairy industry and less so in the beef industry. In dairy cattle, yeast supplementation has improved the gut environment and animal health. However, with so many variations of yeast supplement product types and strengths and differences in animal and diet types used in experiments, animal responses can be inconsistent in the abundance of studies. Very little research has examined yeast supplementation in beef cattle fed high-grain diets with monensin, none of which assesses the impacts of yeast on the permeability of the gut wall.
What did we do?
This experiment was conducted at the University of Saskatchewan in the Livestock Research Barn from February to May 2022. This experiment aimed to examine the effects of supplementing live yeast to finishing cattle on a high-grain ration on feed intake and behaviour, gut pH trends, gut wall permeability, and immune response in the event of an induced reduction of gut pH (acidosis). This was done by adapting cattle to a diet containing 83% (on a dry matter basis) steam-flaked corn and then inducing a ruminal acidosis challenge to reduce gut pH. The challenge was conducted by introducing a large amount of barley pellets into the gut through a rumen cannula (Figure 2) that would readily be digested and cause the pH to drop.
Before, immediately after, and up to 15 days after the challenge, measurements were taken to assess feed intake and behaviour, rumen pH and fermentation through measurement of volatile fatty acids (VFAs), gut wall permeability, and immune response. Feed intake and sorting behaviour were measured using samples of daily feed refusals, and immune response was measured as concentrations of inflammatory markers in the blood. Gut wall permeability was evaluated by adding markers in 2 locations of the gastrointestinal tract (rumen, omasum) via the rumen cannula, allowing it to pass through the gut wall and into the bloodstream to be measured. An increase in the marker appearing in the blood indicates increased gut wall permeability and, therefore, likely increased gut wall damage. Infusing the second marker in the omasum allowed us to assess differences in permeability in the entire gut and just the hind gut.
What did we find?
It was found that yeast supplementation did not impact any of the outcomes that were measured. Therefore, the live yeast supplementation did not affect feed intake or particle sorting behaviour, gut pH or volatile fatty acid concentrations, gut wall permeability or immune status. There were no differences in measurements after the acidosis challenge suggesting that relative to diet containing monensin, yeast supplementation did not improve recovery from acidosis in this study.
What we did find was that regardless of treatment group, cattle that adapted to a high-grain diet experienced a temporary reduction in feed intake, gut pH, and immune response markers in the blood when subjected to acidosis. These cattle each recovered somewhere between 2- and 15-days post-challenge. The hind gut marker of permeability was highest during challenge and was lowest in recovery period three -15 days after acidosis challenge.
What does this mean?
The results of this experiment suggest that for cattle adapted to a high-grain diet, the supplementation of live yeast does not play a role in the recovery of gut wall permeability or gut pH after an acidosis challenge. It is thought that perhaps the expected impacts of yeast that guided the experiment’s hypothesis are circumvented by the adaptation to a high-grain diet containing monensin in the first place. The prevalence of monensin in all diets also may also be masking yeast effects. However, regardless of treatment, it took up to 15 days for animals to recover from the acidosis challenge. Further research into the similarities and differences between gut changes related to high-grain diet adaptation and yeast addition to low and high-grain control diets would be required to confirm this theory.
Dairy and Beef Cattle Production 