On many intensive beef finishing units, the inclusion of a yeast is now commonplace to help maintain optimum rumen pH, improve feed conversion efficiency and reduce the incidence of sub-acute ruminal acidosis (SARA). However, recent research has highlighted just how important it is that the yeast used is in a live, metabolically active form.
“Rumen fermentation is compromised any time rumen pH drops below 5.8, with optimum performance around pH 6.0,” explains our EMEA Ruminant Technical Director Dr Derek McIlmoyle. “How low that pH drops, how long it spends below the pH 5.8 threshold and how quickly it then recovers all have a direct impact on the rumen fermentation and, ultimately, feed conversion efficiency.
“For intensively reared beef cattle, the rumen will typically spend long periods below pH 5.8 unless yeasts or rumen buffers are used to maintain a balance in the rumen. And it’s a particular problem for those fed large amounts of rolled cereal, high starch silages – such as maize and whole crop cereal silages – or starchy concentrates.”
If the rumen drops below pH 5.5 – the threshold for SARA – fermentation efficiency is even further disrupted as the normal fibre-digesting microbes begin to die off. This delays performance recovery when rumen pH rises, whilst also releasing lipo-polysaccharides (LPS) which can cause inflammation of the rumen lining.
“Reducing the extent and time of any drop in rumen pH is therefore critical if feed conversion efficiency, feed intake and growth performance are to be maximised,” adds Dr McIlmoyle.
A recent independent trial carried out by Agriculture and Agri-Food Canada (a department of the Canadian government) compared rumen pH fluctuation in beef heifers receiving either 4g/head/day of a metabolically active live yeast (Vistacell), the same dose of yeast deactivated (killed) by autoclaving at high temperature, or no yeast at all. All heifers were fed a non-acidotic mixed ration containing wholecrop cereal silage and a cereal-based concentrate to give an overall 50:50 forage-to-concentrate ratio.
Rumen pH was then monitored for seven days before and seven days after an acidosis ‘challenge’, which took the form of a large meal of barley grains following a day during which only 50% of the normal ration was fed. The results are shown in Table 1, and it was clear that whilst both forms of the yeast significantly increased average rumen pH, raised minimum rumen pH and reduced time spent below pH 5.8 on the standard diet, it was the active live form that was most effective following the acidosis challenge.
Table 1 – Effect of yeast addition and viable state on rumen pH in beef heifers (Source: Agriculture and Agri-Food Canada, 2013)
“Although all treatments were affected by the acidosis challenge, dry matter intake (DMI) was maintained only by the active live form of the yeast, with DMI for the inactivated yeast and control groups falling by 5% and 15% respectively during the 24 hour period following the challenge,” Dr McIlmoyle explains.
“The cattle receiving the active live yeast form also appeared to recover from the challenge more quickly, tending to have a higher post-acidosis average ruminal pH and less time spent below pH 5.8. In addition, both the cattle receiving the inactivated dead yeast and those on the control diet required a supplementary dose of 250g sodium bicarbonate in order to return the rumen to a balanced state.”
For commercial beef finishers, the implications are substantial. According to the research, either type of yeast would provide a feed efficiency advantage during periods when the risk of SARA is relatively low, yet the yeast needs to be metabolically active (live) to be fully effective in aiding recovery from any significant acidosis challenge.
“Yeasts have a hugely important role to play in intensive beef rations, and are relied upon to help stabilise rumen pH by beef producers throughout the world,” Dr McIlmoyle continues.
“Yet this research shows just how much impact yeast viability can have on efficacy under truly acidotic conditions. For beef cattle regularly challenged by large meals of highly fermentable feeds, the difference in feed efficiency and growth performance is likely to be substantial.”
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