Can Yeast Replace In-Feed Antibiotics?

Background

Severe liver abscesses can negatively affect animal health, performance and carcass value. Rumen acidosis negatively impacts the gut, allows gut microbes to enter the bloodstream, and predisposes cattle to liver abscesses. In-feed antibiotics can reduce liver abscesses, but non-antimicrobial alternatives and a better understanding of how feedlot cattle respond after a bout of acidosis are needed.

Rumen acidosis and liver abscesses are more common on high energy diets. This makes it a challenge to improve gut health, reduce liver abscesses and maintain feedlot performance at the same time. Some studies have shown that direct fed microbials (e.g., yeasts) can benefit rumen health. Although the greatest incidence and severity of ruminal acidosis typically occurs late in the finishing period, little research has studied whether yeast can improve gut health at this point. Variability in yeast form, dose, and diet factors may help explain why different studies have shown conflicting results regarding the value of yeast in feedlot diets. A publication from this group saw no difference in ADG but a 30% reduction and 25% less variation in day-to-day feed intake, 50% better F:G, , and potentially better rumen pH compared to controls. However, the control treatment in this project was a “natural” production system (no ionophores or implants) rather than a conventional system using in-feed ionophores to antimicrobials to maintain rumen health and control liver abscesses.

The overall goal of this study was to understand the modes of action of yeast fed to finishing beef cattle and how they influence growth performance and gut health, and further characterize animal response and recovery to a ruminal acidosis challenge.

Objectives

• Determine the impact of different forms of yeast (live active, heat killed, rumen protected) on steer performance and efficiency, rumen pH, total tract digestibility, gut health and economics.
• Determine the impact of supplementing cattle fed a high grain diet with dry active yeast on in vivo gut barrier function, fecal starch, rumen pH and fermentation following an acidosis challenge and recovery.

What they Did

A metabolic trial in Saskatoon fed 15 cannulated Charolais-cross cattle (seven or eight head per treatment) acorn-based finishing diets containing either 33ppm monensin (control) or 33ppm monensin plus 3g /d (60 billion yeast cells). After a month of adaptation to the diet and five days of baseline measurements, intake was restricted by 50% for a day, then cattle were overfed grain for five days to induce acidosis. They then measured rumen (pH, VFA, lactate), fecal (starch, VFA) and blood (immune markers) parameters during the challenge and recovery periods.

A feedlot trial in Guelph fed 120 steers one of four diets (30 head per treatment) for 100 days before slaughter. The base diet was comprised of high moisture corn (88-90%) and haylage (10-12%) and included the following treatments: (1) control (including monensin), (2) positive-control (tylosin+monensin), (3) monensin + dry active yeast, and (4) monensin + heat killed yeast. They measured ADG, F:G, feed intake, feeding behavior, and variability. Rumen pH was continuously monitored on 10 head per treatment along with rumen and fecal VFA concentration. Intestinal health, liver abscess score and grading data were collected on all animals at slaughter.

What They Learned

In the Saskatoon trial, the yeast product did not impact the severity of rumen pH, measures of gut barrier function or the recovery process of an induced ruminal acidosis challenge at the doses used. No changes were seen in daily dry matter intake, rumen or fecal pH measures, or VFA concentrations within the rumen or feces before, during or while recovering from the acidosis challenge. Despite limited impacts of yeast on parameters measured, some new insights into recovery following an acidosis bout were observed. Hindgut markers of barrier function were elevated during the challenge and the first two subsequent recovery periods relative to baseline. This may indicate that post-ruminal barrier function may be compromised for up to 15 days after an acidosis challenge. Feed intake was reduced during the challenge period but was similar across baseline and recovery periods. Steers also sorted feed to select for larger particles between recovery period 3 and the challenge period.

In the Ontario trial there were few animal performance, feeding behaviour, diet digestibility, or carcass impacts from adding tylosin, live yeast, or heat-killed yeast compared to feeding monensin alone. Adding live yeast to the diet improved ADG compared to the tylosin diet but not the killed yeast or monensin-only (control) diets. Meanwhile, adding heat-killed yeast to a basal diet containing monensin significantly reduced ruminal pH and increased time spent in an acidotic state. This suggests that live and heat-killed yeast may act differently in the rumen. Compared to monensin alone, heat-killed yeast may increase the risk of acidosis in cattle and may not be ideal for supplementation. Better characterizing the yeast products used in published literature would help to understand why animal responses to yeast supplementation are so variable between experiments.

What it means

This study suggests that neither live or heat-killed yeast had a benefit on gut health, animal perfromance or grading outcome in corn-based finishing diets that contain monensin and/or tylosin. However, supplementing live yeast may potentially be better for rumen health compared to offering heat-killed yeast.