Single drench of virginiamycin shows cattle need more than sheep against acidosis

Why grain poisoning was the target#

Acidosis, often called grain poisoning, happens when ruminants eat more starch than their rumen microbes can safely process. Lactic acid accumulates, the rumen environment turns dangerously acidic, and in severe cases animals die. The standard control has been to mix an antibiotic called virginiamycin through the daily ration so it is consumed steadily, but the authors note this is not always achievable in the paddock. Sheep grazing a stubble with spilled grain, or cattle nosing through a crop paddock, cannot easily be fed a measured, blended ration, and on many properties there is simply no mixing equipment on hand.

A drench, delivered as a single dose straight into the mouth, avoids that difficulty. Godfrey and colleagues had already demonstrated that virginiamycin works when blended into feed, and other cited trials had tested it in cattle rations to allow rapid introduction to grain. What remained unresolved was whether a one-off oral dose could protect sheep on its own, and whether the same approach and dose rate would carry across to cattle. Thorniley and colleagues had earlier reported that drenching wethers at a particular rate per kilogram of liveweight stopped deaths in weaners fed unrestricted wheat, and that half that amount still cut losses meaningfully. This study extends that earlier work, asking how the drench behaves across several dose levels and, importantly, whether cattle respond the same way as sheep.

The question mattered commercially because lotfeeding and opportunistic grain feeding of grazing cattle were both becoming more common, and a reliable one-off treatment would remove the need for specialised mixing gear in situations where sheep or cattle might otherwise be left unprotected, particularly on properties without the infrastructure to medicate a whole ration consistently.

Testing the single drench of virginiamycin in two species#

The study ran two parallel experiments at Agriculture Western Australia. In the sheep experiment, twenty Merino wethers weighing roughly 45 kilograms were treated against internal parasites, settled into individual mesh-floored pens, and fed wheaten chaff fortified with urea and a mineral-vitamin premix. In the cattle experiment, twenty young bulls crossed between Angus and Friesian breeds, weighing around 338 kilograms, were likewise treated for parasites and housed in individual feedlot pens on a basal diet of pasture hay, later adjusted to a set proportion of their liveweight once treatment began.

Animals in both experiments received one oral dose of virginiamycin, mixed as a suspension and given by syringe. Four dose levels were compared in each species, ranging from no drug up through three increasing amounts, allowing a direct look at how response changed with dose across a fourfold spread. Rumen fluid was drawn by stomach tube before dosing and again six hours afterwards, then repeatedly over the following ten days. Each sample was strained, its acidity measured, then incubated for twenty four hours with added glucose to gauge how much lactic acid the rumen population could still generate. Two forms of lactic acid, labelled L and D, were measured separately with an automated analyser and enzyme test kits, since only the L form was expected to track most directly with an animal's risk of developing acidosis.

Housing animals individually and stratifying them by liveweight before allocation to treatments was intended to even out starting differences between groups, so that any change in lactate production after dosing could be attributed more confidently to the virginiamycin treatment itself rather than to pre-existing variation between animals.

What the rumen samples showed#

In sheep, every dose of virginiamycin cut L-lactate production to a similarly low level within six hours of dosing. The lowest dose wore off soonest, with lactate production back to untreated levels within three days. The two higher doses kept L-lactate suppressed for five days, meaning the largest dose tested bought no extra days of protection over the middle dose. D-lactate moved in the opposite direction, rising after dosing at every level, for roughly the same span that L-lactate was held down, although greater variability made that pattern harder to confirm statistically for as long a period.

Cattle produced a less consistent picture. L-lactate again fell to a common reduced level within six hours regardless of dose, but the effect faded earlier. At the two lower doses, cattle lactate production had returned to control levels within two days; at the highest dose, suppression lasted three days. D-lactate in cattle showed no clear overall treatment effect, though there was a tendency toward lower D-lactate at the top dose. Rumen acidity in cattle was notably variable and often ran high, with some samples showing contamination from saliva that the authors suspected added noise to the cattle results. Feed intake, liveweight change and rumen acidity did not differ significantly between treated and untreated sheep, and cattle generally ate all the feed offered with only occasional refusals.

Weighing up how firm the result is#

The central comparison is straightforward: in sheep, the middle dose held down L-lactate for four to five days, matching the rate already known from earlier work to prevent deaths in unrestricted wheat feeding. In cattle, even the top dose tested managed only three days of suppression. If the number of days of reduced lactate production genuinely determines how well an animal is protected from grain poisoning, the authors reason that cattle would need an amount beyond what was tested here.

That conclusion rests on a chain of assumptions worth setting out plainly. The link between laboratory lactate suppression and actual survival during a grain challenge was established separately, in earlier sheep feeding trials, rather than tested again directly in cattle facing a genuine acidosis episode in this study. We think this is the main limit on how far the findings can be pushed: the cattle results describe rumen chemistry under controlled hay feeding, not whether a larger drench would truly stop cattle dying when they break into grain. The authors are open about this gap, treating the lactate measurement as a stand-in supported by earlier unreported work from Murray and colleagues, who had shown in sheep on a roughage diet that virginiamycin suppressed lactate production for a set period at a comparable dose rate.

Why cattle and sheep might differ#

The paper offers several possible explanations for why cattle needed more of the drug for a shorter effect, without claiming to have confirmed any one of them. Drawing on Parra, the authors point out that cattle typically carry a larger rumen relative to their size than sheep do, which could dilute a dose calculated per kilogram of bodyweight more quickly. Drawing on Van Soest, they add that fluid passes through the cattle rumen faster than through the sheep rumen, which would shorten the time any given dose stays in contact with the microbial population responsible for producing lactate.

Other possibilities raised include differences between the species in the kinds or numbers of lactate-producing bacteria present at the moment of dosing, and the chance that part of a liquid drench passes straight through the rumen into the omasum and abomasum without lingering where it needs to act. None of these mechanisms was checked directly in this trial, so they stand as plausible explanations rather than proven causes. The authors also note that an earlier trial had recorded reduced feed intake following a virginiamycin drench, an effect that did not appear in either experiment here, possibly because the restricted feeding pattern used in both trials masked any change in voluntary intake by the animals involved.

What this means for grain feeding management#

For sheep producers, the practical signal is reassuring. A single oral dose of virginiamycin at the rate already shown to prevent mortality in earlier wheat-feeding work also suppresses rumen lactate-producing capacity for several days in this study, giving a workable option when sheep are about to graze a stubble or crop carrying spilled grain and a mixed ration cannot be arranged.

For cattle producers, the message is more cautious. The same per-kilogram amount that worked well in sheep did not deliver comparable protection in young bulls on a hay-based diet, and even doubling it fell short of the benchmark set by the sheep result. The authors stop short of recommending a specific cattle dose rate, instead flagging that further testing would be needed to find an amount that actually protects cattle exposed to grain, rather than assuming the sheep figure can simply be carried across. Anyone managing lotfed cattle or stock newly introduced to grain should treat a cattle drench rate as an open question rather than a settled recommendation until that work is done.

This trial was published among the ASAP conference proceedings held in this index, within the wider body of work gathered under the Australian Society of Animal Production community, which brings together many Australian feeding and nutrition trials from this period including studies on starch digestion in horses and on glycogen metabolism in sheep and cattle.

Sources and further reading#

• Australian Association of Animal Sciences: successor to ASAP
• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body
• Feedipedia animal feed database: an open-access database of animal feeds

Questions#

— The Livestock Library team