Muscle glycogen recovery in sheep and cattle after exercise and feeding

Why glycogen levels at slaughter matter#

Muscle glycogen is the fuel that drives the natural acidification of meat after an animal is killed. As glycogen breaks down anaerobically it produces lactic acid, which lowers the pH of the muscle from around 7.2 down to an ultimate figure near 5.5. When animals arrive at the works with depleted glycogen, because of poor nutrition or stress beforehand, that acidification stalls and the ultimate pH stays high, commonly above 5.8 to 5.9. The paper notes this produces dark, firm and dry meat, a quality fault that costs cattle and sheep industries significant amounts worldwide.

The practical idea behind the work is straightforward. If animals at risk of this problem could be identified before slaughter, they might be held back and fed in a way that lets muscle glycogen recover, avoiding the downgrade. But that depends on knowing how nutrition actually drives glycogen repletion, and at the time of writing the authors say there was little information on how different levels of feeding affected the speed of recovery. This paper sets out to fill that gap, comparing sheep and cattle directly.

The same research group has examined related ground before; an earlier summary on this site, glycogen metabolism and meat quality, covers some of the underlying physiology that this paper builds on.

How the experiments measuring glycogen recovery were built#

To study glycogen under controlled conditions, the authors used a biopsy technique taking samples from two muscles, the M. semimembranosis (SM, from the topside) and the M. semitendinosis (ST, the eye round). These were chosen because they are easy to sample and represent different fibre types: the SM is a fast red muscle with relatively high glycogen stores that resist depletion, while the ST is a fast white muscle with lower glycogen that depletes more readily under stress.

Glycogen depletion was induced by exercise. Cattle were made to trot through five separate quarter-hour sessions, and sheep through four such sessions, each pace held around 8 to 9 kilometres per hour with rest periods worked in between, at an intensity the authors equate to roughly 65 percent of maximum oxygen uptake. This reliably dropped glycogen by around half. Biopsies were taken before and immediately after exercise, and again at set points afterwards, either 36 or 72 hours later depending on the experiment, to measure how fast glycogen rebuilt.

Four separate experiments are reported. The first compared roughage and grain-based rations in 40 Angus steers and 90 Merino wethers. The second and third tested hyperglycaemic drenches and then a glycerol and propylene glycol water supplement for their effect on blood glucose. The fourth tested that same water supplement's effect on actual muscle glycogen repletion after exercise, in Hereford cross heifers and Merino wether lambs. All animals had access to their experimental diets for six weeks before and after the exercise challenge, with feed intake recorded daily.

Roughage versus grain: a clear split in cattle, less so in sheep#

In cattle, the hay-fed steers and the silage, barley and maize groups' ST muscle showed very little glycogen repletion 72 hours after exercise. The SM muscle, however, repleted markedly more on the higher energy grain rations than on hay (P<0.001). This produced a clear positive linear relationship between SM glycogen repletion and metabolisable energy (ME) intake per unit of metabolic body weight in cattle (R2 = 0.37, P<0.001).

Sheep told a different story. All three rations, hay, maize and barley, produced a marked lift in glycogen in both muscle types examined, with no meaningful difference showing up between the diets. Repletion in the ST was about half that of the SM, again underlining that white muscle responds more sluggishly to short-term nutritional change than red muscle. Because of this contrast, the authors focused their later energy comparisons on the SM.

The authors interpret the absence of a diet effect in sheep cautiously. It might mean that even the lowest-energy ration tested, the hay diet, already supplied enough ME to maximise repletion in sheep, so extra energy from grain was unnecessary for this particular muscle depot. Equally, they acknowledge that the range of ME intakes tested in sheep was narrower than in cattle, so the experiment may not have properly tested where sheep's response curve flattens out. Sheep nonetheless repleted glycogen faster than cattle at similar ME intakes per unit of metabolic body weight, a difference that recurs through the rest of the paper.

Finding a hyperglycaemic drench, then testing it in water#

Having established that ME intake matters in cattle, the authors looked at whether a hyperglycaemic supplement, something that raises blood glucose directly, could speed glycogen recovery without needing a full ration. This idea borrows from human sports science, where carbohydrate drinks taken soon after exercise boost glycogen recovery in athletes. Because ruminants ferment most dietary carbohydrate in the rumen rather than absorbing it as glucose, ordinary sports drinks were judged unlikely to work the same way in sheep and cattle. Glycerol, already used in the dairy industry to treat ketosis, was identified as a candidate hyperglycaemic agent.

The first test compared glycerol, propylene glycol, glucose, and a combined glycerol and propylene glycol mixture, each drenched directly into the rumen of sheep, against a water control. Blood glucose was tracked over nine hours using indwelling jugular catheters. The combined drench and the glucose drench produced the largest rises in blood glucose (P<0.001). Glucose was ruled out as a practical water additive because it would attract insects, leaving the glycerol-propylene glycol combination as the candidate for drinking water, with the added possibility that propylene glycol helps preserve trough water quality.

When this combination was then offered as a dilute supplement in drinking water to sheep on either low-energy roughage or higher-energy pelleted rations, water intake stayed much the same across treatments. Plasma glucose rose steadily over the first nine hours, with a bigger rise among the roughage-fed animals compared with those eating pellets (P<0.01), before falling back to control levels by 24 hours.

Does the water supplement actually rebuild muscle glycogen#

The more important test was whether this hyperglycaemic effect translated into actual glycogen repletion in exercised muscle. Forty Hereford cross heifers and 30 Merino wether lambs were put through the same exercise and biopsy protocol, this time with feed withheld during the recovery period so that only the water treatment, with or without the glycerol and propylene glycol mix, could drive repletion.

Control animals offered plain water showed no significant glycogen repletion at all without feed, cattle and sheep alike. In sheep, the water-borne supplement significantly lifted repletion in both muscles studied by 48 hours (P<0.05). Cattle showed a similar but smaller trend that did not reach significance. Even so, the gains were modest next to what a full ration achieves. Using an assumed gross energy value of 16.8 MJ/L for the supplement (treating gross energy as equal to ME, since no ME value exists for it), the ME intake it supplied to each lamb worked out at roughly 1.05 MJ per day, compared with 12.7 MJ ME per head per day from a barley ration fed to lambs after exercise in the earlier experiment.

Per unit of ME supplied, though, the supplement was far more efficient. Expressed as glycogen gain per unit of energy consumed relative to body size, the rate for the water supplement worked out at 0.068 g/100g per hour, against 0.011 g/100g per hour for the barley ration, a sixfold difference in efficiency of conversion. When these assumed ME values were used to replot the relationship, ME intake per unit of metabolic body weight again correlated strongly with the speed of glycogen rebuilding across both species, and the slope for sheep was steeper than for cattle, indicating sheep responded more sensitively at these lower ME levels.

A proposed model and where the evidence sits#

Pulling the four experiments together, the authors propose that glycogen repletion rises with ME intake along an exponential curve, rising quickly at low ME and then levelling off, for both sheep and cattle, with sheep reaching their plateau at a lower ME intake than cattle. They say the top of this curve, the maximum possible rate of repletion, is less certain for sheep than for cattle, since more data at higher intakes would be needed to confirm it.

One side finding worth noting is on fluid intake. While the water supplement did not change drinking volumes in the post-exercise trials, the authors report that in a lairage-type scenario before slaughter, fluid intakes at least doubled across all the relevant trials with cattle and sheep. This suggests the supplement would not discourage drinking, and might even encourage it, in animals held before processing.

It is worth being measured about how far these results travel. The experiments used specific breeds, body weights and a standardised exercise protocol rather than the stress conditions animals actually experience in transport or saleyards, and some of the sheep comparisons rest on a narrower range of ME intakes than the cattle data. We would treat the roughage-versus-grain findings as a reasonably solid guide for feeding decisions, but the efficiency figures for the water-based supplement as indicative rather than something to build a precise feeding program around.

What this means for feeding animals before slaughter#

The clearest practical message is that roughage rations appear sufficient to maximise glycogen recovery in sheep and cattle differently, with sheep needing less energy density to reach their ceiling while cattle need higher energy grain-based rations to reach their own maximum rate of repletion. This matters for anyone managing stock identified as being at risk of dark, firm and dry meat before slaughter, since it points toward different feeding strategies for the two species rather than a single blanket recommendation.

The glycerol and propylene glycol water supplement is presented as a genuine but limited tool. It raised blood glucose convincingly and, in sheep particularly, produced a measurable lift in glycogen repletion compared with plain water. But because it cannot supply anywhere near the ME of a proper ration, it is described as no substitute for feeding when time and facilities allow. Its niche, as the authors frame it, would be as a supplementary measure during the curfew or lairage period immediately before slaughter, when full feeding is not practical but a water-based additive could still help.

This work sits within the broader set of papers collected in the RAAN Conference Proceedings series, and connects to the same authors' later work on fat deposition described in the physiology of marbling in cattle, which examines a related aspect of meat quality driven by nutrition.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body
• Australian Wool Innovation: wool industry research and marketing body
• Feedipedia animal feed database: an open-access database of animal feeds

Questions#

— The Livestock Library team