Grazing rest period

Why does moving a flock off before it has finished the grass leave more feed at the end of the season than letting it graze down?

A field of grass is a stock of feed, and the obvious way to get the most out of a stock is to take all of it. Leaving a third of the sward standing looks like waste.

Yet a grazier who pulls the flock off early, and returns only after the sward has recovered, can finish the season having harvested more grass than the neighbour who grazed each paddock bare. Both fields grew under the same sky. So what does the standing residue do, that leaving it should be worth more than eating it?

The mistake is in the first sentence: a pasture is not a stock, it is a factory. What is standing there is not only feed, it is the apparatus that makes next month's feed. Ask what that apparatus is, and the answer is leaf area — the surface intercepting sunlight. Take the leaf and you have taken the means of replacing it.

So what happens after a hard graze? The plant has almost no leaf and must rebuild it from somewhere. It draws on reserves: water-soluble carbohydrates stored in stem bases, stolons or roots, plus nitrogen remobilised from remaining tissue. That is expensive and slow, and the regrowth curve begins with a lag — days of very little visible growth while the plant spends capital.

Leave a residue instead, and the sward restarts with leaf already intercepting light. There is no capital phase to fund; photosynthesis resumes immediately. The same days that were a flat lag in the hard-grazed paddock are productive days in the other one.

Now follow the curve further, because this is where the timing question lives. As leaf area builds, the sward intercepts a rising share of the light falling on it, and growth rate climbs. But interception cannot exceed all of it: once the canopy closes — conventionally when about 95 percent of light is caught — more leaf adds no more capture, and the new leaf shades the old, which stops paying its way, senesces and dies. Growth rate falls again, and the sward loses tissue at the bottom while it grows at the top.

So the daily growth rate over a rest period is humped: slow at first, fastest in the middle, declining at the end. The manager's real choice is which part of that hump the animals occupy. Return too soon and you graze plants still in their lag, repeatedly resetting them to zero leaf and forcing them onto reserves that never get repaid — the classic way to thin a sward until weeds take the gaps. Return too late and you are harvesting grass that has begun to die, eating stem rather than leaf.

Perennial ryegrass gives the rule of thumb: a tiller carries about three live leaves at once, and when the fourth appears the first dies, so grazing near the three-leaf stage takes leaf that would otherwise have been lost.

Two refinements connect the steps. First, the residue does two jobs at once: it is standing leaf area, which shortens the lag, and it stands proxy for the reserves left in the plant, which fund the lag that remains. A sward grazed hard once in spring recovers far better than one grazed hard repeatedly, because the second has no capital left.

Second, "rest" is not a number of days. The plant does not count days; it counts leaves. The same paddock needs perhaps three weeks of rest in a warm moist spring and two months in late autumn, because the interval that matters is however long it takes to climb the hump. Managing by the calendar is what turns rotational grazing into a worse system than simply leaving the gate open.

And here the literature deserves honesty rather than enthusiasm. Rotational grazing is often sold as raising production per hectare in its own right, and the grazing-trial evidence does not support that as a general claim: a well-known 2008 review of the experimental record by Briske and colleagues found rotational systems did not consistently outproduce continuous grazing at equal stocking rate, and the argument over that finding is still live. The physiology above is not in doubt; what is in doubt is whether imposing a rotation captures it, because a continuously grazed sward at a modest stocking rate also leaves residue, and a rotation run on the calendar may not. What is doing the work is the residual leaf area and the recovery interval, not the fences.

The mechanism gives its own test. If regrowth speed is set by remaining leaf area rather than by elapsed rest, then two plots cut to different residual heights and rested for the same number of days should differ in growth from the first day, and the deeper-cut plot should show a measurable drop in stubble carbohydrate. If instead regrowth rate depended only on days of rest, and stubble reserves were unchanged by cutting height, the account here would be wrong — and the case for leaving residue would collapse to a matter of species composition, not of production.

One complement worth naming: a sibling walk-through of grazing and grassland richness asks a related question in a different currency — how many kinds of plant persist, by relief from light competition. Here the currency is kilograms of feed grown and the mechanism is the regrowth curve's shape. The regimes that maximise the two need not coincide, and generally do not.

• Why a sward stocked continuously at a light rate can achieve the same residual without any rotation at all.

Every term the collection defines is gathered in the glossary.