The drought this year has left most livestock producers with very short forage supplies, so many are cutting hay fields this autumn regardless of the calendar or weather forecast. Hay harvesting across Ohio the past few weeks has led to questions about management guidelines and the impact of late cutting or grazing on forage grass and legume stands. The biggest management concern is with legume stands.
When significant regrowth occurs after a fall cutting of tall legumes (alfalfa, red clover, birdsfoot trefoil), energy reserves in the roots and crowns will be depleted and there may not be sufficient time for their replenishment before a killing frost. This is often the case with cuttings made the first 10 to 12 days in October. Fall cutting dates that allow only a short regrowth period will leave the plant in a lower energy status going into the winter. Energy reserves are important for winter survival and regrowth early next spring. This is why we recommend the last cutting be taken early enough (early September) to allow at least 6 weeks of fall regrowth so energy reserves can be built up to a high level going into the winter. An alternative is to delay fall cutting until a time when regrowth will no longer occur, although this is not recommended on heavier soils. The best way to ensure no fall regrowth is to cut after a killing frost.
A killing freeze for alfalfa or red clover is generally defined as temperatures of 24 - 25F over a period of at least 4 hours. A lesser freeze event may burn the top of the plant, but growth will still continue from the green, unburned leaf area below. However, the short day length and lower temperatures going forward from this point in time should prevent substantial regrowth from occurring. So we would expect there to be little regrowth when cutting or grazing during the last few days of October into early November, even if a killing freeze has not yet occurred.
So, does this mean that all the alfalfa we have seen around the state that has been cut before a killing frost is going to die over the winter? While some stands will indeed be hurt by fall cutting, other stands will not, or they will be impacted in a minimal way. That is because there are other factors that play a role in determining winter kill. We have already mentioned the role of cutting date in the fall as a factor in winter injury risk (and degree of regrowth that occurs), but there is less winter kill risk when a fall cutting is taken on a young vs. an old alfalfa stand, and less risk when the stand is planted on a well-drained site. In addition, stands where good soil fertility has been maintained, especially with good levels of soil potassium and a soil pH close to 6.8 will have reduced risk of winter kill. Growers who use improved varieties with good disease resistance and winter hardiness will have reduced risk of winter injury. Finally, cutting frequency during the season can play a role in the effect of a fall cutting. The more frequent the cutting, the more chance there will be for winter injury from cutting during the fall; so a fifth cutting taken in the fall will have much higher risk of winter injury than a third cutting taken in the fall.
There is one important exception to the safety of making a very late fall harvest when regrowth would not be expected. That exception is for heavy soils prone to shrink/swell cycles that can result in heaving of taprooted legumes like alfalfa. Late fall cutting results in lack of residue cover that leaves the soil more exposed to the prevailing air temperatures. So the soil will be more susceptible to freeze/thaw cycles and will substantially increase the risk of heaving damage. A study on a Wooster silt loam soil showed that cutting alfalfa in early November resulted in heaving of 50% or more of the stand compared with less than 10% heaving where no fall cutting was made. A late cutting of alfalfa or other taprooted forages is only recommended on well-drained soils that are not prone to heaving events.
Sometimes the question is asked if too much top growth can lead to smothering out of the stand over the winter. For alfalfa this is not an issue because the leaves will dry up following a killing freeze, become brittle and drop off the plant. The stem that remains standing is not a concern for smothering the stand. Tall grass plants, however, can mat down, providing a favorable habitat for disease development that could thin out the stand. For this reason, it is recommended that a grass hay field with tall growth be cut or grazed before winter. This is especially a concern for perennial and annual ryegrass; those stands should be cut to 2 - 3" going into the winter.
With our shorter days and cooler temperatures it becomes very difficult to get a cut legume or grass to dry down enough to bale as dry forage. Wrapping wilted forage or harvesting as haylage is the best mechanical option.
Grazing a hayfield is usually a more economical option as compared to mechanical harvest at this time of year. Use of temporary electric fencing can facilitate the grazing of a hayfield. While forages such as alfalfa, clovers and cool-season perennial grasses do not produce toxic compounds after a frost, bloat can be a concern when alfalfa or clovers are grazed after a frost.
The risk of bloat is higher one to two days after a killing frost. The safest management practice is to wait a few days after a killing frost before grazing pure legume stands. At that point the forage will begin to dry down from the frost damage. If animals are not accustomed to grazing high legume content stands or when grazing the stand before a killing frost, it is a good idea to fill them with dry hay before turning into the legume field. Move animals into the legume field in the late morning or early afternoon after they have been grazing another pasture so that they are not entering with an empty rumen. Maintain access to dry hay or corn stalks while grazing alfalfa, or swath the alfalfa ahead of grazing and let the animals graze the dried forage in the swath. Bloat protectant compounds like poloxalene can be used effectively if fed to animals in a way that ensures consumption of the compound in sufficient and uniform quantities each day by each animal. Finally, when grazing alfalfa stands, restrict grazing when soils are firm to avoid treading damage to the plant crowns.
Source: Rory Lewandowski, Extension Educator, Wayne County and Mark Sulc, Extension Forage Specialist, The Ohio State University