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.