The next two weeks are the best time to take the last alfalfa cutting of the year while maintaining stand productivity. We recommend the last harvest to be taken by September 7 in northern Ohio and September 15 in southern Ohio. This will allow a fall rest period for alfalfa, which is probably more important than usual this year due to the stressful growing conditions we’ve had with drought conditions and heavy rainfall later in the season.
Cutting schedules of alfalfa have been greatly disrupted in some areas of Ohio this year. May/June drought conditions decreased yields followed by heavy rains that made harvest difficult. Some of these fields have only had 2 cuttings harvested and are not tall enough to harvest now. This is a tough situation because cutting later (September 15 to October 30) will add significant additional stress to fields that are already in poor condition. Alfalfa needs about 500 growing degree days from its last cutting before a killing frost. For most falls this is about 3 or 4 weeks, but the exact date of a killing frost can be hard to predict.
The fall period is when alfalfa and other tall legumes like red clover undergo many physiological responses to the cooling temperatures that prepare the plants to survive the winter. Carbohydrate and protein reserves are accumulated in the crowns and roots during the fall. Cold-hardening processes also occur that increase plants' resistance to cold temperatures. Interrupting those processes by cutting could result in the plants having inadequate cold hardiness along with lower energy and protein reserves for good survival through the winter and for initiating vigorous regrowth next spring.
Fall cutting is a stress to the plant, and its effects will be more severe in fields that are currently not in a vigorous condition. Several factors affect the level of risk incurred with cutting during the critical fall period. These include overall stand health, variety of disease resistance, insect pest stress during the summer, age of stand, cutting management, fertility, and soil drainage.
A vigorous, healthy stand is more tolerant of fall cutting than a stressed and weakened stand. The most significant factor this year affecting alfalfa was drought followed by excessive soil moisture. Alfalfa fields that were stressed by wet soil conditions, along with leafhopper feeding, are in a compromised condition. The fall rest period will be very important to their recovery and future productivity.
Alfalfa varieties with high disease resistance and good levels of winter hardiness will be more tolerant to the negative effects of a fall cutting because there is less total stress on the plant. Adequate fertility, especially soil potassium levels, will improve plant health and may increase tolerance to fall-cutting effects. A high soil pH of 6.8 to 7.0 will also reduce the risk of fall cutting. Stands under 3 years of age are more tolerant of fall cuttings as compared with older stands where root and crown diseases are setting in.
Alfalfa that has been cut three or more times before a fall harvest has a higher risk factor for injury from fall harvesting than does a stand cut only twice so far this year. In other words, the cutting frequency during the growing season can affect the energy status of the plant going into the fall. Frequent cutting (30-day intervals or less) results in the plant never reaching full energy reserve status during the growing season. This makes the critical fall rest period more necessary for plants to accumulate adequate reserves before winter.
A final factor is soil drainage. Alfalfa stands on well-drained soils tolerates later fall cuttings better than alfalfa on moderately or poorly drained soils. Low plant cover going into the winter from late cutting increases the risk of winter heaving on many Ohio soils. We often observed significant heaving in Northern Ohio on fields that are cut in the fall with limited regrowth for ground cover.
Cutting alfalfa during the critical fall period is tempting due to the need for high-quality forage and the disrupted cutting schedules we experienced this year. But before deciding to cut alfalfa after September 15, carefully consider the condition of the stand and the risk factors discussed above. If the stand suffered excessive soil wetness this year and is lacking vigor, consider the risk from fall cutting to be greater this year than usual. Do you need the forage this fall more than the need to maintain the vigor of the stand for next year? Can you risk losing productivity of the stand come next spring?
Another consideration is harvesting after the winterization period; technically, it should be safe to take a cutting at this time. This correlates to cutting after a killing freeze (23-24°F for several hours) after the plant is dormant. This is not as stressful to the plants as cutting during winterization and can be a viable option for those who need feed and do not want to risk next year’s stand. However, remember that you should leave 5-6” of stubble, which leaves some plant tissue and helps to reduce erosion and provide some insulation during freezing and thawing cycles.
If you choose to accept the risk of a fall cutting, then leave some uncut strips in different areas of the field so you can compare the regrowth next spring in cut and uncut areas. That will provide a comparison that will inform your future fall-cutting decisions.