Considerations for late-season alfalfa harvest

Deciding whether to take a fall alfalfa cutting requires a careful balance between immediate forage needs and long-term stand persistence. While low summer yields or terminal stands make a late-season harvest tempting, cutting during the critical fall rest period can severely deplete the root carbohydrate reserves required for winter survival and vigorous spring regrowth. Here are some considerations for fall alfalfa cutting that can affect winter survival and next spring’s success.

Fall dormancy in alfalfa

Alfalfa is a perennial plant that depends on fall dormancy to prepare for winter survival. Shortening day length and declining temperatures in the fall trigger alfalfa to undergo a dormancy reaction that decreases vegetative production and develops crown buds that serve as the source of the first spring regrowth. Alfalfa metabolism also changes in the fall as the plant stores carbohydrates in the roots.

Disrupting this dormancy reaction by cutting in September to mid-October (depending on region) affects alfalfa winterhardiness and survival. Plants cut during this sensitive time stop storing energy and instead expend energy on regrowth. Cutting causes fall regrowth of crown buds that normally overwinter and regrow in the spring. Unfortunately, this doesn’t allow enough time for the vegetation to regrow and normal dormancy reactions to resume.

Timing is everything

For most regions of Minnesota, a final summer harvest the first week of September will allow sufficient time for alfalfa to undergo the dormancy reaction before killing temperatures arrive. We are now in the middle of what some call the “critical fall period” (Sept. 7 to Oct. 15) when it is risky to cut alfalfa. Note that these dates differ regionally based on the killing frost date.

We can minimize risk by delaying final harvests until later in October. This coincides with the date of the first alfalfa-killing frosts (23 to 24 degrees) in many regions. Beyond this date, vegetative regrowth will be minimal.

Alfalfa growing degree day research is another method that can be used when considering a late-season cutting. Fall alfalfa harvest research is based on a window of 500 to 200 GDD (base 41 degrees). If fall-cut alfalfa accumulates at least 500 GDD before going into the winter, it will have good carbohydrate reserves.

This data matches well with the rule of thumb to harvest at least five to six weeks before the killing frost.

A late-fall cutting can be taken if the alfalfa does not accumulate more than 200 GDD before a killing frost. This will still allow the alfalfa to enter the winter with good carbohydrate reserves. The tricky part is to guess when a killing frost will occur and what dates will fall within 200 GDD prior to the killing frost date. Therefore, if the hay is needed, most will wait until at or after the killing freeze to cut.

Field growth has its advantages

Sometimes farmers are concerned that the fall growth will smother and kill the stand over winter. Experts say this does not happen with a legume such as alfalfa. Leaves freeze and eventually drop off the plant. The old fall stem growth may impact forage quality in the next year’s first cutting, but if it’s harvested early enough, the quality reduction is minimal.

Leaving the fall growth over winter is beneficial because it catches and holds snow cover and moderates soil temperature fluctuations during winter and early spring. Alfalfa may break dormancy too early or experience plant heaving due to extreme soil temperature fluctuations.

Harvesting past mid-October removes stubble from the field. Higher alfalfa cutting height (ideally 5 to 6 inches) is recommended to aid in winter survival, as it collects snow, which can insulate the soil. Even without snow, this stubble can insulate the soil from the sun’s energy that can warm the soil and expose the plant to excess cycles of thawing and freezing. Stubble can also disrupt ice sheeting and prevent smothering of plants, which all contribute to winter survival. Increasing the mower height to allow for more stubble is one solution. Some producers leave uncut strips within the field to help catch snow, which may help as well.

Proceed with caution

Taking an alfalfa cutting in late fall always comes with inherent risk, but that risk may increase if the field has already been stressed by intensive summer cutting, pest issues, low soil fertility, drought stress or poor soil drainage.

Bottom line: A vigorous, healthy stand is more tolerant of fall cutting than a stressed and weakened stand.

Cutting alfalfa during the critical fall period can be tempting due to the apparent high quality of the forage in the fall and sunny fall conditions. This is especially true if farms are short on feed or trying to reduce expensive non-forage byproducts. If you decide to cut this fall, work with your agronomist and nutritionist to determine the best time for your geography.

Barry Visser is a nutritionist for Vita Plus.