By Craig Sheaffer and Jared Goplen
Fall cutting can disrupt the fall dormancy reaction of adapted alfalfa varieties. Alfalfa is a perennial plant that depends on fall dormancy to prepare for winter survival. Shortening daylength and declining temperatures in the fall triggers alfalfa to undergo a dormancy reaction that decreases herbage production and develops crown buds—the source of the first spring regrowth (Figure 1). Alfalfa metabolism also changes to increase starch storage, increase sugar content, alter protein metabolism, and decrease free water in the plant cells all in preparation for winter (Figure 2).
Disruption of the dormancy reaction by cutting or grazing in September to early October (depending on region), affects alfalfa winterhardiness and its survival. Plants cut during this sensitive time stop storing energy and instead expend energy on regrowth. Crown buds that would normally overwinter and regrow in the spring begin regrowth. Unfortunately, there isn’t enough time for the herbage to regrow to allow resumption of normal dormancy reactions.
When is a safe harvest date in the fall?
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. We are past this time now and in the critical fall period (7 September – 15 October) when it is risky to cut or graze alfalfa.
When final harvests are delayed until around mid-October we can minimize risk. This coincides with the date of the first alfalfa killing frosts (<25 F) in many regions. Beyond this date herbage regrowth will be minimal. Even after these first frosts we have observed that leaves near the soil surface often survive and can stay green and conduct photosynthesis into late November.
Advantages of stubble going into winter
Harvesting in mid-October does remove stubble from the field. Having more than 6 inches of stubble in alfalfa fields provides advantages to 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 advantages in surviving the winter. 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.
How does the summer drought affect fall harvest in 2021? The drought of 2021 has resulted in considerable variation in alfalfa growth and development within the state. Often harvests in July and August were minimal or did not occur because alfalfa plants went into drought-induced dormancy. Alfalfa is a hardy plant and now has produced lush herbage regrowth following the late August and September rainfall that was received in much of Minnesota. During September the plant is recharging its root storage carbohydrates and should have adequate reserves to survive the winter as long as this process is not disrupted by untimely harvests. A harvest around October 15th or later should be late enough to avoid stimulating regrowth and not affect alfalfa winter survival.
Figure 1. A 1 year old alfalfa crown showing numerous crown buds formed during the dormancy reaction in late September and October. These crown buds located below the soil surface are very winter hardy and are the source of spring regrowth. Untimely defoliation in September will cause the plant to use energy reserves to regrow from these crown buds and expose the plant to greater risk of winter injury.
Figure 2. Changes in alfalfa root carbohydrates from September through March. During the dormancy reaction following an early September harvest, alfalfa stores energy into the roots in the form of starch and sugars. Starch is converted to sugars which allows the plant to live during the winter. Cutting anytime in September will disrupt storage and lead to insufficient energy supplies for the plant to make it through the winter. Delaying cutting until mid-October will allow for high levels of carbohydrate reserves while minimizing the likelihood of winter injury.
Source : umn.edu