By Craig Sheaffer
Much of Minnesota’s alfalfa growing region has had periods of drought or above normal temperatures this growing season. Some level of drought continues in many regions, according to the Drought Monitor.
Fall moisture deficits do not harm alfalfa and can actually increase winter survival compared to well water conditions. That’s because if moisture stress occurs during regrowth, additional carbohydrate energy is sent to the crowns. Alfalfa can therefore survive prolonged dry periods by becoming dormant.
Irrigation of alfalfa in the fall is not advised as it can stimulate regrowth, interfere with the dormancy reaction which reduces free water in the plant cells, and stimulate use of stored energy reserves.
Before any harvest of alfalfa
- Consider winter survival - Consider the risks that fall cutting poses for alfalfa winter survival. There are many interacting variables affecting the outcome of fall cutting including stand age, variety traits, and soil fertility level. One of the more important factors is number of harvests/season . For most of Minnesota, a fourth or even a fifth cut in the fall will be riskier than a third cut. We describe these risk factors in Fall cutting of alfalfa in 2021.
- Consider forage needs and determine if the yield is enough to be worth the cost of harvesting. Although fall forage quality can be high, forage yields following three or four spring/summer harvests often are one ton or less per acre. Plus, fall harvests have been shown to reduce yields the following spring.
- Minimize risks by delaying harvests until average air temperatures reach about 24 F and the chance of regrow are minimal.
- Leave stubble - Plan on leaving at least a 6-inch stubble on fields to reduce risk of winterinjury. Alfalfa stubble in the fall is beneficial to insulate the overwintering crowns. Stubble can catch and hold snow which can insulate the soil surface.
- Consider drying times -Recognize that fall weather and morning dews can greatly increase drying times compared to the summer. Harvesting as haylage and baleage are recommended to reduce drying times.
The fall dormancy reaction
In response to reducing daylength and air temperatures, adapted alfalfa varieties undergo a dormancy reaction which reduces herbage production and stimulates the formation of crown buds that provide regrowth in the following spring. Physiologically, changes include increases in stored carbohydrates, certain amino acids, and lipids in the crown and root. Following an early September harvest, the amount of starch decreases while the amount of sugars increase because sugars provide for energy during the winter. Untimely harvests that remove herbage can stimulate regrowth that results in elongation of crown buds and utilization of carbohydrate reserves.
Wait till 24°F air temperatures
Alfalfa will continue to store energy until the onset of freezing temperatures in late fall. Alfalfa usually requires several days of to completely kill its terminal buds and leafy canopy. Temperatures above 24°F will cause visible damage, but the plant can continue to grow using the remaining leaf area. From central to southeast Minnesota, there is a 50% chance of the temperature falling to 24°F or colder after about mid-October. Producers can determine the information for specific regions at MN DNR's First fall freeze and frost dates. On these dates it is generally considered safe to harvest alfalfa as little regrowth will occur.
Another way to more precisely determine the final cutting date for alfalfa is based on growing degree days (GDD) as described in UW-Madison's Late summer cutting management of alfalfa. Cutting when <200 GDD remain in the growing season ensures that there will be insufficient regrowth to use root carbohydrates.
Growing degree days for alfalfa are determined by averaging the high and low temperatures for a day, then subtracting the base temperature of 41°F. (Alfalfa growth is negligible at 41°F and below). For example, if the daily high was 70°F and the daily low, 42°F, the average is 56°F. Subtract the base 41°F and you have 13 alfalfa GDD accumulated for that day.
At constant or decreasing temperatures and can safely cut about 15 days before the average date of 24°F air temperatures. Therefore, there is flexibility on the date of the final cutting, and we do not need to wait until killing temperatures of 24°F. Given the air temperature example above, if the average date of 24°F air temperature occurs in mid-October, in an average year cutting can occur as early as 1 October without risk. However, it is advisable to first find the average date for 24°F air temperatures and monitor the air temperatures in the region.
As we previously mentioned, unharvested stubble is valuable to catch snow. In addition, it can block solar energy from reaching the soil surface and buffer against premature midwinter warmup of the soil. Unharvested stubble will not smother alfalfa plants. Winter weather causes leaves to die and fall to the ground while remaining stems desiccate. These stems degrade by the time of the early spring harvest and do not affect forage yield or quality of the first harvest.Source : umn.edu