Planting season came early for many across Wisconsin this year. As farmers make their way through corn silage harvest some may be asking “When should I cut my fourth/fifth crop alfalfa?” The “right” answer for each farm depends not only on existing and future forage stocks and needs, but also an assessment of each individual alfalfa stand’s risk for injury based on field and weather conditions.
Alfalfa fall harvest decisions should not be taken lightly. Decisions made now not only impact short-term forage quality and quantity, but potentially affect the long-term viability and productivity of the stand. Many parts of the state have struggled with alfalfa winterkill the past few years due to unfavorable weather conditions during the winter and spring months. Suboptimal fall harvest and poor soil fertility management decisions further diminish alfalfa plant health and vigor.
Combating potential winterkill and promoting rapid spring green-up of alfalfa begins with ensuring the plant accumulates sufficient root carbohydrates and proteins before entering dormancy. This is accomplished by cutting the alfalfa early enough in the fall so the plant can replenish root energy reserves expended during the regrowth period or delaying cutting until the plant is likely to enter dormancy shortly thereafter.
Fall alfalfa harvest timing is typically based on the number of growing degree days (GDDs, base 41°F) remaining in the season. Harvesting alfalfa with more than 500 GDDs or less than 200 GDDs before a killing frost is recommended to limit potential future winterkill, regrowth, and yield challenges. In general, the middle of September through the middle of October is the riskiest time to cut alfalfa in southern Wisconsin. In northern Wisconsin, harvesting alfalfa the last half of September is the riskiest with low probability of either more than 500 GDD or less than 200 GDD accumulation. Waiting until mid-October is often safest for all areas of the state.
The National Oceanic Atmospheric Administration (NOAA) 30-day outlooks for temperature and precipitation suggest an enhanced probability (increase of 50-60%) for above normal temperatures and a slightly enhanced chance for above normal rainfall for October 2021 (maps published September 16, 2021). If this forecast holds true, farms may be able to stretch fourth crop harvest a bit further into September and still accumulate 500 GDD before a killing frost. However, farms need to weigh the potential risks and benefits of pushing the harvest window. Following the historical “no-cut” window may provide the highest probability of avoiding winterkill and ensuring vigorous spring regrowth for those who are risk adverse.
If the farm does not need additional alfalfa forage, leaving current growth on the field is a sound practice. The plant material will help catch and hold snow on the field, assisting with crown insulation. Alfalfa fields where grass is a significant percentage of the stand composition (30% or more) can be at risk for losses if there is a significant amount of regrowth. While alfalfa generally does not smother itself, grasses can smother themselves and alfalfa. If a field contains more than 30% grass and fall regrowth is substantial enough to cover the crowns of surrounding plants in some areas of the field, one may need to consider a late season cut. Adjust the cutter bar high enough (approximately 6”) to limit injury potential to the crowns of all species in the mix.