By Dan Undersander
Now is the time to be thinking about getting alfalfa to survive the winter. While we lost much acreage to this past year to weather related factors, clearly some stands were more susceptible to winterkill than others. Thus, while Mother Nature has the final say, management can play a key role in alfalfa winter survival.
Optimize soil pH
A major factor reducing alfalfa winter survival, often ignored, is less than optimum soil pH. Alfalfa needs a soil pH of 6.8+0.2 for optimum growth. The growth resulting from optimum soil pH helps the plant survive the winter as shown in figure 1. Often fields are planted with less than optimum pH for alfalfa, but those fields with soil pH in the desired range will grow more rapidly, have healthier roots and therefore better winter survival.
Another factor increasing alfalfa winter survival is potassium. It is needed for growth, but it is also key to winter survival as shown in figure 2. Farmers can still apply potassium yet this fall. The approach is to fertilize alfalfa to the optimum soil level as indicated on a soil test report and then annually to replace any potassium removed by alfalfa. Alfalfa normally contains 50 to 55 pounds K2O per ton of hay, so calculating the fertilizer that needs to be replaced in the fall is easy. Potassium fertilizer can be applied as long as the ground is not frozen. It is very soluble and moves into the soil with the first rain and is rapidly taken up by the plant. A fall application of potassium will also provide adequate potassium for first cutting the next year so that the spring fertilizer application can wait until after first cutting when the field is more firm.
A third factor enhancing alfalfa winter survival is a fall rest period. Alfalfa must build a good store of protein and carbohydrates in the crown and taproot to survive the winter. Alfalfa normally uses root reserves for early growth and then replaces them as the plant matures for the next growth. If alfalfa is cut, regrows to 6 or 8 inches and freezes, it will suffer more winter injury and death than if more or less growth occurs before dormancy.
Late fall cutting clearly stresses the alfalfa. It removes top growth which provides insulation for the crown, allowing the crown to be exposed to colder air temperatures. Alfalfa will generally be killed when the crown reaches -15oF. This seldom happens because the crown is insulated from colder air temperatures by snow, crop residue and the soil. Thus, while I generally recommend taking a cutting of alfalfa just prior to winter (late October, early November), there is some risk of winterkill associated with this. If soil pH is adequate, soil potassium is optimum, a very winterhardy variety planted and properly rested before late fall harvest, we see about a 20% increased risk of winterkill – less risk if good snow cover, more if open winter. With the high price of hay and general hay shortage, many farmers will take this risk in 2013.
I recommend taking the late fall cutting, about 10 days prior to the average killing frost. We do not need killing frost before harvest but need sufficiently cold weather after harvest to reduce any fall growth and use of the root reserves. Weather is generally more favorable for hay making prior to a killing frost than after.
Lastly, when selecting an alfalfa variety for planting next spring select varieties with adequate winterhardiness. The best varieties have a winterhardiness score of 2 or less. Those with lower scores will better tolerate late fall cutting with less effect on next year's yield; they will survive adverse winters better, especially periods without snow cover; and will green up with less winter injury (uneven shoot growth) in early spring.
Undersander is a University of Wisconsin Extension forage agronomist.