Dr. Dan Undersander
The freezing rains and ice accumulation in parts of Wisconsin have raised many questions about potential ice damage to the alfalfa stands.
How does ice cause damage? The freezing temperatures of the ice do little damage directly to the alfalfa. Temperatures below 15o F in the crown region (1 to 4 inches into the soil) are required to damage alfalfa. Ice damages alfalfa because alfalfa roots need oxygen during the winter. The oxygen comes from air above ground diffusing into soil. A solid layer of ice restricts air diffusion and suffocates alfalfa. This is the common reason for loss of alfalfa in low spots in fields.
The conventional recommendation has been that alfalfa covered by ice for 3 to 4 weeks will likely suffer injury or death. However, sometimes alfalfa will survive much longer under ice depending on a number of factors.
Thus when determining whether or not ice sheets will cause damage to alfalfa stands we must consider whether the ice is in a solid sheet. If the ice is not uniform, cracked, or has holes, it will not completely restrict air movement into the soil and result in little to no damage. Additionally, alfalfa stems sticking up through the ice will help create air holes in the ice.
What is the status of the alfalfa stands now? First, alfalfa generally had good hardening last fall (2006 similar to 2017). Hardening has decreased the water content outside of plant cells, increased bound water within cells, increased cell solutes, and increased cell permeability. All of which increase the ability of alfalfa to withstand cold temperatures.
What can be done? The short answer is nothing. Some ask about broadcasting fertilizer on top of the ice but this is not a good idea. The thought is that salt in the fertilizer will melt through the ice and causing holes to let air into the soil. However, if the ice is too thick to break up naturally, fertilizer generally does little good because it does not melt all the way through the ice to create holes. Additionally, surface applied fertilizer could be a runoff problem if the fertilizers contain nitrogen or phosphorus, since a rapid melting or a rainfall event would be likely to cause substantial runoff of N and P. For this reason, the current nutrient management standard prohibits most N and P fertilizer applications on frozen or snow-covered soils, except for winter grains. Broadcast applications of potassium, while not prohibited, on an ice cover can also be problematic. A runoff event could cause the distribution of the nutrient to be quite variable and off-site losses would represent an economic loss to producers.
Thus while concerned about the ice, nothing can be done to minimize any potential damage to stands. Remember that several years ago, we had ice sheeting across central Wisconsin for 8 or more weeks and suffered very little winterkill or injury.