Does Ice and Cold in Northern New York Harm Alfalfa?

From the Northern New York Cornell Cooperative Extension Field Crops Team:

Cornell Cooperative Extension offices in Northern New York have fielded several questions regarding ice and risk of winterkill in alfalfa recently.  

It’s a good topic for discussion right now as much of the Northern New York landscape was covered with an inch or two of solid ice.

The ice coating outdoor surfaces in early January was the result of freezing rain and sleet that fell just a few days before Christmas. 

The storm caused lots of immediate damage in the form of downed tree limbs and power outages and it continues to bruise knees, but many are now wondering if the ice also poses an additional risk to alfalfa.

Ice sheeting is associated with increased risk of winterkill in alfalfa, but the conditions now in the North Country are probably not likely to worsen winterkill and may actually be a protective feature for alfalfa.

Alfalfa is an enormously important perennial legume across Northern New York because of its high yield potential and high feed value for high-producing dairy cattle.

With proper management, alfalfa stands can last up to 5 years or more and a more persistent, longer lifespan generally means a better economic return.  All too often though, alfalfa stands can become thin and less productive after the third year and harsh winter conditions can exacerbate this decline.

Historically, risk of winter damage to alfalfa has been linked to several environmental and management factors: age of the stand, variety, disease pressures, soil pH and K, cutting schedule and soil moisture and drainage.

The risk of winterkill increases with advancing age of the alfalfa stand. Younger plants (1-2 years) are generally more stress tolerant compared to older plants (2+ years) because they’ve been exposed to less physical damage and disease organisms.  

Alfalfa varieties vary in winter hardiness as well as resistance to diseases and pests, and choosing a variety that resists diseases such as Phytophthora root rot and Verticillium, bacterial and Fusarium wilts, will reduce the risk of stand loss due to winterkill.

Soil pH below 6.0 stresses alfalfa plants and can exacerbate winter injury.  A soil pH of 6.6 or greater removes this risk.

High exchangeable soil K (>160 ppm) reduces risk of winterkill and conversely, a low exchangeable K (<80 ppm) can make winter injury much more likely by reducing root health and fall carbohydrate storage.

The timing of cutting (or grazing) affects the persistence of alfalfa too. Several factors come into play - stage of maturity, frequency and timing of cutting, and the timing and stubble height of the last fall cutting.  

Harvesting alfalfa at late bud to early flowering stage will optimize both forage yield and quality and results in better stand persistence. Leave a 6 to 8 inch stubble in the fall to effectively catch a layer of snow for insulation.

Finally, let’s talk about ice and its effect on winterkill. We know that alfalfa stands typically do not survive well on poorly drained soils. Poorly drained areas can suffer 2 types of winter injury as a result of the combination of cold and excess moisture.  

Where soil remains wet, frost can heave alfalfa plants out of the ground in late winter and early spring.  Heaved plants may have broken taproots, or the crown may be left up above the soil where it is exposed to cold, dry winds and mechanical injury later in the season. Diseases will often invade the weakened root and the plant dies during the summer.

If water accumulates and freezes on the plants and soil surface, this is referred to as ice ‘sheeting.’  Ice sheeting can occur in low areas as the result of a mid-winter thaw or from sleet and freezing rain as we’ve recently experienced. Ice sheets cause winter injury by smothering plants and by rapidly conducting heat away from the plants and soil surface leading to very low soil temperatures.  The longer the ice remains on the soil surface, the greater the risk of economic damage.

This winter, however, the 1 to 2 inches of ice is above several inches of snow that fell before the freezing rain and sleet just before Christmas. Right now, alfalfa crowns and roots may actually be protected by the ice because it has held an insulating layer of snow on the plants during some very cold temperatures and high winds. 

If ice formed directly on the alfalfa crowns and soil, no such protection would occur, and the plants would be at risk of smothering and loss.  In lab research trials, alfalfa plants directly covered with ice began to die after about a week, and most were dead within a month.

Our present situation here in Northern New York, though uncomfortable for humans, may be just fine for alfalfa.