What causes alfalfa winterkill?
By Doo-Hong Min and Richard Leep
Weather can be a real challenge, especially to alfalfa in the winter months. In order to reduce the risk of alfalfa winterkill, it is important to know what role factors like ice, soil drainage, and soil nutrients play. By understanding the factors affecting winterkill of alfalfa, producers may be able to reduce winterkill damage. But when winterkill leaves you without the forages you’d planned on, it’s important to act quickly. Small grains can work well for late-spring planting, and summer annuals can be a good early summer alternative.
Ice encasement is one of the most critical factors leading to winterkill of alfalfa. Heavy snowfall and moderate, fluctuating, subzero winter temperatures tend to create periodic ice formation, particularly in low-lying, poorly drained areas that collect water. Ice can form as the snow melts and then refreezes which frequently occurs following midwinter thaws. This ice generally penetrates into the soil surface and completely encases the upper regions of the alfalfa root and crown buds which does not allow diffusion of gases such as carbon dioxide.
Solid ice can kill the plants very quickly by suffocating alfalfa from lack of oxygen, but granular ice with interconnecting air passes is less severe. Alfalfa covered by ice for three to four weeks will most likely be injured or die. The presence of stubble from the previous crop is one way to reduce the risk of ice encasement because crop residues or stubble tend to create channels and cracks in the ice through which gas exchange may occur. In addition, stubble tends to catch and retain snowfall which results in an insulation effect for the alfalfa crowns.
Good soil drainage is essential to prevent winterkill. In wet, poorly drained clay soils, alfalfa is much more subject to frost heaving during the late winter and early spring. Heaving can break the taproot, and more often it forces the crown out of the ground, exposing it to drying winds and to mechanical injury during harvest. These weakened roots can be invaded easily by disease, and sometimes these damaged alfalfa plants may die during the summer.
Tile drainage can correct a poor drainage problem, and surface drainage ditches in very level areas may be required to remove surface water in winter when soil is frozen. In addition, leaving enough stubble in the fall will provide more snow cover and less fluctuation in soil temperature.
Low soil potassium levels in the soil can be one of the major factors resulting in alfalfa stand loss. Why?
Because low soil potassium reduces storage of carbohydrates in the roots and crowns, high carbohydrate levels are needed to keep the alfalfa plants alive through the winter months, as well as for new growth in the spring. Therefore, it is important to evaluate the soil potassium level in late summer or early September by obtaining a soil sample for analysis and applying a fertilizer, if necessary.
The risk of winterkill goes up as the stand ages. Stands of alfalfa 4 or 5 years old are more susceptible to winter injury or winterkill than 1- or 2-year-old stands when subjected to the same cutting schedule. This is because younger plants have lower disease infections and have been exposed to less physical damage.
Variety selection is one of the most important factors affecting winter survival of alfalfa because varieties differ in winter hardiness and tolerance of disease or insects. Varieties with resistance to several diseases (Phytophthora root rot, verticilium wilt, bacterial wilt, and fusarium wilt) and high winter hardiness will have a lower chance of winterkill than those that are not winter hardy or not resistant to disease nor insects.
The timing of harvest (whether by cutting or grazing) can affect potential winterkill and persistence of alfalfa. Several factors are involved:
• Stage of maturity at cutting
• The frequency of cutting
• Timing of fall cutting
• And cutting height of the last harvest going into winter
If alfalfa is harvested at the vegetative stage frequently, the stands are going to be weak since the plants don’t have enough time to accumulate carbohydrate reserves in the roots. This is important for regrowth after each cutting and for new growth in the spring. Therefore, it’s important to harvest alfalfa plants at late-bud to early-flowering stage to meet the goals of both forage yield and quality which results in better stand persistence. Leaving a 6- to 8-inch cutting stubble in the fall can be effective in catching snow for insulation.
Growing degree days (GDD) can be a useful tool in aiding late-season cutting decisions. A GDD is the average of the minimum and maximum daily temperatures minus 41.
Recent Quebec research has shown that, if 500 GDD accumulate after the last cutting in late summer, there will be enough regrowth for good carbohydrate accumulation before a killing frost. So growers can cut alfalfa in September as long as enough warm weather remains before a killing frost. The Quebec research also showed that cutting later in the fall was acceptable as long as there was less than 200 GDD accumulated after cutting. When less than 200 GDD accumulated, there would be little regrowth to use up valuable stored carbohydrates and proteins in the alfalfa roots. This would result in good winter survival of the alfalfa plants.
Deer and other grazing wildlife can seriously damage alfalfa stands by grazing the alfalfa down to the crown areas, thereby making them more susceptible to winterkill. Simple solutions are not available to prevent alfalfa damage from deer. Herd population control conducted in cooperation with the state DNR wildlife biologists is the best long-term solution.
Fencing also can be an effective way to reduce deer damage, but it is expensive. Legume-grass mixtures may be an option to reduce the alfalfa damage from deer.
Once you notice that most of your alfalfa plants are winterkilled, a decision needs to be made on what forage options are available for your situation. A small grain such as oats and peas, spring triticale and peas, barley and peas, oats, spring triticale, and barley are all good choices for planting mid-April to mid-May for those needing forage in early to midsummer. Whether the small grain is planted alone or mixed with peas, harvesting forage should be based on the maturity stage of the small grain (late boot for lactating dairy cows and soft dough stage for heifers and beef cattle).
If you miss the window…
If you miss the spring planting time for small grains after winterkill alfalfa, summer annuals would be an option. Forage choices for planting mid-May to mid-June are corn silage, sorghum-sudangrass (for hay or grazing). These crops can do well in drought conditions, too. The seeding rate for sorghum-sudangrass and sudangrass is 20 to 30 pounds per acre, and they can be cut whenever they reach about 30 inches height for high-quality forage or 36 inches height for heifers and beef cattle.
Best choices for planting mid-June to early July will be sorghum-sudangrass hybrids, forage sorghum, and sudangrass for highest yield for the remaining portion of the season until killing frost. When planted this late, forage sorghums will likely be killed by frost to dry sufficiently for ensiling. Harvesting within one week of a killing frost is recommended to reduce potential for prussic acid poisoning. Forage sorghums may not be the best choice if the year is average to cool in temperature because forage sorghums perform better in warm temperature (90 to 95 degrees Fahrenheit). Previous studies on annual emergency crops have shown that corn silage results in the highest dry matter yields with the least risk of crop failure.