Preventing Alfalfa Winterkill
Alfalfa has been put to bed, and now farmers are hoping and praying that their stands will make it through the winter in good shape. The severe winterkill experienced last winter by farmers in Wisconsin and Michigan reminds us that sometimes conditions are so severe that few fields are spared, regardless of how conservative farmers were in managing them. However, under less severe conditions there are a number of factors that can mean the difference between success and failure.
Several factors affecting winter survival of legumes are almost unavoidable, and in some cases completely unavoidable. One is insect damage, primarily due to clover root curculio, but also alfalfa snout beetle in those few areas in the northeastern U.S. unlucky enough to have this devastating pest. The larvae of both species feed on the taproot of alfalfa, and in the case of clover root curculio, of clover as well. In some cases, this damage severs the taproot, but more commonly it exposes the taproot to disease organisms that will eventually kill the plant.
This past fall, some alfalfa fields in St. Lawrence County, N.Y., were so hard-hit by snout beetle larvae that alfalfa plants could be lifted out of the ground by hand - there simply was no taproot left. Cornell University entomologists have discovered a "friendly" nematode that kills snout beetle larvae, offering the first hope of controlling this pest, since insecticides aren't effective.
Another factor farmers have no control of is snow cover. The old saying that snow is the poor man's fertilizer has a basis in fact, because snow provides insulation against subzero temperatures. In 1998, when parts of the Northeast experienced the worst ice storm of the century, alfalfa fields covered by a few inches of snow (before being covered by several inches of ice) fared much better than fields in the stricken area that had no snow cover.
A third factor over which farmers have no control is soil topography. Flat fields are more susceptible to ice sheeting, which can kill alfalfa and other legumes.
There are many factors affecting alfalfa winter survival over which farmers have at least some control. We'll focus on these, since it's more productive to consider what we can control versus what we cannot.
Fall dormancy - Alfalfa varieties vary in fall dormancy, with the dormancy ratings provided by seed companies and/or land-grant colleges. Ratings vary from 1 to 9, but here in the Northeast we normally use varieties ranging in fall dormancy from 2 to 5. A variety with a fall dormancy of 2 "goes to sleep" for the winter much earlier in the fall than one with a fall dormancy of 5.
If you routinely take a fall harvest, you may want to choose varieties with a fall dormancy of 4 or 5. However, pushing fall dormancy too far risks winter damage. Planting a variety with a fall dormancy rating of 8 or 9 almost guarantees that the alfalfa will winter-kill and, therefore, act as an annual, not a perennial.
Winterhardiness - Winterhardiness is not the same as fall dormancy, and alfalfa varieties vary widely in this characteristic. Rely on university trials, farm seed catalogs and your seed dealer for advice on winterhardiness.
Soil fertility - Soil fertility is a biggie, particularly potassium, which is antifreeze for alfalfa. Soil pH is also important, since it influences the availability of soil nutrients. Alfalfa that goes into the winter with low potassium levels will often suffer severe damage.
Time of seeding - I don't like the term "fall seeding" for alfalfa, because seeding alfalfa on or after September 21 is much, much too late. In most of the northeastern U.S., the later in August that alfalfa is seeded, the lower the winter survival rate. And even if it survives, first-cut yields the following spring are often reduced.
Age of stand - Younger alfalfa stands are generally healthier than older ones, since there's been less wheel traffic damage to the crowns. By the time alfalfa fields are 3 years old, almost every plant has been rolled over more than once by the tire on a heavy piece of harvesting equipment.
Soil drainage - You can do little to change field topography, but you can improve internal field drainage by the installation of subsurface drainage tubing, aka tile drainage. You'll need either a natural outlet for drain water discharge or access to power so you can install a pumping station that will elevate drain water up and over whatever is preventing the natural flow. Miner Institute (Chazy, N.Y.) has two pumping stations that are working very well.
In-season harvest management - The more intensively you manage alfalfa during the summer (such as 30-day harvest intervals), the more reason to conservatively manage these fields late in the season, perhaps including not taking a fall harvest. Some agronomists suggest that the negative impacts of intensive harvest management are cumulative; if you harvest one cutting at an unusually short harvest interval, you should give the next one a few extra days to recover root reserves.
Fall harvest management - Much has been written about this topic, but fall harvest management (actually, late-season management) has a big influence on winter survival. The primary goal of fall harvest management is for alfalfa to enter the winter with a high level of taproot carbohydrates.
Stubble height - Alfalfa doesn't regrow from the cut stem, so mowing height has no impact on plant health as long as the crown isn't damaged during mowing.
Look at an alfalfa plant a few days after harvest: The cut stems are dead, with the new growth coming from crown buds. In fact, the only time alfalfa stubble height has any impact on plant health is during the last harvest, and this isn't a direct impact on the plant but a secondary one. A high stubble, 6 inches or so, will catch and hold more snow than a shorter stubble. While you have no control over snowfall, you can influence how long the snow remains in your alfalfa fields.
Another benefit of a high stubble is that if the cut stems stick above an ice sheet, they may reduce the amount of damage. A high stubble is no guarantee that ice sheeting won't kill alfalfa, but it can certainly help.
Ev Thomas has worked as an agronomist in New York for 45 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has written our Forages column for 16 years and has been an expert contributor on a number of other topics.