An unusual amount of winter injury and winterkill of alfalfa stands occurred in south central and southern Minnesota. While reports do not represent a detailed analysis of where injury to alfalfa has occurred across Minnesota, they do suggest a need for producers to check on stands and evaluate them for potential winter injury.
Several environmental factors likely interacted to stress alfalfa and cause winter damage. These include the following:
- A very dry fall and early winter that may have prevented alfalfa from accumulating adequate root reserves to survive the unusually long dormant period
- Periods during the winter without snow cover which typically provides insulation against low air temperatures
- Ice sheeting due to snow melting and rainfall in January/February
- The long periods of snow cover and low temperature that prevented spring alfalfa regrowth when reserves were low.
What does winter injury look like?
Slow or no "green up" in all or portions of the field.
It is intuitive that if plants are dead, there will be no regrowth. There may be regions of the field that are green while other areas remain brown. Alfalfa is a perennial that hibernates during the winter. In the fall, energy reserves are stored in the roots and crown buds are formed. The reserves provide energy during the winter and in the spring shoots grow from the crown buds. Injury can destroy the roots and crowns but often some crown buds survive and slowly regrow; however, this may be limited to only a few per plant and these may be stunted or chlorotic (yellow).
Figure 1. Alfalfa taproot on the left is turgid from a live and healthy plant. The alfalfa taproots on the right are brown, dehydrated and ropey from dead or dying plants. Photo courtesy of Dr. Dan Undersander, University of Wisconsin.
Damage to several of the crown buds during the winter will create uneven regrowth. The plant has potential to regenerate new crown buds, but this regrowth is typically delayed. Plants look "scraggly." If the plant root has been damaged, some shoots may die.
Healthy roots are firm and white. Injured roots are spongy, grey, and if twisted, water can be expressed. Over time, injured roots will become diseased, rot and turn dark brown. A damaged root cannot sustain crown bud growth.
We recommend the following:
- First make sure that "dead" spots are actually dead and not just delayed: (Stands can be slow to recover when injured). So don't be in too much of a hurry to till up a stand until you've looked at some roots to determine health).
- Dig up a few plants and check the top four inches of the tap root for color and turgor. Split the root and crown. The tap root should be an off white (like the inside of a potato) and turgid (not ropey).
- Check plants that are putting out small shoots. Sometimes the dying plants will produce shoots one to two inches tall and then die. Again, dig a few plants and look for off‐white and turgid taproots. Regrowth should also be scouted following first cutting as sometimes plants have the ability to put on growth initially but die after being harvested.
- If a small or moderate percentage of the field is affected and you want to harvest forage from the existing stand, go over the affected areas with a drill seeding (10 lb/a) with a 50/50 mix of Italian (annual) ryegrass and perennial ryegrass to a maximum depth of ¼ to ½ inch.
- If a moderate percentage of the field is affected you can immediately interseed Italian ryegrass (10 lb/a), take the first cutting for forage. Corn or soybean can then be planted into the killed forage. An alternative to this approach is to take two forage harvests and seed oats for fall forage.
- If a moderate or large amount of the field is affected seed oats (2 bu/a) and peas (20 lb/a) for haylage harvest. In some regions, corn or sorghum-sudangrass can be planted following the oat-pea harvest.
- If a large percentage of the field is affected, seed corn or BMR sorghum‐sudangrass before July 1 (20 lb/a). These crops can use the nitrogen from alfalfa. Corn will likely produce the most tonnage of any forage. Sorghum‐sudangrass is a good choice if you expect dry conditions and/or above average temperatures (like last year). Alfalfa can be seeded into a different field at 10 to 12 lb/a either alone or in mixture with grass (e.g., 6 lb/a tall fescue and 2 lb/a Italian (annual) ryegrass).
- Once a stand of alfalfa is killed from a winterkill event, auto-toxic compounds are released into the soil environment. How long they remain and what affect they have on a new alfalfa seeding is a function of soil type, temperature, amount of rainfall, and time from tillage to reseeding. Directly reseeding is risky, but the minimum risk of autotoxicity occurs if you till the old stand and incorporate the residue, wait 1 month from tillage to reseeding, and the old stand is one year old. Risk is also less on sandy soils. The University of Minnesota Agronomy department has developed an alfalfa autotoxicity risk spreadsheet (http://z.umn.edu/AlfalfaAutotoxicityRisk) to help growers determine the risk of re-seeding alfalfa in existing alfalfa stands along with appropriate management recommendations.
In situations where producers are making decisions for the long term they should use stem counts to estimate current yield potential and assess root and crown health to determine future yield potential. Stand health based on stem densities per square foot can be assessed in the following manner:
- Greater than 55 stems indicates density will not be a limiting factor,
- Between 40 and 55 stems is understood to represent some reduction in yield but probably more than adequate in years of low inventories and high value, and
- Fewer than 40 stems indicate a poor stand and consideration for termination.
Stands can be slow to recover when injured. So don't be in too much of a hurry to tear it up until you've looked at some roots to determine health. If surviving plants are injured, but have adequate stem density to maintain the stand it's probably best to plan to push the stand with annual forages (i.e. Italian ryegrass or oats) to get the most out of it this year than to plan to terminate after this growing season. If most surviving plants look reasonably healthy (i.e. losses were localized and/or ice-sheeting related) you might consider inter-seeding perennials to stretch the stand beyond this year. Note that legume credits from winterkilled alfalfa fields are the same as if the stand had been terminated last fall.
Article contributions: Dr. Dan Undersander, Department of Agronomy, University of Wisconsin