What will the Winter do to Alfalfa and Grasses?

We are having an unusual winter many places.  How will this affect alfalfa hayfields?  First a few general principles:

1. While forages can be killed or injured by cold winter temperatures, injury is most common, where shoots are killed and the plant is slow to green up the next spring, reducing yield.
2. Most forages can tolerate temperatures in the crown of around 15 degrees.  The crucial temperature is not air temperature but that of the soil in the crown area of the forage (2 to 4 inches below ground).  Soil is a good insulator (especially if dry), so that night (or day) temperatures below 15^o will only slowly reduce the crown area soil temperature.
3. We depend on snow cover for insulation to help the forage survive the winter (more so as we go north).  Eight inches of loose snow insulated against up to 30 degrees of air temperature in January.  Thus we sometimes see snow mobile tracks hay fields as they begin to green up because the snow was packed under the snow mobile and the soil and plant got colder, injuring the plant.  We see the snow effect particularly where we have good winter survival in the very northern regions with dependable snow cover while winterkill is more common in warmer regions where snow comes and goes.
4. Some varieties of alfalfa and grasses are more winterhardy than others.  One should check for adaptation to the region where the forage will be grown.  For example, many grasses are bred in other parts of the world with milder winter temperatures.  The pictures shows the results of an orchardgrass variety trial at Arlington, Wisconsin where some varieties green up well and some were severely injured.  The brown plots generally have some green shoots and will recover but first growth will be lost.
5. The drought last summer generally meant that forages entered the winter will lower root reserves than normal and will be more susceptible to stress over winter.

The most severe stress for forages over winter is when they begin to green up and then freeze back.  Following such a frost, the forage has to start new shoots which delays growth in the spring and uses some of the supply of root carbohydrates.  Most forages can withstand this once or twice in a winter if plants were healthy when entering the winter.  This green up and freeze back is most likely to occur in “mild” winters when snow is melted and warm 3 to 4 day periods occur, followed by a cold period with temperatures below 24^o. 

While we hope we will have little injury or kill of forages it is best to be prepared, especially since most need forage next spring.  The most important thing is to check hayfields and pastures early to determine if overseeding or stand replacement is necessary. 

What does winter injury look like?

Root damage.  Healthy roots are firm and white (like the inside of a potato).  The first sign of injury in the spring is often roots where the top 4 inches are spongy and brownish.  Over time, injured roots will become diseased, rot and turn dark brown as shown at right. 

Slow or uneven “green up” in all or portions of the field.  In the fall, energy reserves are stored in the roots and crown buds of alfalfa are formed. The reserves provide energy during the winter and in the spring shoots grow from the crown buds.  While injury damages the roots and crowns, 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).  Alfalfa may show uneven greenup where few stems begin to grow from surviving buds and new buds from but result in delayed growth.  If you see shoot starting, watch them carefully as severely injured alfalfa plants may put out shoot and then die when shoots are 1 to 2 inches tall due to lack of root reserves.

Late frost on alfalfa. Late frost (<24^oF) may damage or kill leaves.  If severe enough to cause terminal buds to turn brown (in approximately 3 to 5 days) then shoot will not grow and new shoots must form.

We recommend the following:

1.  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) 
2. Determine the percentage of field affected and manage to meet your goals:

a) If a small 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.

b) If a large portion of the field is affected seed oats (2 bu/a) and peas (20 lb/a) for haylage harvest to replace first cutting or corn for season-long high tonnage.  In some regions, corn or sorghum-sudangrass can be planted following the oat-pea harvest.  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). 

c) Alfalfa releases autotoxic compounds into the soil that inhibit germination and/or growth of new alfalfa seedlings.  The recommendation would be not to reseed alfalfa into alfalfa fields where the killed stands was over 1 year old for 1 to 2 years.

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.