Check Alfalfa Stands for Signs of Winterkill

Icy snow cover during winter has damaged some alfalfa fields.

A number of farmers in northeast Iowa and elsewhere in the state are concerned about how their alfalfa survived the harsh winter because of the extended icy snow cover. There are some problems, and they will become more visible in the next week or two.

"As alfalfa overwinters it continues metabolic activities at a slow rate," explains Brian Lang, Iowa State University field agronomist at Decorah in northeast Iowa. "The importance of good fall management, allowing alfalfa to store carbohydrate reserves before winter is to have these carbohydrates to carry out the overwinter metabolic activities."

This function also releases waste products such as carbon dioxide, which escape as gases from the proximity of the plant's root and crown, he notes. When significant amounts of ice cover the ground, these gases can't escape. Too much waste product build-up around the root and crown is toxic to alfalfa. Alfalfa can usually tolerate about three weeks of significant ice cover before being overcome by the build-up of these waste products.

Areas in fields where alfalfa is dead

In January, we had a significant snow melt that caused ponding and subsequent "ice skating rinks" in low areas of some fields. These "ice ponds" are thick enough and have lasted long enough that alfalfa underneath is dead. "Fortunately there are not a lot of these areas and they are not very large," says Lang.

The bigger question is the concern over larger areas of fields that have had one or more wet snows that have settled and remained most of the winter. How icy are these areas? Any porosity should let the metabolic waste gases escape and not be a problem, he says. Stubble that poked through the icy snow cover would also avoid this problem.

But, were some icy-snow areas dense enough, long enough to cause some damage? "It's not possible to make this call from the office," says Lang. "Aside from density and duration of icy-snow cover, other stress factors include: no fall stubble, higher metabolic rates for less cold tolerant varieties, lack of disease resistance, stresses from lower soil fertility, more intense harvest schedules, age of stand, field drainage, and more. In a week or two weeks, we can start evaluating alfalfa fields to find out."

When, how to do stand evaluations

"It will be early April before I dig up my first alfalfa plants to assess overwintering of alfalfa," says Lang. "However, if you want to check your fields sooner, here are some tips."

Realize that some stand reduction (plants per square foot) is expected every year. Aside from plant-to-plant competition other stresses can take a toll (wheel track damage, stand age, advancing disease problems, lack of attention to soil pH and fertility, minimal stubble left in fall, etc.). So even if winter survival is generally good, some stands will still need to be evaluated this spring to estimate their potential productivity for the season.

Plant counts assessment can be used now

The old procedure to assess alfalfa stands has been through plant counts, explains Lang. The basic recommended plant counts per square foot for a pure alfalfa stand are: > 20 plants in fall for the seeding year, > 12 plants in spring for the 1st production year, > 8 plants in spring for the 2nd production year, and > 5 plants in spring for the 3rd production year.

"The plant count method seems to works fine for young stands (new seedings to one year old stands), but it does not correlate very well with older stands," he says. "You would also need to cut the taproots in half to assess root tissue. Firm, white tissue is excellent, brown tissue is dead, yellow tissue is dying. Color diagrams to help with this assessment are in the University of Wisconsin publication A3620, Alfalfa Stand Assessment. To get this publication, go to: learningstore.uwex.edu/. At this page, type into the search box A3620. From here you can get a pdf file of the publication.

Stem counts assessment used mid-April

A better method to evaluate alfalfa stands is the use the stem count method, says Lang. This works regardless of plant age. Older stands have less plants per square foot, but older plants produce more stems than younger plants. However, this method requires alfalfa topgrowth to average at least 6 inches tall before the method can be used.

"The way this spring is progressing," says Lang, "it'll be at least mid-April before we can use this method. Count stems per square foot in 4 to 6 representative areas in the field. Count only those stems expected to be tall enough to mow. Use the following table to estimate the yield potential of the stand. The yield potential is the context of realistic yields normally expected from that field. Actual yields can be less depending on in-season problems with precipitation, insects, diseases, soil fertility, and harvest losses."

Table 1. Alfalfa stem counts and corresponding estimated yield potential.
Stem counts per square foot: >54 50 45 40 35 30 25
Percent yield potential:     100 90 81 72 62 53 44

Table 2. Recommendations based on the stem count method.
(Stems/sq.ft. - Recommendation)
>54 - Not yield limiting.
40-54 - Usually keep. Some yield reduction.
<40 - Consider replacing. Significant yield reduction. May still keep it if significant grass forage is present.

"Whether or not you should keep an alfalfa stand is an individual decision that depends on many factors, including hay supply, available land, cash flow, etc.," says Lang.

Table 2 provides general recommendations on whether or not to keep a stand. However, these recommendations are for pure alfalfa stands. Alfalfa-grass mixtures have a grass component that contributes to yield. If the desired stand was a 75:25 alfalfa:grass ratio, and assessment of alfalfa stem counts is about 41 stems per square foot (about 75% of a full stand), then the overall stand should provide 100% yield potential because the grass component makes up the other 25%.

Should you control winter annual weeds?

"Every May I get phone calls on outbreaks of weeds in alfalfa fields like Shepherds purse, Field pennycress, and/or Pepperweed," says Lang. "However, by that time there isn't much of anything we can do about it. These are among the most common winter annual weeds that show up in alfalfa fields. They're not very palatable for cattle and are rather low in quality."

Winter annuals germinate in the fall, develop a rosette-type growth (like dandelions), overwinter as the rosette, and then bolt (rapid growth of upright stems) in early spring producing seed for next fall. Once these plants bolt, herbicides are not very effective.

The time to control these weeds is while alfalfa is still dormant, he says, using herbicides such as Pursuit or Velpar. The only way to know if these weeds are present is to scout the fields. This can be done in early spring while you assess stands for how they survived over winter. Weed growth at this time is still in the flat rosette stage. "However, even if you find some winter annuals, realize that most winter annual weed situations are not heavy enough to significantly interfere with yield and quality of the overall forage, so the herbicide application is not usually recommended," says Lang.

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