Rory Lewandowski, Extension Educator Wayne
County
As alfalfa stands break dormancy
and begin growth, growers should make plans to take some time to evaluate the
health of those stands and determine if there was winter injury. Some early bud growth was observed the last
full week of March in the southern half of Ohio. This evaluation is especially important in
those areas of the state where we had periods of near zero to below zero
temperatures this winter combined with little to no snow cover during some of
those cold temperatures. After doing a
quick literature review, it appears that there is general agreement that
temperatures in the 5 to 15 degree F range as measured at the alfalfa crown can
begin to damage the plant and prolonged exposure to these and lower
temperatures can kill the plant. Generally,
the soil temperature at a 2 inch depth is associated with the temperature of
the alfalfa crown. Snow cover is an
important component of protecting an alfalfa plant from sub-zero temperatures
since even a cover of 4 inches of snow can provide 10 to 15 degrees of
protection. Once again, the concern is
for those areas that experienced periods of zero and subzero temperatures
without a 4 inch or greater snow cover.
For many areas of the state, however, soil temperatures at the 2-inch
depth remained at or above 26F even through the coldest days this winter.
An alfalfa
stand health evaluation and winter injury assessment needs to be done by
getting out into the field and doing a combination of stand counts and digging
up some plant roots. Generally that evaluation
should be done when there is 3 to 4 inches of growth from the plant. Evaluation involves selecting random sites
throughout the field and counting the plants in a one foot square area. Check at least one site for every 5-10 acres,
and like soil sampling, more random sampling is better. In addition to counting the plants per square
foot, a count of the total stems per square foot is also useful because healthy
plants can often produce more stems per plant thereby compensating for
potential yield loss from fewer plants per square foot. After counting the plants, dig up all the
plants in a one foot square area for every 5 to 10 acres and examine the crown
and roots of the plants.
The winter
survival rating determined by the plants per square foot is based upon the age
of the stand. The following table is
from a 2012 article on the Iowa State University (ISU) Integrated Crop
Management web site by Stephen
K. Barnhart, ISU Extension forage specialist.
|
|
__________________Plants Per square foot_____________________
|
||
|
Stand Age
|
Good
|
Marginal
|
Consider reseeding
|
|
Year after seeding
|
+12
|
8-12
|
Less than 8
|
|
2 years
|
+8
|
5-6
|
Less than 5
|
|
3 years
|
+6
|
4-5
|
Less than 4
|
|
4 years and older
|
+4
|
3-4
|
Less than 3
|
As
mentioned previously, counting the total stem number in a square foot is
another method of evaluating winter survival and yield potential of a stand and
has been promoted by Dan Undersander, Extension forage specialist at the
University of Wisconsin. Here is a
summary of that system:
|
Stem number/square foot
|
Expected result or action
|
|
Over 55
|
Stem density not limiting yield
|
|
40-55
|
Some yield reduction expected
|
|
Less than 39
|
Consider stand replacement
|
Source: Undersander et al 2011, University
of WI Extension publication A3620
While plant and stem counts are
useful, to get a true determination of stand health, plants must be dug up so
that crown and root tissue can be evaluated.
To do this you must split the crowns/roots. The inside should be a creamy white
color. If it is yellowish brown to
chocolate brown color the tissue is damaged or dying. If more than 50% of the roots show these
symptoms, reduce your stand counts and yield potential.
One other weather condition that can
have a detrimental impact on alfalfa stands is freeze/thaw cycles. These cycles
that typically occur in February through March often present the
greatest danger of winter injury in Ohio.
There is the potential during these cycles to physically lift or heave alfalfa
plants out of the soil. This heaving
exposes the crown of the alfalfa plant making it more susceptible to
temperature and physical injury. In some
cases, heaving breaks the root system, effectively killing the plant. Heaving tends to be more of a problem in wet,
saturated soils and clay soils.
Although winter temperatures and
snow cover amount are primary driving factors affecting alfalfa winter
survival, there are also management factors that growers can control to
decrease the chance of winter injury.
Those factors include:
·
Selecting
varieties with good winter hardiness and disease resistance.
·
Maintaining
soil fertility levels. Potassium in
particular is associated with enhancing alfalfa tolerance to winter injury.
·
Improving
soil drainage.
·
Harvest
management: more cuts is generally associated with a higher risk of winter
injury, particularly if the last fall cut falls in that mid-September to mid-October time frame.
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