Assessing winter wheat
Extreme temperatures this winter have many wondering how the winter wheat crop is faring. Is it at risk for winter kill? It’s too early to tell, said Bob Fanning, South Dakota State University (SDSU) Extension Plant Pathology Field Specialist.
“As one looks across a dormant winter wheat field, it is impossible to know if the plants are alive and will come out of dormancy when spring arrives and the soils warm,” Fanning said.
Fanning explained how soil moisture factors in winter wheat condition. Moist soil has higher specific heat than dry soil, so is not as sensitive to temperature fluctuations, both high and low. Also, winter wheat plants that are adequately hydrated are much better able to withstand low temperatures than those under moisture stress.
“Soil temperatures are highly important in speculating on the condition of winter wheat following cold periods because we know that most of the winter wheat varieties grown in South Dakota can withstand temperatures at the crown level down to about 5 degrees,” Fanning said.
He added that this can vary, depending on the winter hardiness rating of the variety, soil moisture, time of year the plants are exposed to those temperatures, etc.
“We know that soil temperatures fluctuate much more slowly than air temperatures, so low temperatures that occur for a short period of time do not drastically affect soil temperatures, even only a few inches deep, where the winter wheat crown resides,” he said.
Soil temperatures are also buffered from low air temperatures when crop residue is left on the soil surface, particularly when some of the residue is left standing, as in the case with no-till practices.
“This residue provides insulation, slows down the wind at the soil surface, and traps snow, which is an excellent insulator,” Fanning said.
Winter hardiness is a priority in the winter wheat breeding programs in the Northern Great Plains and winter hardiness ratings are a prominent characteristic in the winter wheat variety trial reports from SDSU. To view reports, visit: igrow.org/agronomy/wheat/winterwheat-variety-trial-results/. If there is concern about a variety regarding winter kill, Fanning said the recommendation is to plant in protective cover to improve winter survival.
“Again, winter wheat varieties with ‘good’ or better ratings for winter hardiness are able to withstand temperatures at the crown level down to about 5 degrees if adequately hydrated. The soil temperature at the 2-inch depth at one of the coldest locations in South Dakota dropped to near 10 degrees in early February for a few days, and has since come back up,” he said.
At this point, Fanning said all this is speculation, and the only way to tell if winter wheat is alive now is the bag test. In preparation for a bag test he recently conducted, the dry, loose soil was brushed away from the row, revealing green leaf tissue, a good sign. When the plants were carefully dug out of the frozen soil and exposed, the crowns appeared white and healthy, also a good sign.
To conduct the “bag test,” the frozen soil and winter wheat plants must be thawed and washed off to remove the soil, the leaves cut at about 1.5-inches above the crown and the roots just below the crown. The crowns are then placed in a plastic bag which is inflated, tied shut and placed in a lighted room, but not in direct sunlight.
After a few days, healthy crowns should show a halfinch or more of new growth, which occurred in bag tests of samples that were collected from several fields in central and south-central South Dakota recently. The inherent limitations of the bag test are that it constitutes a very small sample, so is very susceptible to error in properly representing the field, much less the area; is quite labor intensive; and is only an indication of the field’s condition at the time the sample is taken.
It should also be noted that if wind erosion occurs, soil can be lost, exposing the crowns to desiccation, and increasing winter kill potential.
“To accurately assess a winter wheat field, or the winter wheat in a given area, one will need to wait until the field or fields begin to break dormancy. Fortunately, that typically occurs early enough in the spring to make alternate plans if the stand is inadequate,” Fanning said.
For information on how to assess your winter wheat stand, visit igrow.org/agronomy/wheat/assessing-yourwinter-wheat-stand/. — WLJ