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Crops & Conservation Managing Winterkill/Winter Injury Winter wheat genetics, crop residue, seeding date
There are a number of factors that influence winter wheat winterkill or survival including cold acclimation, soil temperature at the winter wheat crown, standing and surface residues and their affect on soil temperature, varieties, seeding date and depth, and phosphorous fertilizer at seeding. Desiccation from low soil and plant water content and ice, which limits oxygen, can also result in winterkill. Plants that enter winter with well developed crowns (area at the base of the shoot from which secondary roots develop, see photo 3) are most desirable. Ideally, plants with 3 to 4 leaves that have a tiller or two are ready to achieve winter wheat’s maximum yield potential next spring. However, plants with 2 to 3 leaves by freeze-up are not usually disadvantaged. Let’s take a closer look at factors that affect survival. I will be referencing material from Brian Fowler, breeder at the Crop Development Centre, University of Saskatchewan, Saskatoon, Canada, one of the foremost authorities on winterkill in winter wheat. Other sources quoted are North Dakota and Minnesota Extension Service winter wheat publications. Cold Acclimation Cold or winter hardiness is not a static condition. For example, the Canadian variety Norstar’s minimum survival temperature is normally near 27˚?F at the beginning of September, but -2˚?F or lower by the end of October in Saskatchewan. Death of the crown tissue will result if the soil temperature falls below the plant’s minimum survival temperature. Maintenance of cold hardiness can occur provided crown temperatures remain below freezing and the plants have an adequate energy supply. Prolonged exposure of fully acclimated plants to winter temperatures above freezing results in a gradual loss of cold hardiness. Winter Wheat Crown Soil Temperatures and Varieties Table 1. Winter cereal minimum survival temperatures (MST).
The NDSU Extension Service winter wheat bulletin indicates that the length of cold temperatures at the winter wheat crown is also important (Table 2). Table 2. Maximum number of day’s winter wheat can survive at different crown temperatures.
Soil Temperature Management
Your variety selection pool broadens based on the field’s ability to retain snow following the seeding operation. When asked by a grower what variety they should seed, I generally respond first with the questions: What prior crop will you be seeding into, the stubble height and density, and the seeding method? Then I offer a group of varieties that will fit that system. Growers who use standing stubble and no-till can justify seeding less winter hardy varieties. For those of you seeding into soybean stubble, select those varieties with greater winter hardiness. Photos 1 and 2 depict the difference in winter wheat survival that surface residue can make in a winter when low temperature stress impacts survivability. Surface residues accumulated in Photo 1 from the preceding no-till seeded crops allowed the field to produce a profitable winter wheat crop, while the winter wheat field in Photo 2 was destroyed. Continuous no-till seeding allowed for the accumulation of surface residues in the field in Photo 1. The fields were seeded by the same grower within two days of one another with all the same inputs. Photo 1: Winter wheat seeded continuous no-till in flax stubble. Photo 2:
Winter wheat seeded no-till in conventionally seeded flax with straw removed. Through December 2006, the lowest soil temperature at the 2” soil depth at the ARS Lab near Mandan, N.D. had been 20ºF. At the DU/NDSU research plot east of Ellendale, Dickey Co. extension agent Gene Elhard reported the lowest air temperature at -13.4ºF, while the soil temperature at the 1” depth was 10.2ºF in the flax stubble with little soil surface cover. The wheat stubble’s low soil temperature was 14.3ºF at the 1” depth at the same time of reading. The difference of 4.1 degrees was due to crop residue on the soil surface. It is important to remember that the prior crop residues need to be managed to allow for proper winter wheat seed placement. A minimum of 6” to 8” tall stubble is recommended for prior small grain and flax crops. However, a 12” minimum stubble height is recommended for prior crops such as canola. A 12” to 16” stubble height is a good goal for all fields being planned for winter wheat. Seeding Date and Depth Photo 3 demonstrates the importance of seeding depth on plant and crown development. Plants 1 and 2, as identified by the numbers under the picture, were seeded at approximately a 1.25” and a 1.5” depth, respectively. They are both at about the 2.5 leaf stage with tillers just starting to form. The crown is very visible about ½” above the seed on plant 2. The crown on plant 3 is about 1” above the seed and is less developed than desired, but the seed was placed 2” deep and the plant is one leaf behind in development. Plants 4 and 5 – ¾” and 1” seeding depths respectively – have well developed crowns at the seeding depth and have 3 to 3.5 leaves with 1 to 3 tillers. This is the stage I strive to achieve prior to dormancy. Photo 3: Depth of seeding impact on seedling growth and development. Plant # 1 2 3 4 5 The minimum soil moisture required for germination of wheat is quite low. In contrast, temperature has a large influence on rate of seed water uptake, speed of germination, and rate of plant emergence. Increases in seeding depth result in delays in emergence that can be magnified by reduced soil temperatures associated with late seeding. For these reasons, it is usually advisable to seed at the optimum date as indicated by soil temperature regardless of the soil moisture conditions. Phosphorous Spring Stand Count Desired and Needed Table 3. Winter wheat yield at different stand densities (Lafond and Gan, 1999).
Standard recommendations are to establish 21 to 23 plants/ft² in the fall and to increase the desired stand to 23 to 25 plants when planting is delayed or seedbed conditions are unfavorable for rapid emergence. I generally recommend 25 to 28 plants/ft² to compensate for seeding problems, reduced soil moisture conditions, late seeding, and the fact that winter wheat seed is reasonably priced. If the stand is uniformly reduced in the spring, stands as low as 6 to 8 plants/ft² have produced remarkable yields. The recovery is very dependent on the environmental conditions when the winter wheat breaks dormancy and the period immediately following it. If stands are reduced, apply early nitrogen to encourage tillering of the remaining plants, and be aware that plant competition will be reduced and delayed. As a result, wild oat competition will need to be monitored. It seems that stand loss is usually in patches and a decision needs to be made as to reseeding, leaving them as is, or destroying the whole field. Many of the growers we work with seed the patches to spring wheat when the occasional loss occurs, unless the losses are extensive across the field. My experience tells me that losses are generally an exception rather than the norm if proper management practices are followed. Winterkill losses can be minimized by prior crop selection, managing the residue of the prior crop, seeding during the proper time period, seeding shallow (3/4” to 1.5”), using an adequate seeding rate, applying phosphorous as a starter fertilizer, and selecting the appropriate variety. Vander Vorst is regional agronomist with Ducks Unlimited, Inc., Bismarck, N.D. Find more information about DU’s agronomy program, including winter wheat
performance data and production information, online at
More information on winter wheat management online
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by Blake Vander Vorst
