Stubbled-In Winter Wheat Seeding Date and Depth
Winter wheat survives the winter in the seedling stage. In order for winter wheat to attain maximum cold tolerance and produce optimum energy reserves for the following spring, healthy vigorous plants must be established before freezeup. Seeding date and depth both affect plant establishment. Therefore, they have a large influence on the degree of success that can be achieved in the production of winter wheat.
The main factor dictating seeding date is fall soil temperature. For this reason, optimum seeding dates differ among production areas in western Canada (Table 1). In general, optimum seeding dates are earlier as one moves north, and to a lesser extent east, on the prairies.
Winter hardiness peaks at or near the optimum seeding date ( Fig. 1 ). Seeding too early can result in excessive growth in the fall and plants that are less resistant to winter injury. However, early seeding is usually not a problem with no-till seeded (stubbled-in) winter wheat as removal of the previous crop rarely occurs before the optimum date for seeding. Late dates of seeding usually result in poorly established plants that have lower winter-survival potential. Plants that enter the winter with well developed crowns (area at the base of the shoot from which secondary roots develop) are normally most desirable. However plants with two to three leaves by freezeup are not usually disadvantaged.
|Lethbridge, AB||September 9|
|Maple Creek/Estevan, SK||September 6|
|Kindersley/Swift Current, SK||September 3|
|North Battleford/Saskatoon/Wynyard/Yorkton, SK||August 30|
|Meadow Lake/Prince Albert/Nipawin, SK||August 27|
The reductions in winter survival potential with early and late seeding dates are large, but their impact should not be overemphasized. A uniform snowcover that is at least four inches deep (10 cm) should, for most winters, provide sufficient protection to overwinter cultivars with Norstar levels of winter hardiness when they are seeded four to five weeks later than the optimum date. (See Chapter 12 for a more detailed discussion of the effect of seeding date on winter survival).
In addition to influencing winter survival, stage of plant development prior to the onset of winter is of considerable importance in determining agronomic performance the following growing season. Seeding too early often results in a yield reduction (Fig. 2) and smaller seed size. Seeding too late usually results in a significant yield reduction (Fig. 2), delayed heading, later maturity (Fig. 3), reduced bushel (hectolitre) weight and increased problems with weeds.
Figure 2. Influence of seeding date on yield of winter wheat. See Table1 for optimum seeding date.
Figure 3. Influence of seeding date on maturity of winter wheat. See Table 1 for optimum seeding date.
The above observations represent the average response to seeding date for stubbled-in winter wheat. However, considerable variability about the average can be expected ( Fig. 1 and Fig. 2 ) and often this range of response is of concern to the producer. Variables such as soil temperature, soil moisture and seeding depth all interact, making it difficult in some instances to predict rate of germination and seedling establishment. In addition, weather conditions the following growing season can add to the uncertainty by favoring seeding dates that are normally suboptimal. For these reasons, responses to seeding date cannot always be determined by simply looking at the calendar. Admittedly, most of these sources of variability are beyond the control of the producer. However, an understanding of the relative impact of variation in these factors can help in decision making and thereby increase the chance of successful winter wheat production.
Winter wheat undergoes two important physiological changes in the fall. The processes that bring about these changes are known as vernalization and cold acclimation. Vernalization is required before heading will take place the next summer. If seeding takes place after the optimum date, vernalization will be affected and maturity delayed
Four to five weeks growth at temperatures higher than those required for vernalization and cold acclimation is necessary to ensure that plants have sufficient energy reserves available for a quick start in the spring. Seeding when maximum afternoon soil temperature is approximately 18°C
Table 2. Average temperature at a soil depth of 2 inches (5 cm) in stubble fields in western Canada for the 6 week period starting at the optimum seeding date.
|Weeks after optimum date|
|Soil temperature (°C)||0||1||2||3||4||5||6|
|Soil temperature (ºC)||25||20||15||10||5|
|Days to emergence||4||6||8||12||30|
Stubble fields from which crops have just been harvested are often low in soil moisture. In these situations, the winter wheat grower is left with the dilemma of deciding whether to seed immediately or to wait for rain.
The minimum soil moisture required for germination of wheat is quite low. In fact, germination has been observed in soils where the moisture level has been less than the permanent wilting point (soil moisture so low that established plants wilt and do not recover under humid conditions at night). Under these very low soil moisture levels, seeds often take up moisture and cell division starts. Cell elongation may be delayed, but once soil moisture improves, growth is very rapid. For this reason it is usually advisable to seed at the optimum date as estimated by soil temperature unless the soil is extremely dry.
Under dry conditions it is important to ensure that the seeding operation leaves the seed covered with well-packed soil. Loose soil fill is subject to greater drying, and seeds that are not covered will often fail to germinate even when moisture conditions are favorable.
Under optimum conditions, winter wheat should be seeded less than one inch (2.5 cm) below the soil surface into a firm, moist seedbed. Deeper seeding requires additional horsepower to pull the drill. It delays emergence and results in weak, spindly plants that are more susceptible to winter damage ( See Chapter 12 ). Improper seed placement also results in crops that are later maturing and lower yielding. The negative effects of deep seeding are more pronounced with late seeding dates because of slower plant emergence in cool soils ( Table 3 ).
The soil moisture in most stubble fields has usually been depleted by the previous crop, leaving a very dry seedbed for winter wheat. Because soil moisture in a stubble field rarely improves with depth, there is little to be gained by seeding deeper than the minimum required to give good coverage of the seed with soil. Shallow seed placement also allows the germinating seed to take better advantage of soil moisture provided by fall rains. Under poor moisture conditions in the early fall, as little as 1/3 inch (0.8 cm) of rain is often enough to successfully establish winter wheat that has been seeded less than one inch (2.5 cm) below the soil surface.
Seeding depth should be adjusted each time a new field is started to ensure seed placement is shallow. However, even with careful adjustment, the emergence distance (true seeding depth) for winter wheat seedlings is often difficult to establish with accuracy at the time of seeding. Drills often cause considerable soil disturbance, especially if they are equipped with wide, blunt openers or when fertilizer banding is part of the seeding operation. Subsequent furrow cave-in often results in a burying of the seed under more soil than the drill operator anticipated when adjusting for seeding depth. Seedling emergence distance, or the true seeding depth, can be determined prior to freezeup in the fall by digging up winter wheat plants and measuring the distance between the seed and the stem region where chlorophyll (green) first appears ( See Chapter 12 ).
Considerable emphasis has been placed on the effect that weather conditions have in modifying the influence of seeding date. Weather conditions also modify the effects of seeding depth. A mild winter, cool, moist spring, and the absence of heat, wind and drought stresses during the summer all favor winter wheat growth and development. Environmental conditions that favor winter wheat growth and development will minimize the losses associated with seeding at suboptimal dates and/or depths.