By Dennis Pennington and Martin Nagelkirk
Achieving top yields requires a uniform stand of healthy seedlings. This is dependent on seeds being dropped as evenly as possible and at a uniform depth. Good seed placement, in turn, requires that fields are appropriately prepped, and planting equipment receive disciplined inspection, necessary adjustments and deliberate calibrations.
Wheat establishment can be successful under conventional, minimum tillage and no-till systems. Generally speaking, no-till has won favor in recent years. It tends to result in more unevenness in the stand, but it can often provide improved moisture retention and less susceptibility to cold temperature damage. Tillage, even at a minimal level, can be helpful in distributing and incorporating residue, fertilizer and lime; and create a more uniform seedbed. Tillage can also be useful when attempting to reduce disease inoculum borne in crop residue (e.g. corn stubble or stalks infected with Fusarium).
Ideally, winter wheat is planted while the soil and air temperatures are still warm to ensure that seedlings can emerge quickly and in plenty of time to develop a couple of tillers and a strong root system. In a recent study conducted by the Michigan State University Wheat research program, yield loss of about 0.6 bushels per day occurs when planting after Oct. 1.
While the Hessian fly no longer poses a significant threat to wheat in Michigan, the fly-free-date is still a useful reference (Table 1). The standard fly-free-date is during the first week of September in the northern Lower Peninsula, around mid-September in mid-state areas and approximately the third or fourth week of September for southern Michigan. Highest yields are often attained when seedlings emerge within two weeks following the posted fly-free-date, assuming heat unit accumulation is near normal in October and November. When wheat is planted within a few days of the fly-free-date, seeding rates and fall-applied nitrogen rates should be significantly reduced to avoid excessive growth. The goal is to plant early enough to achieve two to three tillers produced prior to the winter vernalization period.
Table 1. Hessian fly-free-dates for Michigan
Oftentimes, weather conditions make it difficult to plant wheat on time. How late a wheat crop can be planted is really a question of how much risk a grower is willing to take. If a grower expects to have the crop insured, then the answer is straightforward because Oct. 25 is the last planting date for crop insurance eligibility. For those trying to estimate the odds of achieving a reasonable yield, it is important to recognize that the challenge to late wheat is not only the inherent constraints on grain yield, but also its greater susceptibility to winter injury.
Planting wheat on time starts with the previous crop. Soybean fields that are to be planted to wheat in the fall can benefit from management considerations such as optimum planting time and maturity group (MG) selection (Figure 1). Selection of late-maturity soybean varieties (e.g., 3.0 versus 2.0 MG) before May 15 planting can lead to 5-8 bushels per acre increase in soybean yield, while still reaching maturity by late-September. This system would provide greater soybean yields while allowing adequate time for an optimum wheat planting date after soybean harvest. However, for later soybean plantings (after mid-May), early-maturity varieties can be used without a soybean yield penalty while allowing adequate time for wheat planting.
Figure 1. Impact of planting date and maturity group selection on soybean seed yield (bu/acre, left panel) and date to reach R7 stage (one mature pod on the main stem, right panel). Add 15-20 days to R7 to estimate harvest date. Data is average of four Michigan site-years.
Attaining a consistent seed depth is important in order to increase the probability of even emergence. Usually, a planting depth of 1 to 1.5 inches is enough in heavy soil. Deeper seed placement may have an advantage when some types of winter stresses occur, but usually this is outweighed by the advantage in more rapid emergence posed by more shallowly placed seed. The exception may be where a coarse soil is very dry. In this case, seed should be planted as deep as possible to reach moist soil.
The recommendation is to plant between 1.2 and 2.2 million seeds per acre. Seeding rates on the lower end of the range should be used when planting within a week of the fly-free-date to avoid overly thick stands that can promote disease development and increase the likelihood of lodging the following season. As the calendar advances, seeding rates should become progressively higher. If planting continues into the second half of October, the seeding rate should be increased to at least 1.8 million seeds per acre.
Table 2 identifies the pounds of seed needed based on the number of seeds per pound and your population target. For example, if seed size is 12,000 seeds per pound and the target seeding rate is 1.4 million seeds per acre, then 117 pounds of seed would be needed per acre. Table 3 is useful for assessing the number of seeds being dropped by each row unit (7.5-inch row spacing) and for evaluating actual seedling density. So, with the 1.4 million target, a 7.5-inch drill would drop approximately 20 seeds per foot and, assuming a 92% emergence rate, lead to an estimated 18.5 wheat seedlings.
Table 2. Relating seed size and target seeding rates to the number of pounds required per acre
Target seeding rates (millions of seeds per acre)
Actual pounds of seed required per acre *
*Target seeding rate divided by seeds per pound = required pounds of seed per acre.
Table 3. Relating target seeding rate per acre to seed and seedling numbers (for 7.5-inch row spacing)
Seeding rate (millions/ac)
Seeds per foot of row
Seedlings* per foot of row
*Projected number of seedlings based on an estimated declining emergence rate as percent.
Source : msu.edu