Issue No. 22, Article 5/September 7, 2007
Producers in Illinois planted 970,000 acres of wheat in the fall of 2006. The freeze of April 2007 resulted in the loss of 160,000 of these acres and damage to most of the rest, but with outstanding spring weather, the remaining acres produced an average of 57 bushels per acre. That's 10 bushels per acre less than the record yield in 2006, but the current price of wheat has renewed some interest in planting wheat this fall. That's especially true in areas where wheat did relatively well compared to corn and soybean.
With harvest of both corn and soybean coming on fast in the southern half of Illinois, many producers will have a choice of whether to plant wheat after corn or after soybean. Planting after soybean can be easier, due to less crop residue and, in some cases, to more uniform, and mellower, soil conditions. Our research in western Illinois has shown a consistent yield advantage of 2 to 4 bushels where wheat follows soybean compared to wheat following corn. This difference might be less in southern Illinois, especially where lower corn yields mean less crop residue and some carryover nutrients, primarily nitrogen. Make sure that herbicides applied to the previous crop, and the rate and timing of application, allow wheat to be planted this fall.
There may be some cases in which planting wheat after wheat is necessary. This is not ideal due to the threat of diseases that can carry over. But if the field is not weedy and seed can be placed well, it is usually acceptable. It is critically important to destroy any volunteer wheat in such fields to prevent carrying viral diseases over to the new crop. Scouting for diseases is also useful in such fields, though not all diseases can be controlled with fungicides if they are found.
Wheat responds more than corn or soybean to phosphorus (P), which should be applied if soil test levels are not high. If P levels are known to be high, it is possible to skip P application for a year. Wheat is less responsive to potassium (K) than corn or soybean, so K application probably is unnecessary if soil test levels are adequate. The wheat crop benefits from a small amount of nitrogen at planting time. If wheat follows a corn crop that produced less-than-average yield in a dry growing season, there will often be enough carryover N to meet the needs of the wheat crop in the fall. Otherwise, 20 to 30 lb of N per acre should be applied before planting.
The decision of whether to till the soil before planting wheat depends on the evenness of the soil surface, distribution of residue, and the ability of the drill to place seed to a uniform depth in the soil. This last consideration--the ability to place seed to a uniform depth of about 1 to 1-1/4 inches--is the most important. If it is not possible to do this without tillage, then tillage might be necessary. While tilling on sloping soils is not ideal, wheat usually establishes cover within a month or so, and heavy rainfall is not typical during the fall, so soil loss is usually minimal. In studies in western Illinois, tilling has often increased wheat yield by 3 to 5 bushels, especially when wheat follows corn. Because soil temperature is not much of a factor in the fall, we think that much of this difference is due to better seed placement following tillage.
Many wheat varieties available from commercial companies are tested in University of Illinois trials, with results at vt.cropsci.uiuc.edu. It is also possible to save seed from a previous crop for planting, though this might be limited in some cases by patents on varieties. Finding seed cleaners has gotten more difficult, and seed from the bin is usually not cleaned, treated, and tested like commercial seed. For many producers, using seed from the bin is thus not a very good gamble.
Wheat should be planted at dates that result in plants that tiller some in the fall, without getting so large that winter survival is compromised. The Hessian fly-free date, which ranges from mid-September at the Wisconsin border to mid-October in the southern extremity of Illinois, is based on average date of first frost, and it is usually the best time to plant winter wheat. In April's freeze, the greatest amount of damage was on wheat in southern Illinois that had been planted more than a week before the fly-free date. While we can sometimes "get away with" such early planting, it seldom results in higher yields, and extra fall growth puts the crop at greater risk for both winter injury and more viral disease carried into fields by aphids.
The target for planting is to establish about 25 to 30 plants per square foot, with uniform emergence. Using a well-adjusted drill and planting about 35 plants per square foot will usually provide a good stand. That is 22 seeds per foot of row, or about 1.5 million seeds per acre. If there are 15,000 seeds per lb, 1.5 million per acre means 100 lb of seed per acre. Adjust this for different seed sizes. If planting is delayed by more than two weeks after the fly-free date, increase the seeding rate by about 5% per week to make up for reduced tillering potential.
Seed treatment fungicide is not very costly, and it provides insurance against some seedling diseases. It will also help to control loose smut, which is carried in infected seed. Seed treatment insecticides--Cruiser or Gaucho--have given good returns when the disease barley yellow dwarf virus (BYDV), which is carried by aphids, appears in fields. The chances of BYDV infection are greater with early planting, when there is volunteer wheat in the same or nearby fields to serve as a disease reservoir, and in southern Illinois, where aphids often live longer into the fall and appear earlier in the spring. Based on our results in recent years, a seed-treatment insecticide should pay for itself in most fields in southern Illinois in most years. Fields can also be scouted in the fall and sprayed with insecticide if aphids are found, but it can sometimes be difficult to spray in time to prevent BYDV infection.
Some producers use herbicides on wheat on the fall, especially if fields are no-tilled. This can be very helpful if there are winter annual weeds that can be controlled, and it also helps with weeds like dandelion, in which fall control is usually better than spring control. In tilled fields, fall-
applied herbicides are often unnecessary if few weeds emerge with the wheat crop.--Emerson Nafziger