Issue No. 1, Article 10/March 23, 2007
Should We Keep That Wheat Stand?
Wheat prices have remained high since the crop was planted last fall, and some producers have sold their crop forward, so the incentive to grow wheat remains high. However, corn prices have risen considerably since then, and so the incentive to replace wheat with corn this spring has also strengthened. Planting conditions for wheat last fall were less than ideal, with delays due to wet soils and late harvest of the previous crop. Much of the Illinois wheat crop went into the winter with growth somewhat less than normal, especially where planting was late. Excessive wetness in places, followed by cold temperatures and uneven snow cover in February, contributed to the problems. Still, the crop has had some chance to recover, and present conditions are about average in most places, despite the earlier difficulties.
Regardless of its appearance, almost any wheat crop with an adequate stand at the time it greens up has the potential to produce a good yield. An adequate stand is one that will produce enough tillers to result in 60 or more heads per square foot. When fall planting and winter conditions are favorable, we normally say that 20 plants per square foot are enough for high yield, with 15 plants per square foot worth keeping if they're healthy, crop leaves are in good shape, and there is good potential for tillers to form.
Though tiller (head) number is the critical factor in determining wheat yield potential, trying to guess how many tillers will form per plant is not easy. If plants are healthy, if leaves cover most of the ground area coming out of dormancy, and weather is favorable (average temperatures and less-than-average rainfall, with a lot of sunshine) for several weeks after greenup, each plant can produce three or four productive tillers, so a stand as low as 15 plants per square foot might be adequate. If the plants are small coming out of dormancy, if the weather stays cold or turns warm too quickly, and if there is added stress from wet soil conditions, then there might be only one or two productive tillers per plant, so even 20 or more plants per square foot might not be enough for high yields.
The interaction of present crop condition and weather over the next few weeks suggests a cautious, wait-and-see approach to deciding whether to keep a wheat stand. In most cases, the crop should be given time to develop tillers before the decision is made. Tiller formation slows when the weather is warm enough for upright growth to begin. Upright growth ("jointing") usually begins in late March to early April in the southern half of Illinois and about mid-April in northern Illinois. If the crop recovers quickly to form good ground cover and good tiller numbers before it starts upright growth, then its potential is good. If it warms up early, jointing will start sooner, and tiller number may be reduced. If plants are small with few tillers as they come out of dormancy, there's much less chance that they will get enough favorable weather to produce a good canopy and good tiller numbers.
In most cases, the decision on whether a wheat crop is worth keeping can be made when the crop is 8 to 10 inches tall, by which time you can usually count what will likely become productive (head-bearing) tillers. Tillers likely to become productive will begin upright growth shortly after main tillers, and so will be nearly as tall as main tillers by the time the crop is 10 inches tall or so. Small tillers at this point have to compete with larger tillers, and so are less likely to be productive, especially if the weather is warm and growth is rapid.
One factor in the decision about keeping a wheat stand is the amount of unrecoverable cost already invested in the crop. For most people, the largest unrecoverable cost by far is seed, which can cost $40 or more per acre. Most of the nitrogen applied as top-dress during the winter should be available to a replacement corn crop, unless there is a lot of rainfall before the corn starts to take up most of its N, which is after it reaches the 6-leaf stage or so. If herbicide has not yet been applied to the wheat crop, it might be wise to wait to apply it until it's clear that the crop is worth keeping. Harmony Extra, widely used for wild garlic and broadleaf control, has a 45-day interval before corn can be planted.
Even if we can get a handle on yield potential within the next few weeks, we need to remember that yield potential is not always reached. The condition of the wheat crop in late March is not very well correlated with final yield, because the crop is so sensitive to weather conditions around the time of flowering and a few weeks after. If the weather is dry during the flowering and early grain-filling period, foliar diseases and Fusarium head scab pressure tend to be low, and yields are often good. On the other hand, we have seen that warm, wet weather around the time of flowering can wreak havoc on the wheat crop. The former scenario has been much more frequent than the latter in recent years, but if it is warm and wet in late April and early May when wheat is heading in southern Illinois, one might want to retain the option to destroy a diseased crop even that late and replace it with corn.
In most cases, using glyphosate to destroy an existing stand of wheat works fairly well, as long as the weather is favorable for growth. It might help to raise the rate some if wheat plants are close to heading. It is possible to take wheat off as forage, but regrowth after cutting an immature wheat crop can be difficult to control. In some recent work in DeKalb, Jim Morrison and Lyle Paul found that wheat about a foot tall (in early May at that location) had only about a ton of dry matter per acre. We would expect that to double by early boot, so it may or may not be worth the expense of harvesting as forage. If the crop is killed and left standing in the field, with the replacement crop no-tilled, be sure to check for the presence of insects (such as armyworm) in the residue. The loss of the wheat host will have such insects very hungry when corn plants emerge.
Where double-cropping is possible, getting a good double-crop is critical to profitability. Unfortunately, double-crop yields are highly variable over years, with no guarantee that there will even be enough moisture at (or after) wheat harvest to get and maintain a good stand of soybeans. Harvesting wheat at grain moisture near 20% (rotary combines work best for this) and drying the grain might allow earlier planting of soybean, which can greatly improve the chances of double-crop success. Wheat harvested at such high moisture needs to be aerated quickly to prevent spoilage at the high temperatures common during wheat harvest.
Some might be considering planting corn or grain sorghum instead of soybean as a double-crop. Neither of these crops is as "proven" as soybean when planted this late, but both might have some potential, especially if planting can be earlier than June 20 or so. While high grain price would be a reason to use one of these crops, disadvantages include high seed and fertilizer cost (especially for corn), sensitivity of sorghum to cool temperatures late in the (delayed) season, less tolerance of corn to periods of dry weather, and the possibility of high insect pressures in late-developing corn.--Emerson Nafziger