When is the best time to topdress winter wheat in this region? We have been working with timing studies since the mid 1980s, and we reviewed our research trials to try to make some sense out of the numbers.
We have data on 28 studies from 1987 to 2008. Over that period, we have compared an early application (early to mid February) to a late application (early to mid March) to measure the optimum timing for grain yield. After summarizing the data, we found in some years, timing did not make very much difference — the early February applications made almost as much as the early March applications.
In one study in 2005, the early application produced almost 15 more bushels of grain per acre than the late one. Rainfall records in 2005 showed a dry period from mid March to mid April. Evidently, there wasn't enough rain to move the nitrogen into the soil profile in time for the wheat plants to use it for growth and development.
Other years, the late applications made 8 or 9 bushels more than the early ones. This was certainly true last year, where late March applications were the most productive.
We assembled rainfall data over the study period (courtesy of farmer Maynard Cheek), and found a common denominator. Years with very wet conditions in March and April favored the March applications. Later applications during those years produced 8 or 9 bushels more than the early February applications. On the other hand, in years where March and April rainfall was less than normal, there was very little difference in grain yield between the early and late applications.
We believe this can be explained by nitrogen loss through denitrification and surface runoff. Denitrification is simply the loss of nitrogen in a gaseous form to the atmosphere. It is the most common form of loss in heavy soils with a low water infiltration rate. Most of our blackland and transitional greyland soils have water infiltration rates of less than two-tenths of an inch per hour. The top few inches of soil quickly become saturated during a heavy rain.
Warm, saturated soils create ideal conditions for a number of facultative anaerobes, which can use the oxygen off of the nitrate ion and release the nitrogen back into the atmosphere. This process is called “denitrification,” and can at times reach 10 pounds of nitrogen per acre per day. Heavy water runoff during this time can also carry soluble nitrogen away from the fields and into the waterways. We do not see these types of nitrogen losses in the drier years. This probably explains why there is little difference between the early and late nitrogen applications over a period of years, but there may be real differences in wheat yields, year to year.
As a producer, how do I use this information to maximize my profits? None of us have the benefit of a crystal ball so we cannot determine the wet years ahead of time. Over the past 20 years, we have been well served by targeting mid-February to begin topdressing our wheat and hope to be finished by the first week in March. In research plots, we have often produced as much wheat by topdressing in mid-March (past jointing) as we have in early March.
As far as nitrogen rates are concerned, 100 pounds of actual nitrogen per acre has been our most cost effective rate for the final nitrogen top dress. We have often seen small yield increases with rates in excess of 100 pounds of nitrogen per acre, but the yield increases have seldom offset the cost of the additional nitrogen. We have also not shown any differences in nitrogen sources; dribbled nitrogen solutions (32 percent), ammonium nitrate, and urea have all performed similarly.
It is customary for producers to put an insecticide in with their final top dress nitrogen and spray it over the foliage as an insurance policy for greenbug control. This “convenience” can significantly reduce yields if foliar burn occurs following the application. The table below summarizes a trial we conducted near Royse City in 1990.
The 32 percent sprayed application caused noticeable burn in this trial. The 9 bushel loss we measured here at a value of $5 per bushel would cost the grower $45 per acre, a hefty price to pay for convenience of combining the insecticide with the nitrogen.
Environmental conditions most likely to cause foliar burn are warm, dry, windy weather during or directly following application. Nitrogen applied in small droplet sizes is also more likely to cause burn than nitrogen in large droplet sizes. Our research suggests that any time you can observe burn following a sprayed nitrogen application, there is likely a measurable yield loss.
A few greenbugs have been observed in area fields, but there is no justification to spray every field just for “insurance.” The fields that warrant spraying for greenbugs or broadleaf weeds should be sprayed separately with an insecticide or herbicide mixed in water. Producers would be better served to dribble nitrogen solutions rather than spray them over the crop. As the weather begins to warm in mid-February, beneficial parasitic wasps will become more active and take out the remaining greenbugs without our intervention.