Growers are always asking us when is the best time to top dress 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 found data on 14 studies from 1987 to 2003. Over that period, we have compared an early application (early February) to a late application (early 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. Other years, the late application made 8 or 9 bushels more than the early ones.
We assembled rainfall data over the same period (courtesy of Maynard Cheek), and found a common denominator. Years with very wet springtime 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 Blackland soils have water infiltration rates of less than 0.2 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, bacteria that operate in the absence of oxygen, which can use the oxygen off the nitrate ion and release the nitrogen back into the atmosphere. This is called “denitrification,” and can at times reach 10 pounds of nitrogen per acre per day. Heavy water runoff during this time can carry soluble nitrogen away from fields and into waterways. We do not see these types of nitrogen losses in drier years, which likely explains why there is little difference between the early and late nitrogen applications.
How should a producer use this information to maximize 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 produced as much wheat by topdressing in mid March (often past jointing) as we have in early March.
James Swart is an Extension IPM specialist, and Don Reid, professor and agronomist, at Texas A&M-Commerce.