Heavy rainfall over the past several weeks has left many producers across the state with few opportunities to side dress their corn with nitrogen. To make matters worse, excessive water means that significant soil nitrogen has likely been lost through denitrification and/or leaching. It’s not uncommon or surprising to see standing corn crops with severe yellowing, indicating some level of nitrogen deficiency. Most of the corn in the state has grown too tall for standard application equipment to pass over without crop damage, and some corn is entering late vegetative stages. Given all this, when is it too late to fertilize corn with nitrogen?
Numerous studies have shown that late season nitrogen rescue applications often pay off. Studies from the University of Missouri and elsewhere have shown that broadcast applications of urea or dribbling on 28% UAN are effective strategies for providing corn with late season nitrogen. Some leaf burn may occur with broadcast urea, but this translates into very little yield loss, if any.
But how late is too late? Trials applying nitrogen as late as tasseling show that this can still be very effective, but after tasseling application has yielded mixed results. Nitrogen uptake slows considerably after tasseling, as the plant shifts focus from soil nitrogen uptake through roots, to translocation of stored nitrogen in vegetative parts into grain.
And what about the rate? If no nitrogen has been applied via side dress, full or nearly full nitrogen rates are recommended. Although it is difficult to determine how much soil nitrogen is lost with wet conditions, additional nitrogen may be applied to counter the losses in those extremely wet conditions.