Issue No. 10, Article 6/June 11, 2010
Providing Nitrogen to Rapidly Growing Corn
Warm temperatures along with good moisture in some parts of the state are making corn grow quickly. Also growing is anxiety among producers who have not yet applied all the nitrogen the crop needs. Overall, side-dress application of nitrogen is a good method of applying nitrogen, and from an agronomic standpoint there are no drawbacks. Many studies have shown that side-dress applications can, and often do, produce the highest yields.
Nitrogen uptake rate is highest when corn is around knee-high to shoulder-high. Doing a side-dress application during that time makes sense because the application provides nitrogen when it is most needed by the crop. In other words, the demand for nitrogen is met by the application. Some might say that nitrogen use efficiency is at a maximum with side-dress application. It is not my objective to discuss that here, but talk about efficiency among farmers always ends up with this question: If the application is more efficient, does that mean I can get the same yield with less nitrogen? I don't know of any studies that have shown a consistent response to support the notion of applying a lower rate with side-dress compared to preplant applications.
As I indicated in the May 14 issue (no. 6) of the Bulletin, all major sources of nitrogen have an equal impact on corn yield when applied at side-dress time, and the most important consideration when selecting a source of N and the method of application is the potential for crop injury. The information in the earlier article is still relevant for current conditions. Since soil is still a little too wet in some locations, producers are looking at alternatives to apply at least a portion of the N needed to help their crops (which in some cases are starting to look a little pale) and to buy some time in hope of better soil conditions for injection applications. Most farmers are considering among several options to supply some N for the time being: broadcast application of urea, because it can be done quickly; applications of UAN along with a herbicide, to save a trip over the field; and foliar products.
Broadcast applications of urea normally work pretty well until the crop is about knee-high, but the sooner the application is done the better to reduce the chance of granules entering the whorl. While granules might discolor the leaves in the whorl, research has not shown yield reductions with this management practice when it is done early in corn development. Beware, though, that the longer you wait to make the application or the higher the rate of application, the more chance you have of crop injury.
While it can result in saving time and trips across the field, I strongly discourage UAN solution application as a carrier for postemergence corn herbicide. Research has shown that application of UAN alone can cause injury. When UAN is combined with a herbicide, even with lower rates of nitrogen application, the probability of crop injury increases. If you are applying a post¬emergence herbicide and your corn is pale and in real need of some N, I would not apply more than 10 lb per acre with the herbicide, and I would do it early in crop development rather than later. That small amount of nitrogen likely will not cause much injury and would help the crop green-up until soil conditions are better for additional nitrogen applications.
Having said that, however, I remind you to always read and follow the label for herbicide applications. Many postemergence herbicide labels allow the use of either ammonium sulfate or UAN, but usually the rate of UAN specified on the label is very low, typically no more than 2 to 4 quarts.
Regarding the idea of applying various foliar products (which I don't consider to be the same as side-dress applications) to help green-up the crop: often foliar products are much more expensive than traditional fertilizers, and they won't help the crop enough to pay for the added cost. Some companies are promoting applying foliar fertilizer to small corn plants or plants that are not growing very fast because they have received too much water. If the plant is small, there isn't much leaf surface area to apply the fertilizer to. If the plant is stressed because of excess water, the problem that exists won't be solved by simply applying nitrogen. In either case, foliar applications don't make much sense and they cost too much money. Finally, some people maintain that foliar applications of nitrogen are more effective at providing nitrogen than traditional applications providing nitrogen through the root system. The truth is that very little nitrogen is absorbed through the foliage, and foliar applications are not more effective at providing nitrogen than traditional nitrogen fertilizers.--Fabián G. Fernández