Spring Nitrogen Management – Form and Timing

Most corn producers have planned their spring N program for 2015, and many have already started to implement their program. Such plans might include fall ammonia application, early spring application of ammonia or another form of N, or plans to apply all of the N at or after planting. In recent years there has been a trend towards more applications per crop, and it’s not unusual today to have N applied three or four times on the same field.

In 2014 we initiated a large study with funding (from the Illinois fertilizer checkoff program) administered by the Nutrient Research and Education Council (NREC) board. One of our goals is to compare yields from different N programs. These included fall versus spring N and early spring versus split N applications in on-farm trials, and comparison of 15 different ways to apply the same rate of N in the spring in small-plot trials at several of UI research centers.

June rainfall at the three sites where we ran these trials in 2014 ranged from 8 to 10 inches, or more than twice normal amounts. This might have meant above-normal N loss potential, though we did not have water standing on these plots. We chose to use 150 lb of N as the rate for comparison; this compares to the MRTN (N rate calculator) rate of about 160 lb for corn following soybeans in central Illinois and of about 140 lb of N in northern Illinois. Using a “medium” N rate was intended to help bring out differences in N availability to the crop.

Yields at the different sites were similar at this N rate, ranging from about 200 to 220 at Monmouth and DeKalb, and from about 215 to 240 at Urbana (Table 1). Included in these trials was a full set of N rates, ranging from 0 to 250 lb per acre using UAN injected at planting time. The maximum yield at Urbana was 238 bushels per acre, reached at 230 lb N; at Monmouth the maximum yield was 235 bushels per acre reached at 224 lb of N; and at DeKalb the yield reached a maximum of 223 bushels at 225 lb of N. The range of yields at the 150-lb N rate applied using different forms and timings included the maximum yield (from higher N rates) in the trials at Urbana and DeKalb, but not at Monmouth.

While there were considerable differences among sites in how treatments ranked in terms of yield, most of the N forms and application times we compared produced similar yields when averaged across sites (Table 1). Over the three sites, the highest-yielding treatment (urea plus Agrotain broadcast at planting) yielded statistically more than the five treatments that yielded 215 bushels per acre or less, while the second-best treatment (all of the N as UAN sidedressed at V5) yielded significantly more than only the two lowest-yielding treatments (ESN and UAN + Agrotain, both broadcast at planting).

None of the other treatments differed significantly from one another, in large part because they changed rank so much from one site to another. When this happens, it lowers the predictive ability of experiments like this, since we have no way to predict how a treatment that did well at one site but not another will perform at either site (or across sites) in 2015 or 2016, or in your field this year or in future years. This is why we do trials at different sites over several years.

The 2014 results do raise the possibility that few if any of these N form and timing treatments may, in the end, stand out as being consistently better or worse than another. This isn’t alarming, but it does provide a hint that the list of “acceptable” ways to apply N might turn out to be a little longer than we might have thought. While we need to be cautious about any predictions, this also hints that some of the treatments that we’ve considered should produce higher N use efficiency – such as sidedress or split N applications – might not always do so consistently.

The highest-yielding treatment – urea + Agrotain all broadcast at planting – has not been a common method of applying N in Illinois, and may not even be considered by some to be a sound method. That we saw it do well in 2014 in no way means that it’s the “best new” way to apply N. But with Agrotain as protection against loss of N from urea due to urease activity, with urea sometimes competitively priced as a source of N, and with the speed and ease of application, this practice could gain some traction if it continues to do well compared to other treatments. It probably makes sense to wait until we see more results before committing to it, although running some strips to compare it against another method of N application might be worthwhile.

It’s dangerous to speculate about why a treatment might have done well at one site but not another based on weather differences between the two sites. In part that’s because the weather among sites was reasonably consistent in 2014 – rainfall was normal or below normal in May and above normal in June at all three sites, July was cooler than normal, and there was little stress throughout the season. It’s also the case that the weather in 2015 will probably be different than in 2014, with some of our more imaginative speculation overturned as a result. Delaying all of the N to sidedress UAN or splitting 100 lb at planting with 50 lb at sidedress did much better at DeKalb and Monmouth than at Urbana, perhaps reflecting more loss from early-applied N at those two sites. On the other hand, dribble-applying UAN at planting worked well at DeKalb but not at Monmouth. It’s not likely that we would have been able to find such differences in either the plants or the soils back at the time of application.

The 2015 trials will include fall-applied NH3, and a fall-spring split. We also added a treatment in which we’ll apply some of the N as late as tasseling time. This is a practice that some seem convinced is on its way to becoming common, given recent observations that newer hybrids take up a greater percentage of their N after pollination than older hybrids did. It is not at all clear why, even if plants take up 40 percent of their N after pollination, soil that was fertilized with N early in the season would be unable to supply that amount. In fact, N mineralization rates in mid-season run 3-4 lb of N per acre per day in better soils, and this would be enough to provide at least 40% of total crop N requirement (of roughly 1 lb per bushel) over the six weeks following pollination, whether or not any fertilizer N were still present in the soil. We’ll see what the data tell us.

Most of us can take comfort from the fact that just about any method we choose for putting N on the corn crop is likely to work reasonably well, though no method is entirely safe from unusual weather or crop conditions. We only need to look back to 2012 to find a year when no method of applying N worked very well; when water (too much or too little) becomes the main limitation for a crop, things like N management may make little difference.

A sound N management program should, though, take costs into account – not just the costs of trips across the fields and of the fertilizer material, but also the indirect costs that include such things as the chance for yield loss or of more expensive forms or application methods we might need to use if we can’t get N on when we expected to. Most changes we are inclined to make in how we manage N today involve increasing the complexity, and this often comes at a cost in time, expense, or uncertainty. Such costs have to be covered by consistent improvement in yields.

Table 1. Yield ranks of N form and timing treatments at three Illinois sites in 2014. The N rate was 150 lb per acre for all treatments. Unless otherwise indicated, UAN was injected 2-3 inches deep between rows, and urea, SuperU, and ESN were broadcast-applied. PT = at planting time; AT = Agrotain®; dribble = surface placement between rows. There was no incorporation by tillage.