Posted on Apr 13, 2017 by Emerson Nafziger
Most corn producers have made plans on how to supply the 2017 Illinois corn crop with nitrogen. But with the stakes high, unusually early N application this past winter and early spring, the delay in fieldwork due to rainfall over the past week, and ongoing pressure to “get nitrogen right,” some might be rethinking plans as the season gets underway.
I presented a webinar on the topic of spring N management on March 30, 2017; the link to the recording can be found at https://ifca.com/. In this article we’ll look at some of the data presented during the webinar and will discuss what these findings mean for spring-applied N. This work is funded by the Illinois Nutrient Research and Education Council, using fertilizer checkoff dollars.
Is fall-applied N still present?
A first question for those who applied N last fall is whether the N is still present and how much of it has been converted to nitrate. Dan Schaefer of IFCA and his group sampled soils at three on-farm sites in mid-November, mid-December, late January, and early March, following application of 200 lb. N as NH3 with and without N-Serve in late October last fall. The amount of N recovered from the top 2 feet of soil hasn’t changed; 240 lb. N was recovered on December 16 and 238 lb. on March 3.
Nitrate as a percentage of the N recovered increased some over the winter, from 55% nitrate in December to 67% nitrate in early March. In 2016, about 60% of recovered N was nitrate when soils were sampled in March, and about 70% was nitrate in April samples. So from what data we have, it appears that, at least in years with relatively mild winters, we can expect more than half of the N to be converted to nitrate by April. Using N-Serve in the fall hasn’t consistently lowered the percentage of nitrate in spring samples, though variability in the samples makes this an imprecise measurement.
Is having most of the fall-applied N in the nitrate form by planting time a problem? Not unless the conditions are conducive to N loss before crop uptake begins. At Urbana, nitrate as a percentage of recovered N reached 80% by early May, and was above 85% by early June in both 2015 and 2016. The amount of soil N recovered stayed constant during May; any N that might have been lost from the soil plus N taken up by the crop didn’t exceed the amount of N provided by mineralization. Most importantly, the N was still there when crop uptake began.
In comparison to fall-applied N, N applied as NH3 before planting in 2016 had low nitrate initially, then nitrate percentage increased steadily through most of May, reaching 80% of recovered N by early June. While this longer retention of ammonium in the soil is a positive in that ammonium doesn’t move and nitrate does, whether or not this affects the amount of N available to the crop in June depends on whether or not soil conditions are favorable for N loss (that is, wet) during May and into early June. If that happens in 2017, our N tracking project should be able to measure changes in soil N, and we’ll make those results available.
Choosing nitrogen rates
While it’s easy to get caught up in questions of N timing and form, we first need to decide how much N to use. The 2016 season brought normal to below-normal June rainfall, little N loss, and high rates of mineralization; as a result, relatively low N rates produced relatively high yields. Adding the 2016 data to the database that powers the N rate calculator (at http://cnrc.agron.iastate.edu/) actually brought the Illinois rates down by a few pounds of N. At current corn and N prices, guideline rates for corn following soybean are 154, 172, and 179 lb N per acre in northern, central, and southern Illinois, respectively, and 200, 200, and 189 lb. N per acre for corn following corn.
The calculator guideline rates and the “profitable” N rate ranges found there represent a good starting point for determining N rate for corn in 2017. The calculator uses actual N response data from hundreds of trials to come up with guideline rates. The calculated rate may not be exactly what is required for a given field, though it takes an N rate trial in the field to know that. Some 60 to 65% of the trials in the database have “best” (most profitable) N rates that are lower than the overall best rate. So an N rate trial in a given field is more likely to show a best rate that’s lower than the guideline calculator rate than it is to show one that’s higher than the guideline rate. Choosing high rates in order to be “safe” carries both economic and environmental costs.
Will the crop run out of N?
One concern that seems to have increased in recent years is the fear that the corn crop will run out of N at some point during the season, even if enough N is applied early. In fact, it’s rare to have the crop run out of N during pollination and later (grainfilling) stages when enough N was applied early in the season and leaves have good color at tasseling time. In 2016, nearly every field had good color at tasseling time.
Any N deficiency symptoms that appear during second half of the season are almost always due to having soils too dry, or, less commonly, too wet; such symptoms almost never come form having too little N in the soil. Water uptake is needed to bring N to the roots and into the plant; under dry conditions, water uptake slows or stops, and so N uptake slows or stops. The “firing” that starts with lower leaves during dry periods is completely due to lack of water, and adding extra N to the soil before the crop fires will do nothing to alleviate it. Only water can fix this problem, and leaf area that fires usually doesn’t come back to healthy green. Under very wet conditions, roots function poorly and may be unable to take up adequate nutrients, including N. Roots standing in water are also unable to sustain the plant in ways unrelated to nutrient supply.
So even if we apply enough N, might the crop still run out of N if yield potential turns out to be higher than expected? Again, we see no evidence of this. The crop typically contains a maximum (a few weeks before maturity) of 0.9 to 1.0 lb. N per bushel of yield, so we know that high yields require that the crop take up more N. But we also know from N rate trials that yields of 225 to 250 bushels are often produced at N rates as low as 150 lb. N per acre or less. The extra N in such fields comes from mineralization of the N contained in soil organic matter. Fields and parts of fields with higher organic matter typically produce higher yields as well as more mineralized N, making it easier for the N needs of the crop to be met. In 2016, we saw yields as high as 180 bushels per acre where no fertilizer N had been applied. It is not at all unusual to have the soil provide 150 lb. or more of N to the crop. In lighter soils with lower organic matter, we would expect this amount to be lower, though yields without fertilizer N can be surprisingly high.
One idea being marketed today is to test or model soil N during vegetative development and to apply more N if the test shows low soil N levels. This seems to make sense, but we don’t have good guidelines to tell us how much N needs to be in the soil at a certain stage of crop development to assure that there’s enough for the rest of the season. Soil N levels drop fairly rapidly as N is taken up by the crop. In 2016, we found that during the 18 days before tasseling, soil N levels dropped by about 3 lb. N per acre per day, to less than 10 ppm nitrate in the top 2 feet of soil, without having the crop ever show deficiency symptoms on the way to high yields. Over this same period, the crop took up almost 6 lb. of N per acre per day, about twice the rate at which soil N disappeared. Mineralization presumably made up the difference. Much of the N in the soil is in the ammonium form, especially when soil N levels are low, so nitrate levels, which are often used to measure soil N, can be as low as 3 or 4 ppm as the corn approaches pollination without any cause for concern.
We know from N uptake studies that some 70% of the crop’s N requirement is taken up by pollination, with uptake rates as high as 6 to 8 lb. N per acre per day right before tasseling, and averaging perhaps 5 lb. N per acre per day for the 30 days before tasseling under good conditions. N uptake rates slow after that, to maybe 2 lb. N per acre per day after pollination to 1 lb. per acre per day or less by mid-grainfill. Mineralization rates may be high enough to supply most of the N the crop needs to take up after pollination, with little need for N supplied (earlier) as fertilizer.
As another way to look at the question of running out of N and the need to apply N late, we conducted N rate studies at several locations in 2016, in which we either applied all of the N at planting or all but 50 lb., which we then applied by dribbling the N solution at the base of the row at tasseling. Figure 1 below shows results from this study at Urbana with corn following soybeans, and Figure 2 for corn following corn. Results were remarkably consistent at the different sites where we had these trials in 2016; optimum N rates and yields at those rates were the same whether we applied all of the N early or kept 50 lb. back to apply late. We may see different results in 2017, but in 2016, keeping back some N to apply into tall corn in mid-season did not cover any of the cost that such an application would incur.
N form, timing, and additives
A major part of our NREC-funded nitrogen work in the past three years has been an evaluation of different ways to apply N to the corn crop. One part of this was a comparison of fall- and spring-applied N, using anhydrous ammonia over a range of rates. Dan Schaefer of IFCA conducted these studies as replicated, field-scale strips in farmer fields. Over ten site-years, it took 18 more lb. of N (169 versus 151) to produce about one less bushel of yield (219 versus 220) using fall-applied N compared to spring-applied N. At current prices, spring-applied N netted $11 per acre more than fall-applied N at the optimum N rate for each. Those are small differences; using guideline N rates (which are higher than optimum rates we found) would have produce virtually identical yields whether the N was applied in the fall or in the spring. Given that we often see a little more loss of N through drainage tile with fall application, those able to apply in the spring may see small gains in terms of better efficiency and less loss of N.
We also evaluated the effect of applying all of the N as UAN at planting versus a split application, with 50 lb. of N at planting and the rest applied using UAN at sidedress. Averaged over ten site-years, optimum N rates and yields at those rates were very similar for these two methods (Figure 3). Splitting the application required 9 lb. more N and yielded 1.6 bushels more, so netted about $2.50 per acre more than applying all of the N at planting. Unlike the fall- versus spring-applied N study, though, optimum N rates in the sidedress study were a little higher than the guideline (N calculator) rates; using guideline (lower rates) would have given a slight edge to planting-time N.
As part of N rates studies completed so far at 10 site over 3 years, we applied the same N rate (150 lb. per acre) using a variety of N forms, timing, and additives. Among the 15 treatments in these trials from 2014 through 2016, only 10 bushels per acre separate the highest from the lowest yields (Table 1). The two highest yields came from applying dry urea with Agrotain® (urease inhibitor) or as SuperU® which incorporates both urease and nitrification inhibitors. We did not include urea without an inhibitor, so do not know how much the inhibitors contributed. Other treatments that yielded more than the average included UAN injected at planting (our designated “check” treatment), 100 lb. N at planting followed by 50 lb. UAN, either injected at V5 or dribbled mid-row at V9, and UAN all injected at V5.
Yield averages not followed by the same letter are significantly different; seven of the 15 treatments did not yield significantly less than the highest-yielding treatment, and five treatments did not yield statistically more than the lowest-yielding treatment. The lowest-yielding treatments included UAN with Agrotain broadcast at planting; UAN dribbled between rows at planting or at V9; and NH3 injected at or before planting, with or without N-Serve®. As an observation, treatments with lower yields were those that included surface application of UAN or application of N in a way that likely meant some delay before plant roots could get access to the N. There may have been some loss of surface-applied N to volatilization, but N broadcast as UAN on the surface may also not have moved down to the roots quickly.
We added several treatments after 2014, and because the 2015 and 2016 seasons differed considerably in June rainfall, we’ll look at the data for 2015 and 2016 separately, across three sites in 2015 and four sites in 2016. With the inclusion of seven of the ten site-years averaged in Table 1, of course, yield levels and trends were similar to those that included the 2014 date. Only 12 bushels per acre separated the highest- and lowest-yielding treatments, and the designated check (150 lb. N as UAN injected at planting) produced 221 bushels per acre, higher than six of the 19 treatments and not statistically less than the highest-yielding treatment (Table 2).
Of the four treatments added in 2015, UAN with Instinct II® (nitrapyrin) injected at planting produced below-average yields, though not statistically less than that of the check (UAN injected at planting.) The other three added treatments included 100 lb. N as UAN injected at planting followed by split-applying 50 lb. as UAN. Dribbling UAN into the row at V5 was a very good treatment, yielding only 2 bushels less than the highest yield. The last two treatments including dribbling the split N between rows or at the base of the plants at tasseling time; these also yielded well, at 221 and 222 bushels per acre, respectively, about the same as the check (Table 2).
Treatments that ranked considerably higher in 2015 (wet June) than in 2016 (normal to dry June) included 100 lb. N at planting followed by either 50 lb. N injected at V5, or by 50 lb. dribbled into the row at VT; and the treatment with all of the N sidedressed between the rows at V5. It’s possible that rainfall in late May and early June moved the sidedressed N to the plant roots a little sooner in 2015, and it’s also possible that enough planting-time N had moved out of the root zone that year to make adding the last 50 lb. in the row at tasseling a little higher-yielding.
Treatments that ranked considerably higher in 2016 than in 2015 included urea + Agrotain broadcast at planting, ESN broadcast at planting, and 100 lb. N at planting with 50 lb. dribbled between the rows at VT. There was enough rainfall in May of both years to move urea into the soil without too much problem, so it’s not clear why these performed better in 2016. But both were good treatments across all sites. It’s also not very clear why dribbling 50 lb. N down the row middle at tasseling was better in 2016 than dribbling it into the row, the reverse of what we found in 2015. Again, these were both reasonably good treatments, but not better than the check (UAN injected at planting.)
Yields levels were relatively consistent among sites and years, ranging from 185 to 248 bushels per acre; we didn’t really see the tough conditions that we know can happen. We also found somewhat lower N responses than we expected; the 150-lb. N rate we chose in order to spread the yields from different N treatments was either more than the optimum N rate or within 20 lb. of the optimum at six of the ten site-years. So the high-loss conditions under which some treatments might be expected to do much better than others were not very noticeable in this study, at least during the first three years.
Given all that can happen when we apply N fertilizer in a way that we think will produce high corn yields, it’s no big surprise that this research has not so far identified clear “winners” or “losers” among the different ways we managed N. With top-to-bottom yield ranges as high as 36 and as low as 12 bushels among sites, expecting treatments to “hold rank” across such different environments may not be very realistic.
The ability to separate yield averages statistically is directly related to how well treatments held rank across sites-years. When a treatment ranks high at some sites and low at others, its overall average is in the middle, and the statistical comparison, which measures how well the results predict future performance, becomes less certain. That’s why so many of the treatment yields averaged over sites (as in Table 1) are followed by the same letter – we can’t be sure that a treatment that yielded 4 or 5 bushels more than another treatment will do that again next time, because it didn’t do that consistently across trials so far.
These results show, though, that just about any way we are managing N now is probably working reasonably well. We did not expect that treatments involving dry urea, protected against loss and broadcast at planting, to perform as well as they did. We don’t think that these results suggest a push towards broadcast urea application, but it is a common practice in many parts of the world, and if costs and availability move us in this direction, it appears to be workable. Treatments that did not do as well as we might have expected included applying UAN solution on top of the soil, whether that was all at planting or at other times. Anhydrous ammonia applied at or before planting also produced lower yields than expected.
These results seem to point to the benefit of having much of the N in the soil into which the roots grow, and to have it there relatively early in the season. Though we didn’t measure soil N in this study, most of the treatments that produced below-average yields were ones that supplied most of the N only at or after the plants had grown for a month or more. Treatments such as UAN dribbled or NH3 injected between rows at planting might have placed the N out of reach of early root growth. In contrast, broadcasting urea or injecting UAN between rows at planting might have resulted in more N in the soil where the roots grew early.
Even if the hypothesis that having more N in the vicinity of the roots holds up in further research, yield differences we found over sites were probably not large enough to justify many changes in how we manage N. As an example, incorporating broadcast UAN, which is normal practice, might be adequate to provide the roots with early access to N. And, if it stays dry for several weeks after planting (which did not happen in these trials), broadcasting urea might not work as well as we saw it work so far.
We might, though, want to consider the need for N near the roots during early growth as we plan N programs. This could be as simple as applying more of the N early and less at sidedress, or of applying sidedress N closer to the row for better access by the roots. As is always the case, weather conditions will have a large influence on how necessary, useful, or successful our best-chosen strategies turn out to be; no responsible N management program is completely safe.
One approach that has appeal, but that adds considerable economic and environmental risk, is to “just apply more” in order to make certain the crop won’t “run out” of N. We have seen how rarely the crop runs out of N when normal N rates are applied. Our work is also showing that loss of N (movement out of the top 2 feet of soil) is less than we expected, especially when we account for the amount taken up by the crop. With the equipment and knowledge we have today, everyone can manage N responsibly and with confidence that the crop will get the N that it needs. As is always the case, good weather helps a great deal to make N work, and we wish good weather for everyone as the season gets underway.