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Integrated Pest & Crop Management


Peter C. Scharf
University of Missouri
Division of Plant Sciences
(573) 882-0777

Nitrogen: Flex and Go Fast

Peter C. Scharf
University of Missouri
(573) 882-0777

Published: April 8, 2019

PNK with N standing out

It's April 4 and raining again in central Missouri. A lot of work planned for fall did not get done. Opportunities to catch up in late winter or early spring have been nil. I expect a spring where the value of good decisions will be magnified.

Everyone is waiting for the starting gun, then it will be an all-out sprint. Long hours will happen, working in marginal conditions will happen, but going fast will be crucial. Getting priorities right will be crucial too.

How can you, or anyone, go faster? Nitrogen management for corn presents a key opportunity.

Anhydrous ammonia is the cheapest N source, and the most resistant to loss when it rains excessively after application. It is also, by a large margin, the slowest.

Moving to dry or liquid on most or all acres would save a lot of time. For those who normally split N, this might be a good year to apply it in a single shot–whenever works best, but more likely at the time of the second application in your split. We won't know the value of that time until afterward, but the odds are that it will be more valuable this year than most years. People who don't already have a lot down and who stick with getting anhydrous done before they plant could come out fine, but it's a risk I wouldn't take. Delayed corn planting not only can cost a lot of money itself, it can have a domino effect on other time-sensitive operations.

The secret truth is that planting and weed control are more time-sensitive than N application. Getting planting and weed control done in a timely manner should be top priority, with N application filling in the gaps.

Myth 1: Early-season N stress will reduce row number on corn ears and cost you a lot of yield. We counted ears on plots that had zero N for 11 years in continuous no-till corn. These plots were 135 bu/acre behind the best treatment, but only 0.3 rows/ear behind. That means that out of every 20 ears, 3 ears had 2 less rows. Your corn is not going to experience the kind of stress that comes from 11 years with no N. If you get no N on until the corn is waist-high, the number of rows on the ears will still be the same.

Myth 2: If you get no N on your corn until it's waist-high, you should start planning the farm auction. I've put together data from 5 states over 3 decades and it all agrees: on average, you'll get the same yield applying N as a single shot at waist-high as at planting. It's not something I'd recommend. But the point is: good timing on weed control and planting are a lot more likely to affect your income than good timing on N. Machinery is more of a limitation than the biology of the corn plant.

These last days before planting would be a good time to get a piece of paper and write out all of your options for N application and decide which you like best now (plan A), which you'll use if plan A doesn't work, and so on. Checking on current and future availability with your suppliers should be part of this. With rivers up, moving fertilizer from the Gulf will be more expensive/dicey. The good news is that fertilizer at the Gulf is currently a good bit below the prices from last fall. High-clearance applicators and planes should be on the list, but for many growers will be low on the list.

With significant residue, I'd suggest broadcast dry N (urea with NBPT or ammonium nitrate) as the best option, before planting or any time up to waist high (but no ammonium nitrate after knee-high due to leaf burn). Broadcast UAN will tend to get tied up on the residue, and injected UAN is slower than broadcasting. With minimal residue, broadcasting UAN (probably with NBPT) becomes a fast and reasonable option. And injected UAN is still considerably faster than anhydrous.

For those who have ammonia applied, I doubt that much will be lost even if it went under water. Mostly it was applied late and soils stayed cold and wet. The bacteria need oxygen and warmth to convert ammonia to nitrate, so that probably has gone slower than normal. My guess is 25% loss or less.

Where no N is down yet, that is probably good at this point in terms of cropping flexibility. It will make it easier to switch fields away from corn if needed. This is also a positive of applying all N after corn planting.

Even lower than N timing on my list of priorities would be tillage and application or timing of P & K.

Time savings is one of the top reasons that no-till has been adopted. Even if you have time to till in a normal year, this may be the year where you should figure out how to skip that step. Unfortunately, there are more ruts and gullies to fill this year than normal.

I haven't seen timing experiments comparing fall or preplant P & K applications to in-season broadcast applications, but I'd be surprised if there was a yield difference. If I wanted to apply P & K but hadn't gotten it done by planting time, I'd plant and come back later with the P & K.

I've also done a lot of on-farm experiments comparing with/without P & K side by side. In about 75% of the fields, profit is higher without P & K. This is not unexpected—when soil test is at the target level, the soil should be able to supply all the P & K needed by the crop. Even below target soil test levels, the soil is often able to supply full crop needs and the unfertilized treatment is more profitable. There will be fields where skipping P & K will cost money. But lots where it won't. On-farm strip trials are the way to know which is which, and I foresee a future where they are automated and run on every field, every year. But that's another article.

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REVISED: February 21, 2017