Field Prep Priorities

Should Nitrogen Take a Back Seat to Weed Control, Planting?

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
Connect with Emily:
A variety of both winter and summer annual weeds are coming up in Midwest fields this month and weed control may need to take priority over other fieldwork during this delayed planting season. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- With major storm systems working their way across the Plains, Midwest and South this week, planting may seem a more distant dream than ever.

For some farmers, it may be time to start prioritizing some fieldwork over others, crop experts told DTN.

"Of the big three -- nitrogen applications, weed control and planting -- nitrogen is the lowest priority, because we can apply that later," said Bob Nielsen, an Extension corn specialist with Purdue University. "Weed control and planting take priority in a delayed spring like this."

Farmers can play catch-up with side-dressed nitrogen applications, but once weeds are four- to six-inches tall, weed control will be a season-long struggle, noted Purdue Extension weed scientist Bill Johnson.

And, while early planting is just one of many factors that affect final bushels per acre, it does have a strong and fairly reliable correlation to yield -- particularly after May 1. Nielsen and other agronomists have shown that a corn crop will lose 0.3% yield every day that it isn't planted in early May and up to 1% per day by the end of that month.


Consider switching up your nitrogen source to help you meet a timely planting goal first, said Peter Scharf, an Extension nutrient management specialist with the University of Missouri. Anhydrous ammonia is a popular option for pre-plant nitrogen applications, but applying it can be a time-consuming operation.

"Moving to dry or liquid on most or all acres would save a lot of time," Scharf noted in an article posted on the university's Integrated Pest Management website. Otherwise, consider moving all or most of your nitrogen to a side-dressing application later in the season. "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," Scharf said.

Both Scharf and Nielsen noted that, except in extreme circumstances, corn is unlikely to lose yield to very early-season nitrogen deficiency. The crop can still yield very well even if it receives no nitrogen until a V6 to V8 side-dress application, Scharf said. "I've put together data from five states over three 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," he said. "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."

Other spring nutrient applications can also safely take a back seat in a delayed spring, Scharf added. "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."


Midwest farmers may face a messy blend of weed types in their fields in the weeks to come, Johnson warned. A cool, rainy March delayed the emergence of many winter annuals, such as marestail, some of which are just now emerging. Yet as April warms and advances, early-emerging summer annual weeds are also starting to sprout.

"There will be mixture of winter and summer annuals to control, which becomes a concern, especially for no-tillers, as you get rank weed growth creating these mats of vegetation that lay on soil and hold moisture in and keep temperatures cool," Johnson said.

Consider spending your first days in the field controlling these weeds, which will quickly outgrow the size at which most herbicides are effective, roughly 4 to 6 inches.

"Most foliar post-emergence herbicides in corn have corn growth stage restrictions -- many are around V6, or when corn is 12- to 18-inches tall," Johnson added. "There are very few herbicides you can use on big corn and then you have a challenge of controlling big weeds in big corn, and that's something you should never try to do on purpose."

At a minimum, Johnson recommends applying a burndown with at least a half- or two-thirds rate of a residual herbicide included. "That allows them opportunity to put more residual on later and spread out the length of time for residuals to work," he said. But if you doubt your ability to make a timely second herbicide application, put on a full rate of residuals with your burndown, he added.

Finally, brace for fast-growing weeds, which will have plenty of moisture on hand when the ground finally becomes warm and dry. "Make sure you have enough product on hand, if you have to increase rates to take down bigger weeds if weed growth becomes excessive," Johnson said.

For those planning on tillage to prep seedbeds and control weeds, an abbreviated plan might be necessary in a wet spring like this one, Nielsen added. "Farmers will either need to be really patient and not run on wet fields and create compaction, or think seriously about cutting back on one or two tillage operations," he said.

For more details on changing up your nitrogen plans in favor of weed control and planting, see Scharf's article from the University of Missouri here:…

See more on the relationship between planting date and corn yield from Nielsen here:…

Emily Unglesbee can be reached at

Follow her in Twitter @Emily_Unglesbee


Emily Unglesbee