Caught With Your N Down, Corn Growers?

Corn growers, especially those harvesting corn silage, would do well to consider a late-season stalk nitrogen test - now.

Published on: Sep 10, 2013

During corn silking or even before, too many Northeast corn fields turned yellow – out of "gas", better known as nitrogen. Fields receiving much higher than usual rainfall suffered much higher than usual N losses, reports Penn State University Agronomist Doug Beegle. A few farmers saw it coming and late-applied extra N.

With today's corn input costs and current grain prices, you can't afford to get caught short on nitrogen, especially during a higher-yielding growing season. So, American Agriculturist queried Beegle and Greg Roth, corn specialist also at Penn State, for their advice on planning ahead for 2014. The lead questions were: How can you tell how much N is left for next year? And, when's the best time to test?

YELLOW LIGHT! This corn blinked its yellow caution signal before ear-fill. Yield potential began fizzling then as it ran out of "gas" – nitrogen.
YELLOW LIGHT! This corn blinked its yellow caution signal before ear-fill. Yield potential began fizzling then as it ran out of "gas" – nitrogen.

Gauge in-field variability
Roth also spotted in-field variability and questions whether any patterns reflect other management factors. A late-season cornstalk nitrate test might help rule out solely blaming nitrogen losses. And it could determine if this year's corn received the proper amount.

"I'd use the results as part of my future N management strategy, knowing that in some years with heavy crops and a wet June what you did this year might not work," cautions Roth. "How often do these conditions occur? Do they warrant bumping up your strategy? Or, are there ways to sidedress late to overcome early wet conditions?"

You can visually assess N status as the crop reaches silage harvest maturity by the number of green leaves below the ear, says Beegle. "If you have at least four green leaves at and below the ear at silage harvest time, the crop had adequate N. That assumes the yellow or brown leaves at the bottom of the plant are due to N deficiency.

Be careful to not confuse N deficiency with disease and dry conditions. The latter also cause lower leaves to senesce (turn yellow).

Nitrogen deficiency is characterized by a distinct "V" shaped pattern starting at the tip of the leaf and going back the mid-rib. Drought senescence and diseases usually don't follow this very distinct pattern, he adds.


Stalk N test a no-brainer?
A Late Season Stalk Nitrate Test is a reliable end-of-season indicator of crop N status, affirms Beegle. The LSSNT is very useful for fine-tuning future N and manure management decisions. But stalk sampling instructions must be followed carefully.

Samples should be taken between ¼-milk line and up to three weeks after black-layer. If you're taking samples after silage harvest, be sure to do it within 24 hours of harvest.

Collect 8-inch long stalk sections starting 6 inches above ground. Collect from at least 10 representative plants in a field. Then cut the sample pieces into shorter pieces to promote drying and sent to a lab for analysis.

Corn must be chopped at least 14 inches high where the sample is to be taken to get the correct sample, reminds Beegle. Some farmers will raise the chopper head occasionally to leave some taller stubble to facilitate sampling later.

If test results fall between 700 and 2,000 parts per million, your N management was optimum, he adds. Below that range, the crop likely ran out of N and didn't achieve full yield potential. Results above that range indicate that the crop had more than enough N.

If N amendments were used, the LSSNT could help indicate how well they worked under this year's conditions. This would be especially helpful for check plot comparisons.

See Agronomy Factsheet #70: Late Season Cornstalk Nitrate Test for all of the details.