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N applied in spring creates grass glut

The story punch line comes first when Rob Kallenbach talks about applying nitrogen fertilizer to pastures in spring.

“Don’t do it! Spring nitrogen doesn’t make economic sense.”

Then, the University of Missouri Extension forage specialist backs up to spell out all of the reasons not to spur an overabundance of grass growth in the part of the cool-season grass cycle, when most pastures grow more grass than the cow herd can handle.

Herd owners coming out of a long, hard winter of feeding hay will be tempted to think about increasing grass growth. It’s true that each pound of nitrogen spread on a pasture will make another 25 pounds of grass. But, the response isn’t immediate. Applied when pasture are bare, nitrogen takes six to eight weeks to grow the grass. That adds more forage at a time when the spring growth cycle peaks naturally.

“It’s rare that we don’t have enough grass from mid-April through June,” Kallenbach says. “And, if there is not enough grass, it’s because there isn’t enough rain. Adding N in a drought won’t make more grass.”

In a normal year, added N makes forage that becomes a problem to harvest. The cows can’t eat it all. And May is a difficult month to make quality hay.

Adding nitrogen for plant growth makes agronomic sense, but it doesn’t make economic sense. “Applying N in spring was never a good idea,” Kallenbach says. “The last few years, with high fertilizer costs, it makes even less sense.”

There’s a second big reason not to use nitrogen now. “When you fertilize in spring, clover doesn’t have a chance,” the specialist adds.

When overseeing legumes into a pasture, you want to reduce competition from grass to allow the tiny clover plants to become established. That means keeping the paddocks grazed down. With added grass growth, cows can’t keep up. Legumes are shaded out.

Kallenbach isn’t against fertilizing pastures. If the soil tests show a need, phosphorus, potassium, and lime can be added anytime. “There’s not a day of the year when you shouldn’t spread fertilizer, if it’s needed,” he says.

Key Points

• Spring is not the right time to apply nitrogen fertilizer to pastures.

• A slow N response adds forage at spring’s grass growth peak.

• Application of nitrogen suppresses legume growth.

Right time for N

For the best economic return, nitrogen should be applied mid-August to boost the fall growth cycle. Fall growth is not as plentiful as spring growth. Boosting fall grass helps even out the grazing over the whole growing season.

The second-best time for an N application is about May 20. That gives the fertility time to add grazing into the summer slump of cool-season-grasses.

Graziers using the grazing wedge to track the dry-matter production in all of the paddocks are now using strategic placement of nitrogen to keep paddocks at expected growth rates.

Nitrogen can help fine-tune management-intensive grazing systems. Still, applying nitrogen when it is not needed just adds to management problems, and adds costs.

“The only person who wins with misapplied nitrogen is the fuel dealer,” Kallenbach says. “It takes fuel to spread it. Then, it takes more diesel to run the brush hog to cut it down.”

“I’d not be a good Extension person if I didn’t urge getting a soil test before applying fertilizer,” Kallenbach concludes. “That will pay.”

MAP and DAP add nitrogen

Rob Kallenbach does not advocate mixing legume seed with fertilizer for spring seeding. “That’s a self-defeating deal,” says the University of Missouri Extension forage specialist. “Fertilized grass will crowd out the legume seedlings.”

It used to be simpler to apply phosphate to pastures. Super phosphate had an analysis of 0-46-0. If you can find it, use it. Now, the widely available phosphate formulations add nitrogen.

DAP, the most common, contains an analysis of 18-46-0. Need it or not, you get 18 units of N with 46 units of P. If you do use a spring application when legumes are seeded, use a rate that adds only about 25 units of N. Fertilizing without the seeding, try to keep spring P rates under 60 pounds per acre.

MAP, another formulation, has an 11-52-0 analysis. This is more useful for spring applications, but is not as widely available.

For best results, Kallenbach says, hold off spreading DAP or MAP until
mid-August, when the added nitrogen helps increase fall stockpile growth.

This article published in the March, 2010 edition of MISSOURI RURALIST.

All rights reserved. Copyright Farm Progress Cos. 2010.