Winter Wheat: Keeping the Stand, Fertilizing With N Timely Issues

By Jane Fyksen, Crops Editor
Thursday, April 12, 2007 11:07 AM CDT

Mother Nature pitched winter wheat producers a curve ball this spring, with unseasonably warm weather followed by a wintry cold snap.

At Agri-View's Monday press deadline, the state of the winter wheat was in question and on producers' minds.

Mike Rankin, Fond du Lac County crops agent, was "still hopeful it'll be okay." Though wheat in eastern Wisconsin had broken dormancy in the earlier warm weather, he notes that, at worst, there could be some leaf-tissue burn-back. Last he'd checked, the wheat was looking "pretty good," and he cautions that one "has to give these things a little time."

While it was colder farther north in the state, the wheat wasn't as far along either, which Rankin views as cause for optimism, too.

High corn prices may cause a few farmers to ponder the wisdom of giving up on their wheat, especially if they suspect a sub-par stand this year.

"An adequate stand is one that will produce enough tillers to result in 60 or more heads per square foot. When fall planting and winter conditions are favorable, we normally say that 20 plants per square foot are enough for high yield, with 15 plants per square foot worth keeping if they're healthy, crop leaves are in good shape and there is good potential for tillers to form," says Emerson Nafziger, University of Illinois agronomist.

"Though tiller (head) number is the critical factor in determining wheat yield potential, trying to guess how many tillers will form per plant is not easy," he continues. "If plants are healthy, if leaves cover most of the ground area coming out of dormancy, and weather is favorable (i.e. average temperatures and less-than-average rainfall, with a lot of sunshine) for several weeks after green-up, each plant can produce 3 or 4 productive tillers, so a stand as low as 15 plants per square foot might be adequate."

On the other hand, if plants are small coming out of dormancy, if the weather stays cold or turns warm too quickly, and if there's added stress from wet soil conditions, then, notes Nafziger, "there might be only one or two productive tillers per plant, so even 20 or more plants per square foot might not be enough for high yields."

Nafziger says in most cases, the crop should be given time to develop tillers before the decision is made whether or not to keep it.

"Tiller formation slows when the weather is warm enough for upright growth to begin," he says of jointing. "If the crop recovers quickly for form good ground cover and good tiller numbers before it starts upright growth, then its potential is good."

"IF it warms up early, jointing will start sooner, and tiller number may be reduced. If plants are small with few tillers as they come out of dormancy, there's much less chance that they will get enough favorable weather to produce a good canopy and good tiller numbers," he discusses.

In most cases, Nafziger says the decision on whether a wheat crop is worth keeping can be made when its 8 to 10 inches tall, by which time, a grower can usually count what will likely become productive (head-bearing) tillers. Tillers likely to become productive will begin upright growth shortly after main tillers, and so will be nearly as tall as the main tillers by the time the crop is 10 inches or so.

"Small tillers at this point have to compete with larger tillers, and so are less likely to be productive, especially if the weather is warm and growth is rapid," he notes.

One important factor in the decision to keep a wheat stand or not is the amount of unrecoverable cost already invested in the crop. For most producers, seed is the big one, running $40 or more an acre. He mentions that it might be wise to wait to apply herbicide until it's clear the wheat crop is worth keeping.

He says even if growers get a handle on yield in coming weeks, they need to remember that yield potential isn't always achieved. The condition of the wheat crop this time of year isn't very well correlated with final yield, because the crop is so sensitive to weather around flowering and a few weeks after. If it's dry during flowering and early grain-fill, foliar diseases and Fusarium head scab pressure tend to be low, and yields are often good, says Nafziger.

"On the other hand, we have seen that warm, wet weather around the time of flowering can wreak havoc on the wheat crop," he says.

Using glyphosate to destroy a stand of wheat works pretty good, as long as weather is favorable for growth. He says it's possible to take wheat off as forage, but regrowth after cutting immature wheat can be tough to control. Work in Illinois found that wheat about a foot tall had only about a ton of dry matter per acre. "We would expect that to double by early boot, so it may or may not be worth the expense of harvesting as forage," he mentions.

"If the crop is killed and left standing in the field, with the replacement crop no-tilled, be sure to check for the presence of insects in the residue," he adds, noting that the loss of the wheat host might make some hungry for the emerging corn.

Nick Schneider, Clark County's crops agent, offers further advice on evaluating winter wheat survival. He says cold hardening begins late fall, with plants gradually building resistance to winter weather, with maximum resistance normally in December and January. The growing point should be below the ground during this time to add further protection. Plants are most susceptible to low temperatures prior to hardening in the fall and throughout spring when tolerance is low.

Numerous factors impact winter survival. Cyclical freezing and thawing increase injury from ice crystal growth in tissue. Mid-winter thaw or prolonged thaw might cause the crown at the base of the wheat plant to be flooded; flooded crowns die at warm temperatures. Ice encasement leads to suffocation because carbon dioxide is trapped and respiration inhibited. Frost heaving pushing roots out of the ground is another factor affecting winter survival. Another factor is desiccation during subzero temperatures. Snow, notes Schneider, acts as an insulator, keeping soil temperatures from dropping below critical levels.

To evaluate a stand, this Clark County agent says to pull up several randomly chosen plants throughout the field. Dig each plant with as many roots attached as possible. Shake each seedling to fee excess soil. If soil adheres to roots in columns, root hairs are alive, as is the plant.

Once "test one" is complete, perform "test two," he directs. Cut into the crown at the base of the plant to expose tissue. If the crown tissue is white or light green, the wheat plant is alive. If it's brownish, it's likely dead.

A third "test" is to dig some plants, pot them indoors and water them to see if growth resumes.

Schneider recommends making the first spring application of nitrogen at the onset of tillering to increase the number of tillers and the potential number of heads. If the stand is adequate (18 or more plants per foot of row), apply 25 pounds of N at that time. However, if the stand is poor, then go with up to 50 pounds of N to increase tillering, he notes.

A second N application can be made as stem elongation begins to increase the number of fertilized seed heads.

Schneider also advises scouting and controlling weeds. A thin stand will be less competitive against weeds, he reminds.

Darryl Warncke, in crops and soils at Michigan State University, says how much winter wheat responds to topdress N varies year to year, depending in part on soil moisture and how it relates to N availability and N source.

Sharing work from Michigan, he says in 2004, Hopewell soft red winter wheat responded well to N applied just prior to green-up. Broadcasting 60 pounds an acre results in an 11-bushel yield increase (in wheat that yielded just over 75 bushels per acre with zero N applied). He says the yield difference between the 60 and 120-pound rates wasn't statistically different, even though the average yield was seven bushels greater (for 120 versus 60 pounds of N).

In the Michigan trial, topdressing 60 pounds of N per acre (a total of 85 pounds applied) resulted in the best combination for economic return and minimal residual N in the soil. In that year, soil moisture turned dry after N was applied. Therefore, splitting the N between green-up and later didn't prove beneficial.

In 2005, it didn't rain much in the spring, so movement of surface applied N into the soil and root zone was limited, he reports. Considerable volatile N loss occurred from urea and UAN (28 percent liquid). "Even so, the yield increase with 60 pounds of N per acre broadcast was similar to that in 2004," he notes of 12 bushels per acre more wheat.

Where ammonium sulfate was the N source, little volatile N loss occurred, and yield with 90 pounds of N per acre was 78.3 bushels per acre, compared to 64.3 for urea or UAN.

In 2006, soil moisture was good throughout most of the spring and summer growing period, so considerable N was released from soil organic matter. "Hence, response to applied N was less," he says, noting that the variety Roan was grown last year. It's a high yielder, but susceptible to lodging. Heavy rains and strong winds caused the wheat to lodge about three weeks before maturity in all of the plots getting 60 pounds or more of N. "The severe lodging probably moderated any potential yield response to N rate," he confides.

"With urea and UAN, applying 30 pounds of N per acre resulted in as good a yield as 90 pounds of N per acre either all prior to green-up or by split application," he adds.

ESN (a controlled release polymer coated urea) applied at green-up last year to supply 30, 60 or 90 pounds of N per acre resulted in less severe lodging and yields of 93.7, 106.1 and 105.7, respectively. "Perhaps this was related to the gradual release f available N," he remarks, pointing out that in the dry spring of 2005, ESN applied to supply 60 and 90 pounds of N resulted in yield similar to those attained with ammonium sulfate, and those yields were higher than yields attained with urea or UAN.

"Apparently, with the warm dry soil conditions in 2005, there was significant loss of N by volatilization from urea and UAN, which did not occur with the polymer coated urea (ESN) or ammonium sulfate," he explains. He says soil moisture and rainfall conditions can greatly influence the release of N and the loss of N from various sources.

Based on three years of results, he says it appears that wheat yield response to N is influenced by both weather and N source. Under dry soil conditions, the potential for volatile N loss is greatest with urea and UAN. The risk of N loss can be moderated by applying some of the N as ammonium sulfate or ESN.

Across all years, topdressing 60 pounds (total of 85 pounds of N per acre for the crop) as urea or UAN resulted in near the best economic return. This was also the case with ESN and ammonium sulfate in 2004 and 2006, he mentions.

With the dry spring of 2005, applying 90 pounds topdress (total of 115 pounds) as ammonium sulfate or ESN resulted in a better economic yield.

Using a combination of urea or UAN with either ammonium sulfate or ESN prior to or near green-up to supply 60 to 90 pounds of N per acre appears to be appropriate, according to this wheat expert.

"This year, with the higher cost of N, staying closer to the 60 pounds per acre topdress rate may provide the best economic return," he remarks, assuming about 25 pounds went on at planting last fall.

Matt Glewen, ag agent in Calumet County, where the number of wheat acres continues to climb, stresses that organic matter has a lot to do with the UW N recommendations for wheat. "This should not be a surprise because we know that once the soil temperature gets to 50 degrees, microbes begin converting that organic matter to a useable form of N, he says.

A rotation that's gained popularity is winter wheat after soybeans. For years, growers simply provided 40 units of N if wheat followed beans on medium or fine-textured soil. The new recommendation calls for 30 units that can be credited against the total N needs for winter wheat.

If soil organic matter in under 2 percent, the basic N rate recommended is 80. It's 60 for organic matter between 2 and 9.9 percent, 40 for 10 to 20 percent and none for over 20 percent.

Glewen says the most common scenario he'd expect to find would be a crop need of 60 units and if it were following soybeans, the amount would fall to 30 units after the N credit or about 65 pounds of urea (46-0-0).

"Likewise, a winter wheat crop on a medium or fine-textured soil not following a legume would benefit from about 130 pounds of urea this spring," he concludes.

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