Rice Farming

Preflood N Bump

Rice fertility experts discuss hybrid response to
increased nitrogen on heavy clay

By Carroll Smith

Increasing yields on heavy clay soils by boosting preflood nitrogen (N) rates is not a hybrid-specific event. For years, it has been a well-documented observation in conventional rice by the university systems.

With that in mind, why bother testing hybrid rice response to pre-flood N on heavy clay soils? Why not just apply the same recommendations used for varietal rice?

Tim Walker, associate agronomist at the Delta Research and Extension Center in Stoneville, Miss., says, “Any type of rate response work that we do, whether it’s a hybrid or a conventional variety, we go well below and above where we think the recommendation should be,” he says. “That gives us an opportunity not only to see the N responsiveness with respect to grain yield but also evaluate factors such as lodging and how much N is too much.”

Walker says the first time he observed a yield increase following an increased preflood application of N on hybrid rice planted on heavy clay was with XP710.

“The latest hybrids we’ve looked at are Clearfield XL745 and Clearfield XP746,” he says. “Both of these hybrids have continued to respond to the higher rates, primarily on the clay soils.”

Fertilizer application at heading
Walker notes that there is a fine line between continuing to raise yields and having rice on the ground. The Mississippi agronomist says he feels comfortable with 135 units of N preflood, followed by a late-season application of 45 units.

He also points out that some growers will stack up all of their N between preflood and what is traditionally called mid-season instead of putting out any fertilizer at heading.

“I like the heading fertilizer application because I think of it as insurance against lodging, whether it adds to the yield or not,” Walker says. “Hybrids scavenge N from the stems and leaves for grain fill because they have such huge heads. This really taxes the plant because it’s transferring N from the leaves and stems for grain fill.”

He notes that hybrids typically take up more N than varieties do late in the season.

“That late season N application replaces N in the stalks to help the plant stay greener and stronger, which helps prevent lodging.”

Arkansas weighs in
“When we began testing the hybrids, we realized they needed a higher rate of N just like the conventional varieties on clay soils,” says Rick Norman, professor of soil fertility in rice, University of Arkansas.

“Our clay soils are not N ammonium fixing clays and actually have more N in them than our silt loam soils,” he adds. “The reason our rice needs a higher rate of N fertilizer when grown on clays is because the diffusion or movement of ammonium-N (the form of N taken up by rice) is much slower in clays than in silt loams. Thus, to ensure the rice plant can get enough N during vegetative growth, we have to put a higher rate of N on the clay soils to make a higher concentration of N available in the soil. The higher concentration of N enables the N to move to the plant roots at the rate and amount required during the vegetative growth stage.

Norman says the goal of applying higher N rates preflood is to achieve optimum hybrid yields with minimal lodging. The results of his research show that Clearfield XL729 and Clearfield XP746, when grown on clay soils, require 150 pounds of N preflood, followed by 30 pounds at boot, whereas Clearfield XL745 requires 120 pounds preflood and 30 at boot.

“Applying preflood N above these rates no longer produces a significant grain yield increase,” he says. “The N application at late boot does not always directly lead to a yield increase, but does so indirectly by minimizing lodging.”

In conclusion, Norman notes, “Hybrids have certainly been part of our yield increase for the past few years, but don’t plant more than you can harvest in a timely manner. When they want out, you’ve got to go get them because they shatter and lodge much easier than our conventional varieties do.”

Contact Carroll Smith at (901) 767-4020 or csmith@onegrower.com.

RiceTec considers site-specific N recommendations

Van McNeely, RiceTec’s technical services manager, has worked with both Tim Walker and Rick Norman in the hybrid rice fertility arena, as well as heading up the company’s own N treatment evaluations.

“Internally, our N trials go from zero units up to 180 units pre-flood,” McNeely says. “That’s a pretty wide range.

“On the heavy clays, we can look at multi-year data and see the same trend for increased yield response,” he adds.

“The same type of response has been documented in varietal rice for 10 to 15 years. On the heavy clays, not as much of that N is available, so you have to compensate for it.”

McNeely notes that RiceTec is beginning to look at site-specific recommendations – varying the N rates – taking into consideration soil types, geographic areas and even the hybrids themselves to some extent.

“We’re not to the point that we will come out with, say, 15 different N recommendations across the region,” he says, ”but we are looking at getting more specific to different areas on our N rates.

“Right now our standard recommendation on low response soils is 90 units pre-flood and 30 units at late boot. On high response areas, we recommend 120 units preflood and 30 units at late boot.

“However, if a farmer has a hybrid on heavy clay soil, it’s definitely not going to hurt to bump that preflood rate by another 30 units or so.”