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Don’t get my drift
Off-target herbicide takes toll on rice in reproductive
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By Vicky Boyd |
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Joe Street and Mark Kurtz don’t want to relive another season
like 2003 when the Mississippi State University researchers spent much
of their time traveling the state diagnosing glyphosate drift injury
to rice. Drift of the popular broad-spectrum herbicide wasn’t limited to
Mississippi, either. University of Arkansas Extension weed specialist
Bob Scott says many Arkansas growers began noticing straighthead-like
symptoms as the rice began heading in 2003. But the malady frequently
turned out to be damage caused when glyphosate that had been destined
for nearby Roundup Ready soybean or cotton fields drifted. “The drift problem is pretty much all over the Arkansas Delta,”
Scott says. “Anywhere Roundup Ready soybeans are grown next to
rice, there is the potential for drift.” Once the damage is done, farmers can do nothing to correct it. The
only solution is to prevent glyphosate drift. “Basically, people should use common sense and avoid conditions
where the winds are blowing toward rice,” Scott says. While most of the complaint calls he received in 2003 were about glyphosate
drift damage, he says they still represent a small percentage of the
acreage. Arkansas had about 1.3 million acres of rice, and Scott says he was
aware of about 500 acres that were affected by glyphosate drift. About half of the complaints were related to drift from aerial application
whereas the other half were associated with ground applications. “A lot of guys who use ground rigs figure just because they are
using a ground rig means they are not going to drift,” Scott says. Either application method has a potential to cause drift if the applicator
isn’t careful, Scott says. The Arkansas State Plant Board is currently investigating the complaints. New Mississippi regulations Under a supplemental label, you need a special permit to apply glyphosate
in 19 Mississippi counties from the middle or end of March through the
end of April, depending on which county you farm in. The new label reduced
complaints to about a dozen annually until 2003. But it’s after the moratorium has ended that growers are applying the bulk of glyphosate to Roundup Ready soybean and cotton crops. A new type of Roundup Ready cotton, which allows growers to spray past the four-leaf growth stage, could extend glyphosate applications well into the rice growing season. Later stages more susceptible And as the plant development progresses, less glyphosate is needed
to reduce yields significantly. Kurtz, an MSU weed scientist, and Street, MSU Extension agronomist,
conducted field trials for four years trying to simulate the effects
of different concentrations of glyphosate that might drift onto rice. They used four different herbicide rates on four different growth stages.
Although Cypress, Lemont and Priscilla were used in the studies, observations
by the Mississippi Bureau of Plant Industry found rice cultivars respond
similarly to glyphosate drift. After the full and half-rates of glyphosate killed the rice, they were
eliminated from the trials. Kurtz and Street found that while rice in the three- to four-leaf stage
may exhibit visible symptoms from drift, the plants tended to grow out
of it, and yields only suffered mild reductions at the highest rates. At rates of 280 grams/hectare (8 fluid ounces per acre) of simulated drift onto rice at the mid-tiller, panicle initiation (PI) or boot stage, yields will likely be reduced. Even at a lower rate—70 grams/hectare (2 fluid ounces per acre)—of simulated drift at the boot stage, Street and Kurtz found significant yield reductions in two of the four years. Don’t let looks fool you Visible rice injury doesn’t necessarily correlate to yield reductions
except at the high rate of 280 grams/hectare at the mid-tiller and PI
growth stages, the researchers found. Yield losses were consistently
higher at the boot growth stage, yet visible rice injury was the least. On the other hand, visible injury was most pronounced at the three-
to four-leaf stage at 140 (4 fluid ounces per acre) and 280 grams/hectare,
but there were no consistent yield losses. UA Rice Extension specialist Chuck Wilson conducted similar
trials in Arkansas using the variety Drew and recorded similar results. Contact Vicky Boyd at (209) 571-0414 or vlboyd@att.net. Take precautions to prevent glyphosate drift onto rice Andy Kendig, University of Missouri Extension weed specialist with
the Delta Research Center in Portageville, provides these tips to reduce
potential drift. Is it straighthead or drift? Depending on when drift occurs during rice reproduction, symptoms can vary, says University of Arkansas Extension weed specialist Bob Scott. They range from death of some tillers, yellowing and stunting early to almost no symptoms execpt some stunting later until seedheads and flag leaves emerge. Then the short, stunted flag leaves and “fish hooked” or “parrot-beaked” seed and under-developed seed heads can be seen. Many of the staighthead symptoms are caused when drift occurs shortly after panicle initiation. To differentiate between true straighthead and a drift-induced malady, examine the entire field. True straighthead occurs only in the paddies and not on the levees. True straighthead also does not affect the flag leaf as herbicide drift
does. With severe glyphosate drift, the flag leaf size will be reduced
significantly and may be as short as 2 to 3 inches. The flag leaf in glyphosate-affected rice may be yellowed or lighter
green and can even be curled together or rolled. Plants also may be
stunted. If the field has no straighthead history or is on clay soil, then glyphosate drift should be considered, Scott says. Internet Hotlinks University of Arkansas Soybean Notes— Potential crop injury from
herbicide drift University of Arkansas— Some Arkansas rice fields damaged by glyphosate
spray Louisiana State University—Managing herbicide drift through new
technologies in spray equipment
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