Rice remains sensitive to soybean harvest-aid drift late into the season, according to MSU research.
By Vicky Boyd, Editor —
For the past few seasons, Mississippi State University Extension Rice Agronomist Bobby Golden and MSU graduate student Justin McCoy have received phone calls from growers asking them to come out and examine fields affected by suspected herbicide drift. But it is not the type of off-target movement that has grabbed recent headlines.
Instead, what the two researchers frequently observed were rice fields past heading that appeared to have been injured by harvest aids applied to adjacent soybean fields.
With scant information in the literature about what, if any, potential yield reduction the harvest aids might cause to later-season rice, McCoy began two research projects in 2016 at the MSU Delta Research and Extension Center to address data gaps.
One trial looked at possible yield reductions based on when drift occurred; the other examined possible varietal differences.
Also involved in the projects is MSU Extension Weed Specialist Jason Bond.
McCoy, who is conducting the research as part of his doctoral thesis, plans to continue it through this season. Regardless of the projects’ outcome, he says, “When you have these adjacent fields, you need to be careful with these applications just as you do earlier in the year.
“You can reduce yields all the way out to late in the season. Just be safe, follow proper procedures, and watch your wind and drift potential just like we do early in the season. You just need to be aware that in these later growth stages, rice is still sensitive to off-target movement.”
Research conducted by MSU’s Joe Street and Mark Kurtz in 2002 reported glyphosate applied at the rice boot growth stage at reduced rates that simulated drift reduced yields 30-98 percent. But Golden says their application timing was earlier than when suspected drift occurred on the fields he has examined. Out of the more recent farmer complaints grew Golden and McCoy’s projects.
They chose paraquat because it is widely used as a harvest aid for early soybeans. They also included glyphosate because it is registered as a harvest aid for grain sorghum. In addition, they added saflufenacil and sodium chlorate because they both are labeled for soybean dessication.
The products were applied at 10 percent of labeled desiccation rate to simulate what might occur during drift. For glyphosate, that meant a rate of 3.2 ounces per acre; for paraquat, it was 1.6 ounces per acre.
The first trial involved one variety —CL163. Applications of individual products were made beginning at 50 percent heading, with subsequent applications made at one-week intervals for a total of five applications.
“With saflufenacil and sodium chlorate, we didn’t see any differences from the untreated check,” McCoy says. “When we looked at the glyphosate and paraquat across two years, we did see glyphosate caused yield reductions all the way up to one week before harvest. Paraquat caused yield reductions up to the day of draining.”
Applications of glyphosate or paraquat made at 50 percent heading caused the greatest yield decreases, he says.
“With the first three timings, we saw yield reductions in the 10- to 20-percent range, which is pretty significant,” McCoy says.
But applications made 28 days after 50 percent heading did not result in yield differences from the untreated check.
Differences among cultivars
The second trial involved two hybrids — CLXL745 and CL753 — as well as three inbreds — CL163, Rex and Jupiter. It also included individual treatments of either reduced-rate glyphosate or reduced-rate paraquat as well as an untreated check. All applications were made at 50 percent heading, the plots taken to harvest and yields compared.
“Interestingly enough, across both years, we’ve seen significant differences between the hybrid responses and the inbreds,” McCoy says. “We see much less yield response (in the hybrids) than we do the inbreds.”
Regardless of the herbicide, yields were only reduced 4-6 percent in the hybrids compared to the untreated check. Although yields of the two treated hybrids were numerically different from the untreated check, McCoy says they didn’t differ statistically.
Among the inbreds, the results were much more pronounced. The herbicides caused yield reductions of 10-32 percent, which are statistically different from the untreated check, he says.