A new weed-control option may be the best hope for controlling redvine in soybeans.

Dicamba (Banvel) recently was given EPA approval for use on soybeans late in the season, after the crop has matured. That's been the most effective means of control in three years of research by University of Arkansas scientists.

Agronomist Terry Keisling says that the perennial weed is becoming a bigger problem as conservation tillage gains acceptance. It can entangle tillage and harvesting equipment, but has little impact on yield. That's because it becomes competitive late in the season, after the crop is made. And since its roots run 12-15' deep, it doesn't compete with the crop for moisture.

In 1996, Keisling and weed scientist Dick Oliver began a study to evaluate the interaction between tillage and herbicide for redvine control. They chose a farmer's field near the Northeast Research and Extension Center at Kaiser - a field with 42-52% redvine ground cover. They divided the affected areas into plots, then evaluated various control methods.

Herbicide treatments included Banvel at 2 lbs active ingredient/acre applied two weeks prior to harvest, and two 1-qt/acre applications of Roundup at the V2 and V6 soybean growth stages.

"With an extensive underground root and stem system, redvine is capable of vegetative propagation and requires that substantial concentrations of the herbicide reach the root system," says Keisling.

Tillage trials with and without herbicides included no-till, conventional (spring chiseling or disking), hyperbolic subsoiling and moldboard plowing.

Fall moldboard plowing, which exposed roots to drying and freezing conditions, was the only tillage treatment that gave acceptable season-long control. Plowing sliced off redvine parts 7-8" below the soil surface.

Regrowth from the remaining taproot was hindered, Keisling says. And fragmented stem segments were deposited at the soil surface where they were killed by exposure to cold and wet conditions during winter.

The hyperbolic subsoiler was much less effective. It left many roots and rhizomes intact for regrowth.

Conventional tillage increased redvine stem counts.

Sequential applications of Roundup increased redvine control over tillage alone, except for the moldboard plowing. Roundup provided control for one month after treatment but late-summer regrowth caused final ratings to decline.

Banvel provided excellent control the year after application, with only minimal regrowth late in the season, Keisling reports.

The new registration calls for a minimum two-week delay between Banvel application and harvest.