Controlling weeds in soybean fields this past summer was a challenge for most farmers across the Midwest, often due to untimely rain, and lots of it. Agronomists from WinField share what they saw in fields and talk about the challenges, and some successes, that soybean farmers had with weeds this year.
For soybeans, harder-to-control weeds and resistance were the biggest challenges in 2015. Joe Rickard, WinField agronomist, reports that resistance continues to worsen in this crop. Farmers who have accepted this are using a preemergence herbicide.
Across the state of Ohio, marestail is a continuing problem and Rickard also saw more Palmer amaranth this past crop season in soybean fields.
Even though each operation is different, Rickard noticed farmers who applied preemerge herbicides had cleaner fields than those who did not. While there are a list of good soybean pre-residual products, one key product that Rickard recommends leaving in the tank mix is 2,4-D. “Yes, there is a waiting period to get back in and plant, but the benefits are worth it,” he explains.
Rickard also reminds farmers to spray weeds when they are small, which is crucial for weed control.
Marestail is a growing weed problem for soybean farmers in Michigan, reports Corey Guza. Frequent and at times, heavy rainfall reduced the effectiveness of preemergence herbicides and challenged growers when making timely postemergence weed control applications. Weed size was an issue with farmers as they tried to manage weeds later than normal. In addition to marestail being an issue throughout the state, Southern Michigan had issues with common waterhemp and giant ragweed, while common ragweed and common lambsquarters were issues in central and northern Michigan.
Farmers in Michigan have been adopting strategies to rotate multiple modes of action when using herbicides for weed control. This has slowed the rate at which herbicide resistant weeds are spreading, relative to other states. They are also incorporating pre and postemerence weed control strategies within the same season, which also will improve the chance of success with their herbicide programs, Guza says.
The biggest challenge in 2015 was simply getting herbicide applications on the field, reports agronomist Jason Roth, WinField. In Roth’s territory of northern Indiana, fall conditions in 2014 favored neither tillage nor fall burndowns, and spring conditions in 2015 weren’t any better.
He saw some soybean farmers skip a burndown and residual application, hoping to clean up their fields after planting. However, the onslaught of rain prevented timely, or sometimes any, applications. Areas that experienced heavy rainfall accumulated drown-out spots, which became strongholds for weeds.
Similar to cornfields, many soybean fields were infested with marestail, waterhemp, Palmar amaranth, lambsquarters and giant ragweed. “Some of these weeds are prolific seed producers and the seed bank received many reinforcements for a battle with farmers over the next few years, unfortunately,” says Roth.
Regarding summer annual weeds, farmers with the best weed control and cleanest fields continue to implement multiple passes and overlapping residuals. This includes applying soil residual products around planting, and then making an early post application to extend the soil residual window. “Overlapping residuals is becoming absolutely essential in battling waterhemp and Palmer amaranth, because these weeds continue to germinate and emerge later in summer,” Roth says. The best programs also used multiple modes of action to help combat the development of herbicide resistance, he adds.
Waterhemp continues to be a serious problem for soybean production, says Todd Cardwell, WinField agronomist. Waterhemp together with common lambsquarters made up the bulk of weed challenges for Wisconsin soybean farmers this season.
If farmers who used glyphosate alone on lambsquarters and couldn’t spray before weeds were more than 3 or 4 inches tall, control was difficult. Timing was critical, he adds, since hot, dry weather fostered fast growth of lambsquarters.
Farmers who were successful implemented a weed-control program that used preplant or preemergent herbicides, followed by cleanup spray with glyphosate and additional burner-type chemistries mixed into the tank. This approach, Cardwell says, usually provided a fairly clean field with two passes.
Herbicide-resistant waterhemp has bedeviled South Dakota soybean farmers after taking hold in the eastern part of the state over the last couple of seasons, says Ryan Wolf, WinField agronomist. As you head west in the state, the annual broadleaf kochia has also become resistant, Wolf says.
“Fall applications of Dimetric® (metribuzin) worked well to hold back the weeds early in the spring,” Wolf says. After planting, farmers came back and applied a PPO inhibitor over the top. PPOs disrupt cell membranes and are usually burner-type herbicides. Wolf noted that multiple modes of action with multiple timely applications worked well.
Many factors affected weed control in 2015, says Steve Barnhart, WinField agronomist. Resistance continues to be an issue across Iowa, and farmers who made only a single herbicide application suffered. Weed size was a big factor because frequent rains prevented timely burndown, post plus residual, and/or post herbicide applications. Iowa’s cool evenings didn’t help weed control either, as weeds didn’t effectively take up herbicides during cold conditions observed during the post-herbicide season.
The top four weeds for soybeans were marestail, lambsquarters, giant ragweed and waterhemp. In no-till areas, marestail was a major issue. Giant ragweed was a problem throughout the state, particularly in eastern Iowa. But waterhemp was by far the biggest challenge. In general, soybeans are planted later than corn, sosoybeans are less competitive than corn due to the fact that waterhemp often emerges with or shortly after the soybeans. Rainfall affected this cropping season, delaying soybean planting in southern Iowa until early June, right when peak emergence of waterhemp was also occurring.
Farmers who incorporated a multifaceted approach in their weed control program were successful. Residual products followed by a timely post or post plus residual herbicide application provided the most consistent efficacy.
Glenn Longabaugh, WinField agronomist, reports that there were many weed control challenges for Illinois in 2015. “But without question, public enemy number-one in soybeans was waterhemp, not only for Illinois but the Midwest as well,” he explains. While other weed species were common, waterhemp and Palmer amaranth were by the far the biggest threats.
Similar to corn, timely applications to control these weeds was next to impossible. Sub-optimal conditions due to heavy rain caused delayed crop canopy and poor crop competition.
To control these top two weeds, Longabaugh saw the most success in a two-step herbicide resistance management plan. First, overlapping residual products and two, multiple effective modes of action. Both of these applications had to be made timely, and if they were not, the difference was substantial. “Timeliness is absolutely critical with many chemistries,” he adds.
Above all, waterhemp takes the cake. For the past several years, glyphosate-resistant waterhemp has been a major challenge for soybeans in central Minnesota, reports Mark Glady, WinField agronomist. It’s harder to kill weeds in soybean fields than in cornfields. Because both soybeans and waterhemp are broadleaf crops, selecting a herbicide that has activity on the weed but not the crop is very hard.
This past spring started positively, with early planting and preemergence herbicide applications on soybeans, which is becoming more common. However, May brought cool, wet weather that led to minimal soybean growth that month. While the preemergence applications worked well in May, they wore off in early June and short soybean plants didn’t provide a canopy to shade out weeds. These less than ideal conditions allowed weeds to grow, but not soybeans.
Despite timing issues this past crop season, Glady saw farmers who applied preemergence herbicides and then early postemergence applications gained better control. “Fields with these applications looked significantly better than those without,” he notes. “Compared to doing nothing, these applications drastically improved weed control.”
Glady also saw that farmers were successful when they used multiple modes of action. Using the full rate of a residual preemergence herbicide can be cost prohibitive. But pairing this with dimetric leads to another mode of action and broader-spectrum weed control. Plus this option costs less compared to the full rate.