Weed scientists track species spread, control

Ray Mueller
Waterhemp has been found in 61 Wisconsin counties.

KIMBERLY – The battle with weeds in such major Wisconsin crops as corn, soybeans, and alfalfa continues in various ways, according to reports by Extension Service weed scientists at the 2018 pest management updates.

The University of Wisconsin – Madison's new weed scientist Rodrigo Werle reviewed the results of 12 different herbicide programs in corn plots at Arlington and Janesville and of seven programs with Xtend variety soybeans at Janesville — all conducted in 2018.

At Arlington, all of the treatments — even the pre-emergent ones — provided at least 93 percent weed control and the corn grain yields were 56 to 72 bushels per acre above those for the check plot (no herbicide treatment). Those yields were in a close range of 222 to 238 bushels per acre.

Meanwhile, at Janesville, very poor weed control occurred with the pre-emergent treatments and the corn yields were significantly reduced although the lowest one was still 71 bushels more than on the check plot. Werle said a major difference between the two sites was the high population of giant ragweed and waterhemp at Janesville.

In the soybean plot at Janesville, the six herbicide treatments including a post emergent application all had per acre yields within seven bushels of one another while the pre-emergent only and check plots had respective yields of only 17 and 12 bushels per acre.

Herbicide resistance

What bothers Werle and other weed scientists in Wisconsin and many other states is the continued spread of resistance by some weed species to the active ingredients in glyphosate (Roundup) and herbicides with other modes of action. He noted that waterhemp has been found in 61 Wisconsin counties and Palmer amaranth in nine counties.

Palmer amaranth was found in nine counties in Wisconsin.

The tally in Wisconsin is documented cases of waterhemp resistance to glyphosate in 28 counties and to PPO's mode of action in 10 counties while the ALS mode of action herbicides are not performing well at trial plots, Werle reported. Only one of the 20 waterhemp samples tested in the state during 2018 did not have glyphosate resistance, he pointed out.

Palmer amaranth has established itself in Wisconsin in recent years, most likely coming in the imported cottonseed which is put into dairy rations, Werle observed. It has developed resistance to glyphosate in fields in Dane and Sauk counties, he observed.

Dicamba protocols

Regarding treatments, Werle reviewed the cautions for using dicamba, which controls broadleaf weeds, on soybeans with tolerance to the herbicide. He said applications should be made only until 45 days after planting or before flowering.

Rodrigo Werle

Other precautions to deter drift and damage to adjacent vegetation are to follow the product label, including the buffer distances and the hours of application (one hour after sunrise to two hours before sunset). Werle announced that he will attend a three-day workshop in St. Louis to discuss the problems that have emerged in many states in recent years in the wake of dicamba applications.

What's new on the horizon is the introduction of soybeans with dual tolerance to Liberty Link and glyphosate herbicides, Werle reported. Those soybeans are available from several seed companies.

Soybean cyst nematode

In addition to the surface threats to soybeans, the presence of soybean cyst nematodes (SCN) in the soil causes an estimated $1 billion annual loss to the nation's soybean growers, field crops plant disease pathologist Damon Smith pointed out. He urged growers to take advantage of soil sample tests that are paid for by the Wisconsin Soybean Marketing Board.

Those tests from samples in east central Wisconsin counties are finding a 40 percent positive rate for SCN while the state's average is 29 percent, Smith reported. He noted that only a handful of samples have been submitted by growers in Wisconsin's northeast agricultural district counties.

While it's encouraging that only 4 percent of the tested samples “exceed a do not plant soybeans threshold,” Smith advised growers with a known presence of SCN, which is permanent, to choose soybean varieties with the Peking resistance trait and to switch within those varieties from year to year. He said crop rotation does not get rid of SCN.

Smith pointed out that the test for SCN also scans for the root lesion nematodes which feed on corn plant roots. Unfortunately, he observed, those nematodes are being found in a high percentage of the tested samples.

Alfalfa applications

For the first time in 12 years, there's a new herbicide (Warrant) that's been approved for weed control in alfalfa but only up to the fourth trifoliate leaf growth stage of the alfalfa plant or until about seven days after a previous cutting, according to Extension Service weed scientist Mark Renz. He noted that research on Warrant for this approval dates to 2012.

That research has shown at least 85 percent success in weed control with only one instance of injury to the alfalfa, Renz continued. What's particularly important is Warrant's ability to control waterhemp, he pointed out.

By 2020, Renz expects approval of two new pasture herbicides from Corteva. He said one of them is safe to use on pastures with clovers while the other is safe with white clover but might damage some red clover.

Alfalfa interseeding

Another of Renz's research projects is the early season interseeding of alfalfa into corn. He said the best results (75 to 85 percent success rates) have been with reduced populations of corn, with an extra application of nitrogen, and with the use of a growth regulator to boost alfalfa yields.

Among the risks are wheel track damage when the corn is harvested and the costs of a growth regulator such as Kudos plus fungicide and insecticide applications, Renz observed. To hold the per acre costs of those applications to close to $50 rather than as high as $100, he recommends applying all three in a single trip.

In 2019, Renz hopes to augment the research station studies with on-farm sites. He can be reached by e-mail to or by phone at (608) 263-7437.