by Deborah Jeanne Sergeant
WATERLOO, NY — Marestail, giant ragweed, Palmer, waterhemp and pigweed represent problematic plants no farmer wants to see on the farm’s soybean fields. Mark Loux, extension specialist in weed science from The Ohio State University, recently spoke at the Soybean & Small Grains Congress on how to prevent the spread of these weeds.
While spraying herbicide liberally seems an easy way to control weeds, Loux said these plants develop herbicidal resistance, “many times because of our simplistic approaches.”
These weeds are very competitive, fast-growing, adapted to our production and tillage systems, resist many herbicides, and generally require more complex management than spraying.
“There are no easy fixes, even with the new herbicide resistant crop technology.” Loux said.
To battle these difficult weeds, Loux recommends a multi-faceted approach. Applying herbicide randomly isn’t the solution.
“If you put one herbicide on that doesn’t work with another one that doesn’t work, it still won’t work,” Loux quipped.
It’s ideal if farmers can plant in a field free of herbicide at planting by using a combination of fall and spring herbicides.
“A good canopy can suppress smaller weeds that come up later,” Loux said.
Letting weeds go until they’re too tall won’t work.
Cover crops, such as ryegrass and rye, left in long enough can also help suppress weeds.
“Studies are showing pretty much the same thing: the longer you leave it in, the more biomass and the better control you have over weeds,” Loux said.
After planting, farmers need to consistently scout for weeds all season. As they spot a weed, they should remove it immediately.
As an example, Loux said Palmer or waterhemp plant has up to 1 million seeds per plant. Its seed longevity is four years. By letting just one plant go to seed, a farmer can experience many seasons of weed-ridden fields. Just one Palmer or waterhemp plant can multiply to 31,250 plants by the following season, assuming only 10 percent viable seeds and 25 percent germination of those seeds and 99 percent control of those plants.
Or, farmers could just remove that first plant.
“Spend time going around your soybean field,” Loux said. “Ideally, pull any weeds out of the field.”
Plants should be bagged to avoid spreading seeds as farmers move through the field. Weeds larger than six inches are very difficult to kill through spraying. Considering Palmer can grow three inches in a day, farmers have “a small window for control,” Loux said.
Cutting taller weeds before they bear seeds may work, providing they are taller than the soybean plants; however, many of the weeds may grow back. Plus, cutting weeds requires more labor hours.
Farmers should also consider alternating the type of herbicides they use.
“Anything you use for three years will be into resistance and that’s with a year of corn in between,” Loux said, referencing crop rotation between corn and soybeans. “You still have to prevent seed production.”
How farmers till can make a difference in next season’s weeds. Four- to six-inch plowing can help prevent weed seeds from emerging, since most would end up too deep to germinate. Or, going no-till leaves them on top of the soil where they can’t grow. The weed seeds that are just covered up to four inches under the soil are the ones likely to germinate. Avoiding that danger zone can prevent many weeds from ever growing.
Cornell’s Northwest NY Dairy, Livestock and Field Crops Program hosts the annual Soybean & Small Grains Congresses, which offers identical presentations in Batavia and Waterloo, NY.