Recently-published research has reinforced that “milling around” with seed impact mills can help keep escaped waterhemp from replenishing the weed seedbank in your fields.
Research conducted by Alexis Meadows (Iowa State) and Dr. Ramawatar Yadav (Ohio State) in the 2021 and 2022 growing seasons found that seed impact mills reduced the viable waterhemp seeds in soybeans by 87%. This study also found that at least 82% of waterhemp seeds had distinct visible damage after passing through the impact mill, a factor that hasn’t previously been studied. “We wanted to see the percent visual damage…nobody has tested the level of [weed seed] damage.” explains Yadav. Understanding weed seed damage helps determine how likely a weed seed is to survive in soil and germinate after passing through the impact mill.
The study was carried out in a field near Gilbert, Iowa. Enlist E3 (2,4-D-choline, glufosinate, and glyphosate-resistant) soybeans were planted for this experiment, and a preemergence application of S-metolachor and sulfentrazone was made. No postemergence herbicide was applied so that weeds would be present at harvest. The harvesting and seed impact mill equipment consisted of a John Deere S680 combine equipped with the Redekop Seed Control Unit.
Seed Damage
In 2021, waterhemp plants in the study field held 17,000 to 29,000 seeds per square meter, with 82% of seeds having moderate to severe damage after passing through the impact mill. In 2022, waterhemp plants in the study field held 1 million to 1.1 million seeds per square meter, with 96% of seeds having moderate to severe damage after passing through the impact mill.
“We wanted to sort them into different categories, like completely destroyed or pulverized [severe damage], or moderate damage which is a significant portion of the seed being damaged, or just a tiny chip on the seed [little-to-no damage].” Yadav explains.
Seeds with moderate damage had between a 12% (2022) and 23% (2021) chance of germinating. No seeds with severe damage germinated.
Despite some seeds still being viable, the impact mill greatly reduced waterhemp seeds returning to the weed seedbank. “Although the impact mill did not severely damage all of the waterhemp seeds, moderate damage to the seeds was effective in reducing germination and viability in controlled conditions,” Yadav and Meadows wrote.
Weather Factors
Temperature and precipitation are two factors that researchers believe may influence seed mill effectiveness. These factors could impact how many seeds naturally drop prior to harvest, and wet seeds may not get as damaged as dry seeds when passing through the impact mill. While Yadav and Meadows recorded temperature and precipitation data for their study periods, Yadav stressed that more research is necessary to pinpoint the extent of temperature and precipitation impacts when using harvest weed seed control methods. (See research from GROW on how, among other factors, chaff moisture affects seed impact mill efficacy.)
Yadav suggests that harvesting earlier could reduce the amount of weed seeds naturally dropped from weeds. “If you are anticipating high winds or precipitation, you might want to harvest early because a big precipitation event or frost or high winds can shatter a lot of seeds.” Yadav says.
At the time of harvest, 51% (2021) and 61% (2022) of waterhemp seeds remained on the waterhemp plants that entered the seed impact mill. The viability of seeds that shattered naturally ranged from 84% to 94%. Yadav and Meadows also noted that while disturbance from the combine can cause some weed seeds to drop, the seed impact mill successfully caught the vast majority of seeds remaining on waterhemp plants at the time of harvest. “It can damage seeds as tiny as waterhemp, and we expect that with bigger seeds, it [seed impact mill] would be even more effective.”
Because farmers have to wait until harvest to use seed impact mills, impact mills should not be a farmer’s only method of fighting weeds. “You cannot rely on it because you have to wait until the weed has already caused damage for the current year,” Yadav explains. “You are trying to prevent weeds in the following years, so you have to be careful and still use other practices.”
Dr. Yadav’s research reinforces that using harvest weed seed control methods like seed impact mills can reduce the amount of reliance that a farmer places on herbicide applications, even on small-seeded plant species like waterhemp.
Visit GROW’s website to learn more about herbicide-resistant waterhemp and harvest weed seed control.
Text by Amy Sullivan, GROW; Header and feature photo by Dr. Ramawatar Yadav, Ohio State University.
