In response to the increased herbicide-resistant weed population, researchers are focusing on nonchemical methods to control these weeds. Herbicide-resistant weed populations initially start in isolated patches within a field, then spread across the field — primarily with the help of combines. Efforts to limit this type of weed seed spread has led to the birth of several harvest weed seed control (HWSC) methods.
While it is fairly new to the United States, HWSC has been used for many years in Australian small-grain production. HWSC may also be useful for Iowa row crops, as waterhemp holds a high percentage of its seed on the plant at the time of crop harvest.
How does harvest weed seed control work?
The most common methods of HWSC are chaff lining and weed seed destruction. Both methods rely on a separator baffle inserted behind the sieves to separate chaff from the straw, because weed seeds move through the combine primarily in the chaff. Chaff and weed seed are diverted to the HWSC tools, whereas straw exits the combine as usual and is spread across the field.
In chaff lining, a chute attached at the rear of the combine concentrates the weed seed-bearing chaff fraction into a narrow strip. These narrow strips (chaff lines are 12 to 20 inches wide) are then kept undisturbed during the following growing season. The next year’s crop can be planted as usual, keeping the chaff line between two rows. Chaff lining relies on the mulch effect of concentrated crop chaff, which is less favorable for weed seed germination and survival. Since chaff lines must remain undisturbed, this practice is best suited for no-till growers.
With weed seed destruction, the weed seed-bearing chaff fraction passes through impact mills integrated at the back of the combine. These impact mills pulverize the material and leave weed seed non-germinable. There are currently four impact mills commercially available, including the iHSD Seed Destructor, Seed Terminator, WeedHog and Redekop Seed Control Unit.
What does the data say?
In chaff lining studies, Avery Bennett, a former Iowa State University graduate student, found the chaff liner concentrated more than 99% of waterhemp seeds that entered the combine into the chaff lines. This means those waterhemp seeds were confined to less than 5% of the field area. In the following year’s corn, chaff lines delayed waterhemp emergence by one week and reduced growth by 60%, compared to waterhemp growing outside the chaff line. Additionally, an application of postemergence herbicide was needed only inside the chaff line area, resulting in reduced herbicide use overall.
In weed seed destruction studies, the weed seed destructor finely ground or severely damaged 90% of the waterhemp seeds that entered the combine. An additional 5% of waterhemp seeds showed some levels of visible injury (moderately damaged). None of the waterhemp seeds germinated from the 90% category, while waterhemp seed germination was reduced by half in the moderately damaged category.
Implementation of HWSC technology in Iowa cropping systems is not a replacement of existing weed-control tactics but rather an expansion of the weed management toolbox. These results show chaff lining and weed seed destruction have significant potential to help manage herbicide-resistant waterhemp and diversify weed management programs.
Yadav is a postdoctoral research associate with the Department of Agronomy at Iowa State University.