Cover crops researched to gauge use in weed control

Managing Editor

GEORGETOWN, Del. (July 4, 2017) — Cover crops get a lot of attention for their help in nutrient management but ongoing research in the University of Delaware Weed Science Program is focused on finding out how using cover crops can benefit weed control.
During last week’s Weed Science Field Day at the Carvel Research and Education Center, Dr. Mark VanGessel, Extension weed science specialist, said their weed control research relating to cover crops started about six years ago, initially focusing on its use in organic production.
In those first years, the research was helpful in seeing how cover crops fit into weed management, but focusing only on organic production was limiting, he said.
For the last two years, the weed science program has expanded its cover crop research to investigate what impact managing cover crops for weed control can have on chemical herbicide use.
“Most famers here on the Eastern Shore and Delaware are using cover crops for nutrient management,” VanGessel said. “What we’re looking at though is how can we use them to get other benefits, in weed control particularly.”
In several trials, the research looks at variables including cover crop terminate date, planting date and pre-emergent herbicide application.
One trial focuses on cover crop termination at different times and measuring the weed pressure.
With cereal rye as the cover crop, the trial has kill times of 30, 20 and seven days before planting the cash crop. Within those time intervals, you can see a “huge difference in biomass” from the cover crops which may be significant in crowding out weeds leading up to planting a cash crop, VanGessel said.
“The thicker they are and the heavier they are, the heavier weed suppression you get,” VanGessel said, noting that letting a cover crop grow longer can bring other issues like performance of equipment.
Letting cover crops grow longer in the season for soil health benefits was the subject of a recent series of University of Maryland demonstration field days called “Delay the Burn.”
Benefits touted at the events held in May include moisture retention during dry spells and increases in organic matter, root action, infiltration and the amount of nitrogen retained in the soil profile.
A trial led by UD postdoctoral researcher Kurt Volmer aims to measure the extent at which cover crops at different growth stages intercept pre-emergent herbicide, keeping it from getting to weeds.
“That’s been a concern for a lot of growers,” VanGessel said, that longer living cover crops may reduce a herbicide’s effectiveness.
In corn, VanGessel said farmers have had concerns that following a grass cover crop with corn can suppress corn growth. So one trial is looking at corn performance behind cereal rye and barley cover crops with different termination dates.
What impact different cover crop conditions has on weed size leading up to a post-emergent herbicide application is the focus of another trial in the research. In other words, could certain cover crop methods slow weed growth enough to effect post-emergent timing and application rate?
In a general note on herbicide use, VanGessel told the field day crowd, made up mostly of crop protection company representatives, to be diligent about helping growers use the right product for the right problem.
While glyphosate may be most popular for weed control, it’s not a silver bullet, and fickle spring weather can cut down on a herbicide’s effectiveness, needing a back-up plan.
“Make sure they know how they’re going to kill it and if that doesn’t work make sure they know what they’re going to follow up with to kill it,” VanGessel said.