Residual Herbicides: What Length of Control is Enough?

WALNUT CREEK, Calif.—February 11, 2011—Preemergence residual herbicides are topping researchers’ recommendations for resistance management and overall weed strategy programs this spring. But with several preemergence residual herbicides on the market today providing a few days of control to several weeks of control, growers may be asking themselves: How long should a residual herbicide provide control of tough weeds? 

University of Missouri Weed Scientist Dr. Kevin Bradley said, “With residuals, the longer the control the better. I think two weeks of control is probably not enough out of any herbicide to really even justify spraying.”

So exactly how much control should a grower expect from a residual herbicide? Bradley suggests at least a month of control is critical for both agronomic and economic benefit.


“Four weeks is a realistic amount of control to expect from a residual—six weeks would be great. You should get at least a month of residual so that your program can include one preemergence residual herbicide and one postemergence spray and you’re done. If you only get two weeks’ control, you are probably going to have to spray a postemergence twice.”

According to a 2008 length of control research trial at the University of Missouri, Valor, Valor XLT, Gangster, Authority, Boundary and Prowl provided at least four weeks of residual control.1

“Valor and Valor XLT have both shown excellent length of control in the field,” said John Pawlak, Valent product development manager. “Even in wet weather conditions last spring, many growers still had four to six weeks of residual weed control.”

When selecting a residual herbicide, Pawlak encourages growers to consider residual products that provide:

  • At least 4-6 weeks of proven residual control
  • Control of a variety of problem weeds, including tough or resistant weeds
  • Rotational flexibility, should crop plans shift close to spring


To maximize profitability, farmers may also want to consider products which participate in rebate or reward programs.