By Donald Stotts

STILLWATER, Okla. – Size matters when it comes to many postemergence herbicides and effective weed control in double-cropping systems, Oklahoma State University experts said.

“Once weeds become too large or crops reach particular growth stages, many weed control options are no longer viable as they may injure the crop or will not completely control target weeds,” said Misha Manuchehri, OSU Extension small grains and canola weed specialist.

For example, when using a synthetic auxin plant hormone such as 2,4-D, control is most effective when weed height is about the length of a credit card. Control greatly decreases once weeds grow beyond that height.

Other issues with large weeds include:

• Crops likely will already have been negatively affected by the ever-growing weeds that have outcompeted them for available soil nutrient and water resources.
• Preemergence herbicide options may be limited because of the diversity in crops present within the system.

OSU Extension fact sheets detailing weed control programs for specific crops are available online and through all county Extension offices.

One of the most effective ways to produce a successful crop is to start with as weed-free a field as possible. In double-cropping systems, this is especially true as there is limited time from harvest to planting to control weeds and get the next crop planted in suitable conditions. Keeping up on weed management throughout the seasons of both crops will prevent so-called rescue herbicide treatments that result in poor weed control and ultimately money lost.

Manuchehri stressed that producers should scout fields regularly for weed species, taking special note of those that are herbicide resistant. Weed height, crop stage and awareness of the next crop to be planted should be used when determining which weed management program to employ.

Producers also need to know their goals and operational cash flow needs, said Trent Milacek, OSU Extension area agricultural economist for western Oklahoma.

“Weed management is a systems approach,” he said. “Skimp on any step and it will compromise total control and wreck budgets.”

Milacek and Manuchehri suggested the following general guidelines:

• Be dedicated in looking after the crop during the summer months. Timely weed-control applications can allow a producer to save money on chemicals by not having to apply maximum rates.
• Recognize that weeds will need to be sprayed. The systems approach of a preemergence and postemergence, when combined with frequent scouting, is the most consistent way to minimize weed pressure effects on yield and profitability.
• Develop a working relationship with custom applicators. Timing is one of the most critical aspect of herbicide efficiency. It is easy to get behind on spraying, even for producers who purchase their own application equipment.
• Always read and follow label directions, being sure you understand how much product and what type needs to be used.
• Accept that more than one prescription to control weeds may be needed if a local supplier has difficulty getting the first product requested. Supply disruptions can occur in the best of times, and especially during a pandemic.

“One positive effect in sorting through possible prescriptions is that a producer can price different control methods and possibly better minimize costs associated with weed management,” Milacek said. “Think of it as an investment in your time rather than an obstacle.”

More information about weed management with double-cropping systems is available by listening to the Extension Experience podcast. Milacek and fellow OSU Extension Area Specialists Josh Bushong and Dana Zook provide weekly insights into western Oklahoma’s agricultural activities through the podcast.