In tests at the Upper Coastal Plain Research Station in Rocky Mount, N.C., researchers have found the bargain price for one family of generic herbicides may not be such a bargain after all.

Metalachlor, patented and sold for many years under the trade name Dual, and more recently Dual Magnum, has been a staple herbicide for row crop farmers for over two decades. In recent years, the patent has gone away and many me-too products have come onto the market.

Whether the generics are the same as the original was put to the test by North Carolina State weed scientists, and so far, the answer is no.

Metalachlor may be a key component of herbicide resistance management programs in cotton. Though it is hard to document the advantages of Dual in cotton, its value in a herbicide resistance program will be to provide more time to allow a grower to be more flexible in timing a lay-by application of herbicides.

The generics tested at the North Carolina research station had about 65 percent as much active ingredient as Dual/Magnum. In cotton, if lay-by applications are timely, this may not be a big problem.

In corn and other crops, where metalachlor is the primary herbicide for full season, broad spectrum weed control, it can be a big problem.

If a generic has 65 percent as much active ingredient as Dual/Magnum, can a grower use a pint and a half of the generic to get the same activity as a pint per acre of Dual/Magnum?

“You probably can, but you have two problems. First, you are off label. Second, when you put out higher rates of active ingredient you are also putting out higher rates of inert materials at a higher risk of crop injury,” says Alan York, a weed scientist at North Carolina State University.

Managing herbicide resistance is only one role metalachlor may play in cotton production in the Southeast. New, emerging weeds, like tropical spiderwort, must be factored into the overall weed management program. Controlling both the old weed pest and new and emerging weeds in cotton with 65 percent the active ingredient or the original product may not be a good idea.

In Georgia, Dual Magnum was used by researchers in Roundup Ready and Roundup Ready Flex cotton to help manage tropical spiderwort, which has become an ongoing weed pest for producers in the lower South. For growers using generic metalachlor timing of application may be even more critical than with Dual Magnum.

Dual Magnum only provides residual control of tropical spiderwort and should be applied prior to emergence, according to University of Georgia Weed Scientist Stanley Culpepper. “The level of control observed in our treatments was only achieved when tropical spiderwort was small and not stressed from environmental conditions and when rainfall or moisture occurs within 48 hours of application,” Culpepper says.

The only way to determine active ingredient in any herbicide, original or generic is to read the label. Efficacy, legal, economic reasons may enter into a grower's use of generics, but knowledge is a standard. Comparing labels, not cost, is a more realistic way to make good decision on pesticide use.

In the case of metalachlor, York says, “If a grower is pinching pennies, he should look at the cost of two thirds pint of Dual/Magnum versus a pint of the generic. The lower rate of the original material should provide the same control as a pint of the generic, and should be comparable in cost and it will be legal to use,” York concludes.