Roundup Ready technology has changed the nature of weed control in several respects and the nature of weeds crop growers must contend with.

"With fewer residual herbicides being used, winter annuals can get a head start," said Reid Smeda, University of Missouri weed specialist. "They emerge in late summer or fall and can grow into a heavy mat that insulates the soil and may slow soil warming and drying in spring."

Smeda also notes growing evidence that thick stands of winter weeds may harbor crop pests, including cutworms and wireworms. For the past 2 years, Smeda has supervised trials with fall-applied treatments of residual chemicals to control chickweed, henbit, marestail and other so-called winter annuals.

"We're finding that a fall-applied treatment of Canopy-Excel (Authority plus Classic) or Backdraft gives more than 90 percent control of weeds like henbit and horseweed," he said. "And we've also seen some early-season suppression [in spring] of common ragweed, lambsquarters and other weeds. Depending on weather and soil conditions, we may be able to go until early May without a spring burndown."

Smeda waits until soil temperatures get below 50 degrees-usually in November-to make the fall spray. Most weed control chemicals are degraded more readily by bacteria in the soil if the herbicides are applied while the soil is still warm. The result is that the residual effect may wear off by next spring.

"We generally use a burndown with something like 2,4-D added to the tank-mix to knock down those weeds that are already growing," he said. "Depending on the mix used, a fall herbicide treatment will cost about $10 per acre, plus the cost of application.

"Over the past 2 years, we have monitored soil temperatures at a 4-inch depth, both in plots where weeds were controlled and where they weren't," said Smeda. "The temperature didn't vary much all winter long. However, after the application of a spring burndown, it appears that soil may dry out more quickly without the mat of dead weeds on top."

Does a fall spray pay? Maybe not in dollars saved, especially if you have to make a spring burndown in addition. But spring weather is notoriously fickle; you cannot always get a sprayer into the field when you'd like. And if you plant corn in early April, you may be ahead of the game if a fall treatment gets the weeds out of the field.