How important is weed density when calculating your weed-control program?

ANSWER:

Weed density is the number of weeds per square foot or yard. It is the density of weeds in an area that competes for light, water and nutrients. When weeds are 4 inches or less in height, species really doesn't matter. But when they get 8, 12 or 24 inches in height, species differ in size and competitiveness, and how they affect the crop.

Counting plants per square foot and getting an estimate for the field is tedious work. And when plants get over 4 inches tall, that's when you have to begin to consider species. For example, 10 giant foxtail weeds exert the same degree of competition as one giant ragweed.

But Bryan Young, weed scientist at Southern Illinois University, points out that weed size is not the only factor to consider in determining the threshold for weeds in the crop. Weed density, weed species, crop row spacing, crop seeding rate, crop growth stage, soil moisture status (rainfall or irrigation), the cost for control measures and crop commodity price should also be considered, he says.

"Instead of going into the complexities of weed competition models and predicting the pressure for the remainder of the season, weed scientists generally play it conservative and recommend weeds by a certain size," Young adds. "But the risk of any yield loss should be minimized regardless of density, specific weed species, etc."

One tool to help with weed-control decisions is WeedSOFT software (http://weedsoft.unl.edu). It determines the break-even threshold for weed control or when the cost of weed control is less than the financial loss from weed competition.

Young adds, however, that some growers began to ignore the threshold information following the launch of Roundup Ready technology. Glyphosate killed all weeds of varying size for a low cost, and growers fell into the trap that as long as you kill weeds, you protect yield. Of course that isn't true. Postemergence herbicides must be sprayed earlier to prevent weeds from getting too big.

As a farmer and an agronomist, I consider weed size important and try to get the weeds sprayed when they are less than 4 inches tall. However, while weed density doesn't factor in my decision to spray, I take note of it to evaluate the performance of past weed-control practices.

Young adds that he thinks some growers consider weed density on the urgency to spray a field (first), but that the more limiting factor to gain control of certain weeds is size.

The message coming from academia (Extension weed scientists) and the chemical industry in the face of glyphosate-resistant weeds is to control every single weed in the field since the surviving weeds may be those that increase the seed bank with glyphosate resistance, Young continues. "In reality, I'm not sure how we can control every weed in the field today when we obviously weren't able to do it well enough when glyphosate still worked."

A few weeds left in the field will not have a significant impact on yield. However, if weeds are herbicide resistant, the density of these weeds can build quickly and become a problem.

Today, most chemical manufacturers provide a weed-control guarantee. "As a result, fields get sprayed to remove almost any density of weeds, since the company will clean the field up as part of their program," Young concludes.

Dan Davidson can be reached at daniel.davidson@telventdtn.com

(CZ\SK)

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