Weed control issues

BROOKINGS, S.D. -- With high-priced corn, it doesn't take much convincing to justify protecting crops with herbicides. The question is how to evaluate the payoff in a period of rapidly rising herbicide prices.

Mike Moechnig, a South Dakota State University Cooperative Extension Service weed specialist at Brookings, says farmers are concerned, among other things, about an inconsistency they've seen in glyphosate effectiveness.

Usually when weeds get larger they can get less susceptible. Cool, wet conditions can make weeds less susceptible. And certain species like wild buckwheat are harder to control, leading to weed escapes in corn and soybeans.

Application methods and drift-reduction technologies -- spray nozzles and spray additives that reduce herbicide drift -- may contribute to inconsistencies, which part of Moechnig's research work. One puzzle is that SDSU has been getting better control with Roundup than growers are getting on larger-scale.

"We want to look at this in South Dakota conditions, where conditions may be drier, too," Moechnig says.

"What are rate-type options we can use to keep costs in check but still get consistency and control?" Moechnig asked a wagon-load of tour-goers at a recent SDSU field day in Brookings. "Some farmers have gradually moved to higher rates of glyphosate to get consistency of control, but prices of glyphosate are increasing."

Big choices

With more money at stake because of high commodity prices, farmers may have made some changes in weed control in 2008, but they may make bigger choices in 2009.

"It's tough," Moechnig says. "You've got to control the weeds. There's not much rate-cutting you can actually do. I would think the best approach -- rather than trying count on adding something with a 'post' application, it is really important to get that 'pre' out there. It seems that is by far the most consistent programs, with a 'pre-' followed by a 'post-' application."

Moechnig says there is a lot of collaboration between extension and chemical manufacturers on research trials.

He often looks at new products and has flexibility to his own it to his own trials to see whether there are alternative ways of using some of these products. Some of trials at Brookings show "alternative" tank mix partners for glyphosate, or show how using glyphosate as a burn-down in cereal crops may offer residual weed control.

"There's probably an investment to be made (in weed control) despite recent hikes in glyphosate prices," Moechnig says. "Cost is always a concern, but a lot of what we're looking at is geared toward convenience and getting consistency."

One of Moechnig's big tour topics was the prevalence of "volunteer" corn as a weed -- either in soybeans or in corn.

Different infestations

Volunteer corn in soybeans is not a new issue, but there is not a lot of published or recent data to show the effect of volunteer corn on soybean yields -- particularly in drier climates.

"It turns out with a high volunteer corn density you maybe get a 10 percent yield impact from volunteer corn in corn," Moechnig says. "That's not much of an issue, not nearly what it is in soybeans."

He and others are working on recommendations on what level of infestation justifies the cost of a grass herbicide in either corn or soybeans. He hears that occasionally there may be sales programs available in which farmers who buy certain brands of glyphosate will also get a grass herbicide, if needed.

Volunteer corn in soybeans appears to be the bigger issue.

"It only takes about 400 corn plants per acre of soybeans -- about 1 corn plant per 10-by-10 foot area -- to cause a 5 percent loss in the soybean yield," Moeechnig says. "It doesn't look like the volunteer corn is doing a lot (of damage) but it is."

The illusion is that the corn doesn't do a lot of shading. "You don't think the competition is there from the corn, but somehow, through modeling, we model the competition through light availability. But we can't do it, we can't predict the competition based on light alone. There's probably moisture competition and things like that going on in the soil that's an issue."

Moechnig grew up in southeast Minnesota and earned his doctorate and master's degrees at the University of Wisconsin-Madison and did post-graduate work in weed population dynamics at the University of California-Davis. On Sept. 1, 2005, he replaced Leon Wrage, who retired from the post in 2004.has his own flexibility trials to see whether there are alternative ways of using some of these products. Some of trials at Brookings show "alternative" tank mix partners for glyphosate, or show how using glyphosate as a burn-down in cereal crops may offer residual weed control.

"There's probably an investment to be made (in weed control) despite recent hikes in glyphosate prices," Moechnig says. "Cost is always a concern, but a lot of what we're looking at is geared toward convenience and getting consistency."

One of Moechnig's big tour topics was the prevalence of "volunteer" corn as a weed -- either in soybeans or in corn.