Article from Syngenta Crop Protection

Leading weed scientists caution Midwestern U.S. soybean growers not to cut their spring weed control programs short by spending too much of their herbicide budgets in the fall. 

In a recent issue of  The Ohio State University “Crop Observation and Recommendation Network” (C.O.R.N.) newsletter, Dr. Mark Loux, professor in the Department of Horticulture and Crop Science at OSU, writes, “Excess money spent on fall treatments results in less money available for weed control in the crop, where it usually is greatly needed.”

In the newsletter, Loux recommends that growers try to keep their fall treatments in the $6 to $12 range, excluding application costs.  For treatments that cost more than $12, he questions their value. 

“One of the reasons for this is that the use of a fall treatment, even one with residual activity, does not guarantee that only one herbicide treatment will be required in Roundup Ready® soybeans the following year,” Loux explains.  “Our experience has been that the primary benefit of fall treatments is control of weeds that are present at the time of treatment, not residual control of weeds the following year.”

Loux points out that residual herbicides are needed to provide control of summer annual weeds, such as waterhemp and ragweeds, after planting and into late May to increase the effectiveness of post-emergence glyphosate applications and create more flexibility in post-emergence application timing.  Most residual herbicides applied in the fall don’t last long enough to do this, he says. 

Lasting long enough is not the only concern with many fall residual treatments. Two weed species that are increasingly threatening the yield potential of soybeans are waterhemp and Palmer amaranth, which are in the pigweed or amaranth family. Populations of these species are already resistant to ALS inhibitor herbicides, which include some of the most common fall residual treatments like Scepter® and Canopy® EX. 

For growers who want to maintain relatively clean fields in the off-season, Dr. Duane Martin, herbicide brand manager for Syngenta Crop Protection, recommends that they use a cost-effective burn-down product of choice like Touchdown® or Gramoxone Inteon™ plus 2, 4-D in the fall and reserve their residual arsenals for the spring.  

Again, from the OSU C.O.R.N. newsletter, Loux states, “The utility of most residual herbicides is maximized when applied in the spring prior to planting (pre-emergence), not in the fall,” he says.  “A better alternative is use of a non-residual herbicide program in the fall, such as glyphosate plus 2, 4-D, followed by a spring application of residual herbicides.”

Research data from OSU reported in “The Benefits of Preemergence Herbicides in Roundup Ready Soybeans” (agcrops.osu.edu/weeds) verifies just how effective pre-emergence applications of residual herbicides in the spring can be.  In two studies conducted in west central Ohio, the use of pre-emergence herbicides in combination with a post-glyphosate application increased yield by 4 to 9 bushels/A, compared to post glyphosate alone.  Because of the protection that pre-emergence herbicides provide against yield loss from early season weed competition, authors of this publication, who included Loux along with his OSU colleague Jeff Stachler and Purdue weed scientists Dr. Bill Johnson and Glenn Nice, conclude, “You (soybean growers) can’t afford not to use PRE herbicides!”

A major advantage of the pre-plus-post approach, compared to total post, is that the pre-emergence herbicide will often provide enough weed control to prevent the risk of yield loss even if weather delays the post application.

Martin points out that weather delays are commonplace in the Eastern corn belt region, which makes the use of a pre-emergence herbicide even more critical.  “The use of an effective residual herbicide will allow you to time your glyphosate application post-emergence with a lot more flexibility,” he says.  “If you apply a pre-emergence material upfront and run into a situation where the weather is bad and you can’t get into the field to spray post-emergence, your yields are protected until field conditions improve.”

When choosing an effective pre-emergence treatment in soybeans, OSU and Purdue weed scientists recommend in the “Weed Control Guide for Ohio and Indiana” that growers choose a material offering “activity on key broadleaf weeds that also provides at least some early-season control of grasses.”

New for 2007 is a herbicide option that meets this criteria – Prefix®. The science behind this material helps explain why.  Prefix contains two proven active ingredients, S-metolachlor and fomesafen, delivering two different modes of action for control of broadleaf and grass weeds.  These alternate modes of action work together against species of ALS- and glyphosate-resistant broadleaves, such as waterhemp and Palmer pigweed, making Prefix a key player in the battle against resistance. 

Looking ahead to the 2007 season, battling herbicide resistance and protecting profits will continue to be top priorities for soybean growers.  Martin predicts that despite the projected boom in corn production, the economic outlook for Midwestern soybean producers is positive.  “While ethanol demand may decrease soybean acreage to an extent, the prices are likely to stay very strong,” he says, “so growers should take all the steps required to maximize their profits.”

One of those steps is reserving residual herbicide treatments for the spring when they can make a real difference in a grower’s bottom line.

Touchdown®, Gramoxone Inteon™ and Prefix® are trademarks of a Syngenta Group Company.  Gramoxone Inteon is a Restricted Use Pesticide.  Always read and follow label instructions before buying or using this product.

Roundup Ready® is a registered trademark of Monsanto.
Scepter® is a registered trademark of BASF Corporation.
Canopy® EX is a registered trademark of E.I. du Pont de Nemours and Company.