You plan years ahead for crop rotation, fertilizer  and machinery acquisitions. Since the rise of glyphosate-resistant weeds, you should be doing the same for your herbicide  program, says Lisa Behnken, a Minnesota Extension crops specialist.
She urges growers to devise a multi-year plan to increase herbicide diversity. Adding multiple, effective modes of action and rotating herbicides are “key steps to prevent resistant weeds from gaining a foothold on your farm.”
Purdue University weed scientists Travis Legleiter and Bill Johnson suggest a similar exercise to help diversify weed control. “We encourage growers to go after herbicide management from a long-term standpoint,” says Legleiter.
Look back 2 or 3 years at your past chemical use, Behnken suggests. “Look for red flags,” such as repeated use of the same products and over-reliance on a postemergence program.
Next, Behnken says, write out herbicide plans for the next two seasons, targeting the worst weeds in each field. Study herbicide performance charts from your local Extension service or herbicide distributor.
Then record the site of action for each herbicide you’ve selected. That’s the specific biochemical pathway that the herbicide uses to disrupt plant growth. North Central weed scientists publish a “Corn and Soybean Herbicide Chart” that lists sites of action for individual herbicides and premixesat http://bit.ly/1aIeCIj 
This information is also on herbicide labels. Tally the sites of action your plan deploys: How many are effective against your target weeds?
Now take a critical look at your plan, Behnken says. A diversified herbicide program should:
- Use at least two or three effective sites of action against your worst weeds
- Avoid overusing any site of action in a single growing season
- Rotate herbicides so you are not using the same sites of action consecutively
An ideal herbicide program maximizes the number of effective sites of action without using any site of action more than twice in a growing season, Legleiter says. “Three or more passes of the same site of action really raises selection pressure.”
Missouri farmer Ryan Britt is working to diversify his weed program, but “it’s very difficult,” especially in soybeans, he says.
Britt raises corn, soybeans, wheat, alfalfa and cattle with his father and brother-in-law near Salisbury, Mo. Like many Corn Belt farmers, the Britts are battling ALS- and glyphosate-resistant waterhemp and marestail. Adding to their challenges, the Britts’ no-till cropping system includes some soybeans after soybeans fields, which limits their ability to rotate herbicide sites of action on those acres.
In soybeans, they had been applying a one-third to one-half “set-up” rate of Authority XL with their burndown of glyphosate and 2,4-D. Recently, they’ve had to step up to the full residual rate before planting. The past two seasons, they’ve followed with a postemergent application of glyphosate plus a tank mix of Ultra Blazer or Marvel.
This program deploys four sites of action (SOA): groups 9, 4, 2 and 14. But only two groups — 4 (2,4-D) and 14 (Authority XL, UltraBlazer or Marvel) — are considered effective on herbicide-resistant waterhemp, the Britts’ primary target weed.
To beef up control in 2013, Britt had planned to apply a full rate of Authority XL with the burndown, followed by an early postemergent application of glyphosate plus Prefix (SOA 14, 15). That would have provided overlapping residual control of waterhemp, and an additional site of action, group 15.
But late planting scotched that plan. The Britts’ agronomist, Jason Young, Agrivision, Salisbury, Mo., suggests a fall application of Anthem (SOA 14, 15) to control fall-emerged glyphosate-resistant marestail and winter annuals.
This past season, Britt got decent control of resistant waterhemp. But he worries about the selection pressure being placed on the PPO inhibitors as a result of repeated use, both preemergence and postemergence, in consecutive years. “We’ll have to be very careful that we don’t get resistance.”
Some of this selection pressure could be relieved by rotating to a Liberty Link system and substituting glufosinate (SOA 10), Young says. But the selection of Liberty Link soybean genetics for Missouri is limited, he adds, and the herbicide is also in short supply. “We’ve not used Liberty Link technology yet,” Britt says, “but it’s an option.”
Rotate to succeed
Because the PPO inhibitors are among the few postemergence herbicide options for resistant waterhemp in soybeans, you should avoid using them in corn, says Mark Bernard, an independent agronomist from New Richmond, Minn. “If you use a PPO in beans, then stay away from herbicides such as Sharpen (SOA 14) or Verdict (SOA 14, 15) the next year in corn.”
Fortunately, corn offers more opportunities to switch up herbicide families and still control waterhemp and common and giant ragweed — the main target weeds for Bernard’s southern Minnesota growers. Depending on your weed spectrum, he says, you could shift to products such as Hornet (SOA 2, 4), Harness (SOA 15), Sure Start/Triple Flex (SOA 2, 4, 15), or group 27 products, such as Callisto, Laudis or Impact.
Yet, even if you rotate crops annually, “it can be tough to change sites of action each year,” says Kevin Bradley, University of Missouri weed scientist. For example, common soybean residual herbicides such as Dual and Warrant have the same site of action — group 15 — as widely used corn herbicides, including TripleFLEX, Verdict and others.
But “If you use multiple sites of action each year and rotate crops, you are more likely to get adequate diversity,” Legleiter says.
The Britts put down Guardsman Max premix (SOA 5, 15) before planting corn, followed by an early postemergent application of glyphosate plus more atrazine (SOA 9, 5). They came back with a second postemergent pass of Armezon (SOA 27) to extend waterhemp control. Counting the burndown (SOA 9, 4), their corn weed program supplies five sites of action, four of which are effective on waterhemp.
Preemerge adds sites of action
Your plan should marshal at least two — and preferably three — sites of action for key weeds, says Mark Storr, BASF technical services representative, Nevada, Iowa. The best way to add sites of action: effective, preemergence residual herbicides, he says.
“Effective” is the watchword. You haven’t diversified your chemistry if, say, one of the two sites of action deployed doesn’t control the primary weed in that field, he says. “That’s often lost on people.” Also, “efficacy varies” within the same chemical family, Storr adds. “You need to understand which products control which weeds, so you can pick the right products for your target weeds.”
That starts with proper weed identification, says Young, the Missouri agronomist. In Roundup’s heyday, it wasn’t as important to target specific weeds “because Roundup killed them all.” Go after the most troublesome weed first and get it under control, Bradley says. Do this field by field.
Although a field’s overall weed spectrum usually changes fairly slowly, sometimes a new weed can dominate in short order. “We’re seeing that happen now in Indiana, Ohio and Michigan, where all of a sudden, they’ve got Palmer pigweed,” Bradley says.
So “plans need to be flexible,” Bernard says, “but the cornerstone is a preemergent application before both corn and soybeans.”
Postemergence glyphosate tankmix partners should also be part of your diversification strategy, Bradley says. These treatments “must be planned, they cannot be applied as a rescue. You can’t wait until you see 6-inch waterhemp,” for example, “because you’ve lost the battle by then. It’s all about timing.”
Think about herbicide resistance risk
You should also try to include sites of action with a relatively lower risk for developing resistant weeds, Storr says. For example, group 15 long chain fatty acid inhibitors have been on the market more than 40 years and have shown great resilience, Storr says. In the U.S., just one weed species has overcome it, making this chemistry a lower-risk choice for resistance management. Contrast that to Group 2 ALS inhibitors: in 30 years, 44 U.S. weed species have become immune to this class of chemicals.
Farmers are eagerly awaiting new herbicide-tolerant seed technologies, such as dicamba- or 2,4-D-tolerant soybeans, which will offer additional postemergence options. But if growers aren’t careful, Behnken warns, these new seed traits will soon face the same threats as Roundup Ready technology. “You will still need a plan for herbicide diversification, or we’ll develop resistance to these new tools, too.”
Cover crops, rotation part of grower’s integrated plan
It’s the first week of September, and Ryan Britt is seeding a cover crop of annual ryegrass, tillage radish and crimson clover into wheat stubble. Britt raises corn, soybeans, winter wheat and cattle in north-central Missouri. He is battling herbicide-resistant waterhemp and marestail in his continuous no-till operation. He plants cover crops “primarily for weed management.”
Annual ryegrass helps suppress waterhemp, Britt says. And the residue shades the ground, inhibiting waterhemp germination during the growing season, says his agronomist, Jason Young of AgriVision, Salisbury, Mo. University of Arkansas 2009 and 2010 research found that a fall cereal rye cover crop significantly reduced glyphosate-resistant Palmer amaranth emergence in soybeans the next season. Palmer amaranth is a close cousin to waterhemp.
The cover crop doesn’t eliminate the need for herbicides, Britt says, but it does cut weed pressure. He also plants soybeans in 15-inch rows to help manage weeds. Two other rotation crops, wheat and alfalfa, also aid weed management.