Pigweed's a puzzler

Few farmers are reporting it, yet the scientists say the weed's spread indicates rising resistance

By TOM BUTTON

Herbicide-resistant pigweed is the weed kingdom's version of the soybean cyst nematode. It's here, but nobody's letting on they've got it.

That's OK, weed experts say, but only if the growers recognize they have a potential epidemic on their hands and are dealing with it, and as long as they aren't letting combines or tillage equipment spread the seed to unsuspecting neighbours.

More dangerous is for growers who'll get a surprise lesson in weed resistance this spring. Yield losses - together with expenses for rescue sprays - are almost always highest in the year of discovery.

There's growing consensus, meanwhile, that growers who plan a diversified weed control strategy can expect to keep all their weeds in check without losing sleep over resistance.

Scientists found Group 2 resistant pigweed spread across seven counties in 1997. Of the 42 suspicious fields they sampled, 30 had pigweed that could survive label rates of Pursuit. Three had pigweed resistant to flumetsulam, one of the active ingredients in the Broadstrike family of weed killers.

Since then, there's been little agreement on the scale of the pigweed puzzle. Brent Schmidt, soybean product manager for Cyanamid, which makes Pursuit, says his company didn't get any calls last spring from growers with weed control failures that could be blamed on herbicide resistance.

"If there was a lot of Group 2 resistant pigweed out there, we would know about it," Schmidt says.

Yet Gabrielle Ferguson believes resistant pigweed has infested thousands of acres. Ferguson, former OMAFRA weed expert, is completing a masters degree at Guelph on weed resistance.

"There are roads in Lambton county where over 30 per cent of the fields have suspicious pigweed escapes," Ferguson says. Counties such as Elgin have flare-ups, including near Iona Station.

"We don't know why farmers aren't reporting," Ferguson says. More likely than not wanting to admit to it, Ferguson believes, is farmers simply saying to themselves: "It's here. There's no sense wasting time calling in somebody to tell me what I already know."

University of Guelph weed scientist François Tardif says it would help scientists understand the nature of weed resistance if farmers reported instances of possible Pursuit-proof pigweed. It could help the farmers, too.

Tardif's lab has confirmed at least three cases where pigweed is resistant not only to imazethapyr, the active ingredient in Pursuit, but also to atrazine and other triazine herbicides, including Sencor. They're testing Patriot, Cyanamid's new pre-blend of atrazine plus imazethapyr, recently registered for use with imi-smart corn hybrids.

In most cases, growers can survive resistance and still spray Pursuit, providing they opt for a tankmix with which both partners hit the pigweed. Normally, the imazethapyr plus atrazine program would be a good choice in corn. In fields with dual resistance, Tardif warns, pigweed may shrug off the spray and then go on to produce a seed crop that will ensure the problem persists for years. When these fields are planted to soybeans, Pursuit plus Sencor or Lexone may fall flat.

Cyanamid says soybean growers can quell the resistance threat by using Pursuit in tankmixes. "Our belief is that Group 2 resistance is a very rare occurrence," Schmidt says. "A program of overlapping tankmixes will make it extremely unlikely."

The key is to hit weeds in at least two ways, agrees Marvin Faber, research agronomist for Dow Agro, which makes flumetsulam-based Broadstrike products for soybeans, along with Fieldstar and Striker for corn.

The two-pronged approach ensures that any weed that's resistant to the Group 2 herbicide would be killed by the partner before it gets a chance to set seed, Faber explains.

All of Dow Agro's flumetsulam products come in pre-blend mixes. In soybeans, for instance, the metolachlor in Broadstrike Dual will provide 70 to 80 per cent control of pigweed, so any plants that escape the flumetsulam will still get hit. The trifluralin in Broadstrike Treflan is somewhat less effective on pigweed, but Treflan merits a good rating in the Ontario weed guide.

With those partners, Faber believes that growers in a two-year corn-soy rotation could use Broadstrike Dual, for instance, followed by Fieldstar or Striker. "After a couple of cycles through the rotation, it would probably be a good idea to throw in a herbicide resistant crop for added safety."

The best solution to weed resistance is prevention, Faber says. The key to prevention, he adds, is picking the right tankmix partners.

The ideal anti-resistance tankmix, Faber says, consists of partners that control the same weeds, have the same effectiveness, share the same amount of residual control and belong to different chemical families.

Tardif suggests considering non-chemical controls, too: "Cultivation will kill weeds that are resistant to the herbicide. Narrow rows will shade them out."

And when it comes to chemicals, build diversity into the program. "We need to keep these Group 2 materials," Tardif says. "They're very effective, and we mustn't put ourselves in a situation where we have to rely on Roundup Ready for everything."

© copyright 1999 Agricultural Publishing Company Limited.


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Mercury rising

Those pigweed escapes may not be pigweed at all. Creeping into farm fields is a new weed that, in the seedling stage, looks almost identical to pigweed.

Three-seeded mercury is common through most of Ontario's soybean belt. Until recently, however, it wasn't common in soybean fields.

Instead, three-seeded mercury has been a weed of ditchbanks, wet fencerows and river bottoms. But as few herbicides can tackle three-seeded mercury, it's moving into fields that, because there are so few other weeds, are an ideal breeding ground.

Peter Johnson, OMAFRA crops specialist based at London, calls three-seeded mercury "an excellent indicator weed of fields that have had too many Group 2 herbicides in the past.

"This one will sneak up on you," Johnson adds. "It looks like pigweed as a seedling, but Pinnacle, Pursuit and all those other Group 2 herbicides don't touch it."

At the University of Guelph, weed scientist Clarence Swanton has tested a wide range of herbicides on three-seeded mercury. Most - including Group 2 products such as Pursuit and Pinnacle - give poor results. In soybeans, Broadstrike Dual and metribuzin applied pre-emerge score a six out of 10. Post-emerge applications of Blazer and Reflex each get a five.

Best option for soybeans, Swanton says, is to plant Roundup Ready varieties. In his tests, a litre of Roundup per acre provided 80 per cent control. Next best, he believes, is the pre-emerge Sencor or Lexone spray.

There are better choices in corn. Atrazine gives effective control, as do post-emerge sprays of 2,4-D. Marksman and Distinct have also scored well.

In Swanton's tests, however, Liberty has provided an average of only nine per cent control.

© copyright 1999 Agricultural Publishing Company Limited.


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Simple escape or dreaded resistant?

Resistant weeds are masters of the surprise attack. Typically, growers see a few escapes one year, and then are inundated the next.

So how do you know whether the pigweed escapes in this year's soybean crop are resistant to Group 2s, or if they're ordinary escapes?

The answer, explains Dow Agro research agronomist Marvin Faber, lies in the following checklist:

* Is pigweed the only escape? It's highly unlikely that resistance will develop in different species at the same time. If other weeds are involved, consider whether the blame goes to poor weather or spray timing.

* Is the pigweed in patches or in strips? Resistant weeds spread with the distribution of seed, so they're most often in circles or in patterns created by the combine. If the weed escapes are in strips, the more likely cause is a plugged sprayer nozzle.

* Is the pigweed thickest where the combine enters the field? Resistant pigweed could have hitched a ride from the previous field. Generally, normal spray escapes are more evenly distributed across the field.

* Has the weed control program relied on Group 2 herbicides to knock out pigweed. Research shows it takes three to four years for a resistant pigweed population to build up, and considerably longer if the Group 2 has been backed up with a tankmix partner.

© copyright 1999 Agricultural Publishing Company Limited.


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Winning the white mould war

The disease is unpredictable, but basic precautions can keep it at bay

BY TOM BUTTON

Come mid-August, it's a sure thing many Ontario soybean growers will be scratching their heads, wishing they'd listened to the warnings about the rising risk of white mould. Harder to predict is which fields will have been hit, and how many bushels they'll lose.

Also impossible to predict is how many growers will have heeded the warnings and set up a white mould defence system, rotating out of soybeans, when they could have grown a perfectly healthy soy crop without any white mould precautions at all.

If it sounds like a gamble, disease specialists such as Greg Boland at the University of Guelph have figured out a way to tilt the odds in your favour.

"We can get ahead of white mould in soybeans," Boland says. "Fortunately, strategies such as no-till and rotations make a lot of sense from many perspectives, not just white mould control."

Provincial disease expert Albert Tenuta believes it's important that growers listen to the advice they're getting from white mould watchers. "All I can tell you is, no matter what the weather, we'll get more white mould this summer than we had five years ago with the same weather," Tenuta says. "This is definitely a disease on the rise."

Boland blames factors ranging from non-stop continuous soybean production to the expansion of soybeans into areas with other white mould susceptible crops, including white beans and canola. Tenuta often sees fields suffering losses of 10 bushels per acre and more.

Because white mould in Ontario is rarely spread from field to field by the wind, growers can manage their crops on a field-by-field basis, Boland says. Even if - from a white mould management perspective - one grower does everything wrong, neighbouring farmers still have a reasonable shot at keeping their fields clean.

Start with crop rotation, Boland advises. The severity of a white mould outbreak is related to the number of spores in a field. That in turn is related to how many soybean, edible bean, canola and sunflower crops have been grown in the recent past.

The best option is to insert a grass crop, such as corn or wheat. "Rotations always make sense for pest control," Boland says. "They'll reduce the risk of other diseases, too, not just white mould."

Also helpful is to opt for no-till instead of a chisel-plow based conservation tillage system, he adds. University of Guelph trials show that no-till cuts spore loads roughly in half.

Boland thinks no-till helps because the pellet-like over-wintering bodies called sclerotia don't survive as well when they're left on the soil surface.

Because wind dispersal isn't a big factor in Ontario, Boland stresses, strategies such as rotations and no-till will not only prevent white mould pressures from climbing, they can also reduce them. Don't expect overnight results, but even a field with some history of white mould losses can be made safer.

Until the pressure is reduced, it may still make sense to choose seed with white mould in mind. Companies often have white mould ratings for their varieties, although the information isn't in the provincial variety guide or vetted by third-party experts.

In fields with a history of severe white mould losses, it may also be worthwhile to plant varieties that are 200 to 300 heat units shorter than typical main crop for the area. These varieties may cheat white mould by racing though the sensitive early flowering stage before the spores are released.

© copyright 1999 Agricultural Publishing Company Limited.


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