The toughest weed to control in wheat is also the toughest on yields.

Wild oat, Aventa fatua, is a bad weed. Distinguished from cultivated oats by a round "sucker mouth" callus at its base, it can pose a threat to wheat yields that, plant for plant, is several times worse than damage from foxtail. It's understandable, then, that most wheat growers would like to eradicate wild oats from their fields. Last year, producers treated an estimated 1 million acres of wheat in an attempt to control wild oats. Sometimes, the herbicides didn't kill it.

Amazingly, the moisture-loving weed virtually vanished during the droughty period that stretched from the late '80s to early '90s, and wheat growers began to believe wild oats had become extinct. But wild oat seed can remain dormant in the soil for 15 years and perhaps even as long as 40 years. So when cool, moist springtime conditions returned by the mid-'90s, wild oats germinated again.

Resistance comes quickly. Richard Zollinger, weed specialist at North Dakota State University, says that using the same herbicide modes of action on wild oats year after year let populations become resistant relatively quickly. "One of our first discoveries was that eight to 10 consecutive applications of the herbicide Hoelon, an ACCase inhibitor, resulted in resistant populations of wild oats. Then we found that these resistant biotypes also resisted Puma, even though Puma was brand new at the time."

Nearly epidemic in Canada, ACCase-resistant wild oat populations also are becoming more common in North Dakota and Minnesota. Today, ACCase herbicides include old standbys as well as some of the best and newest post-applied wild-oat herbicides on the market, including Achieve, Cheyenne, Dakota, Hoelon, Puma, Tiller and newly released Discover. All are susceptible to wild oat resistance. If there's a bright spot, Zollinger says, it's that all ACCase inhibitors are not alike. "We can break the ACCase inhibitors into the `dims' [chemical families with names ending in `dim,' such as sethoxydim] and the `fops' [chemical families with names ending in `fop,' such as fluazifop]. Wild oats resistant to dims might not necessarily be resistant to fops, so it's important to read the labels and consider each herbicide individually."

Lest you think that Far-Go, Buckle and Avenge are immune, Zollinger says wild oat biotypes resistant to these lipid synthesis inhibitors have been identified in Montana and North Dakota. "When we're looking at $2 wheat, there just isn't sufficient financial incentive for chemical companies to come up with lots of new chemistries," Zollinger laments. "I think wheat growers are starting to realize that we might not get something totally new anytime soon, so we've got to rotate, manage and preserve the tools we have."

Although not a new mode of action, Discover herbicide from Syngenta is the newest ACCase inhibitor to control wild oats and foxtail in wheat. The company says this clodinafop uses a unique crop safener called cloquintocet-mexyl to allow maximum weed control and crop safety. Discover is labeled for spring wheat, but not winter wheat or barley.

Emmett Lampert, technical support representative for Syngenta, says he expects Discover to continue gaining popularity across northern wheat-growing areas. "Last year about 150,000 acres of wheat were treated with Discover and we had practically no complaints on performance. Plus we have three years of university and cooperative trials that show 96% control of wild oats, considerably better performance than competitors' herbicides tested in the same trials."

Rotate modes of action. Lampert agrees with Zollinger on the importance of resistant weed management. "Growers should rotate herbicide modes of action whether they are rotating to other crops or following wheat with wheat," Lampert says. "And with a prolific seed producer like wild oats, it's best to strive for the highest level of weed control possible when weeds are present. Fewer escapes mean better yields and fewer weed seeds for the coming years. Switch chemistries, rotate crops and cultivate to get maximum control."

One option to avoid overuse of ACCase inhibitors is to use ALS-inhibitor products. Still, Zollinger points out that you don't want to overuse ALS herbicides either. "Everest is the new ALS herbicide for wheat, and yet we found wild oats in Walsh County, ND, that are resistant to it. We suspect this is because another ALS inhibitor, Assert, had already been overused in that region."

Al Scoggan, regional research and development manager with Bayer, says that though Everest is an ALS inhibitor, its new flucarbazome chemistry is somewhat different from that of Assert and most sulfonylurea herbicides. "So far, our testing shows that cross-resistance with Assert has been rare and we would expect it to stay that way because of the differences in chemistry," Scoggan says.

"While cross-resistance seems to have happened in Walsh County, it's interesting to note that wild oats in that county have shown an unusually high incidence of resistance to several types of herbicides," he continues. "It's hard to say exactly what is going on there. But overall, it's definitely good management to rotate modes of action as you rotate crops. We surveyed wheat growers and found that more than 63% are concerned about ACCase-resistant wild oats, which helps make ALS inhibitor Everest herbicide an especially attractive option. Everest is the first ALS inhibitor for wheat that controls both wild oats and foxtails. And it provides a two- to three-week residual, so you can spray earlier with less worry of missing emerging weeds."

Scoggan says growers need to keep good records and look at the herbicide and weed history of each field in order to identify patterns of herbicide overuse. "You may determine that the best strategy is to use tankmixes with different modes of action. Everest is highly tankmix compatible with most grass and broadleaf herbicides, but we recommend against any tankmix with dicamba, as it can cause some interference with Everest performance."

No guarantees. Herbicide resistant or not, wild oats are a challenge to control. While the active ingredients of many herbicides have basically the same mode of action, growers often experiment to find the particular product or products that work best for them. Formulations for each herbicide differ to some degree, resulting in different levels of control and crop safety in particular soil and weather conditions. As conditions differ each season, there's no guarantee that one particular herbicide will work best year after year. "Growers do seem to be moving to postemergence herbicides to control wild oats," Zollinger says. "If it's a dry spring, wild oats won't be a problem and the preemergent product goes to waste. Sometimes nature skips spring in the Dakotas, going straight from winter to summer. Hot and dry means foxtail will be the biggest problem, which may require different herbicides for the best control."

Timing is everthing with postemergence herbicides. Though some of the newer herbicides can provide control up to the six-leaf stage of wild oats, it's usually best to hit the wild oats weed when it is small (before the two- to three-leaf stage) for optimum control. The tricky part is not spraying so early that you miss the bulk of emerging weeds.

Count the leaves. If you go with a postemergence strategy, an accurate leaf count can maximize control with each herbicide. Check the herbicide label, then count the leaves on the main stem of wild oats and disregard the tillers. The youngest leaf is counted as a full leaf only when another leaf becomes visible. Lower leaves, which may have died from various stresses such as frost, also should be counted in the total leaf number. For crop safety, each herbicide label has specific recommendations on precisely what crop growth stage to spray at. That means reading the label carefully and getting out in the field to count emerged leaves and internodes.