Pigweed offensive called for
BY TOM BUTTON
Does it matter if you get Pursuit-resistant pigweeds in your fields? The answer is, "maybe." It all boils down to the strategy you put in place to counteract the threat.On most farms, in fact, the damage is worst in the year the resistance is first discovered, when growers are taken unawares by a sudden flush of pigweed competing with the crop through the critical weed control period.
For many Ontario soybean growers - perhaps hundreds - that year is 1998.
"I believe we're going to see an explosion in pigweed escapes," says Gabrielle Ferguson, researcher for the federal agriculture department. Ferguson's lab tests have confirmed that resistant pigweed has infested farms in 10 townships over seven counties.
Results so far are preliminary, but the news is likely to get worse. Ontario government crop advisers collected seed samples from 42 southern Ontario fields last fall. It wasn't a random sample: The seed came from fields with pigweed escapes, not necessarily from fields where resistance was suspected.
Of the 42 samples, 30 are resistant to label rates of Pursuit. Three of those are also resistant to flumetsulam, one of the active ingredients in Broadstrike.
How many acres are affected? "Thousands," Ferguson says. "We'll find a lot more this summer."
Growers who see pigweed escapes should call their provincial soil and crop advisers right away, she says. The advisers will look at a number of variables, including:
* Pigweed as the only escape. If other weeds have also broken through, the problem is likely a total weed control failure (for example, too little rain for activation of soil herbicides) as opposed to resistance.
* The pigweed is in patches or areas, suggesting it is spreading out from where a single resistant pigweed set seed that was spread by the combine and tillage a couple of years previously. Alternatively, the patch could be on the headland where the combine entered the field last year after harvesting another field that was contaminated with resistant pigweed.
* Whether the weed control program relies on Group 2 herbicides for pigweed.Ferguson recommends keeping an eye on soybean fields and calling in the experts when suspicious escapes are still very small, so there's time to re-spray before they start hurting yields.
She offers two other good reasons for keeping a close eye on fields. It takes a few years for resistant pigweed to build up to epidemic proportions. Most growers don't know when 30 per cent of the pigweed in their field are resistant. The next year, when 90 per cent are resistant, they're impossible to ignore. Growers who spot the problem at the 30 per cent stage, however, have a better chance of stopping the spread before they lose any bushels, or have to shell out extra dollars for rescue sprays.
As well, 1998 is likely the last year that growers will be able to get their pigweed seed tested for free.
Herbicide companies are telling growers not to over-react. George Annette, research manager, Eastern Canada, for Cyanamid, which makes Pursuit, says the resistant pigweed can be controlled with tankmixes.
Registered Pursuit tankmixes with partners that rate excellent for pigweed in Publication 75, the Ontario weed control guide, range from Treflan to Sencor and Lexone, and Basagran and Pinnacle.
Annette points out that atrazine is still one of the most popular corn herbicides, more than 20 years after the scare over triazine resistant lamb's-quarters. "It's a question of value," Annette says. "Growers see the value in atrazine and have learned how to manage it. The same will happen with Pursuit."
Dow Agro rep Jerry Olechowski points out, too, that the Ontario tests have been conducted with flumetsulam, which is only one of the active ingredients in the Broadstrike programs. Growers who apply Broadstrike Treflan will get excellent control from the trifluralin, and those who apply Broadstrike Dual can expect the metolachlor to control 70 to 80 per cent of pigweeds, greatly reducing the odds that resistance will develop.
Olechowski says Dow resisted pressure from farmers to introduce a pure flumetsulam product for soybeans. It opted instead for pre-formulated mixtures that reduce resistance risks.
DuPont is also introducing new Group 2 products, including Classic. And like the other companies, it is telling growers to plan an anti-resistance strategy based on herbicide and crop rotation.
At the same time, DuPont is telling growers not to stampede away from Group 2 products, which are some of the most effective, lowest-rate herbicides in the world. Company rep Kerry Teskey notes DuPont's Glean herbicide was the first Group 2 weed killer to be linked to herbicide resistance, when Prairie farmers had trouble controlling kochia and chickweed more than a decade ago. "We thought it might take over the West," Teskey says. "Instead, 10 years later, resistance is restricted to 20 or 25 per cent of the acreage."
If resistance is confirmed, Ferguson is telling growers to be sure to rotate their fields - and to stay away from Group 2 products, probably for a couple years. When Group 2s are used again, make sure they're tankmixed with a partner that will kill the pigweed, she says.
© copyright 1998 Agriculture Publishing Company Limited.
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Track herbicide cost-yield advantage
When does it pay to spray? Which herbicide has the most profitable combination of low cost and high crop yield?Those are the toughest herbicide questions. Now, a federal agriculture department computer program promises to give precise answers for growers and crop advisers.
After its introduction last year, the program is on the market in two versions, reports Susan Weaver, weed scientist at the federal research station at Harrow. An updated version of last year's DOS program is called Ontario HERB. A new Windows version is called HADSS, short for herbicide application decision support system.
The program runs through a number of steps to produce a herbicide recommendation based on the cost of the herbicide, the expected value of the crop, the effectiveness of the herbicide for the weeds in the field, and the yield reduction that can be expected if any of the weeds escape the treatment.
To use the program, growers scout their fields and input the average weed emergence, along with economic factors and the yield that would be expected if the field were weed free.
The program then searches through its memory banks of performance results of registered treatments and ranks them in order of economic value. (See PC Weed Control).
The program can also rank the herbicide options in order of which will do the best job of killing the weeds, regardless of cost.
Crop consultants such as Bill Stevens, who works for Cargill at Essex, are lining up for their copies.
"My job depends on making the recommendations that make the grower the most money," Stevens says. "Sometimes a grower will want to use a favourite treatment, but with the program you can print out in black and white what the costs and benefits are going to be."
Stevens sees the program as helpful for growers considering a rescue treatment for weed escapes. "There isn't much sense spending the money if you aren't going to get a yield benefit," he says. "The program helps identify that benefit."
Both versions cost $160 and can be ordered by calling Weaver at (519) 738-2251 ext. 478, or via the station's website at www.res.agr.ca/harrow.
Current versions are restricted to post-emerge herbicides, based on weeds emerging at the same time as the crop. -Tom Button
© copyright 1998 Agriculture Publishing Company Limited.
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Explaining biotech
Farmers understand biotechnology much better than most people. Here's a simple "grammatical" way to explain it to non-agrarians.Think of a plant's DNA as a long, detailed sentence that exactly describes all of its characteristics. Plants have many of the same words in their sentences to describe common characteristics, i.e. roots, leaves, chlorophyll. Corn plants have nearly all the same words, with a few unique words describing the different characteristics of each variety.
Scientists have not only identified the "word" that describes the toxin in the bacteria Bacillus thuringiensis (Bt) but have also removed and inserted it into a corn variety's sentence. One of the main challenges here is to ensure that the added or "novel" word or words do not adversely affect the variety's sentence or any of the other words. Depending on the species, these sentences can be millions of words in length.
© copyright 1998 Agriculture Publishing Company Limited.
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We used to think of winter wheat as a trouble-free crop. In recent years, with headlines about fusarium and bunt, more growers are wondering whether the crop has suddenly become more vulnerable to diseases.In truth, most of the wheat diseases we commonly see in Ontario crops have been with us probably as long as we've been growing the crop: We just haven't paid much attention to them, mostly because we're preoccupied with other farm jobs.
Many growers also believe that it takes a lot of training to identify wheat diseases, so they are not able to interpret the disease ratings in the variety charts published in Publication 296, the province's Field Crop Recommendations guide. A number of disease and disease-like symptoms are easy to see, however. The most common are described below.
Yellow strips between the veins, about a half-inch long and a sixteenth-inch wide, appearing in May, often indicate infection by the virus that causes wheat spindle streak mosaic. Symptoms will disappear as temperature increases. Generally, yield is not significantly reduced.
Yellowish, dwarfed plants with yellow leaf tips may be caused by a different virus, appropriately called the yellow dwarf virus. It is transmitted by aphids the previous autumn and may be serious if widespread. There is no control possible at this stage. Crops should be safe from the disease if planted after frost has killed the aphids.
Gray mouldy spots on lower leaves, but sometimes reaching as high as the head, are caused by the fungus powdery mildew. Fungicide application may be warranted on susceptible varieties, especially the soft red 2510, but for most varieties the yield reduction is not often great, and few growers resort to fungicide application.
Yellow oval flecks on leaf surfaces could be caused by septoria, which can also cause spotting on the chaff. This is rarely a serious concern in Ontario, and there is little difference in variety tolerance. Last year, a phenomenon called physiological spot produced similar symptoms, especially on Freedom and to a lesser extent Ruby. The damage is like sunburn and is linked to toxic activity of solar ultraviolet light. It appeared in 1997 when crops grew rapidly through a number of cloudy days and then were scorched by a sudden switch to intensely sunny weather.
Pre-mature head bleaching is a sign of fusarium head blight. Closer observation may reveal salmon-coloured fuzz in the heads. At this stage there is no effective control. If the level of infection is less than a few heads per thousand, the yield reduction will likely be insignificant and the accumulation of vomitoxin will likely be below the permissible threshold. Fusarium infection is related to wetness during flowering. Except for avoiding planting wheat following either corn or wheat, there is little growers can do to control the disease. Federal breeders hope to develop a source of genetic resistance within the next five years.
Dark red to black spots often appear just prior to ripening and are caused by stem rust. If these appear in early July after the plants have started to dry down, the effect on the crop may not be significant. If the disease appears earlier, it may reduce yield and cause shrivelled kernels with low test weight, quality and grade. Stem rust can be controlled by fungicide, but spraying is rarely warranted. Leaf rust may produce orange-red spots on the leaves, produced by infections that start with spores that blow north from the U.S. Again, spraying is rarely warranted.
Black powder in the head that soon drops off is a sign of loose smut. This fungal disease rarely appears if the seed has been treated with Vitaflo. Loose smut rarely reduces yield.
Dr. Teich is former wheat breeder for the federal agriculture department at Harrow and is currently research consultant for the Ontario wheat marketing board.
© copyright 1998 Agriculture Publishing Company Limited.
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Spray-rate findings in
Corn is the best crop to start with for growers who want to try shaving their herbicide rates below label recommendations.Even better, keep the cultivator hitched to the tractor, say weed scientists at the federal agriculture department's Harrow research station.
The researchers have just finished a worldwide search of scientific studies that compare label rate and reduced rate applications, including results from some 2,000 treatments over 20 years.
"There's a much greater opportunity to reduce herbicide rates in corn compared to soybeans," reports Harrow scientist Susan Weaver. "It's probably due to the canopy in corn providing more effective competition.
"It appears we can cut our herbicide rates quite a bit in corn and still get at least 80 per cent weed control," she says. Best trial results came from pre-emerge treatments in corn, followed with inter-row cultivation, Weaver says. Newer post-emerge products also worked well at low rates, but performance depended on variables including weed species, growth stage, environmental conditions and adjuvants.
Researchers at Harrow and the University of Guelph have launched a series of trials trying to find ways growers could spray at what are called "biologically effective rates." Each herbicide has strengths and weaknesses. Growers may need only a whiff of one herbicide to kill lamb's-quarters, for instance, but the full label rate to knock out velvetleaf.
For post-emergent sprays, weed control also depends on weed stage. Many herbicides are effective at low rates against tiny sprouts, but need the full rate for bigger seedlings.
The research team is aiming to give growers advice on how much herbicide they need for specific weed scenarios. Weaver warns, however, that low-rate herbicide spraying isn't free. Growers may save on their chemical bills, but they'll have to invest more in learning about weed identification and herbicides, and spend more time scouting their fields.
Even so, many growers are already partly prepared, she thinks. "The level of grower expertise has definitely improved in recent years." -Tom Button
© copyright 1998 Agriculture Publishing Company Limited.
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