Since giant ragweed is resistant to multiple herbicide sites of action, researchers at the University of Minnesota set out to determine the impact of alternative control strategies on both the emergence of giant ragweed and the number of giant ragweed seeds in the weed seedbank. They evaluated six, three-year crop rotation systems, including continuous corn, soybean-corn-corn, corn-soybean-corn, soybean-wheat-corn, soybean-alfalfa-corn and alfalfa-alfalfa-corn.
Researchers found that corn and soybean rotations were more conducive to giant ragweed emergence. Thirty-eight percent fewer giant ragweed plants emerged when the crop rotation system included wheat or alfalfa.
They also found that adopting a zero-weed threshold can be a viable approach to depleting the weed seedbank, regardless of the crop rotation system used. When a zero-weed threshold was maintained, 96 percent of the giant ragweed seedbank was depleted within just two years.
"Since the ragweed seedbank is short-lived, our research shows it is possible to manage fields infested with giant ragweed by simply eliminating weeds that emerge before they go to seed," says Jared Goplen, a member of the research team.
Herbicide-resistant giant ragweed is rapidly becoming a major threat to corn and soybean production in the Midwest and elsewhere. This research will help growers utilize crop rotation as a much-needed additional strategy for managing this weed.
In the past six decades since these discoveries, weed scientists have documented more than 250 weed species with some form of herbicide resistance. These span 23 of the known 26 herbicide modes of action and impact 86 different crops across 66 different countries. As a result, the cost of weed control across the nation’s crop fields has tripled in recent years as growers are being forced to employ more herbicides per season, increase application frequency, and spend more on fuel costs to achieve some measure of control. | READ MORE
“Back in 2007, we decided to ask people what their worst weeds were, just to see what their concerns were. Then in 2016 we followed up with another survey,” says Dave Bilyea, a research associate in weed management at the University of Guelph’s Ridgetown campus. “Things are always evolving in agriculture, so we thought it would be interesting to see how things have changed given the almost 10-year span between the two surveys.”
Bilyea worked on the 2016 online survey with his Ridgetown colleagues Kristen McNaughton and Christy Shropshire. More than 300 people from 31 counties participated in the survey, which was publicized by various agricultural and government agencies.
Respondents were asked to identify and rank their five worst weeds from a given list. If their own worst weeds weren’t on that list, they could add their choices. They weren’t asked to give reasons for their choices. From all the votes, Bilyea determined the top 10 weed problems for Ontario-east, Ontario-west and Ontario-wide (see tables). Since more of the respondents were from the southwest than the east, the Ontario-wide results are tipped slightly toward weed concerns in the southwest.
Bilyea emphasizes that the survey results are just for people’s interest, providing a way to create conversations about weed issues. Although the collected data are not comprehensive enough to draw any definitive conclusions, it’s interesting to speculate on what lies behind the results.
In the 2016 survey, the Ontario-wide five worst weeds were, in order: lamb’s-quarters, Canada fleabane, common ragweed, eastern black nightshade and pigweeds. All five of these are broadleaf annual weeds with at least some herbicide-resistant populations.
Lamb’s-quarters Chenopodium album, which was in fourth place in the 2007 survey, is a very common weed that can grow up to 200 centimetres tall. “We can only surmise why people think certain weeds are the worst. In the case of lamb’s-quarters, it is probably one weed that touches all types of operations across Ontario whether they are horticulture or field crops, or even orchards and things like that,” Bilyea says. “It’s so pervasive; it’s everywhere.”
He thinks herbicide resistance might be an additional factor contributing to this weed’s top ranking, but it’s likely not the major reason. Ontario’s 2016 maps of herbicide-resistant weeds show lamb’s-quarters populations with resistance to Group 5 herbicides (e.g. Aatrex, Sencor) or Group 2 herbicides (e.g. Pursuit, Pinnacle) have been found in 36 counties. But Bilyea points out that just because some populations of a weed species in a county are resistant, that doesn’t mean all populations are.
In the top 10 list, lamb’s-quarters was followed very closely by Canada fleabane Conyza canadensis. Canada fleabane is a winter or summer annual and can be up to 180 cm tall.
“Canada fleabane is obviously a major problem now in Ontario, but in 2007 it wasn’t even on the list,” Bilyea says. “Canada fleabane is not a new weed; it has always been around. It’s a problem particularly for growers who have no-till because it likes undisturbed ground. And now we have a certain part of the population that is resistant to glyphosate [Group 9 herbicide].”
Glyphosate-resistant Canada fleabane populations have been spreading rapidly in the province. Glyphosate-resistant biotypes were first identified in Essex County in 2010. By 2012, they were found in eight counties, and now they’re in 30 counties. Some counties have populations with multiple resistance to both Group 9 and Group 2 herbicides.
Bilyea thinks glyphosate resistance is very likely the key issue that earned Canada fleabane such a high ranking. In fact, in the survey column where respondents could add their own weeds, many respondents specifically stated resistant Canada fleabane was a concern, rather than ordinary Canada fleabane. “Glyphosate resistance makes Canada fleabane control very challenging for a lot of growers because glyphosate – the Roundups, the Touchdowns and herbicides in that group – are the major keystone for weed control across Ontario in corn and soybeans.”
Respondents indicated glyphosate-resistant populations were the issue for common ragweed Ambrosia artemisiifolia, the third-place weed in 2016, and giant ragweed Ambrosia trifida, in eighth place. “Not all giant ragweed is resistant and not all common ragweed is resistant, but there are significant numbers of fields with glyphosate resistance,” Bilyea says.
He adds, “In the 2007 survey when I mentioned ‘ragweed,’ we were just thinking of common ragweed. But now Ontario has giant ragweed, as well as common ragweed, which has always been in fields.” Common ragweed can be up to 150 cm high; giant ragweed can be up to about four metres high.
Fourth-place eastern black nightshade Solanum ptycanthum is another weed that can grow in many types of habitats. “Eastern black nightshade is especially an issue for growers who have beans. You can’t have nightshade in your bean crop for export for food-grade beans,” he notes. The juice from the nightshade berries can result in a discoloured coating on the beans, which is very difficult to clean off. “Also, after some of the early herbicide sprays have stopped doing their job, spots of nightshade will come up. I think the weed’s high ranking is also because nightshade goes right across the province, so it’s a very common, problematic weed for soybean and edible bean growers.”
The survey didn’t distinguish between different pigweed species (Amaranthus genus). Bilyea explains, “Green, redroot and smooth pigweeds are nearly impossible for most growers to tell apart. Also, for the most part, the control measures for them are similar.” These troublesome weeds can grow to between 150 and 200 cm tall. Redroot and green pigweeds are often in the same field. Many of the counties that have herbicide-resistant redroot pigweed also have herbicide-resistant green pigweed; resistances are to Groups 2, 5, 6 or 7.
Bilyea suspects better weed identification has influenced the changes in some weed rankings from 2007 to 2016. “People now have cell phones and they can look up online on their cell phones in the field and identify a weed or at least send a picture to somebody to have it identified.” He thinks misidentification of grass species might have contributed to the very high ranking of quackgrass in 2007, with some people identifying any grassy weed as quackgrass. “Quackgrass has completely disappeared off the [Ontario-wide] top 10 list in 2016, and some of the answers about the kinds of grasses that people have are a little more definitive, at least in the east.”
For people who would like to improve their weed identification skills, Bilyea maintains the Weed Identification Garden at the Ridgetown campus. “It’s a self-touring garden of common weeds, not just for rural people but for urbanites too. It has 208 pots set up in four rows, including lawn weeds, wild flowers, problem weeds, poisonous weeds, and a lot of weeds that people don’t even realize are in the area.” This year is the garden’s 40th anniversary. It is open to the public from May to October so people can examine the specimens and learn more about the weeds’ properties.
Some people may be disappointed that their own particular weed nemesis didn’t make it into the top 10. “Each grower has their own concern,” Bilyea says. “Just as an example, someone from the east was saying that they have a lot of bedstraw. Bedstraw is not a widespread problem, but for those growers in eastern Ontario who have the weed, it’s a huge problem. So that would be their number one problem, but there just aren’t enough of them across the region to put bedstraw into the top 10.”
The idea for this project was sparked by the growing use of night spraying. “GPS-guided autosteer has given farmers the ability to spray during the night. So we wondered, is night spraying as effective as daytime spraying?” says Ken Coles, the general manager of Lethbridge-based Farming Smarter.
Coles led the three-year project (2012 to 2014), which was funded by the Alberta Canola Producers Commission and Alberta Barley Commission. The project compared early morning (4 a.m. to 5 a.m.), midday (noon to 1 p.m.), and night (midnight to 1 a.m.) timings for herbicide applications. This small plot, replicated research project included a pre-seed burndown study and an in-crop study, and involved a range of herbicide groups.
The pre-seed burndown study, which took place at Lethbridge, evaluated herbicide efficacy for controlling natural weed infestations in the plots. The treatments involved glyphosate and several products that were becoming popular as tank mixes with glyphosate (see table).
The in-crop study was carried out at Lethbridge by Farming Smarter, at Bonnyville by the Lakeland Applied Research Association, and at Falher by the Smoky Applied Research and Demonstration Association. The crops included wheat, peas, Liberty Link (LL) canola, and Roundup Ready (RR) canola. They were seeded on two seeding dates each year to try to make sure the herbicide applications would occur in range of weather conditions. Simulated weeds were seeded in the plots: tame mustard for a broadleaf weed, and tame oats for a grassy weed. Herbicides from various groups were compared (see table).
Highlights of findings
“Herbicides are registered to work in a wide range of conditions so I didn’t really expect to see a tremendous difference in efficacy between the different timings. But what I started to see right away was often the early morning application was the least consistent of any timing,” Coles says.
“Having grown up in southern Alberta, I found that particularly interesting because it is almost inbred in our culture that we wake up really early to spray, to beat the wind. But the project’s results show the early morning timing is often coming at a cost for herbicide efficacy.”
In both the burndown and in-crop studies, the most effective timing was usually midday, followed by midnight. Coles says, “Since night spraying was usually more effective than dawn, night spraying could be a good option when daytime opportunities for spraying are limited.”
For canola, Liberty and Roundup (Vantage Plus Max II) usually performed best at midday and worst in the early morning. He notes, “I was surprised that Roundup had a strong time-of-day effect. We expected that for Liberty – it is well known that you should spray Liberty in the heat of the day because it needs the heat to activate it as a contact herbicide.”
The story was somewhat different for wheat and peas. “We didn’t see nearly as strong a correlation with time-of-day on the wheat herbicides; generally, they worked the best under most conditions. The pea herbicides, Odyssey and Select, tended to work better when sprayed at night.”
So Coles suggests a general guideline would be to spray wheat in the early morning, canola in the middle of the day, and peas at night.
Generally, broadleaf weeds tended to be more sensitive to the time-of-day effect than grassy weeds.
On grass control, Coles points to another interesting finding: “Liberty is known for not having the greatest control of grassy weeds, but when we sprayed Liberty at night, its grass kill improved. This type of information could be helpful when dealing with herbicide-resistant weeds. Let’s say you’ve got Group 1 and 2-resistant wild oats and you don’t have a lot of options left to kill them. Spraying Liberty at night on Liberty canola might help keep those resistant wild oats under control.”
Looking into results
The results showed some strong patterns overall, but not every site in every year followed the general trends. To better understand the reasons behind the results, Coles and his team looked closely at the weather data for the trials because of the profound effect weather can have on herbicide efficacy.
“Usually, our conditions were pretty dry, so during the day, the temperature rose and the humidity dropped. Then at night, the temperature dropped and the humidity rose. But, for instance, if we had a rain event with lots of moisture and maybe no wind, then that pattern really got jumbled up. It was harder to predict the [herbicide efficacy] results when more moisture was present,” Coles says.
The project team was able to identify the likely causes for some of the plot results that bucked the general trends. For instance, the results suggest that plants stressed by very dry soil conditions might have reduced herbicide translocation, resulting in poorer herbicide efficacy. Moisture stress may also change a plant’s form and structure, causing such things as leaf rolling or thickening of the protective waxy covering on the leaf surface – changes that could reduce the amount of herbicide entering the plant. It’s also likely that a heavy rainfall event shortly after spraying in one of the experiments washed away the herbicides, so those applications were almost totally ineffective.
The link between time of day and changing temperature and humidity conditions got Coles interested in Delta T. “Delta T is the wet bulb temperature minus the dry bulb temperature. The dry bulb is just a regular thermometer, and the wet bulb is essentially a thermometer with a wet sock on it. It measures the evaporative cooling effect, which is the same effect as if you jump out of the shower and stand in front of a fan – you feel cold.”
Delta T can be used to determine if conditions are optimum for spraying. The general guideline is that Delta T should be between two and eight or 10 (the upper limit depends on whether the spray is fine or coarse).
“When I mapped all of the Delta T values for all of our data, the poorest herbicide performance was between zero and two,” Coles says. So he dug a little deeper into Delta T.
According to Coles, very little academic research has been done on Delta T for spraying decisions. And most of that research has been in Australia, where growers use Delta T mainly to determine if conditions are too hot and dry for spraying. As Delta T rises above 10, the air gets very hot and dry, causing spray droplets to evaporate faster and volatile pesticides to vaporize faster, so herbicide effectiveness tends to be reduced.
If Delta T is below two, then the air is very moist. In the Australian literature, the reason given for not spraying when Delta T is below two is that the high relative humidity causes the spray droplets to be very slow to evaporate. So fine droplets tend to last a long time, increasing the risk of spray drift if a temperature inversion occurs.
A temperature inversion is when the air near the ground is cooler than the air above it. To check for an inversion, compare the temperature at the top of the crop canopy with the temperature at about eight to 10 feet above the canopy. The air in an inversion is very stable because the lower, cooler air is denser than the warmer air above. So there’s no vertical movement of air parcels; the airflow is horizontal. Although coarse spray droplets will fall to the surface fairly quickly, fine droplets will take a long time to fall and may float for long distances.
Inversions tend to be strongest and deepest just before sunrise, so they may be a factor in the generally poorer performance of the early morning applications in the project. Coles explains, “We always talk about not spraying when there’s a temperature inversion because of the risk of spray drift. But it’s also possible that inversion issues are resulting in poorer weed control on our own fields – basically not all of our fine droplets are hitting our targets. If that is the case, that might explain why we’re not getting as good control in the early morning: we have less spray coverage, especially with the finer droplets which are sometimes more easily absorbed in plants.”
However, Coles thinks the effects of early morning weather conditions on plant physiology might be even more important than the effects of early morning inversions on spray coverage.
One physiological factor could be that most metabolic process in plants increase with warming temperatures. So, as the day warms up from the relatively cool conditions at dawn, herbicides tend to become more biologically active. (However, if conditions get too hot, then plants will start to reduce their metabolic activity, slowing the rate of translocation and metabolism.)
Coles also suspects evaporative cooling could be important. “Early morning is when we have the highest humidity and the lowest temperatures. Then the wind comes in with really dry air and it dries off the plants really quickly. So my premise is that the evaporative cooling effect is sucking a lot of heat energy out of the plant, which stresses the plant. And a stressed plant is not going to uptake herbicide.”
From his initial look at Delta T, Coles thinks it might be a useful tool to help farmers make time-of-day decisions on herbicide spraying. Small hand-held units are available for measuring Delta T, so it’s easy to do. “It looks like avoiding spraying if Delta T is between zero and two is more important [in Alberta],” Coles says. “I think I would go somewhat higher than 10 [for the upper end of the optimum spraying range] and be comfortable.” However, he’d like to see more research on Delta T, especially on why spraying is less effective when Delta T is below two.
“We don’t have it all figured out, but it has definitely taken us onto an interesting path trying to understand the why,” Coles says. “And the more we know about herbicide effectiveness, the better job we can do at managing herbicides to increase yields, decrease weed seeds in the seed bank, and deal with herbicide resistance before it becomes an even bigger issue.”
June 13, 2016 - Crops in Manitoba benefitted from the warmer temperatures and drier weather conditions earlier in the week and allowed producers to make progress on weed control operations.
Excess moisture conditions in some areas of the province are impacting crop growth, particularly in the lower areas of the fields. Crop yellowing is evident. Wet field conditions also continue to hamper some field operations.
Weed control, and fungicide applications where warranted, will remain a priority for producers as crop development advances. Fungicide applications in winter wheat for leaf disease control and suppression of fusarium head blight has started.
The widespread evolution of multiple herbicide resistance in annual weeds infesting Australian cropping fields has dramatically depleted the available herbicide options and forced investigation into the development of alternative weed control strategies. One approach that is now widely adopted in Australian cropping systems is referred to as harvest weed seed control (HWSC), which targets weed seed during grain harvest to minimize seed inputs to the seedbank.
We’ve put incredible selection pressure on wild oats for resistance. It’s our driver weed. It’s a weed that makes most of our herbicide decisions. In 2006 over $12 an acre on average was spent in Western Canada on wild oat herbicides, about $500 million annually. That’s more than double any other weed species as a weed target.
I am struck by how fast wild oat resistance occurred. Hugh Beckie’s three surveys in 2001 to 2007 to 2011 showed Group 1 resistance went from 11 per cent of our fields to 39 per cent to well over 50 per cent of our fields in Alberta. That’s very rapid and is indeed cause for concern.
Dale Fedoruk, an agronomist in central Alberta, illustrated the problems with herbicide resistance. He gave me some data on wild oat resistance from wild oat patches that had been treated with herbicides. The samples were tested for three Group 1 herbicides and three Group 2 herbicides. He also has crop rotation and herbicide application history.
Field 3 had 11 years of field history. Eight of the 11 years had a Group 1 herbicide applied. Ten of the 11 years have a Group 1 or a Group 2. In 2014 the field received both Groups 1 and 2. In 2010 there was some Fortress (Group 3 and 8). In 2009 it had glyphosate in Roundup Ready canola.
Resistant testing on the wild oat seeds from Field 3 found very high levels of resistant seeds with all of the Group 1 or 2 herbicides screened. None of the major Group 1 or 2 herbicide chemistries would be effective on these wild oat patches in wheat. That’s fairly sobering to think about because we always talk about resistance as something that’s coming. This isn’t the only field that’s like this so essentially we are losing a lot of herbicide tools.
What this means in wheat is that there are no post-emergent wild oat herbicides that would be effective. He still has soil-applied Avadex (triallate Group 8). However, in Alberta, John O’Donovan confirmed 34 sites with triallate resistant wild oats from 1990 to 1993. These fields had an average of 17 years of Avadex. O’Donovan has been back to some of these sites recently, and if Avadex hasn’t been used since, he thinks that there’s probably two to three years of susceptibility to Avadex before resistance builds back up.
There are integrated production practices that lower selection pressure for resistance. Crop and canopy health is important for weed competition, and is even possible in so-called weakly competitive crops like peas. Seed shallow for canola to get a good crop stand. You may not need to do a second in-crop herbicide application with a competitive canola crop.
Agriculture and Agri-Food Canada (AAFC) Lacombe did a study in central Alberta where a lot of barley silage is grown continuously. Three-year rotation of all Seebe barley, three different barley varieties, a rotation with triticale and a rotation with an oat variety were compared. The trial showed biomass of wild oats could be reduced simply by having a more diverse rotation, and this rotation wasn’t very diverse at all.
Some of the Prairie rotations can look pretty good. Wheat-canola-pea sounds good but these rotations are all just summer annual crops. Wild oat and many other problem weeds are summer annuals so unless we do something different and introduce a silage crop or introduce a winter wheat, we’re really just telling these weeds they can just continue as normal. They have no trouble adapting or thriving in those systems.
Although barley silage is a summer annual crop, it adds diversity because it is cut earlier than a crop harvested for seed. Winter cereals are even better. They start growth in the fall, and by the time the wild oat is ready to emerge in the spring many growers don’t even need a wild oat herbicide. This takes away wild oat selection pressure with herbicides for an entire year. Doing the same thing over and over whether it’s winter cereals or summer annuals is not going to get us where we want to be. We need to rotate and mix things up. Perennial forages do that pretty well because they compete very well and before wild oat gets a chance to set seed, you cut them off.
Another integrated weed management trial at Lacombe compared continuous barley versus a barley-canola-barley-pea rotation, as well as short versus tall barley cultivars and normal versus 2X seeding rates and different herbicide rates. Cumulative effects of these treatments in year five were measured. (See Fig. 1.)
The integrated practices provide additive benefits. Using tall versus short varieties reduced wild oat biomass. Doubling seeding rate reduced wild oat biomass. Using both tall varieties and double seeding rates provided a further reduction. Putting those practices in a crop rotation led to even further reductions. By doing one thing right, you can get a two to three times reduction in wild oat biomass. Do two things right and you get a six to eight times reduction, and doing all three together gives a 19-fold reduction in wild oat biomass.
But the problem with that rotation is that it uses all summer annual crops so it’s not really diverse. Another study put some real diverse rotations to the test. This was done at three sites in Alberta, two in Saskatchewan, and one in each of Manitoba, Ontario and Quebec.
It compared canola-wheat-canola-wheat to more diverse rotations. The typical canola-barley and canola-barley-pea-wheat rotations were included as treatments. More diverse rotations included early-cut silage and winter wheat along with canola and spring wheat. Herbicides were either applied at full rate or not applied. Seeding rates were 1x or 2x normal rates. The rotations ran for five years from 2010 through 2014.
Five of six treatments with no wild oat selection pressure in three of five years did as well (wild oat emergence) as canola-wheat-canola-wheat with full herbicide regime. Similar results for wild oat biomass were observed, where five of the diverse treatments did as well as putting on tremendous selection pressure with wild oat herbicides on the more conventional canola-wheat rotation.
Canola yield was measured in the fifth year. Canola-wheat-canola-wheat with full herbicide throughout the five years was one of the lowest yielding, probably because of low crop diversity and too much canola in the rotation.
In this study some of the wild oat seedbank numbers with treatments not using herbicides did increase, but there were four treatments with zero wild oat herbicides three years in a row where it was not significantly greater than the canola-wheat-canola-wheat rotation with full herbicide applications.
Overall, combining 2x seeding rates of early cut silage with 2x seeding rates of winter cereals and excluding wild oat herbicides for three to five years often led to similar wild oat density, above ground wild oat biomass, wild oat seed density, wild oat seedbanks and canola yield compared to a repeated canola-wheat rotation
under a full wild oat herbicide regime.
Wild oat was also similarly managed after three years of perennial alfalfa without wild oat herbicides.
A cautionary conclusion in the study was that forgoing wild oat herbicides in only two of five years in exclusively summer annual crop rotations resulted in higher wild oat density, biomass and seedbanks. But where there was a winter cereal and high seeding rates, we were able to manage wild oat effectively.
In summary, some herbicides are being overused. Weed resistance continues to increase at a rapid pace and many popular wild oat herbicides are already less useful than they were a few years ago. In some cases you could say all of the popular in-crop herbicides are not available anymore for wild oats. Few or no new herbicide modes of action are being registered. Low diversity rotations are dominant and that’s the biggest reason for the situation we have. Herbicide-resistant canola did give us a reprieve but if we overuse that system we’ll also get resistance from different groups.
Economics is why people say they do what they do. So far weed resistance has not driven much integrated weed management adoption in Western Canada – that could change. In Australia, how many guys wanted to pull a chaff cart? Zero. How many do pull a chaff cart? Quite a few. How many want to burn their stubble? Zero. How many do in Western Australia? Fifty per cent of growers burn their stubble.
In terms of herbicide resistance, some fields are in serious trouble with more trouble on the horizon. We still have time to act. Those with vision will make some sacrifices now to preserve precious herbicide tools that are a relatively non-renewable resource.
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