Weeds
In 2013, two University of Guelph weed scientists began collaborating on alternatives to herbicides for weed control. The report, by Francois Tardif and Mike Cowbrough, was released in 2016.
In wheat, in canola or in pea, the message is the same: control weeds early for highest yields. Those messages have been repeated in the past and now new research highlights the need to repeat that same message with respect to wheat crops.
Largely overlooked and previously not studied a lot in Canada, weed seed predation provides the second-largest loss of weed seeds from the seed bank, second only to germination. Although research has been almost exclusively carried out in Europe and the United States, recent research at the University of Saskatchewan proves weed seed predation is occurring in western Canadian cropping systems and can be measured.
Glyphosate-resistant (GR) waterhemp was first found in Ontario in 2014, but it already has a foothold in three counties in the southwest of the province. Fortunately, Peter Sikkema’s research group at the University of Guelph’s Ridgetown Campus has made a good start on finding effective options for controlling this challenging weed.
Originally from Asia, woolly cupgrass has been in the United States since about the 1950s and has caused problems in field crops across the corn belt. This annual grassy weed was first found in Canada in 2000, when it was discovered in Quebec. Since then, government agencies and producers have been working to prevent the weed from getting out of hand, and researchers have been learning about the weed and its management under Canadian conditions.
Another weed control tool bites the dust. A field in southwest Saskatchewan was confirmed to have Group 4-resistant kochia in the fall of 2015. The durum field had been sprayed with OcTTain herbicide (2,4-D and fluroxypyr; both Group 4 active ingredients) and it had little effect on the kochia population.
The key to controlling tufted vetch in soybeans is to try to maximize control in all crops in the rotation and in all kinds of windows. That’s the advice of Mike Cowbrough, weed management specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). He has been investigating options for tufted vetch control for about 14 years so he knows just how difficult this weed is to conquer.
Researchers writing in the latest issue of the journal Weed Science provide important insights on the control of herbicide-resistant giant ragweed - a plant shown to produce significant yield losses in Midwest corn and soybean crops. 

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.
Weed control challenges are becoming even more difficult as the number of herbicide-resistant weeds in pulse crops continues to grow. With more than 60 unique cases of herbicide resistance identified in Canada and some weeds developing resistance to key pulse herbicides such as Pursuit (imazethapyr, Group 2) and Solo (imazamox, Group 2), the challenges will become even more daunting in the future.
A new factsheet from the Weed Science Society of America is now available for free download, exploring research of weed seed longevity. It highlights the unique ways weed seeds can travel (earthworms can collect and move them into their burrows), their viability (moth mullein seeds buried in 1879 were able to germinate more than 130 years later), ways they can be eliminated (carabid beetles can consume large quantities of weed seeds that drop to the soil) and more. Download the factsheet here and visit www.wssa.net for more information.
When it comes to controlling weeds in emerged winter wheat during the fall, 2,4 D is not recommended. Joanna Follings, the cereals specialist at the Ontario Ministry of Agriculture, Food and Rural Affairs, breaks down the reasoning behind this on FieldCropNews.com. | READ MORE
The year 2017 will mark the 60th anniversary of the discovery of the first known herbicide-resistant weeds in 1957 — a spreading dayflower found in a Hawaiian sugarcane field and a wild carrot variety found in Ontario, both which showed resistance to up to five times the normal usage dosage of synthetic auxin herbicides.

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
What’s your worst weed? Pigweed? Canada fleabane? Field horsetail? Ontario farmers recently had the opportunity to vote on which weeds are the most troublesome. The results provide an intriguing glimpse into changing weed challenges in the province.

“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 time of day when you spray often makes a significant difference to herbicide efficacy. That’s one of the overall findings from a recent Alberta project. The sometimes-surprising results are leading the project team to explore the reasons behind the data and to look at a possible tool to help producers make time-of-day spraying decisions.

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.”

Ongoing research in Western Canada is looking at alternate weed control technologies that do not utilize herbicides to target weeds.

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