Destra IS is a post-emergence corn herbicide with one-pass broad-spectrum knockdown and residual control, and adds two additional modes-of-action to a glyphosate tolerant system – there’s also residual control and multiple modes of action. The herbicide will allow growers to control hard-to-kill broadleaf and grassy weeds and to keep corn weed-free during the critical weed-free period.
Destra IS has a wide window of application, allowing growers to apply up to the eight-leaf stage, with excellent crop safety and a broader geography, including short season areas. It offers a smaller, easy-to-handle package and compact dry formulation, and is the only dry mesotrione formulation on the market. It offers a faster pour and bottle cleanout.
Fore more information visit Dupont.ca
The Canadian Weed Science Society (CWSS) recently honoured several individuals for their extraordinary contributions to the field of weed science. The awards were presented during the organization’s 70th annual meeting, held this year in Moncton, N.B.
Fellow Award (sponsored by CWSS)
CWSS presented the Fellow Award – its highest recognition – to Eric Johnson, who was until recently a researcher employed by Agriculture and Agri-Food Canada in Scott, Sask. Johnson now works at the University of Saskatchewan. He is recognized internationally for his research on weed management, including his work on mechanical and chemical weed control. He is also active in the areas of organic production systems and cropping systems/agronomy. Johnson has been an author or coauthor on more than 80 peer-reviewed publications, seven book chapters, and multiple conference proceedings. He has maintained an active collaboration with the University of Saskatchewan where has taught a pesticides course, given more than 30 guest lectures, and has served on the supervisory committees of several MSc students. He has also served in various capacities within the CWSS/SCM, including as a board member for over 10 years, and as president in 2015.
Excellence in Weed Science Award (sponsored by Dow AgroSciences)
CWSS honored Dr. Robert Gulden, an associate professor with the University of Manitoba in Winnipeg. Gulden's research focuses on weed biology and management. Dr. Gulden has served on the board of directors for CWSS/SCM and currently serves as an associate editor for the Canadian Journal of Plant Science. Gulden has published more than 60 peer-reviewed manuscripts, several monographs and book chapters, and supervised or co-supervised nine graduate students. In addition, he has received multiple awards for teaching excellence at the University of Manitoba.
Excellence in Weed Extension Award (sponsored by Valent)
CWSS honored Dr. Peter Sikkema, who has been involved in applied weed research and extension in field crops for the past 20 years at the University of Guelph, Ridgetown Campus. Sikkema has published more than 250 manuscripts in various national and international peer-reviewed scientific journals. He has presented more than 450 oral extension presentations in the province of Ontario, more than 50 extension poster presentations, written more than 70 popular extension articles, conducted more than 325 extension/research tours of weed management plots and has obtained more than 137 minor-use registrations through the Pest Management Regulatory Agency. He has also served in various capacities within the CWSS/SCM, including as president in 2011.
Outstanding Industry Member Award (sponsored by CWSS)
CWSS honored Al McFadden, research scientist with Dow AgroSciences, based in Guelph, Canada. McFadden has a strong track record of interaction with CWSS, CropLife Canada, and the Ontario Weed Committee (OWC), through various presentations at various scientific meetings. Al has served as the industry representative on the CWSS Board of Directors, the Ontario Provincial Council Chair of the Technical and Education and National Biology sub-committees of CropLife Canada, and was the industry representative for the Ontario Weed Committee. In addition to the various presentations he has given at various scientific society meetings, his name appears in peer-reviewed journal manuscripts based on his willingness to involve himself in graduate student training.
Presidential Award (sponsored by CWSS)
CWSS honored Dr. Rory Degenhardt, research scientist with Dow AgroSciences, based in Edmonton. His primary responsibilities are as biology team leader for Canadian Cereal Herbicides. Degenhardt has published several peer-reviewed manuscripts, and has served as principal biologist for discovery of herbicides such as Arylex, as well as for nitrogen stabilizers. He has a strong track record of interaction with CWSS since becoming a member in 2002, including his most recent appointment as local arrangements chair for the CWSS annual meeting held in Edmonton in 2015. Degenhardt played a key role in the co-ordination of this meeting, and his strong leadership abilities were a tremendous asset to the society and to the local arrangements committee. He remains an active member of the CWSS.
Student Scholarships and Travel Awards
Travel Award for PhD student (sponsored by Monsanto) was presented to Charles Geddes from the University of Manitoba. Geddes' research covers optimization methods in canola to reduce populations of volunteer canola in subsequent soybean crops. He works under the direction of Dr. Rob Gulden.
Travel Award for a M.Sc. student (sponsored by Monsanto) was presented to Moria Petruic from the University of Saskatchewan. Petruic's work focuses on expanding weed management options in flax. She works under the direction of Dr. Christian Willenborg.
Travel Award for a M.Sc. student (sponsored by Syngenta) was presented to Felix Marsan-Pelletier from Laval University. Marsan-Pelletier’s work focuses on herbicide resistance in wild oat and common ragweed. He works under the direction of Dr. Anne Vanasse.
Travel Award for a M.Sc. student (sponsored by Dow AgroSciences) was presented to Taiga Cholette from the University of Guelph. Ms. Cholette's work focuses on the interaction between cover crops and herbicides. She works under the direction of Dr. Peter Sikkema and Dr. Darren Robinson.
Travel Award for a M.Sc. student (sponsored by CWSS) was presented to Meghan Grguric from the University of Guelph. Grguric’s work focuses on management of giant hogweed in Ontario. She works under the direction of Dr. Francois Tardif and Mike Cowbrough.
Travel Award for a M.Sc. student (sponsored by CWSS) was presented to Mike Schryver from the University of Guelph. Schryver's work focuses on the distribution and control of glyphosate-resistant waterhemp. He works under the direction of Dr. Peter Sikkema and Dr. Darren Robinson.
Mechanical weed control, using some form of tillage, still has an important role in weed control in Eastern Canada. Generally, though, it isn’t getting much attention from weed science – as compared to research with herbicides.
South of Lake Ontario, Charles Mohler, a senior weed research associate at Cornell University in Ithaca, N.Y., recently completed semi-basic ecological research on the ability of five annual weed seedlings to recover from burial.
That work has some direct implications for shallow tillage in rows after planting for growers in Ontario and Eastern Canada, says Mike Cowbrough, the field crops weed management specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, in Guelph, Ont.
Most Ontario corn still has some tillage for weed control, although the form can vary, Cowbrough says.
“Tillage is very much crop specific. The majority of corn has either a single pass or double pass. The vast majority of wheat acres are no-till, and soybeans probably fall somewhere in the middle for weed control,” he says.
As for the weeds, common lamb’s-quarter is the most significant annual weed in Ontario, being both abundant and very competitive. Pigweeds (redroot and green), common ragweed, green, yellow and giant foxtails and velvetleaf also are common annuals. Among perennial weeds, perennial sowthistle is most abundant, he says.
“There’s no question that tillage is a critical component of weed management. However, species react in different ways, so the best strategy varies with the weed species,” Cowbrough says.
In mid-state New York, with agricultural climate and soils similar to Ontario, Mohler chose to investigate how well five species could recover from burial. The five were common lamb’s-quarters, Powell amaranth (or green pigweed), velvetleaf, giant foxtail and barnyard grass.
Greenhouse experiments began in 2006 and ended with field experiments in 2013.
Farmers have three basic ways to kill a seedling weed with tillage: burial, dismemberment and uprooting.
Mohler says, “Burying seedlings is a very effective way to kill them, but you’ve got to get them completely buried. When we cultivate corn or soybean, we throw four or five inches of soil into the row. You can bury some pretty good-sized seedlings with that much soil.”
In practice, however, burial may not work out. The trials in New York, with more than 35,000 weed seedlings, revealed:
• Recovery often exceeds 50 per cent if a small portion of a seedling is left exposed.
• Recovery from complete burial (by two centimetres of soil) ranges from zero to 24 per cent, but recovery greater than five per cent is rare.
• No seedlings recover from four cm of complete burial.
• Large-seeded species tend to recover from complete burial better than small-seeded species.
• If the soil remains dry after burial, the recovery rate is low or may even be zero.
• Small seedlings are easier to kill by burial, with less soil.
In a controlled greenhouse setting, weed seedbeds were watered daily at first. After burial, some received no water; some were watered immediately after burial and some had daily watering for up to two weeks.
“In one experiment, we got re-emergence of seven seedlings out of thousands that were buried. In another experiment, we watered every day and something like 12 per cent of the velvetleaf re-emerged. Recovery of other species was lower,” Mohler says.
Among the five weeds, generally, velvetleaf was the most successful at recovery.
The final experiment purposely left a small, consistent fraction of leaf area exposed while burying the rest of the seedling, including the growing point, with two cm of soil.
Despite almost complete burial, more than 35 per cent of seedlings recovered in every species. In most cases, more than 50 per cent recovered. Velvetleaf and giant foxtail recovery was 60 to 80 per cent.
That’s the basic science.
Mohler says, “I try to teach, don’t run through a field without thinking about what you’re doing. You need to target the kind of damage you’re trying to inflict. That depends on the soil conditions, the weather conditions, what your implement can do and how you set it up to use it.”
In practice, Cowbrough says, effective tillage needs strategy as well as complete weed burial.
First, there’s a need to distinguish perennial from annual weed control. In-crop tillage is better suited for annual weeds in most cases.
Cowbrough says, “Doing a shallow primary tillage pass in spring on dandelions is good eye-candy for a couple weeks, but that’s all. For effective perennial weed control, you really have to get rid of the entire root or the plants can grow back.”
Some annuals, such as eastern black nightshade, germinate very late. Those weeds arise too late for effective in-crop tillage. Canada fleabane is a common annual weed in Ontario and often glyphosate-resistant.
“Tillage is very good at controlling Canada fleabane, if it is aggressive and early. It has to be effective enough to knock the soil off the roots so the fleabane has zero chance of recovery,” Cowbrough says.
The second point is about timing. Secondary roots develop as the fleabane gets beyond seedling stage.
“Secondary roots hold soil, too. Tillage may uproot the plant but it can eventually recover and grow again as long as it has some secondary roots holding onto soil,” he says.
For ragweed control, there are strategy variations. For an infestation of common ragweed, the best strategy is complete burial.
But giant ragweed is different, including glyphosate-resistant giant ragweed.
“The best strategy is to leave the seed of giant ragweed on the surface in a no-till environment,” Cowbrough says. “That seed is prone to predation and degradation, and it’s unable to germinate without a bit of disturbance along with moisture and light. Tillage is not an asset for managing that species of ragweed.”
The disc hiller can bury more advanced weeds ahead of planting.
And, after crop emergence, a treatment with the sweeps on a row crop cultivator can throw a lot of soil within the rows to bury and destroy seedling annuals.
• Determine the one or two most prominent species to control.
• Pick the tillage tool that’s best for that job. No one tool will address all the situations.
• If there’s opportunity, adjust the tool to its best advantages.
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
Harvest began the previous week in parts of western and central Ontario. Yield reports have been variable ranging from single digit to upper 60 bushels per acre (bu/ac). Low yield reports, not surprisingly, appear to correspond closely with the precipitation received throughout the season. However, some producers have been pleasantly surprised with how well soybeans have yielded even with less than ideal amounts of precipitation. Seed size appears to be smaller than normal, although quality has been good to date. Although the presence of green stems is more prominent this year than others, it has not seemed to significantly reduce harvesting efficiency. It is estimated that between 40 and 45 per cent of the provincial acreage has been harvested.
An early soybean harvest for some has provided an opportunity to plant wheat into very nice conditions with emergence occurring in less than a week. A reminder that a proper planting depth of one inch is extremely important and planting too shallow is often the cause of stand issues the following spring as a result of frost heave and winterkill. Regardless of planting date, seed placed starter fertilizer provides an additional eight bu/ac of grain yield. Seedling Canada fleabane, which in many cases is glyphosate resistant, has already emerged, and there is an opportunity to apply Eragon pre-plant or pre-emergence to manage this problematic weed. Otherwise, a post-emergence herbicide application will be necessary to control Canada fleabane in winter wheat. If chess (aka cheat grass) or downy brome has been a problem in your cereal fields in the past, there are now two options to deal with these grassy weed species. The first is called Focus and must be applied pre-plant or pre-emergence to the wheat crop (and before weed emergence), while the second is called Simplicity and can be applied post-emergence to both crop and weed in the spring.
A very small amount of corn has been harvested although grain moisture is dryer than it is normally at this time of year, prompting some producers to start taking off high-moisture corn especially when rain has delayed soybean harvest. A significant number of producers and agronomists have noticed western bean cutworm (WBC) damage in mature cobs. When damage is significant, consider harvesting early to stop mould growth. Adjust your combine to discard lightweight mouldy kernels and dry mouldy corn as soon as possible. Normally hot, dry conditions are not good environmental conditions for ear mould fungi like Fusarium graminearum (Gibberella), and Fusarium verticillioides. However, rains or humid conditions along with hybrid susceptibility, incomplete pollination, and cob damage by WBC has resulted in pockets of infection in some areas of the province that could result in mycotoxins being produced, especially deoxynivalenol (DON or vomitoxin). Growers should be assessing fields for ear mould infection and harvest fields first with 10 per cent or more ear mould.
Harvest progress ranges anywhere from 50 to 80 per cent done. The least amount of harvest progress has been made with Adzuki beans since they mature later than other market classes, while significantly more progress has been made harvesting cranberry and white beans. As with many crops in 2016, dry conditions have impacted grain yield. Harvest yield reports have been variable with the larger seeded coloured beans yielding average to below average. While white bean yield reports have been average to below average. Overall bean quality has been good with seed size being somewhat smaller than normal.
“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.”
Unfortunately weather conditions around the time of application can be quite variable and can influence a herbicide's effectiveness. Let's go through "top tips" to make the most of this application window.
- Choose the most effective products, rate and tank-mixes for the perennial weed that you are targeting. Table 1 outlines what public researchers in Ontario have found to be most effective at controlling perennial plants in the fall.
- Apply when air temperatures are above 8 C for a minimum of two hours after application. This is best accomplished by applying during late morning or mid-day so that the targeted plant is taking up glyphosate during the heat of the day.
- After a frost event, wait two to three days before evaluating weed growth and if the target plants look fine and air temperatures are above 8 C then resume applications. For example, milkweed is very sensitive to frost. Figure 2 shows a milkweed plant three days after an evening where the air temperature reached a low of -3 C. It would not make sense to apply glyphosate on a weed species in that state since its leaves are unlikely to absorb any herbicide. Alternatively, dandelion and wild carrot were not affected by the same frost event (Figure 3 and 4) and one could resume fall applications to those species based on the condition of their leaves.
- Wait a minimum of 72 hours after application to perennial weeds if you want to till the soil. The longer that you can wait after application before making a tillage pass, the more the herbicide will translocate within the plant and do a more effective job controlling the species.
Click here to view images.
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.”
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Royal Manitoba Winter FairMon Mar 27, 2017
Employee Selection WebinarMon Mar 27, 2017 @10:00am -
Cultivating the Great Clay Belt Agriculture SymposiumThu Mar 30, 2017
Hiring Employees WebinarMon Apr 03, 2017 @10:00am -
Spring Workshop on Organic ResearchFri Apr 07, 2017 @ 8:30am - 04:00pm