Canola growers gain a new tool to help control cleavers as the CODEX Alimentarius Commission (CAC) formally adopts a Maximum Residue Limit (MRL) for the active ingredient quinclorac in canola. Growers can now use quinclorac products in their canola to control cleavers.
Corn planting is essentially complete. Due to variability in rainfall and soil fitness this spring, there is a wide range of crop stages. Early planted corn is now past the V6 stage and approaching row closure in some fields, while late planted corn on heavier textured soils is in the emergence to early V stages. Side dressing continues in many areas. While stands look good overall, there have been some comments about non-uniformity now showing up in some fields where soil conditions may have been pushed at planting, particularly in areas which have remained dry. Corn replants, mostly due to excessive rainfall in 2018, are reported to be below normal. There have been reports of some fields with heavy weed pressure where weed control has been delayed due to high demand for other sprayer activities. Good weed control from emergence to 6 leaf stage is critical for protecting from yield loss in corn.
On June 6, OMAFRA completed its annual Pre-Sidedress Nitrogen Test (PSNT) sampling survey at its zero nitrogen trial locations. This survey assesses soil nitrogen status by measuring natural background nitrogen mineralized from the soil. Average soil nitrate results came back at 12.7 ppm. This is slightly above the long term (2011-2017) average of 11.7, suggesting nitrogen mineralization processes appear normal this spring. The full report and details on the nitrogen status in 2018 corn fields is available online. This survey serves as a general guide. Soil nitrate results are highly field specific, and growers are encouraged to sample their own fields before making any nitrogen decisions.
Most areas of the province have completed planting, though some still continues on very heavy soil textured areas such as Niagara. Due to variability in planting date, crop staging ranges from planted to early emergence for later planted field to 3nd trifoliate for early fields. Thin stands are being observed in many areas. Issues range from soil conditions at planting, heavy rains after seeding resulting in crusting, seed corn maggot feeding, root rots, or extremely dry conditions. Fortunately soybeans are able to compensate for thin stands within reason. Leaving a stand of 90,000 plants per acre on medium textured soils is usually more profitable than replanting. (110 000 plants per acre on heavy clays) For those fields with very poor stands replanting is still a viable option at this date. Feeding from Bean Leaf Beetle has been reported so monitoring is recommended.
A large amount of hay has been cut over the past two weeks. While most dairy hay is complete, some first cut continues this week. Limited rainfall has been conducive for dry hay production as well. Yields have been reported to be good. Quality has been good, with the only challenge reported to be early season grass growth staying ahead of alfalfa due to the cool start in April. Alfalfa weevil has been observed in many areas and scouting is encouraged after first cut this year. Temperature models suggest alfalfa weevil development may be delayed, and could still pose a risk to early regrowth. Control is warranted if there are two or more active larvae per crown, or 4–8 larvae per 30 cm by 30 cm (1 ft2).
The winter wheat crop is progressing well; however, some wheat stands are variable and considerable discolouration is still evident. In the driest areas wheat is starting to flare up due to a lack of moisture. Fusarium Head Blight fungicides (T3) have been going on as wheat progresses through the anthesis stage. A number of acres in the far southwest of the province did not receive a T3 fungicide as warm temperatures moved the crop quickly through the ideal application window. Stripe rust was reported on June 5th in the St. Mary’s area on a susceptible variety. A second field has since been reported in the Clinton area. Disease levels continue to remain low and as temperatures continue to increase, stripe rust becomes less of a concern. If growers are still considering a late T3 fungicide application for stripe rust control, pre-harvest intervals must be considered.
Canola is progressing well, with earlier planted fields in southern and eastern canola growing regions now at green bud stage or bolting. In northern regions the crop is approaching full rosette and expected to start bolting in the next week or two. Flea beetle is being reported, though pressure is generally low. Flea beetle must feed on the seedlings to be exposed to seed treatment insecticides. Foliar insecticide application is not warranted until at least 25% foliar feeding is observed, and once the crop is at the 4 leaf stage it can likely outgrow the feeding damage. Swede midge is now being observed, although populations are reported to be relatively low. Swede midge pheromone traps should be in place and checked every few days through to bolting. With some early reports of Cabbage Seedpod weevil, monitoring is recommended.
A large portion of edible bean acres have been planted, with many growers done or expecting to be done within the next week. Planting continues in some localized areas where rainfall had delayed field operations, or for shorter season beans such as Cranberry beans. Early reports suggest good stands in most cases, with the exception where seedbeds were overly dry. Lack of heavy rainfall events over the past two weeks has limited crusting and other rainfall related plant loss issues.
Group 14 herbicides are part of a group of chemistries that require light to be effective as an herbicide. In Canada, one of these compounds is called Heat (saflufenacil), and is a protoporphyrinogen oxidase (PPO-inhibiting) herbicide. There are other light-dependent herbicides, as well. Photosystem II (PS II) is a chemistry that interferes with photosynthesis and disrupts plant growth. An example would be AAtrex (atrazine) (Group 5). There’s also inhibitors of PS I, another part of photosynthesis, including compounds like Gramoxone (paraquat) (Group 22). These two chemistries are related and affect the transfer of electrons within photosynthesis.
Plants also need chlorophyll and carotenoids for photosynthesis to occur, and there are compounds that are inhibitors of PDS like Solicam (norfluzaron) (Group 12). Another compound inhibits one of the precursors to the carotenoid pathway such as Command (clomazone) (Group 13). Some of the new chemistries, the HPPD inhibitors like Callisto (mesotrione) (Group 27), are also part of this class of chemistries. All of these are called light-dependent herbicides because they affect one aspect or another of photosynthesis, either through the transfer of electrons or the synthesis of the pigments, and require light to be active.
I’ll be talking about PPO inhibitors, an enzyme that is involved in porphyrin and chlorophyll synthesis. Why do we care about these compounds? When they work they work really, really well. PPO-inhibiting herbicides were first commercialized in the 1960s and their market share in the U.S. reached about 10 per cent in the late 1990s. A lot of herbicides have been synthesized that target this enzyme or pathway. About 100,000 compounds may have been synthesized that can inhibit this enzyme. Of course not all of them make it to be an herbicide.
These PPO-inhibiting herbicides were initially used mostly as post-emergent, broad-spectrum weed control in soybean fields. That’s how they were primarily used for the longest time. Some like carfentrazone (Aim in Canada) were developed for cereal crops. Some were so active that they were used as non-selective herbicides.
Mode of action
When the herbicide is applied, it lands on the leaf surface and then goes through the top layer, called the cuticle. It goes through the epidermis, and then has to get to the target site. There it inhibits an enzyme that produces a compound called Proto IX. Proto IX is supposed to be in the chloroplast, but when you apply the herbicide, Proto IX accumulates outside of the chloroplast. When the sun comes out, Proto IX reacts with sunlight, what’s called reactive oxygen degradation, and basically destroys the cell structure of the plant. Within a few hours the plant dries up. It becomes paper-thin and completely dehydrates. Injuries like leaf cupping, crinkling, and bronzing appear on some plants, and then typically necrosis and completely dead tissue within a few hours. It’s a pretty fast-acting herbicide, and it works really well under the right circumstances.
Some plants are very sensitive because they can’t metabolize the herbicide. Some plants are very tolerant because they metabolize the herbicide very quickly. Since some plants can metabolize it very quickly, a plant can become resistant by developing the ability to metabolize this chemistry, which would be non-target site resistance.
Most PPO inhibitors degrade very quickly in the environment. Most compounds have a very short half-life and have very poor pre-emergence activity. However, a compound like sulfentrazone (Authority; Authority Charge) can have a very long half-life, 280 days. In the south US that may actually affect rotation of your crops because of the long residual activity of some of that chemistry.
[Ed. Note: In Canada, carfentrazone has a short half-life and when used as a pre-seed treatment, there are no cropping restrictions. Sulfentrazone’s longer half-life means it can be used as a pre-seed surface application that provides residual weed control, but also means there are re-cropping restrictions.]
The PPO inhibitors are very rapidly metabolized and don’t stick around in water. They’re considered to be a pretty safe chemistry.
A resurgence in use
There used to be a lot of use of the PPO chemistries in the 1990s. In 1996, the first Roundup Ready crops were introduced and their use dramatically decreased. Where PPOs were used extensively for weed control in soybean, it was replaced by glyphosate. But the use has picked up again because of glyphosate resistant weeds. It is a great tool to manage glyphosate resistant weeds in the south and the Midwest as well. In Canada it might be a good tool in the future as you see more and more glyphosate resistant weeds.
Some plants have become resistant to PPO chemistry. For most of them we don’t know the mechanism. But for waterhemp, Palmer amaranth, and ragweed, we know there have been mutations on the target site gene. That’s similar to what happens with ALS inhibitors and ACCase inhibitors. That’s what happens with some glyphosate resistance in some cases.
At the target site, there are two genes that make two proteins. One goes to the chloroplast; one goes to the mitochondria. When the plant became resistant, many scientists sequenced the gene for the protein that goes to the chloroplast because that’s where the herbicide works by preventing chlorophyll synthesis. However, no mutation was found at that location. Dr. Tranel at the University of Illinois sequenced the other gene that goes to the mitochondria. He found that there was a mutation where a whole amino acid was removed, and that was kind of unusual. But there was also something added to the gene, and that was the first time this was reported to happen in plants.
This was very unusual. The herbicide is supposed to inhibit the chloroplast enzyme, but that little bit of DNA that was added to the sequence made the mitochondrial gene also go the chloroplast. So now you have a plant cell where a resistant trait is in both locations – the mitochondria and the chloroplast.
That’s important because these resistant plants now have the capacity to do the deletion and develop resistance, and have the capacity to move it to both locations. This has proven to be true in Palmer amaranth, water hemp, and ragweed. There’s no other herbicide so far that we know where plants have become resistant by this mechanism.
We looked at many genetic sequences to look for all the potential plants that have the same gene structure that could have a deletion. One of the plants is kochia. Kochia is a big weed in Colorado and in Canada. We now know that kochia is already predisposed to that mutation. If we keep using PPO chemistry the way we’ve been doing it and try to control kochia, most likely kochia will become resistant to that chemistry in exactly the same way that Palmer amaranth has become resistant. If you know a weed is predisposed to the mutation, then you should be scouting for weed escapes when you use that herbicide.
Now because you have resistance doesn’t mean you have resistance. What? Some interesting research was conducted by Peter Sikkema in Canada where fleabane escaped control by PPO chemistry. He demonstrated in the greenhouse that those seeds he collected in the field were resistant. What’s interesting is he went back the next year to the same field, applied the same herbicide and had 100 per cent control. An escape does not mean that your field is infested with the resistant weeds. In this case, it could be that the resistant weeds did not over-winter very well. So be on the lookout, but don’t freak out. If you have an escape it could be just something that’s a freak accident. But always be on the lookout for those escapes because we know that it can happen.
I’m not very familiar with the Canadian system, so suggested management strategies come from Arkansas where they deal with PPO resistance all the time in soybean. These may not necessarily be applicable to Canada. Use two active ingredients at planting, typically metribuzin (Group 5) and a Group 15 such as acetolachlor. Both are needed for successful residual activity. Then 21 days later use a post-application of glufosinate (Group 10), dicamba or 2,4-D (Group 4s) tank mixed with Dual (s-metolachlor; Group 15) for additional residual activity. In Arkansas, glyphosate is not useful because most major weeds including PPO resistant biotypes are already resistant to glyphosate. ALS herbicides are not useful in Arkansas either, as about 50 per cent of weeds have resistance to this group.
For more stories on this topic, check out Top Crop Manager's Focus On: Herbicide Resistance, the first in our digital edition series.
In order for harvest weed seed control (HWSC) to be effective, weed seeds still have to be retained on the plant at the time of harvest. If they’ve already dropped to the soil, they’re already in the seed bank. The weed seeds also need to be at a height where they can be collected by the combine. For example, chickweed is very low growing and its seeds are very low to the ground. Most producers don’t cut that low to the ground because of risk of damaging their equipment, so chickweed would not be a good candidate for harvest weed seed control.
Harvest weed seed control also means being able to get the weed into the combine. An example is a big tumbleweed, such as kochia. If the tumbleweed won’t feed into the combine and goes over top of the header, then you won’t be able to get the seeds into the combine for harvest weed seed control.
There are different methods of harvest weed seed control. Some of them have been scientifically evaluated in Australia. One of the most common methods is narrow windrow burning. The straw and chaff are dropped into windrows using metal chutes that are attached to the back of the combine. It’s cheap and easy to implement. But there are environmental impacts because it does involve burning. From a practical point of view, it may not work in western Canada, but it is used a lot in Australia.
Chaff carts were originally developed in Canada. The Australians have modified Canadian chaff carts and use a conveyer system instead of a blower system to move the chaff to the cart. They’ve also adopted new technologies to make burning or collection easier and more efficient. Some of the chaff carts are programmed with GPS to dump the chaff in a certain area of the field to be grazed or burnt.
There was one Australian producer that commented he’s been using a chaff cart for 15 years, and about 10 years in he started seeing annual ryegrass that was much shorter, much lower to the ground and was dropping its seeds much earlier. So this is still a selection pressure. You will select for resistance to these methods if it’s what you’re relying on to control your populations.
The bale direct system bales chaff and straw directly behind the combine into a square bale. The square bales are removed from the field, taking the weed seeds with them. The loss of the residue from the field can be detrimental in terms of nutrients loss. And there is potential for transport of weed seeds in the bale from one region to another, potentially moving herbicide resistant weeds with the bale. The other issue in Australia is one producer started doing this and he saturated the entire market. The bales can also be pelletized to produce pelletized sheep feed, but again it’s a relatively small market. So market can be an issue with this methodology.
The Harrington Seed Destructor uses a cage mill to grind the chaff and weed seeds. The cage mill has two counter-rotating plates that spin very fast in the opposite directions. The weed seeds go in to the middle of the mill and have to move from the inside out to continue to move through the system. The straw moves along a conveyor belt and goes through a spreader at the back. Only the chaff is processed through the cage mill. The disadvantage is that the first model was towed behind the combine and required a lot of horsepower.
The tow-behind model was always intended as step one. The Integrated Harrington Seed Destructor (iHSD) is mounted on the combine and uses the same cage mill system. The integrated model had several improvements. Instead of having the two counter-rotating plates there’s only one rotating plate and one stationary plate, but that rotating one turns twice as fast. It is a hydraulically driven machine and takes about 80 horsepowers from the combine to run this machine.
A new combine mounted seed impact implement was first announced January 2017. The Seed Terminator is competition to the Harrington Seed Destructor. It uses a slightly different type of mill called a multi-stage hammer mill, but it works on essentially the same idea of crushing or grinding those seeds so that they’re dead and can’t grow the next year. This is mechanically driven rather than hydraulically driven. In terms of price differences, the original tow behind Harrington Seed Destructor was about $200,000. The integrated Harrington Seed Destructor is somewhere around $150,000. The Seed Terminator is about $100,000. So what you’re seeing is as these competitors come to the market that price point is dropping, and we do expect that to continue.
Chaff deck or chaff tramlining works in a controlled traffic system. The idea is to put chaff on the permanent tramlines so if weeds grow there isn’t much impact on overall yield. The chaff in the tramline is also driven over multiple times, which can impair weed growth, and there is potential for seed decomposition in those tramlines. What farmers have seen is that there are fewer weeds growing in the tramlines, but it hasn’t been scientifically evaluated at this point.
Chaff lining can still be used outside of a controlled traffic system. The chaff is placed in a narrow row to decompose instead of spreading the seeds across the entire field. However, there is potential for some seeding or emergence issues if you’re seeding through this concentrated chaff row. It hasn’t been researched, but a lot of producers are adopting this in Australia as their first step in harvest weed seed control because it’s inexpensive and easy to implement.
In Australia, a 2016 survey of 602 growers were asked about their adoption of narrow windrow burning, chaff carts, chaff tramlining, the bale direct, and the HSD. The Seed Terminator and integrated Harrington Seed Destructor were not released at the time so they don’t show up in the survey.
The Australian experience
Across Australia 43 per cent of producers were using some method of harvest weed seed control. Narrow windrow burning was the most common. In Western Australia that number goes up to about 63 per cent. Western Australia is essentially where all of these methods were developed. Western Australia is really the epicentre because of herbicide resistance, and harvest weed seed control is spreading out from there.
The adoption of chaff tramlining this past harvest has skyrocketed. There is a lot more discussion about different systems on social media, and a lot more discussion about what works and what doesn’t work than we’ve see in past years. If that survey was to be redone I think we would see some of the tramlining and chaff lining skyrocketing.
Results from the same survey show that 82 per cent of producers said they expected to adopt some form of harvest weed seed control in the next five years with 46 per cent expecting to use narrow windrow burning. More producers would like to be using the iHSD, but they had concerns about the cost and the perception that it was unproven in terms of weed kill. The perception of unproven control of weed seeds is interesting because weed kill is where there is the most research.
Research has been done in Australia to show how effective harvest weed seed control was on controlling annual ryegrass populations in “focus paddocks” or “focus fields.” The research compared crop rotations where harvest weed seed control was used in 38 per cent of crops compared to rotations where it was only used in 11 per cent of crops. The ryegrass population was managed far more effectively where harvest weed seed control was used, and it has stayed very low.
Effects of HWSC in Australia:
Potential in CanadaIn Western Canada we’ve believed that the physical impact implements that destroy seeds are most likely to have the best fit. They don’t require the burning, and it has some scientific testing behind it that shows it’s effective. So that’s where researchers have focused efforts in terms of testing a method for Western Canada.
We looked at the top 10 weeds in Saskatchewan and gave them a seed retention rating -- how well does the weed holds onto those seeds until harvest. A number of weeds are in the good or fair to good retention rating, and that’s promising. Green foxtail gets a good retention rating while buckwheat gets a fair to good. Volunteer canola is rated good. The unfortunate ones are the poors: wild oat, spiny annual sow thistle, narrow-leaved hawk’s-beard. Those have poor retention and are unlikely to be primary targets for harvest weed seed control because a lot of their seeds are already gone by harvest.
Looking at some small plot experiments, seed retention of wild oat, cleavers, and volunteer canola was looked at. Volunteer canola retained most of its seed by the end of September, cleavers was intermediate and wild oat retained about 20 per cent of the seed by the end of September.
Kochia has good seed retention. Their seeds only mature after harvest, so most of the seed is still there at harvest, but the concern is that below the cutting height, typically six inches, there can still be over 5,000 seeds below that cutting height. So even though a lot of seed is collected by the combine, there could still be a lot missed and left in the field. At this point we aren’t sure what impact harvest weed seed control would have on kochia.
As part of my PhD research, we looked at running samples through the Harrington Seed Destructor in a stationary format set up in the shop. We mixed buckets of chaff with weed seeds and ran them through to determine how many are destroyed. We looked at five weed seed species: kochia, green foxtail, cleavers, volunteer canola, and wild oat. We put 10,000 seeds of each of those species into a five-gallon pail of chaff, put it into the Seed Destructor and assessed how many lived when they came out the other side.
A second study looked at weed seed size. Weed seed species are all different shapes, sizes and seed coat types. We took canola seeds and we hand sieved them to get thousand kernel weights between 2.2 grams per 1,000 and 5.8 grams per thousand.
We also looked at weed seed number by comparing 10 canola seeds up to a million canola seeds in the same volume of chaff. We also looked at chaff volume, so 10,000 canola seeds going through with no chaff or up to eight five-gallon pails of chaff in the same timeframe. And we also looked at chaff type, so barley, canola, and peas.
When we looked at weed seed species we did find significant differences in terms of control but our lowest level of control was still over 97 per cent killed. It worked really well on all the species that we tested.
In terms of canola seed size, we expected to see an increase in control as the size of the canola seed went up, and we did. But again, we’re within a percentage point of 98.5 per cent control so weed seed size isn’t a big factor in control.
Looking at weed seed number, once you have over 100 seeds going through, we were back up at that 98 per cent control.
As we increased the amount of chaff going in, initially our control increased, which may be that there’s more deflection within that mill. Those seeds get hit an extra time or two, and then it started to taper off. But again, we are in the 98 to 99 per cent control so it’s not going to have a huge impact in the field.
There was a similar story with chaff type. We did have less control in our canola chaff but we were running volunteer canola seeds through the seed destructor so there was likely a background presence of volunteer canola in our canola chaff that we did not account for. But again it’s by one-half per cent and we are still getting 98 to 98.5 per cent control.
In summary, what we found with the seed destructor was if you can get the weed seeds into the seed destructor you’re going to kill most of them – greater than 95 per cent.
The big question now is how does it work in the field? The answer is we don’t know yet. We have an ongoing study with the seed destructor in 20 producer fields where the seed destructor is in the field at harvest time. We harvest with the seed destructor and compare it to a pass with the seed destructor not milling the chaff. We learned a lot of lessons in 2017.
The first is that air velocity is really key. Chaff needs to be moved from the sieves, up and into the input of the tow behind Harrington. In order to get the chaff from the sieves, it has to go up into an input tube, and takes a fair bit of air velocity. If your air velocity is too low, your machine will plug. And if you don’t catch the plug fast enough, you end up with burning belts.
Greener, wet material also doesn’t work. We know it takes a lot more effort for the combine to thresh green or wet material. It’s a similar story with the mills. You need higher air velocity, and without it the green, wet material can plug where it forms a nice solid block of really hot, wet chaff in the blower. Green, wet material doesn’t grind well, either. So if you have green material in the field desiccation or swathing is going to be needed to dry the material down.
The other complication the tow behind HSD is a big machine that has problems with hills. The integrated seed destructor or the Seed Terminator makes a lot more sense for Western Canada. The research that’s been done in Australia shows that the tow behind unit and the integrated unit are very similar in terms of their control, so it’s still a valid test for those integrated units in Western Canada.
An example from a single field in 2017 shows some interesting results, although very preliminary. We compared photos from an untreated and treated Seed Destructor pass. There was substantially less volunteer canola in the treated pass after harvest. There is still some volunteer canola, but there’s substantially less.
We hope to start seeing benefits in the spring of 2018, but it is a three-year study. We’ll be back on the same locations for the next two harvests so that we can take into account the seed bank buffering that we’ll see in terms of our treatments.
These are new strategies. There’s always going to be bugs to work out, but they can be very effective in helping us manage the herbicide resistance that we’re currently facing.
For more stories on this topic, check out Top Crop Manager's Focus On: Herbicide Resistance, the first in our digital edition series.
Eric Page, research scientist in weed ecology, and Sydney Meloche, weed science technician, at AAFC Harrow Research and Development Center have conducted planting trials looking into the feasibility of this idea. Although they have yet to work out all of the agronomic issues involved with implementing a winter canola-soybean double crop, the early results are promising.
Soybean and canola production in Ontario is challenging due to a number of factors. Soybean production in Southwestern Ontario has to overcome the prescence of multiple glyphosate resistant weed species. Canola production faces challenges like swede midge and clubroot.
Winter canola and soybean double crop could allow canola production to escape swede midge pressure while helping to stem soybean yield losses in longer season regions of the province.
Read here for more information on the details and results of trials.
All crops have a critical weed control period, which is the time when the crop is susceptible to significant yield loss from weed competition. The critical weed control period for canola is around 17 to 38 days after emergence. Peas can be as early as two weeks after emergence. “Other, more competitive crops, like the cereals, have a less defined critical period,” Brook says. “Corn’s critical period depends more on nitrogen availability than anything else. If you can keep the weed pressure down until the critical period is passed, you minimize yield losses from weed competition.”
Field scouting is essential to giving an edge battling weeds, notes Brook. “Field scouting tells you what weeds are present and their density. Once a field has been scouted and a weed problem identified, the degree of threat needs to be assessed. An example of an early, non- yield threatening weed is whitlow grass. It’s a very slow growing, small plant that bolts and goes to seed, usually before seeding. It’s not a direct threat to the crop. However, if other weedy plants are also present in sufficient numbers and are a threat to yield, you can choose an appropriate control measure.”
Winter annual weeds like stinkweed, narrow-leaved hawk’s-beard, shepherd’s purse, scentless chamomile, and many others can start growing in the fall. They overwinter as a small rosette but are then quickly able to go to seed once spring arrives. “Control of them in the spring requires very early action. You need to know the weeds present to choose the best control method. Crop volunteers from previous years are also an increasingly problematic weed obstacle. Volunteer canola is one of our top weed control issues every year. These and other problem weeds will require additional products when applying a spring burn-off with glyphosate.”
To get the best result from any early herbicide application, Brook says the herbicide must be applied when the weeds are actively growing. “Under cool or cold conditions you can expect poor results from the spray as the target weeds are either dormant or growing too slowly. They cannot absorb and translocate enough active ingredient to kill them. Weeds also have to be large enough to absorb enough herbicide to be killed, yet not too large to have already affect crop yield from competition. Low spray volumes and coarse sprays can lead to insufficient herbicide landing on the plants. Best temperatures for application should ideally be above 12 to 15 C, when the plants are actively photosynthesizing. If it was frosty in the morning, waiting until a warm afternoon will improve efficacy.”
Another tool in the weed control toolbox is the competitive nature of the crop itself. “Highly competitive crops can reduce the effects of weeds on yield. Once a crop canopy has covered the soil, sunlight no longer can penetrate to the ground and weeds stop germinating,” adds Brook. “Heavier seeding rates can also squeeze out weeds. Hybrid canola and barley are our two most competitive crops. You still have to choose a competitive variety. Semi-dwarf barleys are less competitive than regular barleys. Heavier seeding rates always increase the crop’s competitive nature against weeds. Thin crops allow light to hit the ground, stimulating more weed growth.”
For more information, contact the Alberta Ag-Info Centre at 310-FARM (3276).
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