- Contributor: Drew Lyon
- Posted by: Gale Perez
Over the past two years (2022 and 2023), we conducted field studies to evaluate the benefits of using herbicides with different modes of action, both with and without glyphosate, for the control of Italian ryegrass in glyphosate-resistant spring canola. We found that glyphosate (Roundup PowerMax) provided excellent control of Italian ryegrass applied alone or in combination with other herbicides. The use of trifluralin (Treflan HFP) applied preplant incorporated or the early postemergence application of glufosinate (Liberty) reduced the number of Italian ryegrass plants that were subsequently treated with glyphosate. The fewer plants you treat with glyphosate or any other herbicide, the lower the chance for selecting an individual or individuals with resistance to glyphosate. Consequently, the use of trifluralin or glufosinate for weed control in glyphosate-resistant spring canola can potentially extend the useful life of glyphosate-resistant spring canola for Italian ryegrass control.
Despite the good news on weed control, we observed a yield reduction with some of the glyphosate applications in both years. In 2022, the greatest Italian ryegrass control resulted from applications of Roundup PowerMax, which provided nearly 100% control at harvest (Table 1). Applications of Roundup PowerMax were so effective in controlling Italian ryegrass that it was difficult to see added control from Treflan HFP. The early Roundup PowerMax application (3-4 leaves) of 44 oz/A resulted in the highest canola yield of 2590 lb/A. All late postemergence Roundup PowerMax applications (6-leaf to bolting) delayed flowering and reduced yield.
In 2023, a year with a low density of Italian ryegrass (the result of a delayed replanting), all treatments except Liberty applied alone early postemergence provided excellent control of Italian ryegrass (Table 2). Without the competition provided by a high density of Italian ryegrass, we were able to observe a reduction in canola yield with all treatments containing Roundup PowerMax. It did not matter if the Roundup PowerMax was applied early or late postemergence or at a rate of 22 or 44 oz/A. We are unsure if this is a problem unique to the TruFlex spring canola varieties we used (Invigor LR344PC and Invigor LR345PC) or if it should be a concern for all Roundup Ready spring canola varieties. Further research is needed to better understand the yield drag associated with the use of glyphosate in Roundup Ready spring canola.
In the meantime, growers should be aware of this potential yield drag in glyphosate-resistant spring canola. The yield drag appears to be partially the result of a delay in plant development and flowering. This delay often results in increased heat and drought stress during flowering, which reduces yield. Growers are encouraged to avoid late planting of glyphosate-resistant canola varieties; however, if late planting is necessary, growers should consider not applying glyphosate and possibly further delaying crop maturity. Late planting may allow control of major Italian ryegrass flushes before planting, thus reducing in-crop infestations. Growers are encouraged to consider the use of a preplant herbicide such as trifluralin or ethalfluralin (Sonalan HFP) prior to an early postemergence application of glyphosate to reduce selection pressure for glyphosate-resistant Italian ryegrass biotypes.
Drew Lyon is an Endowed Chair and Professor in the Department of Crop and Soil Sciences at the Washington State University.
Original source: Weeders of the West blog :: Nov. 22, 2023
- Author: Konrad Mathesius
Summary Review
Results from this year's Italian ryegrass (IR) herbicide trials helped quantify differences in herbicide resistance among IR populations within the southern Sacramento Valley. The trial took place in Bird's Landing, CA (near Rio Vista), and was replicated farther north in Esparto, CA.
- Trials from this year suggest that Osprey-resistant IR populations often associated with the area around Dixon, CA could extend at least as far south as Bird's Landing with only 26% control of IR by Osprey in the trial site.
- The Osprey-resistant population appears to also be moderately resistant to Simplicity, another herbicide in the same chemical family, which only provided 60% control at the Bird's Landing site.
- Axial (an ACCase inhibitor) worked well at controlling IR in Bird's Landing (92% control).
- In Esparto, IR in the trial was somewhat more susceptible to ALS inhibitors. IR control was only around 80% with Simplicity.
Introduction
For a few years I've heard PCAs mention that Italian ryegrass (IR) populations around Dixon are showing resistance to Osprey (an ALS inhibitor herbicide), but quantification of differences in weed control can help provide a better understanding of what growers are dealing with. IR is notorious for its capacity to develop herbicide resistance to multiple modes of action (MoA). As an obligate outcrossing plant, IR must cross-pollinate in order to produce viable seed. This means that genetic material is regularly exchanged during pollination, which allows populations to respond to environmental pressure. Development of herbicide resistance in IR over the years has caused increasing concern for small grain growers, who are limited in their options for control.
This study is part of a wider range of studies examining different options available to growers for IR control in small grains. This study quantifies the efficacy of various available herbicides on IR populations in small grains both in the northern parts of Yolo County and the southern parts of Solano County.
Methods
Two trials were set up: one in Bird's Landing and one in Esparto. Each plot was 15 x 100 feet and was replicated 4 times at each location. Plots were planted with wheat in late December 2022 after a burndown treatment and were treated in January of 2023 using the maximum label rate of each herbicide along with recommended adjuvants (Table 1). A 20ft x 15ft section of each plot was left unsprayed as an untreated reference. Axial was unintentionally applied at half label rate in Esparto.
Prowl was tested to see if it might improve control by by providing residual control of late germinating ryegrass that escaped preseason control efforts but did not germinate with the first fall rains. IR control was not significantly better in the Prowl tankmix vs Simplicity applied alone in this experiment.
Treatments and Rates Evaluated for Italian ryegrass Control in Two Wheat Sites in 2023 in the Sacramento Valley.
Weed counts were taken in late February in each plot (3 sub-samples). The untreated reference sections within each plot were measured once (1 representative sub-sample). Percent weed control was measured by the difference in ryegrass density within the treated and untreated areas in each plot.
Grain was hand harvested in late July using 3 x 4ft^2 quadrats in each plot. Spikes were collected by cutting the stem at the base of the spike using a sickle. Samples were then air dried. Grain yield was determined by subtracting an estimated chaff weight of 17% from the weight of the harvested spikes (McCartney et. al, 2006).
Results and Discussion
Percent control
Within each location, herbicides varied significantly in terms of the capacity to control IR.
In Bird's Landing (20 miles south of Dixon, CA), Axial provided significantly better IR control than Simplicity, Prowl + Simplicity, and Osprey. Osprey did not reduce IR populations compared to the non-treated control plots. (Figure 2)
In Esparto, a tank mix of Prowl + Simplicity provided better control of IR than Osprey but was not different than Simplicity alone (p = 0.52) or the tank mix of Axial + Simplicity (p = 0.81). Osprey provided only moderate control (58%) and was marginally different from Simplicity (p = 0.084), which provided 79% control.
The differences in control between the two ALS-inhibitor herbicides (Simplicity and Osprey) at the two locations are an indication of the variation among IR populations that are only 50 miles apart.
Yield
Yield results collected from the Bird's Landing site show trends that generally correspond with differences in weed control, although no significant differences in estimated yield were found due to the variability of the data.
Conclusion
Variations in herbicide efficacy are a good reminder that there is a lot of genetic variability in IR populations; even fields only a few miles apart could have important differences in response to herbicides. This means that grower practices can directly impact the development of herbicide resistance in their area. Growers in the Dixon area should incorporate IPM practices listed above and consider the use of Axial as an alternative weed control if they haven't already, and growers farther north should remain particularly vigilant about preserving the efficacy of their ALS inhibitor herbicides by integrating some of the IPM practices listed below.
- Using certified seed (to prevent weed seeds from hitching a ride to entirely different areas of the state)
- Thoroughly cleaning equipment, or operating equipment only within local areas to prevent the spread of weed seed to other parts of the state
- Rotating herbicides within the season and from one season to the next, where possible
- Spraying at the right time (check labels, apply on the early end of the spray window)
- Spraying the right rate (Axial applications in Esparto were sprayed unintentionally at half-rate and provided no control).
- Spraying when weeds are actively growing
- Planting wheat at the right density
- Incorporating mechanical cultivation or Harvest Weed Seed Control where possible
- Checking and calibrating spray nozzles
- Incorporate the use of pre-emergent herbicides labeled in California
- Rotating crops where possible to diversify herbicide programs
- Check for escapes and monitor fields for efficacy
Growers are also encouraged to take advantage of UC IPM resources online, and in-person through their local farm advisors.
References
McCartney, D.H.; Block, H.C.; Dubeski, P.L.; Ohama, A.J. Review: The composition and availability of straw and cha? from small grain cereals for beef cattle in western Canada. Can. J. Anim. Sci. 2006,86, 443–455.
- Author: Konrad Mathesius
Summary note: This is a belated research update from herbicide trials (targeting Italian ryegrass) carried out in the winter of 2021-2022 in wheat fields in the Esparto area. The southern Sacramento Valley saw a record number of consecutive days without rain in that year, which severely limited the efficacy of post-emergent systemic herbicides. Growers should prioritize scheduling post-emergent herbicide applications early in the weed's growth stages and when weeds are actively growing to avoid the severe reduction of herbicide efficacy that can occur as a result of unexpected droughty conditions. This is particularly important given that the weather systems in our area are unpredictable between December and February and will often dissipate as they approach the Sacramento Valley.
...
Hot days would seem to indicate that we are nowhere near wheat season, but unpredictable weather patterns as early as October can make weed management difficult. As we approach winter, it's worth taking a minute to reiterate some of the interactions between herbicide efficacy and drought conditions that we witnessed in what was a very dry winter (2021-2022).
Italian ryegrass: a widespread weed with a track record of herbicide resistance
Italian ryegrass (IR), like Palmer amaranth and horseweed, is known for its capacity to develop herbicide resistance. In order to address resistance issues in IR, the University of California Cooperative Extension (UCCE) has been looking into the various ways in which growers can reduce IR populations through the use of available herbicides as well as mechanical cultivation techniques. Some of these lessons are valuable not only in winter rotations, but also in summer cropping systems.
2022 Herbicide Trials: a perfect storm
If there's one thing we learned in the spring of 2022, it was how severely drought conditions can reduce the efficacy of herbicides. In short: drought-stressed weeds are much harder to control with postemergence herbicides.
Figure 1. Cumulative rainfall from 2022. Flat parts of the curve are indicative of drought periods.The season was characterized by feast/ famine rainfall patterns. Heavy rainfall followed by long periods of drought stress produced conditions that dramatically reduced herbicide efficacy.
Substantial early rains in October of 2021 led to early germination of IR in the trial. Burndown applications were delayed due to wet field conditions throughout most of November and December and wheat was planted during a brief window of time in December.
The herbicides for the post-emergent trial were sprayed in late-January when wheat was at the tillering growth stage (Feekes 2) and ryegrass had 2-3 tillers and was about 6” tall.The ryegrass was still within the size recommendation on the herbicide labels; however, drought conditions had begun to set in three weeks prior to treatment.
The area where the trial was located saw no rainfall for 70 days after the herbicide application. The dry spell lasted 92 days in total, the longest in the region since the 1920s.
Results / Discussion:
The absence of a burndown application allowed for competitive weed growth that stunted the crop
The fact that the planned December burndown spray had to be canceled meant that the early-emerging weeds continued to grow alongside wheat. By January, the impact of the weed competition on the crop stand was severe. Although in-season herbicides suppressed the ryegrass and other weeds, much of the damage from competition had already been done. Most wheat plants were severely stunted and had only one or two reproductive tillers, the wheat spikes were shortened and had far fewer seeds per tiller at harvest than you would expect with an average crop stand.
Drought conditions reduced herbicide efficacy dramatically
It took more than 30 days before the IR in the trials began showing a significant amount of damage in response to the herbicide application. When damage assessments were taken on March 7th (38 days after application), where 0 indicated ‘no damage' and 6 indicated ‘total control', the highest rating was a 4.5 via a tank mix of Simplicity and Osprey. In other words, none of the plots had been controlled after more than 5 weeks.
After a few light showers in late March, the IR resumed growth and rapidly declined as herbicides began to finally take effect. Despite the increased impact of the herbicides, overall control was still sub-optimal, with the highest control ratings being between 71 and 87 percent. Axial, which had performed well in this exact field two years prior ended up with very low levels of control (14%). The reduced efficacy was likely due to drought stress, weeds hardening off, or simply because the herbicide was affected by some amount of decay in the field before it could be fully taken up by the plant itself. This was a good reminder that researchers and growers should consider other externalities before they throw their hands up and say the “R” word (resistance).
In both summer and winter crops, if plants are healthy and actively growing, herbicides will be much more effective and should work within the weed growth stage ranges specified on the label. However, because weather patterns can be unpredictable, the windows for herbicide labels should be seen as somewhat optimistic (written for ideal conditions).
Yields in the trial area ended up being one quarter of what they normally would be in this field due to a combination of early weed competition from the missed burndown treatment and the effects of a persistent drought period which both stressed the crop and reduced the efficacy of the applied herbicides.
Takeaways:
Spray windows on herbicide labels are often somewhat optimistic and assume that plants are actively growing (labels for systemic herbicides will typically specify that applications be made to actively growing weeds). Because drought can't always be predicted, growers should prioritize herbicide applications early in the weed growth stages and should apply herbicides when weeds are actively growing. In cases where irrigation is an option, growers should consider an irrigation event if conditions are excessively dry as it can greatly improve the efficacy of an herbicide (not to mention the fact that a small grain crop will likely be able to utilize the soil moisture as it enters the rapid growth phase in late winter).
The trial results are a strong example of how herbicide efficacy is tied to a number of factors. It is also an example of how researchers can serve their clientele by demonstrating how much things can go wrong when environmental conditions collide with their own optimism and the idealized ranges of herbicide labels.
Figure 2. Percent control of Italian ryegrass in a wheat field experiment in 2022 near Esparto, CA. Treatments were applied in January and these ratings were made on April 15th (80 days after herbicide applications, and following a few small showers in March that provided enough moisture for plants to continue growing). Note that Axial, which had worked well at this location in the previous years was largely ineffective under these conditions. It is highly unlikely that this is due to resistance, but rather is an example of how drought conditions can impact herbicide efficacy.
Figure 3. Plots in Esparto trial area in April (80 days after herbicide treatments). By the time herbicides began to impact Italian ryegrass, wheat was already severely stunted. This is an example of how critical early weed control and burndowns can be. Note again that Axial, which has worked well in this field two years prior, was dramatically less effective due to drought conditions. As an ACCase inhibitor, Axial is a valuable tool for growers looking to diversity their herbicide program, but this is an example of why it is important to prioritize early applications of herbicides on actively-growing weeds.
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- Author: Clebson G Goncalves Ph.D.
- Posted by: Gale Perez
In California, Italian ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] has been around for a long time and is a major weed in orchards, vineyards, field crops, fallow fields, and so on. It grows vigorously in winter and early spring. Italian ryegrass is a short, rhizomatous, that grows from 11 to 35 inches tall, often with erect stems exhibiting purple coloration at the base. That species can be identified by its dark green, glossy, and hairless leaves that are rolled in the bud. Auricles are well-developed and the shape can vary from clasping to blunt, and the ligules are long and membranous. Once flowering occurs, Italian ryegrass is easily distinguishable by alternating spikelets that run along the length of the main seed head stem. The seed heads range from 3 to 12 inches and consist of solitary small stalkless spikelets (Fig. 1). Additional details regarding identification can be found on the UC IPM website.
Also called annual ryegrass, Italian ryegrass herbicide resistance and management strategies have been the topic of discussion among growers, PCAs, UC Cooperative Extension farm advisors, and UC weed science specialists. Because Italian ryegrass has been controlled in crop production systems mainly with herbicides for decades, since 1998, populations of Italian ryegrass have been documented to be resistant to herbicides (Fig. 2). In California, the first report of Italian ryegrass glyphosate-resistant was in 2008 in almond orchards and vineyards, and the evolution and spread of these populations in the state made alternative postemergence herbicides an important management strategy against this troublesome species (published by Jasieniuk et al. 2008, Weed Sci. 2008).
Repeated herbicide use has selected Italian ryegrass populations resistant to multiple of herbicide mode of actions. A recent Weed Science publication by Dr. Brad Hanson's lab confirmed multiple Italian ryegrass resistance to the herbicides paraquat (Gramoxone® 2.0 SL), clethodim (Envoy Plus®), and glyphosate (Roundup PowerMax®). A second study conducted in Dr. Hanson's lab also confirmed that another Italian ryegrass biotype population collected in Hamilton City, CA is resistant to paraquat (Gramoxone® 2.0 SL), clethodim (Envoy Plus®), glyphosate (Roundup PowerMax®), and Mesosulfuron-Methyl (Osprey®). It should be noted that ryegrass populations resistant to fluazifop-butyl (Fusilade® DX), glufosinate-ammonium (Rely® 280), and sethoxydim (Poast®) also have been reported in other cropping systems elsewhere in California.
Worldwide Italian ryegrass has been identified as resistant to 29 herbicides, from 8 different groups of classification with multiple resistance to 1, 2, 3, or 4 sites of action (WeedScience.org, 2022). The most recent Italian ryegrass biotype was identified in a soybean and winter wheat field in North Carolina with multiple resistance to 4 sites of action (clethodim, glyphosate, nicosulfuron, and paraquat).
Management Strategies
For efficient management of Italian ryegrass, integrated weed management (IWM) practices must be adopted that allow the combination of various control strategies, including preventive, cultural, mechanical, physical, biological, and chemical control.
Prevention Control: Avoiding dispersion and introduction of Italian ryegrass into the property and ensuring that seeds and equipment are free of Italian ryegrass contaminants is one of the main strategies in the IWM programs.
Physical Control: Soil tillage may be used as one strategy to be added to the IWM toolbox. Italian ryegrass seeds buried in the soil die quickly and must be close to the surface to emerge. So, this practice can help suppress the population and provide long-term control.
Chemical Control: Italian ryegrass control using PRE and POST herbicide management programs continues to be the most effective. A well-designed herbicide application program with a combination of PRE- and POST-emergent herbicides (e.g., fluazifop-butyl (Fusilade® DX), glufosinate-ammonium (Rely® 280), pyroxsulam (SimplicityMT), rimsulfuron (Matrix®SG), sethoxydim (Poast®), flumioxazin (Chateau®), oxyfluorfen (GoalTender®), pendimethalin (Prowl H2O), and so on are possible options) still been quite effective in controlling Italian ryegrass. Growers should always consult a PCA and a local UCCE farm advisor to help design herbicide application programs and application rates appropriate for their unique growing system. Growers should also keep in mind that they need to avoid repeated herbicide use to prevent resistance in Italian ryegrass populations. Herbicides with different modes of action on a rotational basis should be encouraged to suppress the Italian ryegrass population and provide long-term control.
The growers also may adopt non-synthetic herbicide products for Italian ryegrass control. Among those available commercial products, the active ingredients, including acetic acid (Weed Pharm®), citric acid + Clove oil (BurnOut®), caprylic acid + capric acid (Suppress®), pelargonic acid + related fatty acids (Scythe®), D-limonene (AvengerAG®), ammonium nonanoate (Axxe®) and others, have been used for the non-selective control of that weed. Usually, organic herbicides are recommended for grass and broadleaf weeds in the early stages of growth. For Italian ryegrass already established, organic herbicides have shown poor effectiveness, and recovery approximately two weeks after treatment application (Fig. 3). Studies have shown that organic herbicides applied to control Italian ryegrass may require late sequential application due to the non-systemic characteristics of these products (ongoing trials by Clebson Gonçalves, 2023).
Even with the increase in herbicide-resistant weeds, herbicide control options continue to be the most effective and used option for producers because of time and money. However, diversified management practices should be encouraged as they are crucial for crop productivity, efficiency, saving cost, and sustainability of the systems.
Clebson Gonçalves is the UC Cooperative Extension Diversified Agriculture Advisor in Lake and Mendocino counties.
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- Author: Konrad Mathesius
- Editor: Mark Lundy
- Editor: Brad Hanson
The heavy rains have stopped a lot of growers from getting into their wheat fields, and growers may be wondering what the best move is given the break in the weather. Our forecast for rain in the next 10 days looks spotty, and that has some implications for both nitrogen and herbicide management in small grains.
Nitrogen (N) management.
- Granular fertilizers such as urea or ammonium sulfate need some rain to fall after they are applied in order to be successfully integrated into the soil. A general rule of thumb is that 0.25 inches or greater in the same storm will be sufficient to dissolve the fertilizer and get it into the soil profile. Timing applications ahead of a storm or irrigation event is advisable. If the weather dries up and no new storms materialize, N applied now may not become available to the plant in the near-term. N needs water to be utilized by the plant.
- Much of the wheat in the southern Sacramento valley is still in the early tillering phase (Feekes scale: 1-3). This means that nitrogen uptake has been pretty limited so far this season (around 1-2% of the total for the season). In other words, there's a long way to go.
With these things in mind, growers should continue monitoring the weather closely. Waiting a few more days is a given in most cases anyway as a lot of the acreage is still too wet to drive on. Now would be a good time for some soil nitrate quick tests, which can give growers and idea of the current N status in their soil. If we see another large storm system coming through in early to mid-February, growers will want to consider a top dress, but for now, depending on your crop's maturity, it might be prudent to watch the weather, monitor the crop, and wait until we know more.
The Nitrogen Fertilizer Management Webtool for California Wheat can help growers track their crop growth status over time and will chart N-uptake curves. If you have an N-rich reference zone, small seedlings may not yet be old enough for the model to produce a recommendation because the amount of N taken up by the crop at this stage is so minimal relative to the seasonal demand (Feekes 2-3). If crops are slightly more mature (Feekes 4-5), the N-rich reference zones may provide additional information to inform grower decision-making. By early- to mid-February, most of the acreage in the southern Sacramento Valley should be mature enough to provide data that can be used in the webtool.
Herbicide Considerations:
Italian ryegrass / weeds in general: So far I've seen chickweed in the valleys and Italian ryegrass in the hills, mostly at an appropriate stage for herbicide treatment depending on whether or not your wheat is mature enough. With good soil moisture, warming conditions, and lengthening days, weed growth is accelerating. Pay attention to the labels, but most of the ALS inhibitors (Osprey and Simplicity) and ACCase inhibitors (Axial) labels suggest spraying when weeds are between 2 leaves and two tillers. Some of those herbicide programs have slightly wider windows, but experience from a look-on-the-bright-side herbicide trial last year highlighted three lessons:
1. Spraying herbicides on drought-stressed weeds reduces efficacy, sometimes catastrophically. The same is true of weeds that have matured beyond the ideal growth stage.
2. Treatment windows on the labels are relatively optimistic about efficacy: yes, you CAN spray Italian ryegrass at three tillers in some cases, but herbicide performance on the early or late end of that window may be less dependable, especially under less-than-ideal conditions.
3. Weeds will out-compete your small grains if given the chance: every day that weeds are in there is a day that the crop is going to get edged out. Weeds grow faster and are more competitive and have the ability to completely overtake the crop in the course of a few warm weeks. Even if you manage to wipe out Italian ryegrass, if it's too late in the season, your crop's yield potential will have dropped substantially because so much of its key growth stages were spent competing with weeds for resources.
In terms of Nitrogen management, however, now might be a time to watch and wait until we have a bit more certainty about the weather. Growers will ultimately need to make the call on how to respond but having more information from soil nitrate quick tests and knowing what growth stage your crop is in will help inform those decisions.