- 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.
/h3>/h3>/h3>