If growers in Sacramento, Solano, or Yolo County are interested in collaborating with UC Cooperative Extension Agronomists in trials focusing on control of Italian ryegrass in small grain crops or have related questions, please reach out to Konrad Mathesius ([email protected]).
When growers need to manage herbicide-resistant populations of weeds without tillage, one strategy is to reduce the amount of seed returned to the seedbank by destroying, or removing weed seed caught during harvest operations. Collectively these strategies are referred to as Harvest Weed Seed Control (HWSC). These methods have long been studied in Australia. Researchers at Washington State University, Virginia, Texas, and other parts of the US have been looking into these methods for several years. With increasing herbicide resistance developing in Italian ryegrass populations in California, similar approaches may be worth exploring in the Sacramento Valley.
The caveat to all of this is that in order for HWSC to be effective, fields need to be harvested before weed seeds shatter (falling from the plant to the ground). As of yet the shatter patterns of Italian ryegrass in California are not well known. Additionally, previous research has indicated that Italian ryegrass seed retention at small grains harvest can be highly variable across different locations. For example, researchers from the inland Pacific Northwest have reported Italian ryegrass seed retention rates at harvest of 27-50% whereas a 58% of seed retention has been found in Australia. Furthermore, grains in California are planted at different times of the year and our harvest season occurs at different times of the year relative to Australia (not to mention the Pacific Northwest).
Of course, the best control strategies for herbicide-resistant weeds still include a mixture of different tools such as: the use of herbicides with different modes of action (including pre-emergent and post-emergent types where possible), the use of diversified crop rotations, and well-timed mechanical control. However, a better understanding of the potential for HWSC in California may give growers an edge in reducing the spread of herbicide resistance.
Limitations and project work
What is Harvest Weed Seed Control?
Direct Baling – Collecting straw and chaff immediately into bales, usually using a tow-along baler behind the combine. After harvest, bales can be moved off the field, thereby removing a large proportion of weed seed before it can disperse into the soil. However, removal of this much biomass can be problematic for growers with low organic matter soils that otherwise benefit from maintaining residues on the ground.
In an effort to address this, UC researchers will begin collecting data on the shatter status of Italian ryegrass populations in several areas of the Sacramento Valley leading up to harvest. If we consistently see that a significant portion of the Italian ryegrass seed remains attached to the plant at harvest, then HWSC strategies may offer viable control options for California. Conversely, if ryegrass seed has largely shattered by the time our grain crops are harvested, then California growers may need to consider other options for control.
Dependence on herbicides alone in these systems has resulted in weeds with resistance to multiple modes of action. In Australia, there is one documented population of rigid ryegrass (Lolium rigidum) that is resistant to 15 different herbicides, covering seven different modes of action. Italian ryegrass (Lolium multiflorum), an annual winter species commonly found in small grain production systems in California is also notorious for its ability to develop resistance to entire groups of herbicides. One population collected in California orchards has resistance to four modes of action, but this population is not yet widespread.
Figure 1. Formation of narrow-windrows duing wheat harvest (image credit: Lauren Lazaro)
Figure 2. Narrow-windrow burning implemented during wheat harvest
Italian ryegrass has been a persistent problem in wheat production in the Texas Blacklands. Great adaptability, profuse tillering, and high seed production make this a troublesome weed. Rapid development and spread of resistance to some of the important herbicides warrants the development of additional interventions for its control. As this species reproduces by seed, a seedbank in the soil allows for long-term persistence in crop fields. Italian ryegrass maturity almost coincides with wheat maturity, and any unshattered seed at the time of harvest is collected by the combine harvester and spread across the field. The harvest operation, however, presents an opportunity to collect the ryegrass seed retained at that point and destroy them. These practices are collectively known as harvest weed seed control (HWSC).
The HWSC tactics, developed and widely adopted in Australia, have a great potential to reduce ryegrass seedbank inputs and subsequent field infestations. A key to the success of HWSC is the ability of weeds to retain their seed at the time of harvest. High levels of shattering prior to crop harvest reduces the efficacy of HWSC. Our estimates of Italian ryegrass seed shattering during wheat hearvest window in the Southeast Texas region typically ranges from 35 to 50%, and is highly regulated by the environmental conditions. Proper timing of wheat harvest is vital to maximize weed seed capture at harvest, as delays can reduce the amount of seed retained by ryegrass plants. Our observations indicate that heavy rainstorms can drastically increase ryegrass seed shattering prior to harvest. Nevertheless, mathematical models show that an ability to remove even 50% of the weed seeds can still be very helpful for long-term management.
Among the different HWSC tactics, narrow-windrow burning is relatively inexpensive and has been extensively used in Australia. With this method, a simple chute attached at the rear of the combine concentrates all chaff (mixed with weed seed) and straw exiting the combine into a narrow windrow (Figure 1), which is then burned to kill the weed seeds (Figure 2). The amount of residue matters here because it affects the amount of heat produced and its ability to kill weed seed.
We conducted a USDA-NIFA (United States Department of Agriculture – National Institute for Food and Agriculture) funded field study from 2016 to 2019 in College Station, TX where narrow-windrow burning was integrated with herbicide programs in wheat to evaluate its impact on long-term population size of Italian ryegrass. Results showed that inclusion of narrow-windrow burning tremendously improved long-term Italian ryegrass control compared to a simple herbicide-based program that only included Prowl ® H2O at 32 oz/A delayed preemergence application, approximately 5 days after planting (Figure 3). When narrow-windrow burning was combined with an early postemergence application of Axiom ® 8 oz/A and a mid postemergence application of Axial ® XL at 16 oz/A, excellent control was observed. In fact, ryegrass densities in these plots during wheat harvest were negligible by the end of the fourth year.
Currently, a few variants of HWSC tactics are used in Australia, including narrow-windrow burning, chaff carts, chaff lining, bale direct system, and mechanical weed seed destruction by “impact mills” such as the Harrington Seed Destructor (HSD), Redekop system, and Seed Terminator.
Figure 3. Italian ryegrass infestation at wheat harvest: a simple delayed preemergence only herbicide program without harvest weed seed control (left) and the same program with the inclusion of narrow-windrow burning (right)