Aniruddha Maity¹, Blake Young¹, David Drake², and Muthukumar Bagavathiannan¹

¹Department of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843

²Integrated Pest Management Extension Agent, Commerce, TX

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.

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.

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.

Figure 1. Formation of narrow-windrows duing wheat harvest (image credit: Lauren Lazaro)

Figure 2. Narrow-windrow burning implemented during wheat harvest

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)

Here we demonstrated the benefit of integrating narrow-windrow burning with herbicide-based programs for long-term management of Italian ryegrass. However, it should be noted that burning is not always a preferable method because of fire risk, smoke concerns, and loss of valuable residue. What is demonstrated here is only a proof of concept. Some advanced HWSC methods such as chaff lining and weed seed impact mills are better alternatives and show great potential. In Texas wheat, we are in the process of testing both techniques. An impact mill is currently being deployed in the Blacklands region wheat with a grant support from the USDA-Natural Resources Conservation Service (NRCS). Please stay tuned for updates from this on-farm research.