Diversity is a key concept in most contexts in people’s lives: diversity of ideas, in people’s backgrounds, and cultures. Diversity in weed control strategies is key to any successful weed control program: rotating and mixing herbicide with different mechanisms of action, cultural and physical practices, and preventive measures are a few examples of common approaches that can be integrated in a crop production system to reduce weed interference. Crop residue retention after harvest, when feasible and economically viable, is an important factor in an integrated weed management program. This additional layer that covers the soil surface may reduce weed interference by various mechanisms:
- Suppression of weed seed germination that required activation by light
- A physical barrier to early weed development
- Allelopathy
- Increase weed seed decay
It is estimated that approximately half of the annual weed species in crops are highly dependent on light for germination (e.g. common purslane, crabgrass, junglerice). The other half of the weed species that are not dependent on light to germinate may encounter another barrier in the presence of crop residue: the lack of quality solar radiation. The energy for the initial weed development depends on carbohydrates stored in the seeds; therefore, enough carbohydrates must be available to sustain growth through the crop residue. If the crop residue layer is thick, or if the carbohydrate reserves in seeds are small, the chances to overcoming the crop residue and reaching the soil surface (and solar radiation for photosynthesis) reduce drastically.
In cool-season grass seed crops, the majority of the growers choose to bale the crop residue because it can be sold as animal feed. This brings an additional source of income that is very welcomed. However, crop residue retention may increase the long-term soil health indices, such as increasing soil organic carbon, increase soil microbial and macrofauna activity. In parallel, enhancing biological activity in the soil also increase decay of weed seeds in the soil seedbank due to predation by small animals (e.g. earthworms) and attack by fungi. With these aspects in mind, a few growers choose to keep the crop residue regardless of straw prices.
Now, if you read this text up to this point, you are probably thinking that, if you were a grower, you would be more inclined to bale your crop residue and sell for feed, especially in times when the bale price is tempting. Although this seems like an obvious choice at first, crop residue retention has some other short-term benefits. With the straw removal, macronutrients are also taken away that would otherwise be recycled, in particular potassium, and will need to be replenished before the next growing season. With herbicide resistance evolution in action, crop residue retention seems like a promising non-chemical weed control tool.
To understand how crop residue can serve as a non-chemical weed management tool, we conducted a preliminary study to simulate inhibition of annual bluegrass, roughstalk bluegrass, and Italian ryegrass development under different amounts of crop residue left after crop harvest. We assessed 1,800, 3,600, and 7,100 pounds/acre of tall fescue or perennial ryegrass to understand the weed suppression potential of these crops species. The residues amounts chosen are realistic crop residue amounts found in grower fields, where 8 t ha-1 is the most common. We observed that crop residue can inhibit annual bluegrass and roughstalk bluegrass up to 50% (figure 1), whereas Italian ryegrass germination was not inhibited. We observed that tall fescue had a stronger inhibitory effect compared to perennial ryegrass, which was expected as many other researchers have observed allelopathic effects in this crop species.
Another important consideration when designing integrated weed management approaches is the interaction of herbicides with the additional practices. We observed 50% reduction in weed germination, meaning that 50% of the weeds are still able to develop that need to be controlled. Anecdotal knowledge indicates that the efficacy of preemergence herbicides is commonly reduced in the presence of crop residues. While this may be true for some compounds (e.g. trifluralin and prosulfocarb), some others may be sprayed directly onto the crop residue and still be efficacious (e.g. pyroxasulfone). Field trials are currently underway in our weed science program to identify herbicides that exhibit beneficial interaction with crop residue (i.e. do not reduce herbicide efficacy) that will enable cool-season grass seed growers to implement a more sustainable weed management strategy, reducing weed seed contamination in the final product and reducing herbicide selection pressure. We will be sharing the results from our field studies soon.
Figure 1. Response of annual bluegrass to tall fescue cover at A) 0 lbs/A, B) 1,800 lbs/A, C) 3,600 lbs/A, and D) 7,100 lbs/A.
Special thanks to Seth Abugho, Postdoctoral Scholar, Department of Crop and Soil Science, Oregon State University, and Andrew Hulting, Professor and Extension Weed Management Specialist, Department of Crop and Soil Science, Oregon State University, for their assistant with this blog post.