Several of our most damaging weeds, such as Palmer amaranth, are annuals that complete their life cycle in one growing season and reproduce by seed, thereby potentially establishing large viable seedbanks in the soil because of prolific seed production. If allowed to produce seed, the inevitable consequences will be competition in subsequent cropping years and increased populations of herbicide-resistant species. Reduction of both consequences is the primary goal of any effective weed management program.
For generations of farmers, herbicides have been the most effective tool to control weeds in crops because of ease of application, high efficacy and lower cost. Unfortunately, heavy reliance on herbicides has resulted in the evolution of herbicide-resistant weeds throughout the U.S. with six confirmed in Louisiana today. With reduced options to control herbicide-resistant weeds, adoption of integrated weed management practices that utilize multiple strategies are urgently needed.
Harvest weed seed control techniques use nonchemical practices to allow growers to target weed seed during harvest. This involves the collection and/or destruction of weed seed during grain harvest, thus minimizing or eliminating seed rain from the mature plant, which contributes to the soil weed seedbank. These various techniques can ultimately turn harvest equipment such as combines from a weed seed spreader into weed seed predators.
During harvest, the combine collects seeds retained by weeds above the harvest height along with crop grain but distributes the weed seed back across the field with crop plant residue. With harvest weed seed control techniques, the weed seed are intercepted by the combine, separated from the grain and crop residue, and either removed from the field, concentrated in a narrow band behind the combine on the ground or destroyed.
Harvest weed seed control practices should aim to supplement other measures, such as the use of herbicides, so that problematic weeds can be further minimized in a long-term approach. There are currently six approaches for producers to consider: chaff lining, chaff tramlining, chaff carts, bale direct system, narrow windrow burning and seed impact mills. LSU AgCenter weed science research is focusing on the viability of the seed impact mills and their incorporation into modern grain harvest equipment.
The confinement of just the chaff material, which is plant material separated from the crop grain, into narrow rows during harvest relies on a mulch effect to prevent weed seed germination and emergence. These narrow rows can be either on dedicated wheel tracks (chaff tramlining) or between stubble rows consisting of straw or the crown of the crop left in the ground after harvest (chaff lining). This is accomplished by attachments on the rear of the harvester that collect and place chaff into narrow rows.
The concentration of the chaff material places weed seeds in an environment unsuitable for germination and emergence. For this approach to be effective, the chaff lines need to remain undisturbed by tillage or other field operations as any disruption of the chaff layer creates opportunities for weed seedlings to emerge. Often, a banded herbicide application, which is an application directed to a given row area instead of the entire row width, a technique that can reduce the overall amount of herbicide used, is needed in the spring as the weed seed that do contact the ground are concentrated in a much smaller area.
A noncrop simulated chaff lining trial was conducted by the LSU AgCenter in Louisiana in 2021. Palmer amaranth was seeded inside and outside of the chaff line. Additionally, seed packets that contained Palmer amaranth seed were placed inside and outside of the chaff line. Seed viability showed an average difference of 12% between inside and outside of the chaff line across retrieval timings. Weed emergence was not significantly different.
This relatively simple system consists of a chaff collection and transfer mechanism attached to a combine that delivers the weed seed-bearing chaff fraction into a bulk collection bin, usually a trailing cart. Once the carts are filled, they are moved outside the field, emptied and often burned to destroy seed viability. The management of the large volumes of collected chaff material is difficult and has been a primary reason for the low adoption and use of this approach.
The bale direct system consists of a large square baler directly attached to the harvester that constructs bales from the chaff and straw residues exiting the grain harvester (similar to baling hay). This system serves a dual purpose of capturing weed seed and baling harvest residues that can be used for livestock feed. There does not appear to be a large market for this method, which has limited the adoption of this system.
Narrow windrow burning involves depositing all field residues, including weed seed, into narrow rows using a chute mounted behind the combine. These windrows are later burned to destroy the weed seed. The chutes used for this technique are relatively inexpensive and can be fabricated and assembled on-farm. A three-year AgCenter trial funded by the Louisiana Soybean and Grain Research and Promotion Board showed no shifts in weed populations or seed shattering time were observed. However, there was a significant reduction in weed density and the weed seed present in the soil seedbank when harvest weed seed control and robust herbicide programs were used in combination.
Seed impact mills are commercial units that are integrated into the combine. All these units utilize metal parts at high velocity to crack the seeds as they move through the mill, thereby greatly reducing their viability. The constraints to this system are the requirement of a combine with significant horsepower and being cost prohibitive for most producers with small to medium acreage. Previous research has shown these mills to greatly reduce seed viability, upwards of a 70% to 90% reduction, but the effectiveness of the system is incumbent on getting all weed seed into the seed mill.
So, which system is best? The answer is dependent on the individual producer’s farming operation due to each having distinctive pros and cons. Their individual effectiveness, when combined with that of herbicides, has the potential to greatly reduce the soil weed seedbank and ensure longevity of effective herbicide usage by reducing herbicide-resistant weed seed viability.
Lauren Lazaro is a senior agronomist at Blue River Technology and an adjunct professor in the LSU AgCenter School of Plant, Environmental and Soil Sciences.
This article appeared in the summer 2022 issue of Louisiana Agriculture.
The bale direct system. Photo provided by Lauren Lazaro
A chaff cart. Photo provided by Lauren Lazaro
Chaff lining. Photo provided by Lauren Lazaro
A combine harvests a wheat field, leaving behind a narrow windrow. Photo by Lauren Lazaro.