Harvest Weed Seed Control

Weed seedbanks are the primary source of annual weed infestations. Reducing seedbank replenishment is, therefore, critical for effective weed management. Frequently, weeds survive herbicide applications or other control tactics and set seed. These escaped weeds can often be seen poking out above the canopy. Weed seeds can still be managed before they replenish the soil weed seed bank with the harvest weed seed control (HWSC) tactics discussed below. The success of HWSC practices depends on the biological attribute of seed retention at crop maturity: HWSC is only effective for weed species that do not shatter (meaning drop their seeds) prior to mechanical harvest.

“Soil seedbank” refers to weed seeds present in the soil. The soil seedbank serves as the source of next season’s weed problems. The majority of seeds in the soil were deposited by plants that escaped control in previous years. Due to seed dormancy, weed seeds can remain viable in the soil for many years and often emerge over several weeks during the growing season. Seed dormancy, prolonged viability, and extended emergence periods make it easier for weeds to survive across a wide range of environments and management tactics. Current management recommendations target weed seed production to prevent additions to the soil seedbank. Recommendations include controlling weeds while they are small and susceptible to weed control tactics, removing weeds that escape control, and managing weeds that produce viable seeds to minimize their impact on the soil seedbank. These late-season practices are not new, but due to wide-spread herbicide-resistance they are being revisited with new technology. In this video Dr. Mandy Bish explains the importance of understanding the weed seedbank and why preventing weeds from producing seeds is so important.

Currently there are no economical methods to kill weed seeds once they enter the soil. Weed management practices focus on controlling weeds early in the growing season with herbicides and cultivation, then following up to manage those weeds that escape control and prevent seeds from entering the soil. Studies have demonstrated that preventing seeds from returning to the soil can rapidly reduce the seedbank and resulting weed densities. Allowing weeds to produce viable seeds, however, can cause rapid increases in weed densities. Stopping seeds from entering the seedbank can be achieved by:

There has been a renewed emphasis on increasing seed mortality, mechanically removing seeds from the field, and altering the placement of seeds. These tactics were used before the wide-spread availability of herbicides. Because of technological advances in machinery, these tactics are again being researched and adapted for use in the US and around the world.

The most common tactics to reduce the soil seedbank include crop rotation and tillage. Diverse crop rotations allow for a diversification of other supporting IWM tools, such as herbicides, tillage, and crop competition. Diversified crop rotations are rarely used to manage the soil weed seedbank. The use of only one (or a few) cash crops ultimately allows weeds with similar traits to thrive, replenishing the weed seedbank annually. Tillage can both increase the soil seedbank (by burying seeds) and decrease it (by unearthing buried seeds that may germinate early and die). The effect of deep tillage on the soil seedbank can be gauged by knowing the main weed species in a field and understanding how long the seeds of the species stay viable: deep tillage is a better choice for short-lived seeds. Grass weeds tend to have less persistent seeds than broadleaf weeds. Strategic deep tillage affects seedbanks depending on weed traits such as germination and seed size. A specific tillage tactic to manage the seedbank is the stale seedbed method. Stale seedbed management uses tillage to promote weed germination so that another management tactic can then be used to kill weed seedlings prior to crop planting, reducing the size of the soil seedbank. Crop rotations and tillage are fundamental for weed management, but management decisions tend to prioritize economic and logistic factors over effects on weeds.

Hand-pulling: 
Sometimes weeds escape chemical controls and set seed, making hand-pulling the only option in many cases if HWSC has not been planned ahead of time. Weeds such as Palmer amaranth and waterhemp set viable seeds in a short period of time and so should be completely removed from the field in order to prevent them from adding the soil seedbank.

Harvest Weed Seed Control:
Harvest weed seed control (HWSC) is an approach to managing the soil seedbank that focuses on cultural and mechanical options to reduce the impact of seeds from escaped weeds at harvest time. All HWSC methods/technologies are based on separating weed seeds from the chaff fraction when harvesting the cash crop. The final destiny of the weed seeds will depend on the HWSC method used. Some of them concentrate weed seeds in rows, lines, bales, or dumps, while others destroy weed seeds with a mill before they can drop to the ground.

All HWSC methods are similarly effective, but come with different initial costs, operating costs, and residual costs. Additional pros and cons related to feasibility and level of nutrient removal or redistribution should also be considered when deciding what works best for your system. The best HWSC method for your farm depends on cost and other management concerns.

Table 1: Pros and cons of the six different HWSC methods.

All HWSC methods are similarly effective, but come with different initial costs, operating costs, and residual costs. Additional pros and cons related to feasibility and level of nutrient removal or redistribution should also be considered when deciding what works best for your system. The best HWSC method for your farm depends on cost and other management concerns.

Our research started four years ago with three ongoing large plot field trials in Urbana-Champaign, IL; Beltsville, MD; and Fayetteville, AR. These three locations are examining how the use of different IWM tactics can reduce the soil seedbank and combat herbicide resistant weeds. Tactics under investigation include the use of crop rotations, cover crops, impact mills, and narrow windrow burning.

Several field trials have been conducted testing narrow windrow burning in soybean, grain sorghum, and wheat cropping systems at various locations. More recently, research is being conducted in small plots and on farms to adapt chaff lining (a well-proven technology in Australian wheat) to soybean in the US.

Our efforts are spread all over the country with the hope that region- or state- specific trials will help inform growers of the practices best suited for their farms. As our research progresses, our goals are to help growers improve their use of IWM and to make use of HWSC where needed and feasible to Get Rid of Weeds.