As cropping seasons trend earlier and earlier, we are left to deal with a new flush of annual weed species post-harvest. As your crops are removed from the field, the environment is still exceptional for problematic summer annual weed populations to emerge.
When should I initiate a fall residual program?
Weather patterns may hold for the next 2-3 months that are conducive for weed species emergence. Such as Palmer amaranth, morning glory spp., Barnyard grass, johnson grass, etc. But, not close enough to October to initiate a fall residual program. This timeframe can put you in a difficult situation.
Here are a few things to consider when thinking about post-harvest weed populations.
The goal is controlling the soil seed bank. During this time from August to early October weed seed have the propensity to germinate and still produce seed; however, the length of daylight is steadily decreasing. With reduced daylight hours weed species will reach reproductive growth stages much faster than during the spring growing season. What we often observe are much smaller plants with established seed head or reproductive structures.
Figures 1 and 2: Summer annual broadleaf and grass weed species emergence near and/or post-harvest.
Weeds post harvest example
A prime example is a 4-6” Palmer amaranth that emerged post-harvest and has a seed head on it. Typically, these are not as robust and numbersome as plants that emerged in the spring; however, if that seed head contains 2-300 seed, it is still viable seed that we must contend with the following spring if allowed to mature.
Control Mechanisms
Think about control mechanisms that can be utilized during the time between post-harvest weed control, but prior to fall residual application programs. Some great options include tillage, cutting, mulching, mowing, or chemicals.
Tillage
Tillage is a great option, as it is effective for field preparation. The challenge is that in fields where weed escapes exist, we have effectively used the combine to mulch and seed existing plants existing during harvest.
When we utilize a tillage option post-harvest, we are also covering seed and bringing new seed to the surface depending on what implement is used. Tillage is a foundational weed control mechanism and is a viable option in delaying weed species emergence post-harvest.
Cutting, Mulching, or Mowing
Cutting, mulching, or mowing stalks is also an option for weed control, as the existing crop stubble (specifically grass crops with a lot of residues left after harvest) can act to cover weed seed and prevent emergence.
Chemical Control
There are several chemical options available, such as paraquat (Devour®), glufosinate (Liberty® 280; Fever® Herbicide), and glyphosate (Round Up PowerMax®, Envy™ Brands). However, there are some challenges to keep in mind.
Chemical active ingredient limits per acre, potentially unbudgeted application expense, and dealing with mixed and varied weed population sizes. In these instances, we suggest treating post-harvest applications on a field-by-field basis to manage expenses, time, and weed populations.
The Takeaway
Ultimately the goal is two-fold, manage the soil seed bank and delay summer annual emergence post-harvest. By managing one, you can control the other and vice versa.
As harvest is under way, consider problem weed control areas and what additional steps after harvest and into fall should be utilized to decrease the weed seed bank.
Please consult any of your local Agronomic Sales Enablement Team or your Simplot Grower Solutions Crop Advisor for any further questions and product options.
Authors
Ben Lawrence, Bayou
Jeffrey Mansour, Midsouth
Justin McCoy, Southeast