Targeting Weed Seeds In-Crop: A New Weed Control Paradigm

The widespread evolution of multiple herbicide resistance in the most serious annual weeds infesting cropping fields has forced the development of alternative, non-chemical weed control strategies, especially new techniques at grain harvest, says a recent Weed Science Society Of America (WSSA) publication.

Harvest weed seed control (HWSC) systems target weed seed during commercial grain harvest operations and act to minimize fresh seed inputs to the seedbank. These systems exploit two key biological weaknesses of targeted annual weed species: seed retention at maturity and a short-lived seedbank. HWSC systems, including chaff carts, narrow windrow burning, bale direct, and the Harrington Seed Destructor, target the weed seed bearing chaff material during commercial grain harvest. The destruction of these weed seeds at or after grain harvest facilitates weed seedbank decline, and when combined with conventional herbicide use, can drive weed populations to very low levels. Very low weed populations are key to sustainability of weed control practices. Here we introduce HWSC as a new paradigm for global agriculture and discuss how these techniques have aided grain cropping and their potential utility in global agriculture.

Current Weed Control Paradigm in Global Field Crops: Target Weed Seedlings

The current dominant paradigm for weed control in field crops is PRE or early POST herbicides to remove weed seedlings in young, establishing crops. Crop yield is securedby controlling weed seedlings during this critical ‘‘weed free period’’during which crop plants are particularly vulnerable to weed competition.

However, despite herbicide use, some in-crop weeds inevitably escape herbicide control for a range of reasons (eg. adverse environmental conditions, insufficient herbicide, herbicide resistance, delayed emergence) and in the great majority of situations there is no other feasible, practicable method of controlling these surviving weeds. Hence, these weeds survive to maturity with the crop and produce significant quantities of seed which sustain or build a viable weed seedbank.

It is the ongoing, annual production of weed seeds that perpetuates and amplifies many crop-weed infestations in global field crops. Annual weed seedbank replenishment ensures that each year high weed numbers emerge and are herbicide treated, with the inevitable risk of resistance evolution. It has long been recognized that alternate weed control strategies are needed to alleviate intense herbicide selection.

Globally, there are few suitable alternatives to herbicides and even when alternatives (eg. cultivation, crop competition, delayed seeding) are considered, the focus remains on preventing weed seedlings from interfering with early crop growth. However, weed adaptations, such as seed dormancy, in response to changes in cultivation and cropping practices, have already occurred in several annual species including annual ryegrass, wild oats, brome grass and barley grass.

These evolutionary adaptations in response to a selection pressure further highlight the consequences of neglecting seed production and seedbank replenishment by weeds surviving early season control practices. Clearly, new tools are needed that complement early season weed control techniques by targeting annual weed seed production.

New Weed Control Paradigm: Harvest Weed Seed Control (HWSC)

Widespread multiple herbicide resistance in the very important weeds, annual ryegrass and wild radish, of Australian cropping has forced the development of additional weed control strategies.
Knowledge that the major proportion of an in-crop annual ryegrass population results from the previous season’s seed production led to a focus on minimizing weed seed production.

The biological attribute of seed retention at maturity in annual ryegrass, wild radish, and several other annual crop weed species, means that seeds are attached to the upright plant enabling the weed seeds to be collected (harvested) during grain crop harvest. For example, in field crops a large proportion (up to 80%) of total annual ryegrass seed production can be collected during a typical commercial grain harvest. These weed seeds enter the front of the grain harvester, are processed, and exit the grain harvester in the chaff fraction. An irony is that these ‘‘harvested’’ weed seeds are evenly redistributed across the crop field to become future weed problems.

Thus, grain crop harvest presents an opportunity to target weed seed production, thereby minimizing replenishment/increase of the weed seedbank. As outlined below, harvest weed seed control (HWSC) systems have been developed in Australia to target and destroy weed seeds during commercial grain crop harvest, minimizing weed seed inputs into the seedbank.

SOURCE: Weed Technology, 2013, Weed Science Society Of America, Michael Walsh, Peter Newman and Stephen Powles. Academic citations removed have been removed.

Go here to read the full academic review as a .pdf file.

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