The widespread evolution of multiple herbicide resistance in the most serious annual weeds infesting Australian cropping fields has forced the development of alternative, non-chemical weed control strategies, especially new techniques at grain harvest. 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 Australian grain cropping and their potential utility in global agriculture.

In 2003, a random survey was conducted across the Western Australian wheatbelt to establish the frequency and distribution of herbicide resistance in ryegrass populations infesting crop fields. Five hundred cropping fields were visited at crop maturity, and ryegrass seed was collected in 452 of these fields. Subsequently, each crop field population was screened with herbicides of various modes of action that are commonly used for ryegrass control in Australian cropping systems. Most of these ryegrass populations were found to be resistant to the ACCase-inhibitor herbicide diclofop-methyl (68%) and the ALS-inhibitor herbicide sulfometuron (88%). A comparison of resistance levels in the same agronomic zones surveyed 5 years earlier determined that there had been an increase of 20 percentage points in the frequency of resistance over this 5-year period. This survey also determined that the majority (64%) of populations were found to be multiple resistant to both diclofop-methyl and sulfometuron. The distribution patterns of the collected populations indicated that there were higher frequencies of resistant and developing resistance populations occurring in the intensively cropped regions of the wheatbelt, which had greater herbicide selection pressure. Of concern is that 24% and 8% of populations were found to be developing resistance to trifluralin and clethodim, respectively. Currently these herbicides are heavily relied upon for control of ACCase and ALS herbicide resistant ryegrass. Nearly all populations remain susceptible to glyphosate. Ryegrass across the WA wheatbelt now exhibits multiple resistance across many but not all herbicides, posing severe management and sustainability challenges.

In most world crop-production areas, the evolution of herbicide-resistant weeds is becoming a major issue. This problem has become most severe across the Australian dryland crop-production region, where herbicide-resistant weed populations are threatening crop-production profitability and sustainability across 20 million ha. Widespread herbicide resistance has forced changes in agronomic and herbicide practices. This problem is particularly evident in Western Australia, where the frequency and distribution of herbicide-resistant weed populations appear to be greater than anywhere else in the world. Judicious use of herbicide mixtures and rotations can reduce the selection pressure for evolved resistance to any one specific herbicide. Additionally, agronomic practices, such as the double knockdown (preseeding sequential application of nonselective herbicides), increased seeding rates, and targeting of weed seed production to prevent seedbank inputs, are needed to reduce the selection pressure on all herbicides by reducing in-crop weed populations. However, these techniques are not without problems or limitations, and their weed control efficacy is inferior to that of most in-crop selective herbicides. The adoption by Australian farmers of the current limited technology is clear evidence of the value placed on the use of these alternate crop weed-control practices. The continued evolution of herbicide resistance more than justifies continuing research and development efforts to produce integrated strategies and smarter herbicide use so as to achieve sustainable weed management.