Does cutting herbicide rates threaten the sustainability of weed management in cropping systems?

Occasional mouldboard ploughing slows evolution of resistance and reduces long-term weed populations in no-till systems

“…Through each year there are germination events, where individual seeds have a probability of becoming plants; herbicide spray events, where individual weed plants die with a probability that depends on the applied spray rate and their own 'resistance status', which in turn depends on their individual genetics; and a harvest event, where crop-weed competition is evaluated and the number of new weed seeds is calculated. The relationship between crop density, weed density, weed seed production and crop yield is estimated using the well-established hyperbolic competition function (Firbank and Watkinson, 1985;Diggle et al, 2003;Renton et al, 2011). The genotype of each new weed seed is determined individually based on the laws of Mendelian genetics.…”

“…A full description of the original PERTH model is provided in Renton et al (2011). For this study, specific extensions were made to the original model so it could address our questions regarding occasional mouldboard ploughing and these extensions are detailed below and in Table 1.…”

Optimizing pesticide spray coverage using a novel web and smartphone tool, SnapCard

“…Thus, applications of pesticides should be considered a selection pressure imposed by humans, and the sustainability of a particular pesticide becomes a matter of resistance management: how to kill enough pests to avoid economic losses but at the same time not impose a strong and persistent selection pressure leading to resistance in the target pest population and shortening the useful life of the pesticide. Many factors influence the actual level of pesticide resistance development in a pest population, including the following (Nansen and Ridsdill-Smith 2013;Gassmann et al 2009;Georghiou and Taylor 1976;Renton et al 2011Renton et al , 2014: (1) genetic factors (i.e. frequency, dominance, and expressivity of resistant alleles and their interactions with other alleles, fitness costs associated with resistance, past selection pressures in pest population, and whether the resistance is monogenic or polygenic), and (2) biological factors (fecundity, generation and development times, mating behavior, level of polyphagy, migration/dispersal and mobility, fitness costs of resistance development, and feeding biology).…”

“…Georghiou and Taylor (1977) also examined the effects of threshold-based pesticide applications (only applying pesticides when the pest density is above a certain threshold), and they showed that the potential benefits of this approach was particularly important when resistance was associated with a fitness cost. Regarding resistance in weeds and low-dosage herbicide applications, Renton et al (2011) conducted modeling and demonstrated no effect of dosage on the risk of monogenic resistance but an increased risk of herbicide resistance in certain cases of polygenic resistance, when weeds are exposed to low-dosage herbicide regimes. All genetic population modeling studies focus exclusively in genetic/physiological resistance; however, animals such as insects are also known to potentially develop behavioral resistance or avoidance (Martini et al 2012;Wang et al 2004;Hostetler and Brenner 1994).…”

“…Likely consequences of low spray coverage and target pests developing behavioral avoidance/resistance include reduced pest suppression and therefore an increase in number of applications (because after a spray failure, farmers have to come back and spray again), increased risk of environmental contaminations, and potential loss of agricultural productivity. There are also quantitative analyses highlighting that low dosage of herbicide applications (which is an indirect effect of low spray coverage) may lead to increased risk of weed species developing resistance (Renton et al 2011(Renton et al , 2014Powles and Yu 2010;Mortensen et al 2012).…”