The Impact of Herbicide Application and Defoliation on Barley Grass (Hordeum murinum subsp. glaucum) Management in Mixed Pasture Legumes

Abstract

Barley grass (Hordeum murinum subsp. glaucum.) is an annual weed associated with grain revenue loss and sheep carcass damage in southern Australia. Increasing herbicide resistance led to a recent investigation into effective integrated weed management strategies for barley grass in southern Australia. Field studies in Wagga Wagga, New South Wales (NSW) during 2016 and 2017 examined the effect of post-emergent herbicide applications and strategic defoliation by mowing on barley grass survival and seed production in a mixed legume pasture. Statistically significant differences between herbicide-only treatments in both years showed propaquizafop to be more than 98% effective in reducing barley grass survival and seed production. Paraquat was not effective in controlling barley grass (58% efficacy), but led to a 36% and 63.5% decrease in clover and other weed biomass, respectively, after 12 months and increased lucerne biomass by over three-fold after 24 months. A single repeated mowing treatment resulted in a 46% decline in barley grass seedling emergence after 12 months and, when integrated with herbicide applications, reduced other weed biomass after 24 months by 95%. Resistance to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides observed in local barley grass populations led to additional and more focused investigation comparing the efficacy of other pre- and post-emergent herbicides for barley grass management in legume pastures. Haloxyfop-R + simazine or paraquat, applied at early tillering stage, were most efficacious in reducing barley grass survival and fecundity. Impact of defoliation timing and frequency on barley grass seedlings was also evaluated at various population densities, highlighting the efficacy of repeated post-inflorescence defoliations in reducing plant survival and seed production. Results highlight the importance of optimal environmental conditions and application timing in achieving efficacious control of barley grass and improving pasture growth and biomass accumulation.

Simulating the population dynamics of barley grass (Hordeum spp.) and impacts of weed management strategies in a southern Australian lucerne (Medicago sativa) pasture

“…By contrast, mowing has been associated with legume dominance (Myers and Squires 1970), frequently reducing annual grass seed production when timed to coincide with reproductive development (El-Shatnawi et al 1999;Bowcher 2002;Anderson and Frank 2003;Donald 2006;Brownsey et al 2017). Additional beneficial effects of mowing have been documented considering that repeated mowing may further limit seed production (Donald 2006;Kelly et al 2020), because barley grass experiences difficulty recovering from defoliation after the commencement of reproductive growth (El-Shatnawi et al 1999). These effects are also typically exacerbated in dense populations (Kelly et al 2020), likely resulting from pre-existing competitive stress exerted by neighbouring plants (Weiner 2004).…”

“…Additional beneficial effects of mowing have been documented considering that repeated mowing may further limit seed production (Donald 2006;Kelly et al 2020), because barley grass experiences difficulty recovering from defoliation after the commencement of reproductive growth (El-Shatnawi et al 1999). These effects are also typically exacerbated in dense populations (Kelly et al 2020), likely resulting from pre-existing competitive stress exerted by neighbouring plants (Weiner 2004).…”

“…Despite much research on grass weeds in pasture, information is scarce regarding the long-term efficacy of integrated weed management (IWM) programs against the barley grass seedbank. Recent studies in Australia compared the impacts of single herbicide applications and of mowing with combinations of each on barley grass survival (Kelly et al 2020), with treatment success shown to be dependent on the timing of applications to plant growth stage and seasonal weather patterns. Other studies investigating herbicide combinations with repeated mowing for control of other annual weeds have also shown some success (Donald et al 2001).…”

Herbicide Resistance in Weed Management

“…While this focus has been of great interest as of late, further research on the development of integrated weed management approaches to address herbicide-resistant weeds is required [16][17][18]. We are pleased to report that two papers on this topic are presented in the current Special Issue [11,12]. However, more efforts should be pointed toward this urgent issue in the near future.…”

“…After three generations of selection, the progeny survival rates were higher than those of the parental population, suggesting that reduced susceptibility to glufosinate can evolve in weed populations following repeated applications of low herbicide rates. Kelly et al [11] examined the effect of integrated weed management using post-emergence herbicide application and strategic defoliation by mowing on the control of Hordeum murinum subsp. glaucum in Australia.…”