Herbicides and the water quality conundrum

HERBICIDAL IMPACT ON the health of the Great Barrier Reef (GBR) lagoon came to the forefront in 2009 with the Queensland Government's Great Barrier Reef Protection Amendment Act 2009 and the concurrent review of diuron by the Australian Pesticides and Veterinary Medicines Authority (APVMA). Subsequently, Federal and Queensland government programs have maintained the spotlight on both freshwater and marine water quality. Ambitious pesticide load reduction targets have been set by the Reef 2050 Plan, as one of the means to improve water quality and the resilience of the GBR ecosystem. Photosystem II (PSII) herbicides in particular are targeted under the plan. Gaining sustained industry practice change is paramount to achieving these targets. Progress is being made, although the challenges remain, both on the practice change level and on the technical knowledge level. Weed management practices with demonstrated environmental benefits include timing spray applications to avoid run-off within the 20-25 days following spraying, incorporation of residual herbicides by non-run-off inducing irrigation or rainfall, switching to strategic and/or banded application of residual herbicides, and avoiding the use of residual herbicides on ratoons where trash blanketing provides sufficient weed suppression. Other farming system improvements such as controlled traffic may reduce the amount of run-off, contributing to reductions in overall herbicide losses. Growers are switching to alternative residual herbicides in response to tighter controls on the PS II herbicides diuron, atrazine, ametryn and hexazinone. Relative risk rankings being developed indicate that alternative herbicides can offer reduced environmental risk

Combining weed efficacy, economics and environmental considerations for improved herbicide management in the Great Barrier Reef catchment area

The current Australian sugarcane industry transition toward adoption of an 'alternative' herbicide strategy as part of improved environmental stewardship is increasingly complicated by recent farming system, regulatory and herbicidal product changes. This study quantified and compared the efficacy, economic costs and environmental risk profiles of a range of established, emerging, and recently registered pre-emergent herbicides across field trials in the Wet Tropics region of North Queensland. Several herbicides were effective on certain weed species, but lacked broad spectrum control. Better efficacy results from products with multiple active ingredients (i.e., imazapic-hexazinone) demonstrated the benefits of using mixtures of active ingredients to widen the spectrum of weed control efficacy. All tested pre-emergent herbicides behaved quite similarly in terms of their propensity for off-site movement in water (surface runoff losses generally >10% of active applied), with their losses largely driven by their application rate. Herbicides with lower application rates consistently contributed less to the total herbicide loads measured in surface runoff. Results demonstrated alternative choices from the more environmentally problematic herbicides (such as diuron) are available with effective alternative formulations providing between 4 and 29 times less risk than the traditional diuron-hexazinone 'full rate'. However, considerable challenges still face canegrowers in making cost-effective decisions on sustainable herbicide selection. Additional research and effective grower extension are required to address information gaps in issues such as specific weed control efficacy of alternative herbicides and potential blending of some herbicides for more effective broad spectrum weed control, while also minimising environmental risks. (C) 2020 Elsevier B.V. All rights reserved

Direct comparison of runoff of residual and knockdown herbicides in sugarcane using a rainfall simulator finds large difference in runoff losses and toxicity relative to diuron

Runoff losses of herbicides have rarely been compared simultaneously under the same conditions. Our aim was to directly compare herbicide runoff losses, normalised for the amount present (relative runoff loads) and in absolute terms. Toxicity and runoff concentrations were combined to provide a risk ranking relative to diuron. Four rainfall simulation trials were conducted in sugarcane in the Great Barrier Reef catchment. Herbicides studied were older PSII residuals (atrazine, ametryn, diuron, hexazinone), alternative residuals (isoxaflutole, imazapic, metribuzin, metolachlor, pendimethalin) and knockdown herbicides (glyphosate, 2,4-D, fluroxypyr) and the tracer bromide (Br). Simulations were conducted two days after spraying, before differences due to half-lives were apparent. Two trials had bare soil and two had sugarcane trash. Herbicide runoff losses and concentrations were closely related to the amount applied, runoff amounts and partitioning coefficients. Relative runoff losses and absolute losses were similar for most older and alternative residual herbicides, 2,4-D and Br. Glyphosate and pendimethalin relative runoff losses were low, due to greater sorption. Isoxaflutole, imazapic, and fluroxypyr are applied at much lower rates and runoff losses were low. Herbicides were lost in the dissolved phase, except pendimethalin. There was a large range in toxicity relative to diuron. There is a range of herbicide choices posing less offsite risk than diuron and ametryn, which have high application rates and high toxicity. Herbicide choice should consider application rate, runoff losses, sorption, and toxicity

Spot spraying reduces herbicide concentrations in runoff

Rainfall simulator trials were conducted on sugar cane paddocks across dry-tropical and subtropical Queensland, Australia, to examine the potential for spot spraying to reduce herbicide losses in runoff. Recommended rates of the herbicides glyphosate, 2,4-D, fluoroxypyr, atrazine, and diuron were sprayed onto 0, 20, 40, 50, 70, or 100% of the area of runoff plots. Simulated rainfall was applied 2 days after spraying to induce runoff at one plant cane and three ratoon crop sites. Over 50% of all herbicides were transported in the dissolved phase of runoff, regardless of the herbicide’s sediment−water partition coefficient. For most sites and herbicides, runoff herbicide concentrations decreased with decreasing spray coverage and with decreasing herbicide load in the soil and cane residues. Importantly, sites with higher infiltration prior to runoff and lower total runoff had lower runoff herbicide concentrations