Spray drift
More than 90 percent of Australia’s winter field crops are now grown using “conservation tillage” techniques. Much of the weed control that used to be achieved by mechanical cultivation is now achieved by herbicides. Conservation tillage provides substantial agronomic benefits including reduced soil erosion, improved soil structure, higher soil organic matter, higher soil moisture retention and the opportunity for earlier sowing. However, the higher reliance on herbicides is not without problems, including changes in the weed spectrum and the development of herbicide resistance in weeds.
Control of summer-growing weeds is now a regular practice in cropping programs, to conserve soil moisture and nutrients and to remove the “green bridge” that enables some pests and diseases to carry over from one season to the next. Most of this summer weed control is achieved with herbicides, particularly in continuous cropping programs where livestock has been largely displaced.
The development of GPS guidance and autosteer capability in farm machinery has been a game changer for the efficiency and accuracy of farm operations. Farmers regularly achieved gains in spraying efficiency of 5-10 percent through the elimination of spray overlap and missed strips. However, with no more need for visual guidance of farm machinery, the main change that GPS technology brought to pesticide application was the ability for farmers to spray accurately at night. Night spraying was promoted particularly for summer weed control to improve efficacy, by avoiding hot and dry daytime weather conditions.
For decades, off-target plant damage from the group of herbicides variously referred to as “hormone” herbicides or “phenoxy” herbicides or now Group I herbicides (based on mode of action) has been a recurring problem. The growth of many broad-leaved species can be affected by exposure to Group I herbicides, even at extremely low concentrations. This group of herbicides includes 2,4-D, which was available as various formulations including high-volatile esters (HVE), low-volatile esters (LVE) and practically non-volatile amines and salts.
General opinion was that the HVE products presented the greatest risk of off-target damage because they could vaporise, particularly when air temperature was high and relative humidity was low (i.e. summer) and drift as vapour for significant distances. In October 2006, the AVPMA prohibited the use of 2,4-D HVE products from 1 September to 30 April each year, as a spray drift management practice.
Grape vines are very susceptible to Group I herbicide damage, particularly 2,4-D. In-crop spraying of winter cereals with 2,4-D largely occurs when grape vines are dormant. However, summer weed spraying – that often incorporates 2,4-D and other Group I herbicides – occurs while grape vines are growing actively.
From 2003, off-target herbicide damage to grapevines was reported every summer from at least one of SA’s major grape-growing regions, even though 2,4-D HVE products were no longer used at that time of year. The only region that appeared to not be at risk from such damage was the Southern Vales. In the 2008/09 summer, vineyards across the entire Clare Valley were damaged by exposure to 2,4-D herbicide. The source was generally considered to be summer weed spraying somewhere in the mid-North cropping regions surrounding the Clare Valley. PIRSA Rural Chemicals Program undertook a major investigation but was unable to identify the herbicide source or the particular circumstances that promoted the off-target herbicide movement.
Significant off-target herbicide damage to grape vines in the Clare Valley occurred again in the 2010/11 summer. Largely instigated by two mid-North agronomists, Mick Faulkner and Peter Cousins, a regional working group was formed with field crop and viticultural industry representation to address the ongoing problem of off-target herbicide damage to vines. (At this same time, the cotton industry in northern NSW and southern Qld was also experiencing substantial off-target herbicide damage to cotton crops).
A better understanding of the effects of weather on pesticide application was showing that weather conditions at the time of spraying, particularly whether or not a surface temperature inversion was present, was one of the main risk factors likely to cause off-target herbicide movement. Surface temperature inversions frequently form overnight, becoming strongest just before sunrise. Night spraying of herbicides might have benefits for weed control efficacy but it occurs in atmospheric conditions that present a very high risk of causing off-target herbicide movement.
The activities of the Mid-North spray drift committee were the catalyst for the development of a Code of Practice (COP) for Summer Weed Control that was released by PIRSA in November 2011, and has been updated since as regulations have changed. Important messages in the COP are to spray during the day wherever possible, to not spray when a surface temperature inversion is present and to use nozzles that produce very coarse, or larger, spray quality.
Significant consultation with SA primary industries occurred prior to the amendment of the Agricultural and Veterinary Products (Control of Use) Regulationsin 2012.These amendments required most users who spray specified Group I herbicides to have a minimum standard of training, and to make and keep a record of the application in an approved format.
2,4-D has been under ongoing review by APVMA since 2003 approximately. Spray drift management has been an important component of the review. In August 2013, APVMA removed all 2,4-D HVE formulations from the marketplace and amended the label instructions for the LVE and amine/salt formulations that remained registered. In October 2018, APVMA made further changes to 2,4-D product labels including a requirement to only use nozzles that produce very coarse, or larger, spray quality and a prohibition on application when surface temperature inversion conditions are present at the application site.
Pesticide application has benefited from recent technological advances in weather forecasting and observations. Hand-held weather meters and automatic weather stations are relatively reliable and affordable and various weather apps can be accessed easily on mobile phones and tablets. A network of 40 weather stations generally referred to as the mid-North mesonet, which was supported by funding from PIRSA and other sources, was launched in September 2019. The mesonet is accessible online and provides reliable real-time observations to assist farm management decisions, including pesticide application. The stations have the capability to measure the vertical temperature distance between 10m and 1.2m above ground level, which indicates whether or not a surface temperature inversion is present. A further 30 weather stations will be online in the Riverland and Mallee regions and most of the cropping areas in SA are likely to have access to this infrastructure by the end of 2020.
Author: Michael McManus, Manager, Rural Chemicals Operations, PIRSA
Date: November 2020