Spray Weeds Early To Save Moisture and Money

Author: Toni Somes, GDRC

Date: 20th December 2018


Early and effective control of summer weeds will be high on the to-do list for growers in Queensland and New South Wales with the start of the summer storm season.

Recent rains across much of the region have been ideal for a flush of summer weeds and the Grains Research and Development Corporation (GRDC) is encouraging growers to implement control tactics early to boost water use efficiency, preserve precious stored soil moisture and nutrients, improve herbicide efficacy and pest and disease control and protect the yield potential of future crops.

GRDC Crop Protection Officer North Vicki Green said most growers were aware that early summer weed control could deliver significant economic, agronomic and environmental benefits.

“In summer, the difference between effective and ineffective weed control is a matter of days, not weeks,” she said.

“Weeds can make or break efficient water storage. As a general rule, if summer weeds are allowed to establish, they will start extracting water from depth – more than 100mm depth – within about 12 days of establishment. That is water that could be used to grow a crop.

“Our research has shown that every dollar invested in summer weed control generates an average return of $5, with early control enhancing the potential benefits.”

This has been further backed up by the GRDC’s landmark National Water Use Efficiency (WUE) Initiative that found complete weed control, which involved spraying 10 days after a significant rain event, resulted in the greatest subsequent winter crop yield when compared with late summer weed control and no control at all.

Agricultural scientist James Hunt, who was one of the key researchers leading the WUE Initiative, encouraged growers to manage summer weeds at the three to five leaf stage using herbicides at registered label rates, as herbicide efficacy was generally highest when summer weeds were young and actively growing.

Dr Hunt, of La Trobe University (previously CSIRO), said summer weed control replicated experiments conducted during the WUE Initiative demonstrated average yield improvements in winter crop performance of 60 percent.

However, summer weed control will be important for growers in parts of the northern region where ground cover protecting topsoils is adequate. In areas where ground cover is scant following significant rainfall deficits and failed crops, growers will need to consider the risks of eliminating any existing vegetation too early, including weeds and volunteer crop plants, which can assist with erosion prevention.

So as part of their summer priorities’ list, Mrs Green said it was important growers had spray equipment ready and adhered to spray application recommendations on chemical and water rate, environmental conditions, droplet size and boom height.

“Weed control is generally more effective if plants are sprayed when they are small, leaving them too long can be a costly failure, requiring additional control measures and risking the development of herbicide resistance on farm. The most expensive spray is a failed spray,” she said.

“Efficacy is also impacted by temperature and humidity, travel and wind speed, droplet size and viscosity of spray so it’s important from a cost/benefit perspective to adhere to best management recommendations on spray equipment and conditions.”

She said an additional benefit of early summer weed control was it destroyed the green bridge (weeds and volunteer cereals) that could harbour pests and diseases between seasons, minimising the impact of these on the following crop.

More information on the importance of summer weed control, the latest research on problem weeds in the northern cropping region and advice on spraying – including tips for minimising spray drift – is included in the GRDC GroundCover™ Summer Weeds Supplement.

Growers are reminded to adhere to best practice when spraying summer weeds to reduce the risk of off-target spray drift and to be aware of new restrictions to the use of 2,4-D.

The Australian Pesticides and Veterinary Medicines Authority (APVMA) has suspended the labels of all products containing the active ingredient 2,4-D from October 4, replacing them with a permit.

Key changes for using 2,4-D under the permit include:

*           Applicators must now use at least a Very Coarse (VC) spray quality;

*           When using a boom sprayer, boom heights must be 0.5 metres (or lower) above the target canopy;

*           Downwind buffers now apply (typically less than 50 metres, subject to rate and product being applied) between application sites, downwind sensitive crops and environmentally sensitive aquatic areas.

The new permit also includes an advisory statement for 2,4-D use in cereals, fallow and pasture from October 1 to April 15. These statements advise operators to use an Extremely Coarse (XC) or Ultra Coarse (UC) spray quality and to take steps to mitigate the risk of spray drift such as adopting increased water rates and slower application speeds.

Grain growers and spray operators can access a practical guide explaining how to maintain efficacy when using coarser spray qualities in line with new restrictions to the use of 2,4-D. A ‘Maintaining efficacy with larger droplets’ fact sheet has been developed by the GRDC to assist industry understand the on-farm implications of the new restrictions.

Mrs Green said importantly, to achieve good spray coverage with VC, XC or UC spray quality, especially on small weeds in stubble, growers should consider using robust chemical rates, slowing down and increasing water volumes.

“Suggesting growers slow down and use in excess of 80L/ha of water (and preferably 100L/ha with contact herbicides) may cause angst, but compared to the costs associated with a failed job and potentially having to respray the paddock, it is worth making the changes,” she said.

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