Water quality key to effective herbicides

Author: Andrew McFadyen | Date: 31 May 2016

By GRDC northern panelist Andrew McFadyen

Portrait of Andrew McFadyen

GRDC northern panellist Andrew McFadyen says understanding water quality is important.

The recent heavy rainfall which has fallen across inland areas of the eastern states is a major boon to those farmers who took a risk in recent weeks to dry sow crops, and an opportunity for many growers.

With seed in the ground, growers need to start thinking about their post-emergent herbicides and how their efficacy may be affected by water quality or temperature.

Growers are often adept at paying attention to elements such as target species and mode of action, but understanding how chemicals interact with the environment is also important.

Given many chemicals can be affected by poor water quality, the water used for spraying operations can be critical to ensuring the best spray results.

Hydrolosis, or the breakdown of chemicals in contact with water, is dependent on pH, formulation, type of herbicide, time and temperature.

So when considering if the water quality is suitable for a particular product, you need to have accurate information about what’s present in the water and how it may affect the product you intend to use.

The starting point should always be an accurate water test from a reputable laboratory and a thorough check of the product label and technical information from the manufacturer.

Test the water for pH, hardness, salinity and when you are in the paddock, it’s important to check for dirty water and water temperature.

Dirty water can adversely affect chemicals due to the clay colloids suspended in the water. A good test is to drop a 10 cent coin in a bucket of your water, and if it cannot be seen in the bottom of the bucket the water is too dirty for use with products affected by dirty water.

Increased water temperatures can accelerate the breakdown of some products when the water quality is not suitable. Low-temperature water can lead to solubility problems and gelling in the tank (even in clean water).

Water testing should be done on a regular basis when using bore water, water from streams and rivers, piped water sourced from ground water, and water stored in unlined dams or concrete tanks.

In terms of getting the tests done, growers can check with their local department of agriculture or primary industries, or talk to a local chemical reseller about suitable laboratories in the area.

Some chemical and adjuvant manufacturers also offer water testing through the reseller networks.

For an alternative, basic test strips for hardness are available in hardware stores or through stores that supply pool equipment.

The Grains Research and Development Corporation (GRDC) recommends that if using test strips, it may be useful to compare the results with laboratory tests to check the accuracy of the strips. This can be done by setting aside water collected for analysis.

The strips can then be used to assess if significant changes in water quality have occurred and when another laboratory test may be required.

The GRDC has some useful resources outlining the various chemicals and their susceptibility to poor water interactions, including a Spray Water Quality Factsheet that can be found here.

Andrew McFadyen is an agronomist and manager with Paspaley Pastoral Company near Coolah NSW with more than 15 years’ agronomy and practical farm management experience. He is an active member of the grains industry with former roles on the Central East Research Advisory Committee, NSW Farmers Coolah branch and planning committees for GRDC Updates.  He is also a board member and the chair of Grain Orana Alliance.

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