Chemical weed control

Herbicides registered for use in vineyards are listed on pages 23 and 24 of the Agrochemical ‘Dog book’. Products/active constituents that are underlined may not be approved for use by your winery or grape purchaser. Contact your winery or grape purchaser prior to the use of underlined products/active constituents.

Chemical registrations for herbicides vary between states. Governments around the world set limits for the amount of residue fungicides, insecticides and herbicides that is legally allowed in a food such as grapes or wine. These limits for agrochemicals are commonly referred to as MRLs (maximum residue limits), can be searched for using the AWRI MRLs online search tool, and for Australia they are listed in the Australian New Zealand Food Standards Code.

Herbicides can be chosen that work either selectively on a particular plant species or more generally on a range of plant species (broad spectrum). Their effectiveness may vary with temperature, humidity, soil conditions, sprayer type and set-up, growth stage in the vineyard and health of the target plant.

Pre-emergent herbicides

Pre-emergent herbicides are applied before weeds have emerged from the soil. The herbicide has a residual activity; that is, it stays in the soil and kills seeds as they germinate. The roots of newly germinated seeds need to contact the herbicide for it to be effective. Some pre-emergent herbicides may provide some control of weeds after they emerge from the soil.

Post-emergent herbicides

Post-emergent herbicides are applied after weeds have emerged from the soil. Contact herbicides affect growing plants (foliar application) and kill plant tissue at the point of contact with limited movement within the plant. Symptoms tend to show within a day or two on plant tissue that has been in contact with the herbicide. Good coverage is essential.

Systemic herbicides move around inside of the plant via the xylem and phloem to affect one or more parts of the plant. It may take weeks for the target to show symptoms after a systemic herbicide application.

Herbicide application and herbicide spray drift are important workplace health and safety issues, as many of the chemicals used can be toxic to humans and other organisms and this need to be taken into consideration during application. In addition, systemic herbicides may damage grapevines if they make contact with the vine; however, the damage may not be visible until the next season.

For a range of reasons, many growers are adopting non-chemical weed control approaches which support natural vineyard biodiversity and ecosystems and improve vineyard soil vitality. When aiming to reduce or eliminate herbicide use, it is important that growers develop a weed control strategy that complements their long-term vineyard management goals.

Herbicide chemical resistance

Some weed species can develop a chemical resistance to a herbicide that would normally control it. Resistance may develop after frequent use of one chemical or chemicals from the same activity group. Incorrect chemical use, such as under- or over-dosing or application at the wrong time in the life cycle of a target, can also promote resistance.

Any population might contain a very small number of individuals that are naturally able to survive the application of a particular chemical. If the chemical or chemicals from the same activity group are used repeatedly and exclusively, the susceptible individuals continue to be removed, and those with natural resistance survive and multiply to essentially dominate the population. The chemical then fails in the field. It has been observed in vineyards that despite several herbicides being used over a season, they are often applied at the same time each season. As such, the weed species peculiar to that time are treated with the same herbicide each year, therefore promoting resistance. CropLife Australia maintains resistance management strategies for herbicides.