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Dry seeding conditions can challenge pre-emergent herbicides

Soil moisture is sadly lacking in many grain-growing areas this season, particularly in the southern and western regions of the country. Growers planting their winter crops into relatively dry soils have every reason to be concerned about the effectiveness of their pre-emergent herbicide applications under these conditions.

Industry expert Mark Congreve, senior consultant with Independent Consultants Australia Network (ICAN), says that the dry conditions can impact both effective weed control and crop safety.

The first consideration is whether sufficient rain has fallen to eliminate any carry-over effect from previously-applied herbicides with residual activity. All herbicides will persist in the soil if there is insufficient soil moisture to allow microbial breakdown of the herbicide, he says. Follow the instructions on the herbicide label and consider how many weeks the topsoil (0 to 10 cm) was actually moist over last spring and summer, as this can often be more important than just looking at total rainfall amount.

If the risk of carry-over effects is low, or the following crop is not sensitive to the herbicide mode of action, the next step is to consider the best options to manage early weed growth and ensure crop safety at planting.

Some pre-emergent herbicides, such as Boxer Gold, Overwatch Herbicide or Group 14 herbicides, may improve knockdown efficacy when applied in a tank mix with paraquat as part of a double-knock tactic, says Mark. But, setting up the sprayer for high-coverage with paraquat will result in high levels of stubble interception, increasing the potential for some pre-emergent herbicides to bind to the stubble. Also, the green leaf material will take up pre-emergent herbicide deposited on the weeds. Losses to stubble binding or green leaf uptake mean less pre-emergent herbicide reaching the soil and therefore shorter length of persistence.

Always spray to the coarsest spray quality requirements on the label for all tank mix partners.

Pre-emergent herbicides vary considerably in their relative solubility and tendency to bind to soil and stubble. Mark suggests that growers sowing crops into high stubble loads, such as following a stripper front stubble, chaff lines or stubble flattened by livestock, should choose mobile herbicides so that most of the product will wash off and reach the soil surface post-rainfall.

In lower stubble situations, the behaviour of the herbicide in the soil becomes the key consideration, says Mark. Very mobile herbicides can be difficult to keep away from the crop seed furrow, particularly where the herbicide has been applied dry, the soil type is lighter, and hence there are large soil pores, and the initial rainfall after application is significant. This scenario can increase the risk of crop injury.

Herbicides with less mobility are less likely to move into the planting line, especially when incorporated by the sowing (IBS) operation with a knife point and press-wheel seeder. These less mobile herbicides require more rainfall to adequately wash off the stubble and higher subsequent soil moisture for uptake by the weeds. Consequently, the performance of these herbicides may be compromised if conditions remain dry, he says.

If you choose to burn stubble before seeding to reduce pre-emergent losses to stubble tie-up, it pays to wait for the ash to blow away or wash into the soil before applying any pre-emergent products.

Source: Mark Congreve, ICAN. See more in the webinar below.

Mark Yerbury, FMCs Technical Services Manager for Australia and New Zealand, says growers can modify their spray practices to maximise the deposition of pre-emergent herbicides on the soil surface.

The combination of reduced spraying speed (for most sprayers, speed of < 20 kph results in more droplets travelling vertically to the soil), coarser droplets, and higher spray volume results in better herbicide performance and less spray drift, he says. Once the herbicide is applied, the seeding operation can significantly affect crop safety. Many pre-emergent herbicides will impact emerging crops if the physical separation of the treated soil and emerging crop seedling is compromised during sowing or if the furrow collapses soon after sowing, from heavy rain or strong winds.

Mark says knife-point press-wheel systems are generally considered the safest option. Other equipment, such as disc seeders, trailing harrows and splitter boots, can also work well when paired with appropriate herbicides. Its always important to read and follow the label directions of each herbicide used.

Both soil throw into the inter-row and accurate seeding depth are important factors for protecting the crop seed, he says. The effect of pre-emergent herbicide contact with germinating crops is not always obvious. Pre-emergent herbicide damage from some modes of action may result in lower seedling numbers, reduced seedling vigour, or lower biomass in the root zone, which is less obvious in the field compared to herbicides, such as Overwatch Herbicide, that cause bleaching.

The soil tilth at sowing can also impact the efficacy of pre-emergent herbicides. Mark suggests growers carefully consider their use of pre-emergent products if the soil is excessively dry, cloddy or has been aggressively tilled or ameliorated recently.

There is a higher risk of crop damage if pre-emergent herbicides are applied to soils that have been ameliorated and particularly if the soil profile has been inverted, he says. Pre-emergent herbicides can become hotter because of changes to the soil surface including less organic matter in the topsoil, making the chemical more available than would otherwise be expected, and infiltration of water is generally greater, increasing the risk of moving herbicide into the seed furrow. The risk of crop damage is often less if the soil has sufficient time and rainfall to settle.  

Spray efficacy is one of the WeedSmart Big 6 tactics within a strategy that embraces herbicide and non-herbicide weed control tactics to keep weed numbers low in cropping systems. The latest weed control tactics and technologies will feature at WeedSmart Week in Port Lincoln on 29–30 July 2024.

Key resources

Understanding pre-emergent herbicides and how they interact with the environment factsheet

Soil behaviour of pre-emergent herbicides in Australian farming systems manual

Disc seeding and pre-emergents factsheet

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