Dry seeding challenges pre-emergent herbicide efficacy

Cindy Benjamin, May 20, 2024

Effective weed control through the use of pre-emergent herbicides and crop safety can be impacted by dry conditions in which many growers across southern Australia are sowing this autumn. Photo: Ashley Jacobs, Corrigin, WA

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.

Independent Consultants Australia Network senior consultant Mark Congreve said 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,” Mr Congreve said.

“All herbicides will persist in the soil if there is insufficient soil moisture to allow microbial breakdown of the herbicide.”

Mr Congreve said growers should follow instructions on the herbicide label, and then consider how many weeks the topsoil down to 10cm was moist over last spring and summer.

“This can often be more important than just looking at total rainfall amount.”

ICAN senior consultant Mark Congreve.

Mr Congreve said 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.”

This comes with a warning.

“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.”

Mr Congreve said the coarsest spray-quality requirements on the label for all tank mix partners should be chosen.

Pre-emergent herbicides vary considerably in their relative solubility and tendency to bind to soil and stubble.

Mr Congreve suggests growers sowing crops into high stubble loads, such as those made following a stripper front, chaff lining, or flattening 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.

“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.

Less mobile herbicides require more rainfall to adequately wash off the stubble, and higher subsequent soil moisture for uptake by the weeds.

Consequently, Mr Congreve said the performance of these herbicides may be compromised if conditions remain dry.

If growers 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.

FMC technical services manager for Australia and New Zealand, Mark Yerbury, said 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 less than 20kmh results in more droplets travelling vertically to the soil –  coarser droplets, and higher spray volume results in better herbicide performance and less spray drift,” Mr Yerbury said.

“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.”

Mr Congreve said knife-point press-wheel systems were 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, in accordance with label directions.

“Both soil throw into the inter-row, and accurate seeding depth, are important factors for protecting the crop seed,” Mr Congreve said.

“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 seeding 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.”

Soil tilth at sowing can also impact the efficacy of pre-emergent herbicides.

Mr Congreve suggests growers carefully consider their use of pre-emergent products if the soil is excessively dry, cloddy, or 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.

“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, South Australia, on July 29-30.

For more information about effective herbicide application, visit

Source: WeedSmart


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