Understanding cause and effect of herbicide carryover on pulses

image of Typical symptoms of damage in a pulse crop from carryover of clopyralid
Typical symptoms of damage in a pulse crop from carryover of clopyralid or picloram-based herbicides. Photo: Mark Congreve, ICAN

It is not uncommon for grain growers to experience negative effects in their pulse crops resulting from carryover of clopyralid or picloram-based herbicides that were applied in previous seasons.

To help growers better understand the issues involved, the Grains Research and Development Corporation (GRDC) has produced a fact sheet ‘Rotational constraints for pulse crops following the use of aminopyralid, clopyralid and picloram herbicides‘. The fact sheet can be downloaded from the GRDC website.

Mark Congreve, senior consultant with Independent Consultants Australia Network (ICAN) and fact sheet author, says damage to subsequent pulse crops can result from three particular situations.

“Firstly, herbicide close to the soil surface may not have fully degraded before planting of the pulse crop the following winter,” Mr Congreve says.

“These herbicides are broken down in the soil by soil microbes. Soil microbes benefit from warm and moist soil conditions to build their population. Little degradation occurs during winter as it is often too cold for microbial activity.

“When the following spring/summer is dry, there may be insufficient microbial degradation occurring before planting the following season. In this case, herbicide residues may persist at levels sufficient to cause crop damage at establishment, or growth of the following pulse crop may be compromised.”

Secondly, Mr Congreve says these herbicides are also relatively mobile in the soil, so some of the applied herbicide may also move deeper in the profile following rainfall after application.

“The amount moving deeper in the profile will depend on soil type and rainfall,” he says. “Microbial activity deeper in the profile is significantly less than near the soil surface, so herbicide moving deeper in the profile will take longer to break down.

“Where there are subsoil constraints, such as hard pans or significant change in soil texture/structure, herbicide may concentrate at or above these barriers and be particularly problematic. In this situation, the pulse crop may establish, provided the residues at the surface have degraded, but symptoms may not be seen until later in the crop when roots reach this herbicide at depth.”

Mr Congreve says the third situation in which damage of subsequent pulse crops may also arise is when herbicide applied post-emergent to a preceding cereal crop, has not been fully metabolised by the cereal crop before harvest.

“In this case, there can be herbicide remaining in the dead cereal stubble following harvest.

“Herbicide trapped in crop stubble is then ‘released’ back into the soil as the cereal stubble is placed into contact with the soil and starts to decompose.

“The concentration of herbicide in the stubble depends on the application rate and, importantly, when it was applied to the cereal crop.

“Typically, applications made later in the season are more likely to result in higher levels of herbicide in the cereal stubble than applications made earlier in the year.

“Early season applications will have less interception by the crop, more direct application to the soil, and longer for degradation to occur before the crop matures. Availability of herbicide from the crop stubble, and therefore when symptoms appear, depends on when the stubble decomposes.”

Mr Congreve suggested harvest is particularly timely for considering the possibility of herbicide residues in stubble.

“Many clopyralid herbicide labels recommend burning or mechanically incorporating cereal stubble from sprayed crops immediately after harvest. This is to put stubble into contact with the soil and allow time for the stubble to decompose, however, this is rarely done in no-till farming systems.”

Particular care needs to be taken where harvest weed seed control (HWSC) techniques that concentrate the stubble or chaff are employed, according to Mr Congreve.

“This last winter we saw a few examples of herbicide damage in pulses that was only present in old chaff lines, resulting from cereal crops that were treated with clopyralid, in some cases from over two years earlier.

“Where there is concern that residues of these herbicides may be still present in the soil or stubble, the best solution is to plant a cereal crop or canola, until growers are confident that any herbicide residues have dissipated.

“Always read and follow directions of the registered label of the product you have used. Product labels are a good source of information, as are manufacturer representatives.”

Mr Congreve says the GRDC publication ‘Rotational crop constraints for herbicides used in Australian farming systems‘ is also a useful reference to further understand herbicide carryover constraints for many herbicides.

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