Cropping

Understand the effects of carryover herbicides

By Rodney Woods

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 has produced a fact sheet: Rotational constraints for pulse crops following the use of aminopyralid, clopyralid and picloram herbicides.

Independent Consultants Australia Network senior consultant and fact sheet author Mark Congreve said damage to subsequent pulse crops could 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 said.

“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 said these herbicides were 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 said.

“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 said 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,” he said.

“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.”

The fact sheet can be downloaded at: https://grdc.com.au/rotational-constraints-for-pulse-crops

More information on herbicide behaviour is available at: http://bit.ly/2peRWCp