Herbicide Carryover

With an increasing diversity of break crop species being grown and a trend towards earlier sowing, there is an increased risk of crop damage resulting from carryover of herbicide residues from one year to the next. Of particular concern are the early sown pulse crops such as Field Peas, Lupins and Vetch. However pasture species such as lucerne, clover and phalaris as well as non-Clearfield canola and wheat can also be affected by some products.

Herbicide carryover issues often arise under the following circumstances:

• When herbicides are applied late in the season.

• When limited spring and summer rainfall is received.

• When crops and pastures are sown early in the season following late application of herbicides.

Herbicides which are most likely to cause damage are Group B’s (Ally, Glean, Logran, Hussar, Atlantiis, Crusader, Intervix, On Duty & Sentry) and Lontrel.

Many of these products have a re-cropping interval of 9 months or more, plus a minimum rainfall requirement of up to 250 mm. Therefore, care should be taken when applying these herbicides from July onwards.

Some important points when considering the risk of herbicide carryover include:

• Herbicides break down more quickly in warm moist soils, as microbial activity is higher.

• Risk of carryover will increase with increasing application rates.

• Herbicides break down faster in low pH soils

• Herbicides break down more slowly in alkaline soils

• Imazapic (the active in On Duty & Sentry) has increased root uptake and greater residual activity than imazamox & imazapyr (the actives in Intervix).

Having a plan is crucial to minimising the risk of herbicide carryover. Most of the issues arise when products are applied without considering the crop types programmed to be sown the following year.