Key Points
- Residual herbicides are designed to persist in the soil and kill seedlings between germination and emergence. However if they persist too long, they can prevent the establishment of subsequent winter crops sown after maize.
- To determine the persistence of the residual herbicides atrazine, acetochlor, mesotrione and nicosulfuron, experiments were conducted in the glasshouse (controlled bioassays using mustard and oats) and the field (Waikato, Taranaki and Canterbury).
- In the controlled bioassays, the greater the herbicide dose, the lower the biomass yield of mustard and oats. The detection limit (the dose rate when yield falls below 85% of the untreated) varied between soil and herbicides.
- In the field trials, there was no atrazine, acetochlor or mesotrione detected at any site at harvest. Nicosulfuron was detected in Taranaki at 10 and 15 weeks after application. This affected subsequent growth of mustard but not oats.
- Rainfall appears to be the main driver of herbicide dissipation in the field. Low rainfall is likely during summer so residual herbicides applied from mid December could result in herbicide residues at harvest.