Minor Crop Herbicide Update

In 2008 we received a 24C for use of Dual Magnum on spinach; however, two issues make the current version of the label difficult to use: 1) the plant back interval for lettuce is 12 months which seriously limits its utility in Salinas Valley rotations, and 2) the preharvest interval (PHI) is 50 days. The plant back issue is still not resolved; however progress was recently made on the PHI.

The IR4 program conducted residue studies to change the PHI to 21 days. We just received news about the residue trials: Dual Magnum residues collected at this interval exceeded the current tolerance and a 21 day PHI will not be possible. As a result, Syngenta is currently trying to settle for a PHI of 40 days. This would be an improvement over the current PHI of 50 days, but still leaves Dual Magnum in a grey zone for use on clipped spinach which commonly matures in 30 days or less.

One alternative that we have explored is the application of Dual Magnum prior to planting. The question that comes up is do you lose a certain percentage of the Dual Magnum if it is applied to the top of the bed and it sits for a period of time before being incorporated into the soil with sprinkler irrigation following planting. In San Ardo in 2009 we conducted a trial in which Dual Magnum was applied to shaped beds on July 1 and the field was planted on July 21. We observed good efficacy at the 0.75 and 1.0 pint rates (Table 1). There was little efficacy at the 0.50 pint/A rate when the Dual Magnum remained on the soil surface for 21 days prior to planting; normally we see reasonable weed control at the 0.50 pint/A rate when Dual Magnum is applied immediately following planting and incorporated into the soil with sprinkler irrigation. It therefore appears that Dual Magnum can remain on the soil surface for at least 20 days, but higher rates may need to be used to obtain weed control equivalent to what is needed for at-planting applications.  We will need further evaluations of this application technique to better understand the rates and timing. 

Table 1. Weed counts and phytotoxicity rating on August 6, 2009