Growers are reporting variable control of annual ryegrass with clethodim. Poor performance of clethodim can be attributed to cold and frosty conditions. In 2013 and 2014 trials to examine the effect of frost on clethodim treatment of annual ryegrass were conducted under controlled conditions using a frost chamber to mimic a frost event.
Frost events in the few days before or after Clethodim application can reduce activity of the herbicide on annual ryegrass. Clethodim efficacy tends to be reduced more substantially by frost where ryegrass populations have evolved resistance to this herbicide.