Field, greenhouse, and laboratory studies were conducted to determine the effect of MCPA ester, fertilizer type, and fertilizer rate on feral rye control with imazamox. In field studies near Sidney, NE, increasing the concentration of liquid ammonium phosphate (10–34–0) from 2.5 to 50% of the spray solution decreased feral rye control with imazamox by as much as 73%. Conversely, adding MCPA ester to imazamox significantly increased feral rye control in field studies by up to 77%. Initial greenhouse studies confirmed the liquid ammonium phosphate antagonism effect, but subsequent greenhouse studies were inconsistent with regard to the interaction between fertilizer and imazamox. At least one source of liquid ammonium phosphate was shown not to be antagonistic, and therefore fertilizer source or contaminants may be responsible for initial field observations. Greenhouse studies confirmed the synergistic interaction between MCPA and imazamox. MCPA ester applied at 560 g ai ha⁻¹ decreased the rate of imazamox required to cause 50% reduction in feral rye dry weight (GR₅₀) to 13 g ha⁻¹ compared to 35 g ha⁻¹ for imazamox alone. Although addition of MCPA ester increased ¹⁴C-imazamox absorption by 8% in laboratory studies, less ¹⁴C translocated out of the treated leaf; therefore the mechanism of synergism does not appear to be related to imazamox absorption or translocation.

Nomenclature: Imazamox; MCPA; feral rye, Secale cereale L.