Selectivity and mode of action of flamprop‐isopropyl, isopropyl (±)‐2‐[N‐(3‐chloro‐4‐fluorophenyl)benzamido]propionate, in the control of Avena fatua in barley

The post‐emergence herbicide isopropyl (±)‐2‐[ N ‐(3‐chloro‐4‐fluorophenyl)benzamido]propionate (flamprop‐isopropyl) showed good activity against wild oat with selectivity in barley. The basis for activity and selectivity was similar to that previously established for benzoylprop‐ethyl, and found to be dependent on its rate of degradation to the biologically active acid flamprop. Flamprop stunted the growth of the oat by inhibiting cell elongation and showed a relatively high rate of movement in the phloem, approximately five times that of benzoylprop. Selectivity of flamprop‐isopropyl was dependent on its relative rate of hydrolysis and the subsequent detoxication of the acid to inactive conjugates. However, although the relative rate of de‐esterification of flamprop‐isopropyl was lower than that of benzoylprop‐ethyl similar quantities of the parent ester gave comparable effects on oat. The inherent activity of flamprop is approximately twice that of benzoylprop. The effect of flamprop‐isopropyl was best seen when the compound was applied during growth stages when the crop could offer the most effective competition to the wild oat. Throughout a range of growth stages the rate of hydrolysis of flamprop‐isopropyl was higher in oat than in barley. The metabolism of the compound was not markedly affected when the plants were under stress.