Tillage and fallow period management effects on the fate of the herbicide isoxaflutole in an irrigated continuous-maize field

Water drainage and herbicide degradation and leaching were studied during four years in a continuous maize field managed with two tillage systems and two types of fallow periods. The tillage systems consisted of either a conventional practice with mouldboard ploughing (28 cm-depth) or a conservation practice with superficial tillage (<12 cm-depth). The management of the fallow period involved either a cover crop sown after maize or bare soil. Water drainage and isoxaflutole and its active diketonitrile metabolite leaching were determined under the four cropping systems using fibreglass wick lysimeters installed at 40 cm-depth. Isoxaflutole was rapidly degraded (half-lives <2.1 days) in soil for all cropping systems and years and its leaching was limited (<1% of applied dose). Diketonitrile was more mobile and was quantified each year in soil solution at 40 cm-depth. Cumulated losses of diketonitrile ranged from 1.8 ± 0.6 to 31.5 ± 15.1% of applied dose according to the cropping system and year. Tillage system had no systematic effects on herbicide losses, but, considering the four years, significant lower losses were measured under conservation tillage. In a same way, using cover crops during the fallow period led to a decrease in herbicide losses compared to cropping systems where soil was left bare during the fallow period. The use of winter cover crop in continuous maize production showed promise to reduce diketonitrile losses by leaching.