The plant residues are the main part of the no-till system. These covers reduce density of weed population, but intercept herbicides when applied on its surface. Some people suggest to increase the rate to compensate herbicide losses. Field experiment, bioassays and chromatographic analysis were made using atrazine at the rates zero, 1,25; 2,5; 3,75 and 5,0 kg/ha a.i. on 4.500 and 9.000 kg/ha of oat residues and soil without mulch. Soil samples were collected immediately after atrazine application. Twenty-four hours after atrazine application the field was irrigated with 20 mm and more samples were collected for bioassays and chromatographic analysis. The results showed that before irrigation, the oat residues intercepted close to 85% of any rate applied over 4500 and 9000 kg/ha respectively. Irrigation was enough to leach nearly all atrazine to soil surface. Nomulched soil and without herbicide presented close to 700 plants/m2 of Brachiaria plantaginea. Soil not treated with atrazine but covered with 4500 or 9000 kg/ha of oat residues, presented 20 and 5 plants/m2 of B. plantaginea, respectively. Control obtained with 1250 g/ha of atrazine in both covers was better than 5000 g/ha of atrazine on nonmulched soil.
bioassays; straw mulch; chromatographic analysis; no-till; residues