Erosion control benefits provided by conservation tillage can be diminished if mulch fails or loses its effectiveness. A field experiment was carried out at the Agriculture Experimental Station of the Federal University of Rio Grande do Sul in Eldorado do Sul, RGS, Brazil, on a 0.105 m m-1 slope, sandy loam Ultisol, to investigate mulch failure for different forms of residue management under no-till system. Simulated rainfall (63.5 mm h-1) was applied with the rotating-boom rainfall simulator until steady-runoff rates were obtained, when extra-inflow consisting of clear water was added at the upper end of the plots to simulate longer slope lengths. Ten-levels of extra-inflow ranged from 16 to 197 (10-5) m³ s-1 m-1, with a duration of 7-min each. Treatments consisted of fresh oat (Oct/97) and soybean (May/98) residues, and semi-decomposed corn and soybean residues (150 and 175 days after harvesting, Oct/97-98, respectively), in different rates and management forms. Mulch failure was identified, and critical slope lengths calculated, using visual observations in the field and a theoretical criterion, based on the relationship between erosion and discharge rates at the midpoint of the plot. Calculated, critical slope-length intervals ranged from 29-58 m (3,950 kg ha-1 of unanchored, semi-decomposed soybean residue) to 152-164 m (6,200 kg ha-1 of semi-anchored, semi-decomposed corn residue). There was no evidence of mulch failure for the 5,600 kg ha-1 semi-anchored oat residue treatment. The results proved that slope-length limits may exist under no-till system, and that mulch failure criteria can be helpful for a better RUSLE's C-factor evaluation, as well as for determining terrace spacing in this system, resulting in its increased effectiveness for erosion control. Critical slope lengths, however, will vary with rainfall, soil, slope, and management decisions.
water erosion; conservation tillage; no-tillage; mulch failure