An experiment was conducted at NuPAM/FCA/UNESP, Botucatu-SP, Brazil, to evaluate the dynamics of water retention and the path of a tracer (simulating a herbicide) in different crop residues. The treatments were constituted by monitoring of the tracer FD&C-1 on mulches of barley (Hordeum vulgare), perennial ryegrass (Lolium multiflorum), wheat (Triticum aestivum), harvested oat (Avena strigosa), rolled oat (A. strigosa), corn (Zea mays), millet (Penisetum americanum) and Surinam grass (Brachiaria decumbens), in the following amounts: 3000, 6000 and 9000 kg ha-1, before and after rain simulation. The replications were eight sets of PVC + Funnel + Becker with straw, with water retention and transposition being estimated and the extracted tracer quantified through the rain leached in the straws and the PVC support weight by using spectrophotometric procedures. The different crop residues were found to be similar regarding rainwater retention, with uniformity occurring between the first 7.5 and 15 mm of rain. The formation of "dry points" associated to "preferential running off channels'' induced the smaller capacity of absorption and retention of the rain water by the straws. The maximum mean capacities of rain retention by the straws were 1.22; 1.99 and 2.59 mm for 3,000, 6,000 and 9,000 kg of dry matter ha-1, respectively. The initial precipitations between 10 and 20 mm were crucial for the uniform wetting of the straws and wash off of the tracer to the soil, regardless of type and amount of straw. This behavior indicates the viability of using similar programs of weed control for different types and amounts of straw under no-tillage systems.
mulch; wetting dynamics; tracer FD & C-1
