Effect of crop residues on interception and activity of prosulfocarb, pyroxasulfone, and trifluralin.

Crop residue retention on the soil surface in no-tillage system can intercept pre-emergent herbicides and reduce their efficacy. Three experiments were conducted to investigate the effect of crop residue amount (0, 1, 2 and 4 t ha-1), moisture (wet versus dry), type (wheat, barley, canola, chickpea and lupin) and age (fresh or aged for one year) on the interception and subsequent leaching of prosulfocarb, pyroxasulfone, and trifluralin from the residue into soil. Bioassays, using cucumber and annual ryegrass as indicator plants, were used to assess herbicide activity/availability in the soil and on the residue. Herbicide interception increased considerably as residue quantity increased from 2 to 4 t ha-1. After simulated rainfall, which washed herbicide into the soil, complete control of ryegrass occurred for trifluralin with 0 t ha-1 residue, for prosulfocarb with 0 and 1 t ha-1 residue, and for pyroxasulfone with all residue rates. Therefore, with rain or irrigation, pyroxasulfone was the herbicide least affected by high residue loads. Less chemical leached from the crop residue into the soil after rainfall, when prosulfocarb and trifluralin were applied to wet residue compared with dry residue, but the initial moisture condition had no effect on the leaching of pyroxasulfone from residue. If practically possible, farmers should minimise spraying prosulfocarb and trifluralin onto wet crop residue. Barley and wheat residues intercepted more herbicide than an equivalent mass of canola, chickpea or lupin residue, which was largely due to the increased ground cover with cereal residues. The effect of residue age on herbicide interception and leaching was relatively small and variable. Overall, more herbicide reached the soil when sprayed on one-year old residue than new residue, which was largely due to reduced ground cover with aged residue. A strong positive linear relationship existed between ground cover percentage and growth of bioassay species (r2 = 0.75). This means that there was little difference in the ability of residue to adsorb and retain herbicide between crop residue types and ages, such that farmers can simply use the ground cover of the crop residue to assess interception.