STANDING STEM PERSISTENCE IN NO-TILLAGE SMALL-GRAIN FIELDS

Standing stem residues affect erosion, hydrology, and other processes differently than flat residues, but stem persistence under no-tillage management is not well understood. We developed an equation to predict standing stem number over time, based on precipitation and air temper ature. Crops were field-grown winter and spring wheat (Triticum aestiv um L.), barley (Hordeum vulgare L.), and oat (Avena sativa L.) grown n ear Bushland, TX, on Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll). Fallow-period irrigation treatments produced three decom position environments. Standing stems were counted in flagged quadrats 18, 98, 158, 223, 289, and 379 d after harvest. The daily minimum of precipitation-based moisture or mean air temperature coefficients was accumulated as decomposition days (DD). Standing stem fraction (SF) wa s predicted assuming SF = exp [k(DD - B)[. The threshold, B, was almos t-equal-to 17.5 DD for all crops, and k was -0.284, -0.176, -0.169, an d -0.116 for oat, barley, and winter and spring wheat, respectively. E quation evaluation used data from North Dakota, Oregon, and Texas. Ste m number prediction tended to be high before the B threshold and low l ater. Paired t-tests indicated no significant difference between predi cted and measured stem fraction of spring wheat or barley. Stem fracti on was over-estimated by 0.09 for winter wheat averaged across Oregon and Texas data. Use of DDs improved prediction of standing stem persis tence across diverse climates. Such information is needed for a wide r ange of erosion, water balance, and micrometeorological studies. A qua ntitative index for forces such as strong winds, animal traffic, or bl owing precipitation may improve the model.