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Tracing the vertical Distribution of Simulated Shattered Seeds as Related to Tillage

USDA North Central Soil Conserv. Res. Lab., Morris, MN 56267

P. M. Burford and R. R. Allmaras

USDA-ARS and Dep. of Soil Sci., Univ. of Minnesota, 439 Borlaug Hall, 1991 Buford Circle, St. Paul, MN 55108

W. W. Nelson

Univ. of Minnesota, Southwest Exp. Stn. P.O. Box 428, Lamberton, MN 56152

^* Corresponding author.

Weed control in conservation tillage requires a knowledge of weed seed ecology, but germination and loss of viability make it difficult to trace seed movement during tillage. Ceramic spheres were evaluatedas tracers to determine the effects of chisel and moldboard tillage on the vertical distribution of shattered seeds. Tracers and seeds of foxtail millet (Setaria italica [L.] Beauv.) were applied to the soil surface [Ves loam (a fine-loamy, mixed, mesic Udic Haplustoll)] before fall primary tillage with a chisel or moldboard plow. Soybean (Glycine max [L.] Merr.) and oat (Avena sativa L.) were grown in a strip-plot design. Soil cores (18 mm diam.; 60 cm deep) were taken midsummer, sectioned into 2-cm increments, and composited (12-15 cores per sample) by depth. Soil bulk density, seed counts, and tracer counts were recorded for each sample. Seeds and tracers were found to depths of 12 cm in the chisel plots and 32 cm in the moldboard plots. In the chisel plots, 51 and 48% of the seeds and tracers, respectively, were found within 4 cm of the surface compared to 11 and 4% for the moldboard plots. Germination of applied seeds was greater in the chisel plots whereas germination of indigenous seeds (previously buried by tillage) was greater in the moldboard plots. An abrupt increase in the soil bulk density occurred at the penetration depth of the primary tillage operation and coincided with the maximum burial depth of seeds and tracers. These observations verify the differences in working depth and associated seed placement by the two implements. Ceramic spheres can trace the vertical distribution of shattered seeds related to tillage.

Received for publication November 15, 1989.

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