TIMING AND MEASUREMENT OF WEED SEED SHED IN CORN (ZEA-MAYS)

In west central Minnesota the extent and duration of weed seed shed wa s measured for two years in corn that received cultivation but no herb icides. Percentage of seed production represented by viable (filled) s eeds was about 79% for green foxtail, 68% for wild mustard, 49% for Pe nnsylvania smartweed, 48% for common lambsquarters, and 35% for redroo t pigweed. Percentage viable seeds varied from 11% in 1993 to 59% in 1 994 for redroot pigweed, but was more stable for other species. Seed s hed commenced in late August in a cool year (1993) and early August in a warm year (1994), Average growing degree days (base 10 C) from corn planting until 25% seed shed was 983 for common lambsquarters, 984 fo r wild mustard, 1004 for Pennsylvania smartweed, and 1034 for both gre en foxtail and redroot pigweed, Brief weather events, such as wind sto rms, dispersed large percentages of total seed production within a sin gle day. More than one-fifth of all viable seeds of green foxtail, red root pigweed, and common lambsquarters were retained by the seedheads and dispersed by combines at harvest. In contrast, seeds of early-matu ring species, such as wild mustard, were completely dispersed before c orn harvest in the warmer year, but one-third of seeds were retained b y seedheads at harvest in the cooler year. Measurement of seed shed wa s compared using five seed trap designs. The preferred design consiste d of a 10-cm-diam plastic cup, whose bottom was replaced by a brass sc reen, and the entire unit attached to a small wooden stake for support . This design provided, on average, the highest estimates of seed prod uction, least among-replication variability, highest correlation with weed population density and aboveground dry-weight, lowest assembly co st, and greatest ease for sample access and seed processing.