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Seed size has been associated with early seedling vigor (i.e., germination rate, emergence rate, and growth) in grasses. This study was conducted to compare seedling development over a 60-d period in the field as affected by heavy seed (HS) (0.19 to 0.21 g 100 seed-1) and light seed (LS) (0.13 to 0.16 g 100 seed-1) of 'Blackwell' and 'Trailblazer' switchgrass (Panicum virgatum L.). The experiment was conducted in 1995 and 1996 at Lincoln, NE, on a Kennebec silt loam (fine-silty, mixed, superactive, mesic Cumnlic Hapludolls). The experimental design was a randomized complete block with four replicates. Seed was separated into two sizes using a South Dakota seed blower. Seed was planted into rows at a rate of 100 pure live seed per meter of row. Plants were excavated and evaluated for shoot weight, leaf area, and root weight. Shoot and root systems were morphologically staged four times during the summer. Seed size differences in switchgrass appeared to produce only slight differences in morphological development of shoot and root systems, leaf area, shoot weight, and adventitious root weight from seedling emergence to 6 wk of growth. Adventitious roots formed more quickly on seedlings from heavier than lighter seed, but the advantage to seedling establishment was minimal even when soil moisture appeared to be lacking. By 8 to 10 wk after emergence, growth and development of LS seedlings were similar to HS seedlings. Once seedlings formed two or more adventitious roots, seed size no longer affected establishment and growth. Seed size in switchgrass appears to have a minimal long-term effect on growth and development of seedlings.