A simulation was done to compare harvest costs for large round bales (1.8 m diameter x 1.5 m wide) vs large square bales (1 m high x 1.2 m wide x 2.4 m long). Assumptions were made to give each system an idealized opportunity to harvest switchgrass yields of 4.5, 9 and 18 dry Mg/ha. Using the round bale, total harvest costs (mow-condition, rake, bale and move to an on-farm storage location) were $17.00, $15.90 and $19.15/dry Mg, respectively. For the square bale, they were $13.40, $12.25 and $11.95/dry Mg, respectively. Density of hay in the windrow affected the performance of the round baler; heavier windrows produced lower density bales, thus harvest cost increased as yield doubled from 9 to 18 dry Mg/ha. Comparing at 9 Mg/ha yield, harvest cost was 23% less for the square bale. The square bales must be stored in covered storage ($14.16/dry-Mg-delivered annual storage cost to achieve 3% storage/handling loss), while net-wrapped round bales can be stored outside on crushed rock ($3.20/dry-Mg-delivered annual storage cost to achieve 5% storage/handling loss). Total harvest plus storage costs, per-dry-Mg delivered basis, were $20 for the round bale and $27 for the square bale. Because of the requirement for covered storage, the square bale will have more difficulty competing economically with the round bale in the southeastern United States. However, as yield increases above 9 dry Mg/ha and storage losses for round bales stored outside increase above 5%, the difference in cost decreases. (C) 1996 Elsevier Science Ltd.