A crop rotation of sorghum [Sorghum bicolor (L.) Moench] and soybeans [Glycine max (L.) Merr.] with various weed control treatments was conducted from 1968 to 1975 at Lincoln, Nebraska, in order to improve weed control in these two crops. Poor weed control during one growing season increased weed growth and decreased yields of succeeding crops. Trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 1.1 kg/ha on soybeans showed greater soil carryover toxicity to sorghum planted 12 months later than did atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] at 3.4 kg/ha on sorghum to subsequently planted soybeans. Under this crop rotation green foxtail [Setaria viridis (L.) Beauv.], tall waterhemp [Amaranthus tuberculatos (Moq.) J. Sauer], and large crabgrass [Digitaria sanguinalis (L.) Scop.] decreased; whereas, velvetleaf (Abutilon theophrasti Medic), sunflower (Helianthus annuus L.), and Pennsylvania smartweed (Polygonum pensylvanicum L.) increased. Germinating weed seedlings from soil samples were greater in weedy check plots during the final years of the crop rotation due to a buildup of seed from those species that increased. Handweeding, herbicides, and plowing reduced weed populations, weed competition, and increased sorghum and soybean yields.