Experiments were conducted to control spiny amaranth (Amaranthus spinosus L.) and velvetleaf (Abutilon theophrasti Medic.) in a field of forage corn sown in early, mid-and late spring by using an Italian ryegrass (Lolium multiflorum Lam.) living mulch, and to control swinecress (Coronopus didymus (L.) J. E. Smith) in an Italian ryegrass sward by dense sowing at the Kyushu National Agricultural Experiment Station, Nishigoshi, Kumamoto. Growth spiny amaranth in the corn field depended on weed control treatments. The living mulch adequately controlled the weed in the field of corn sown in early spring, but the weed-controlling effect was reduced with the delay in the sowing date, and the living mulch did not control the weed in the late spring sowing plot. Although the living mulch substantially reduced the fodder yield of corn in the early spring sowing plot, it only slightly reduced the corn yield in the mid-spring sowing plot. Pre-emergence application of atrazine + alachlor adequately controlled the weed irrespective of the sowing dates. Neither living mulch nor pre-emergence herbicide mixture adequately controlled velvetleaf, but the growth of velvetleaf in the field of forage corn depended on the sowing date of corn. At the time of corn harvest, growth in the late spring sowing plot was more reduced than that in the plot sown in mid-spring, which indicates that a shift of the corn sowing date to late spring could be effective for avoiding damage by velvetleaf. Sowing of Italian ryegrass at twice as much as the standard density adequately suppressed the growth of existing swinecress in the ryegrass sward, and dense sowing markedly reduced the amount of the weed both in the first and second croppings of ryegrass without reduction of the crop yield. Cropping systems of forage crops that could effectively avoid the damage caused by velvetleaf, spiny amaranth and swinecress were examined based on the experimental results.