Effects of Interrow Space and Sowing Date on Sudangrass (Sorghum bicolor Moench) Growth and Weed Competition

Abstract

Sudangrass is a summer forage grass, which is adapted to a long sowing period from late April to early August in warm regions of Japan. However, yield loss is sometimes caused by weed competition because early growth of sudangrass is affected by air temperature. Here, we investigated crop–weed competition of sudangrass on 2 sowing dates (late May: mean air temperature, 19.6°C; mid-July: mean air temperature, 27.1°C) and 3 interrow space (17 cm, 30 cm, and 75 cm) by measuring canopy cover degree of sudangrass, dry matter yield, and dry matter weight of weeds. When sudangrass was sowed in late May, cover degree changed on the basis of interrow space. When interrow space was 17 cm and 30 cm, cover degree was about 90% at 28 days after sowing (DAS) and 42 DAS, respectively. However, when interrow space was 75 cm, cover degree reached up to 69%, even at 42 DAS. Interrow space also affected sudangrass yield and weed weight. Sudangrass yield for interrow spaces 17 cm, 30 cm, and 75 cm was 884, 625, and 482 kg/10 a, and weed weight was 3, 36, and 92 kg/10 a, respectively. Thus, for sowing in late May, preferable interrow space was considered to be 17 cm for weed suppression and high yield. For sudangrass sowed in mid-July, cover degree was about 90% at 27 DAS for interrow spaces 17 cm and 30 cm. Even for interrow space 75 cm, cover degree was about 90% at 42 DAS. Sudangrass yield for interrow spaces 17 cm, 30 cm, and 75 cm was 971, 925, and 814 kg/10 a, and weed weight was 0, 0, and 12 kg/10 a, respectively. Mean air temperature was higher in mid-July than in late May; therefore, early growth was superior in mid-July, and suppressed weed growth was observed. These results showed that for sudangrass sowing in mid-July, preferable interrow space is less than 30 cm for weed suppression and a high yield. In conclusion, to suppress weed growth and obtain a high yield, sudangrass interrow space should be set at about 17 cm, especially in a low air temperature season such as May. However, in case the interrow space cannot be set at 17 cm or if there are many weeds, sowing date of sudangrass should be set in a high air temperature season such as July, because high temperatures promote the early growth of sudangrass and reduce weed competition.