Uprooting and shearing resistances in the seedlings of four weedy species

Abstract

For mechanical weeding, the uprooting force of whole seedlings and the shearing force of basal stems were measured in two grass weeds (Setaria faberi and Echinochloa crus‐galli var. crus‐galli) and two broad‐leaved weeds (Chenopodium album and Amaranthus patulus), using balance‐type equipment and a pair of scissors, respectively. Seedlings were grown in plastic pots. Uprooting and shearing experiments were carried out on the 11th, 16th, 21st, and 26th days after sowing. The uprooting force, the shearing force required to fracture the specimen, plant dry matter weight and cross‐sectional area exponentially increased as seedlings grew, but the uprooting stress and shearing toughness maintained similar values after the 2nd sampling time, except for the 3rd and 4th sampling times in C. album. The increases in uprooting force and the shearing force required to fracture the specimen were supported by an expansion of the cross‐sectional area of stems of the broad‐leaved weeds or pseudostems of grass weeds. The higher values of uprooting stress and shearing toughness in C. album were related to an increased area of lignified organs in the basal stems. There was a significant correlation between uprooting and shearing forces. The shearing force was always higher than the uprooting force, so that most seedlings were uprooted and not fractured at the basal stems or pseudostems. The mean ratio of uprooting force to root dry matter weight after the 2nd sampling time suggested a high uprooting resistance at an extremely low amount of root dry matter weight.