Mechanisms of weed suppression by an autonomous weeding robot in organic rice paddy field

This study was conducted to investigate the weed suppression mechanisms of an autonomous weeder (AW) in Ibaraki Prefecture, Japan. We hypothesized that AW suppresses weeds by generating turbidity through stirring of ponding water and topsoil. Field experiments were conducted in organic rice paddy fields, where an autonomous weeder (AW) was employed as the weeding treatment. The following parameters were measured: weed occurrence, turbidity, pH, electrical conductivity (EC), oxidation-reduction potential (ORP), and dissolved oxygen (DO). Mixed linear models revealed that AW treatment significantly reduced the number and fresh weight of the dominant weed species, <i>Schoenoplectus juncoides</i> and <i>Echinochloa oryzicola</i>, compared to the no-weeding treatment; however, analysis of covariance (ANCOVA) did not show significant interactions between turbidity and treatment. The results suggest that weed suppression by AW is driven not only by turbidity but also by other factors. These include reduced light transmission due to increased turbidity, the formation of a surface mud layer by AW operation, stabilization of ponding water depth, lower dissolved oxygen (DO) levels, and direct uprooting of young weed seedlings. Additionally, the fresh weight per individual weed did not differ significantly between the AW and no-weeding treatments, suggesting that the AW treatment reduced weed density but did not inhibit growth after germination. Therefore, future research should focus on the mechanism by which the stirred-up soil accumulates to form a soft layer that buries weed seeds and prevents their germination. This study also explored the relationship between turbidity and ponding water depth and DO. Significant correlations were found, particularly a negative correlation between turbidity and both the ponding water depth and DO.