Mechanical weeding frequency enhanced rice growth by competing with weeds for N absorption in an organic field in northeastern Japan

Organic rice fields face a variety of problems, such as nutrient deficiency, weed growth, and pest infection. Weeds have a greater impact on organic than on conventional rice cultivation. Mechanical weeding with a rotary weeder machine is a common practice employed by Japanese organic farmers. Here, a two-year field experiment was conducted in one organic rice plot located at Yamagata University Farm, Tsuruoka, Japan. This study aims to improve the competitiveness of rice to weeds and the effective application of mechanical weeding in organic rice fields. ‘Sasanishiki’ rice was planted from May to September 2020 and 2021. Different weeding frequencies (0, 2, 4, 6, and 8 times) were employed from 0 to 49 DAT (days after transplanting). Rice and weeds were sampled five times to determine their biomass, density, and nitrogen (N) uptake. The findings showed that weeding 8 times induced the highest biomass, tiller number, N concentration, and N uptake in rice. In contrast, these were significantly suppressed in weeds. For instance, during eight weedings, rice biomass recorded was 2 and 2.6 times greater than that in the control in 2020 and 2021, respectively. Also, the N uptake in the former was 2.5- (2020) and 3-fold (2021) higher than that in the latter. Biomass and N uptake elevated as the weeding frequency increased from 2 to 8 times. This was probably due to the enhanced competitiveness of rice. Additionally, while considering efficient weeding frequency, although six weedings produced biomass lower than eight weedings, it did not vary markedly from the biomass and N uptake of eight weedings. Additionally, the harvest index was slightly higher at 6 times. These findings show that a weeding frequency of 6 times was the most economical and effective with potentially the greatest organic rice yield. © 2025 Japanese Society of Soil Science and Plant Nutrition.