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.

有机稻田面临养分缺乏、杂草滋生与病虫侵染等多种问题。相较于常规水稻种植，杂草对有机稻种植的负面影响更为显著。日本有机稻种植户普遍采用旋转除草机（rotary weeder machine）开展机械除草作业。本研究于日本鹤冈市山形大学农场的一块有机稻田中，开展了为期两年的田间试验，旨在提升水稻对杂草的竞争能力，并优化机械除草在有机稻田中的高效应用方案。本试验于2020年5月至9月、2021年5月至9月期间种植了‘佐々锦’（Sasanishiki）水稻品种。在移栽后天数（days after transplanting, DAT）0至49天范围内，设置了0、2、4、6、8次共5种不同的除草频次。分别于5个时间点采集水稻与杂草样本，测定其生物量、密度及氮（N）吸收量。研究结果显示，除草8次的处理组水稻生物量、分蘖数、氮浓度及氮吸收量均达到最高水平，而杂草的上述指标则受到显著抑制。例如，在8次除草处理下，2020年和2021年的水稻生物量分别较对照组提升2倍和2.6倍；氮吸收量分别较对照组高2.5倍（2020年）与3倍（2021年）。随着除草频次从2次提升至8次，水稻生物量与氮吸收量均随之升高，这一现象可能源于水稻竞争能力的增强。此外，在筛选高效除草频次时，尽管6次除草处理的生物量略低于8次除草组，但其生物量与氮吸收量与8次除草组并无显著差异，且其收获指数（harvest index）略高于8次除草组。综上可知，除草频次为6次时，既经济高效，又可实现有机稻的最优产量。