Abstract: Aim Goosegrass (Eleusine indica (L.) Gaertn.) is one of the world's malignant weeds, a dominant weed in the cotton fields along the Yangtze River in Anhui Province. Cotton is highly sensitive to weed competition. This article aims to clarify the density competition effect of goosegrass on cotton and identifies its critical period for control, providing a theoretical basis for the integrated weed management of goosegrass in cotton fields. Methods From 2010 to 2012, two experiments were conducted at an experimental site in Anqing City, Anhui Province: the goosegrass density competition experiment and the critical control period experiment. In the density competition experiment, the cotton density was kept constant while eight different densities of goosegrass (0, 0.125, 0.25, 0.5, 1, 2, 3, and 4 plants per meter) were tested to evaluate their competition with cotton throughout its growth period. This aimed to determine the effects of varying goosegrass densities on cotton growth and yield. The critical control period experiment involved different durations of weed interference and weed-free periods (0, 2, 4, 6, 8, 10, 12, and 14 weeks after crop emergence) to gather information on how goosegrass affects cotton. Important findings As the density of goosegrass increased, the plant height gradually decreased. Compared to 0.125 plants m-1 goosegrass, the plant height of goosegrass at densities of 3 plant m-1 and 0.25 plant m-1 was significantly decreased. On average over three years, the biomass of goosegrass per unit area increased from 715 kg ha-1 (0.125 plants m-1) to 4148 kg ha-1 (4 plants m-1). This indicates that intraspecific competition among goosegrass becomes more pronounced at higher densities. In 2012, the number of tillers and biomass per plant of goosegrass gradually increased, and the height of goosegrass exceeded that of cotton during the first 12 weeks after crop emergence. With the increasing weed density, the plant height and stem diameter of cotton decreased, while the number of fruit branches and bolls per plant was significantly reduced. The single boll weight was also reduced at high densities of goosegrass, however, there is no significant change in the lint percentage. In 2012, the plant height of cotton significantly decreased at densities of 2 plant m-1 or more. The cotton stem diameter was remarkably reduced at densities of 3 plant m-1 or more in 2011 and 4 plant m-1 in 2012, respectively. Specifically, at the density of 1 plant m-1, the fruit branch numbers, the boll number per plant and single boll weight was reduced by 8.7% to 11.6%, 18.6% to 35.2% and 0.1% to 4.6%, respectively. The seed cotton yields were significantly reduced at density of 0.125 plant m-1 (2010 and 2012) and 0.25 plant m-1 (2011), with reductions ranging from 10% to 18%. The boll number per plant of cotton and the loss rate of seed cotton yields were affected by the goosegrass densities, following a hyperbolic model. It is predicted that goosegrass densities of 2.3 to 3.7 plants m-1 would result in a 50% reduction in the number of bolls per plant, while densities of 0.05 to 0.09 plants m-1 would lead to a 5% reduction in seed cotton yield. As weed infestation increased, the experiment demonstrated a gradual decline in the height and stem diameter of cotton plants. Additionally, there was a significant reduction in the number of fruit branches and bolls per plant, which led to a marked decrease in seed cotton yields, fitting well with a logistic model. Conversely, With an increasing weed-free duration, cotton plants exhibited greater height and stem diameter, along with a notable rise in the number of fruit branches, bolls per plant, and seed cotton yield. The increase in seed cotton yield with prolonged weed-free duration followed a Gompertz model. The critical period for controlling goosegrass at a density of 2.5 plants m-1 is between 35 and 83 days after cotton planting, based on a 5% yield-loss threshold.

Key words: weed competition, goosegrass, cotton, threshold of weed competition, critical control periods for weed