Purple nutsedge (Cyperus rotundus L.) competition with cotton: Species biology and effects of proportion, density, and moisture

Abstract

Greenhouse and field experiments were conducted to determine how proximity factors and water stress interact to influence competition between purple nutsedge and upland cotton. Purple nutsedge produced more total dry weight than cotton in wet conditions but produced less or similar total dry weight in dry conditions. Cotton's ability to extract water from greater soil depth and maintain a high rate of photosynthesis during water stress enabled it to maintain higher RGR (relative growth rate), LAR (leaf area ratio), and leaf expansion than nutsedge. Absolute growth rate (AGR), initial propagule weight, and early shoot production were important parameters for purple nutsedge competition with cotton. Moisture stress affected the relative importance of intraspecific and interspecific competition between species in both greenhouse and field experiments. intraspecific competition was more important than interspecific competition in determining cotton biomass production in wet conditions but drier conditions further reduced the relative importance of interspecific competition. The results of all experiments indicate that greenhouse addition series competition experiments can be applicable to field conditions provided the experimental design takes into account the biological characteristics of the species being studied. Pot size had a large influence on intra- and interspecific competition between purple nutsedge and cotton. In greenhouse experiments, physiological measurements were initiated one hour after irrigation and repeated every 2 h throughout the day. One hour after the cessation of water stress, the photosynthetic rates of both species increased, but photosynthesis recovered faster in cotton than in purple nutsedge. The faster recovery of photosynthesis in cotton was probably due to the osmotic adjustment that occurred in cotton leaves that protected enzymes and other cellular components during water stress. In additive field experiments, seed cotton yield was reduced because the number of harvestable bolls m⁻¹ was reduced as purple nutsedge density increased. Yield was also reduced by cotton seedling death at the highest nutsedge density. The interference of purple nutsedge with cotton cannot be reduced through water management alone. But, based on the growth characteristics of indeterminate cotton varieties, we suggest that delaying the first post-planting irrigation of some cotton varieties could reduce the competition of purple nutsedge with cotton without affecting final seed cotton yield.