Switchgrass (Panicum virgatum L.) plants and switchgrass residue reduce the biomass and density of associated weeds

Competition is one of the main forms of interaction between cultivated crops and their neighboring plants. Allelopathy is a chemical mechanism that gives plants an advantage in competing for limited resources. Switchgrass (Panicum virgatum L.) has recently been introduced to China's Loess Plateau. As a non-native species, the competitive or allelopathic effects of switchgrass plants or switchgrass residue could have an important effect on weed growth in the switchgrass stand. In this field experiment, we investigated the effect of eight switchgrass cultivars (Blackwell, Cave-in-Rock, Dakota, Forestberg, Illinois USA, Nebraska 28, Pathfinder, and Sunburst) on associated weed growth. Weed density and biomass under each switchgrass cultivar were measured on four dates during the growing season. The effect of switchgrass residue on associated weed growth was also studied. Almost all of the switchgrass cultivars suppressed weed growth early in the growing season; however, Cave-in-Rock was the only switchgrass cultivar that significantly suppressed weed growth throughout the entire growing season. There was a significant negative relationship between switchgrass biomass and weed biomass during the middle part of the growing season (i.e., 28 July and 30 August). This indicated that the competitive effects of switchgrass had the greatest effect on weed growth during this stage. The residue of Blackwell, Illinois USA, and Pathfinder suppressed weed growth more than the growing switchgrass plants did. These results have implications for weed management strategies in agroecosystems and provide important information for the introduction of switchgrass to new ecosystems.