Competition between grass pea (Lathyrus sativus L.) and pigweed (Amaranthus retroflexus L.) under deficit water stress condition

Water is one of the most important factors for plants growth and development and both weeds and crops compete for it. To investigate the influence of drought stress on competition between grass pea and pigweed, a greenhouse experiment was conducted as randomized complete block design with three replications based on replacement series arrangement at Faculty of Agriculture, University of Birjand in 2012. Treatments were different planting ratio of grass pea– pigweed (100-0, 75-25, 50-50, 25-75, 0-100%) and four water regimes (100, 75, 50 and 25% soil field capacity, FC). Results showed that a reduction of soil water content from 100 to 25% of FC (pot) caused a 95% (P <0.01) in dry matter production of both species in their pure cultivation. The relative yield index, relative invasion index and relative dominance index of these species indicated that, where both plants were competing, pigweed was more competitive than the grass pea at high soil moisture levels (100 to 50% of the soil (pot) field capacity). Although with further soil water reductions to 25% of field capacity, grass pea showed the superiority over wild amaranth, it seems that growing grass pea is not economically justifiable under these circumstances.