Weeds pose an important biological constraint to crop
productivity. In particular, many weeds produce and
release allelochemicals in order to interfere with the
growth and establishment of crop plants (Belz 2007;
Kong et al. 2007).Wild oat (Avena fatua) and crabgrass
(Digitaria sanguinalis) are widespread weeds in unirrigated
farmland.A heavy infestation of these two weeds usually
occurs in cultivated fields, which results in adverse effects
on the growth and yield of crop plants (Aguyoh &
Masiunas 2003; Valverde 2007; Gallart & Verdu 2010).