Impacts of pasture legume phase on the seed bank, establishment and growth of barnyard grass (Echinochloa crus-galli) in drill sown rice

Water-saving systems such as drill sown rice and delayed application of permanent water are increasingly being adopted by the Australian rice growers to address water scarcity and competition. Terrestrial weeds such as Echinochloa crus-galli (barnyard grass), however, proliferate in these water-saving systems due to the absence and delay in the application of water during the early stages of rice growth. Rice producers have observed that the inclusion of annual pasture legumes in the rotation might provide the opportunity for weed management through reducing the build-up of the weed seed bank.

This research has been designed to investigate the different suppressive mechanisms of winter pasture legumes on the seed bank dynamics, establishment, and early growth of barnyard grass. Suppressive mechanisms such as inhibition/delay of establishment, restriction of growth and reduction of seed longevity were evaluated through laboratory assays, glasshouse pot trials and field experiments to the extent possible due to restrictions imposed by the covid19 pandemic and drought. The effects of environmental factors temperature, light and burial depth on germination and early growth of barnyard grass were investigated to gain understanding of the seed bank dynamics involved.

This study showed that pasture legumes suppressed barnyard grass seed bank build-up through reduced seed viability over two years of rotation and enhanced seed mortality of seeds left on the soil surface. Also, with reduced density and growth of barnyard grass with winter pasture legumes and potentially differential phytotoxic effects on barnyard grass seedlings on the pasture legume species involved.

The phytotoxicity trial and pot bioassay showed that phytotoxic effects of extracts from pasture legume species and winter crops against barnyard grass and rice varied between donor species/cultivars and test/receiver species. Moreover, the expression of the phytotoxic effects was influenced by several factors including extract dose, residues amounts and placements and duration. Shaftal Persian clover extracts were found to significantly inhibit barnyard grass germination and growth, but results were inconsistent with the pot bioassay, with enhanced growth instead. Nonetheless, other pasture legume cultivars residues showed significant barnyard grass shoot biomass reduction in the pot bioassay. Rice germination and growth, however, were also significantly inhibited by the pasture legume extracts but to a lesser extent. Moreover, incorporating Shaftal Persian residues into the soil inhibited rice growth, whereas mulching it on the soil surface enhanced rice growth. The investigation of germination response and dormancy of the barnyard grass seed ecotype showed germination in all temperature regimes used (spring to summer) but delayed germination early in spring with light stimulation. Dormancy was maintained under darkness. Barnyard grass emergence declined with increasing soil depth and ceased completely at 15 cm burial depth. While mature barnyard grass seeds were 100% dormant at harvest, greater than 80% had broken dormancy after 2 months of burial.

The pasture legumes in rotation with rice provided more suppression to barnyard grass than the crops (canola and barley) and fallow. The phytotoxic effects of the incorporation of pasture legume residues against rice can be avoided by no-tillage practice with direct drill sowing of rice. Other weed management tools such as delayed rice sowing, stale seedbed technique, drill sowing implements with the disc, and competitive rice cultivars with early vigour should be combined with the pasture legume rotation.