Recent research conducted over three years (2015-17) at two sites in the south west slopes of Wagga Wagga district, noted arrowleaf clover and biserrula were the most consistent and competitive annual legumes with respect to stand establishment and consistent suppression of winter pasture weed species.
Other legumes assessed in the research included sub clover (Seaton Park), bladder clover, gland clover, yellow serradella (Santorini and Avila) and French serradella.
Mixtures of pasture species were also assessed in combination with lucerne, phalaris and cocksfoot.
Combinations did not perform as well or any better than best single species, with production and competitiveness no better when annual legumes were in combination with perennials.
Sajid Latif (School of Animal and Veterinary Sciences, Charles Sturt University Wagga Wagga), and colleagues Saliya Gurusinghe, Paul Weston, William B. Brown, Jane C. Quinn, John Piltz and Leslie A. Weston, undertook the research.
Past research has generally indicated that the more productive a winter legume pasture, the better able it is to compete against winter weeds.
Aspects associated with pasture competitiveness, such as correcting soil deficiencies and selection of varieties and species suited to a given soil and environment are important.
This study supports these findings, plus confirms an important role for some newer pasture species.
An important outcome from the research was that Santorini serradella did not perform that well from a productivity aspect on this soil type (red Sodosol at both trial sites with pH of 6.2), but was close to the more productive species of arrowleaf clover and biserrula for suppressing weeds.
The researchers suggest serradella varieties like Santorini have mechanisms other than competition for resources (water nutrients, sunlight) affecting weed suppressive ability.
Other research indicates species like yellow serradella, and biserrula may possess phytotoxins produced by leachates from their residues, which suppress weed establishment in the presence of specific soil microbiota.
The underlying mechanism of suppression is likely associated with metabolites produced by plants or microbes.
Generally, the three years of research across the two trial sites showed weed suppression was positively correlated with pasture biomass (except in the case of Santorini).
Arrowleaf and biserrula Casbah were generally the most consistent annual pasture legumes with respect to productivity, yearly regeneration and suppression of annual pasture weed species.
Both arrowleaf clover and biserrula significantly reduced light interception at the base of the canopy and had higher leaf area index than other pasture species, potentially contributing to reduced weed-seed germination.
Other research supports that both leaf area index and pasture biomass contribute to weed suppression.
Previous studies have shown, that early pasture biomass and root-system establishment contributed to weed suppression by competition for resources and/or the release of secondary metabolites by the pasture species in the rhizosphere.
However, because of seasonal factors in the three years of this research, early autumn verses early winter pasture establishment had little impact on pasture species weed suppression.
Additional research is now under way to confirm the mechanisms driving enhanced weed suppression by pasture legumes.
In general, the rapid establishment of pasture species, as well as optimal production of biomass, contributes to weed suppression the authors conclude.
Weed species assessed in the research included barley grass, sow thistle, poppy, ryegrass, fumitory, capeweed and Paterson's curse.
Pasture weeds have been estimated to cost the Australian pastoral industry $2.2 billion annually in management expenses and yield loss.
Winter pasture weeds are an important part of this annual loss and hence the importance of this research.
Next week: Managing for diverse, productive, native pastures.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email email@example.com or contact (0428) 752 149.