Performance and decondary chemistry of selected annual pasture legumes in Southeastern Australia

This project was designed to assess the performance, secondary chemistry and weed suppressive potential of annual self-regenerating pasture legumes in an effort to develop non-chemical weed management strategies for rotational cropping systems in southeastern Australia. Novel annual pasture legumes were introduced to Australia from Mediterranean regions of Europe and Africa in recent decades to improve biodiversity of forage species in Australia while supporting improved pasture production for grazing livestock. These include Biserrula pelecinus L. (biserrula), Ornithopus sativus Brot. (French serradella), Ornithopus compressus L. (yellow serradella), Trifolium glanduliferum Boiss. (gland clover), Trifolium spumosum L. (bladder clover) and Trifolium vesiculosum Savi. (arrowleaf clover). However, their performance, weed suppressive potential and capacity to produce phytoestrogens and related flavonoids have not been fully evaluated under southeastern Australian conditions. Replicated field trials were conducted over multiple years in Wagga Wagga, NSW with treatments established as monocultures and mixed stands. Crop and weed growth were assessed by evaluating several key above-ground competitive traits. Plant tissues (leaf, stem, inflorescence and root) along with rhizosphere soils were collected for the assessment of phytotoxicity against weed seedling indicators and to perform metabolic profiling, with an emphasis on secondary metabolites associated with phytotoxicity and phytoestrogenic activity in grazing livestock.
Choice of pasture species impacted stand establishment, yearly regeneration and suppression of common annual weed species in pastures, with arrowleaf clover and biserrula suppressing annual weeds effectively. Biomass accumulation in pasture species was found to contribute significantly to reduction of weed biomass for the majority of species followed by light interception (LI) at the base of the canopy. Interestingly, yellow serradella cv. Santorini, consistently produced limited crop biomass, while significantly
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reducing weed growth and exhibiting phytotoxicity, suggesting chemical interference as a potential mechanism of weed suppression.
Foliar tissue extracts and soil incorporated crop residues of selected annual pasture legumes inhibited the growth of monocot and dicot weed seedling indicators. Through the development of a comprehensive protocol for non-targeted metabolic profiling using UHPLC QTOF-MS and chemometric analyses, several phytotoxic flavonoids were identified. These included quercetin, kaempferol, isoquercetin, and kaempferol-7-O-glucoside with all present at higher abundance in weed suppressive annual pasture legumes. In addition, the highest, ecologically significant concentrations of phytotoxic flavonoids were detected in field collected rhizosphere soils of biserrula cv. Casbah and serradella cv. Santorini. These results further suggest that plant produced chemical intereference is one of the key mechamisms of weed suppression in those species.
Phytoestrogens that impact livestock fertility, mainly coumestrol, 4’-methoxycoumestrol, daidzein, formononetin and genistein, varied qualitatively and quantitatively among annual pasture species. Our results demonstrated that phytoestrogens present in pasture legumes were at concentrations insufficient to pose a negative impact on livestock reproduction, with the exception of gland clover, bladder clover and lucerne. Metabolic profiling provided unique insights into flavonoid biosynthesis through quantification of both pathway intermediates and end products in selected self-regenerating annual pasture legumes. The results also indicated that most Trifolium spp. and lucerne upregulated coumestan and flavonoid phytoestrogen production under southeastern Australian agro-climatic conditions in contrast to hard-seeded annual legumes.