Underperforming legumes?

Pasture legumes are an integral component of pasture systems throughout the mixed farming and permanent pasture zone. Pasture legumes are grown to provide high quality feed for livestock either in mixed pastures or monocultures, for grazing or conservation as hay or silage. Pasture legumes and their associated rhizobia play an equally critical role in fixing atmospheric nitrogen for use by other pasture components or following crops. Australian agricultural systems have traditionally relied upon relatively few pasture legumes to underpin production. The annual legumes, subterranean clover and annual medics along with the perennial legumes, lucerne and white clover, have been the main legumes used in Australian agriculture. Subterranean clover has historically been highly successful across the permanent and mixed farming zones where average annual rainfall exceeds 400 mm and soils are slightly to moderately acidic. Annual medics have similarly been successful, particularly in the mixed farming zone where rainfall exceeds 250 mm and soils are neutral to alkaline. Lucerne, a member of the medic family, was traditionally seen as a conserved forage species but is increasingly used in mixed pasture situations mainly in the mixed farming zone. White clover is an important legume for the high rainfall (700 mm+) permanent pasture zone. While these legumes have and continue to be important contributors to pasture production in southern Australia, their reliability in achieving the dual goals of high value feed for livestock and providing biologically-fixed nitrogen for pasture and/or subsequent crop growth has come under increasing scrutiny.
Reports of pasture decline began to emerge in the 1980’s (e.g. Carter 1982, Wheeler 1986) and has variously been attributed to factors including: poor plant nutrition, soil acidification, variable climatic conditions, poor grazing management, establishment techniques (such as; timing , companion species and sowing rates), renovation frequency, partial renovations (e.g. surface), increased use of herbicides and increasing stress from pests and diseases. While these factors have often been considered in isolation, in a paddock situation, pasture legumes encounter such challenges concurrently. The question then has to be asked:
‘Are legumes underperforming or are we expecting optimal performance under sub-optimal conditions and management?’
Further, another question should be asked:
If conditions and management are sub-optimal for legume performance, what can or what should be changed to improve legume performance?’
The remainder of this paper will explore some of the factors that may be contributing to sub-optimal legume production and discuss options for amelioration, including whether changing legume species should be considered. Importantly, we will consider not only the legume plant response to possible factors that may impede performance but also their associated rhizobia. Without an effective symbiosis between the host legume and its rhizobia, optimal production and nitrogen fixation will not be achieved.