Abstract
Pasture legumes are crucial to the sustainability of ley farming systems. They provide a disease break for cereals, nutritious feed for livestock and supply biologically fixed nitrogen (N) for subsequent non-leguminous crops. However, there are frequent observations where biological N‑fixation from the pasture phase is significantly less than expected leading to a N-fixation deficit. Previous field experiments have indicated that the application of Group 2 herbicides contributes to this N‑fixation deficit. Pre-emergent and early-season post-emergent herbicides are applied to avoid early-season weed competition. Group 2 herbicides, inhibitors of the acetolactate synthase (ALS), are widely used in the farming systems of the Western Australia Wheatbelt to control weeds. Greenhouse experiments were conducted to quantify the damage of Group 2 herbicides to French serradella ( Ornithopus sativus ) cvs. Fran 2 o and Margurita, inoculated with commercial strain WSM471. Imazethapyr, imazamox and flumetsulam were applied at logarithmic rates to ascertain a dose-response curve and elucidate the tolerance of these cultivars at three different crop stages. Root nodulation of both French serradella cultivars was more sensitive to herbicide damage than above-ground biomass. Imazethapyr applied post-emergent to Fran 2 o reduced nodulation by 50% when applied at 40% of the recommended label rate (140 g ai/ha). In contrast, the above ground biomass was reduced by 50% only after the application of 170% of the recommended label rate of imazethapyr. Imazamox and flumetsulam applied at label rates reduced nodulation of Margurita by 42% and 20% respectively. The implication of this is that although the legume may be observed as unaffected by these herbicides, N‑fixation is being severely affected.