An investigation into the extent and causes of leucaena toxicity in Queensland

Summary

Leucaena leucocephala (leucaena)-grass pastures are productive, profitable & sustainable. However leucaena contains a toxic amino acid called mimosine. The introduction into Australia of the rumen bacterium Synergistes jonesii (the leucaena 'bug'), that detoxifies mimosine & its rumen breakdown product DHP, had solved this problem. However, at the break of the 2003 drought, deaths & severe DHP toxicity symptoms in cattle grazing leucaena were reported in herds previously thought protected by the bug.

With support from MLA a study was conducted to investigate the extent & causes of leucaena toxicity in Queensland. A survey of leucaena growers revealed that many had used inappropriate rumen inoculation methods &/or were unaware of the need for on-going bug management. The toxicity status of 385 animals in 44 herds grazing leucaena was tested. Leucaena toxicity was found to be a significant problem. Subclinical 3,4-DHP toxicity was considered to be limiting animal performance in 20% of these herds. A further 32% were found to be excreting high levels of 2,3-DHP.

Of particular concern, many of these herds had been inoculated with the bug & were considered protected. Method of bug inoculation was linked to herd protection status & direct drenching with S. jonesii culture was most effective. The discovery of partially protected herds with animals excreting large amounts of 2,3-DHP was surprising & highlighted the need for more research into detoxification processes & the ecology of the bacteria involved. Graziers require accurate information about leucaena toxicity & its prevention to enable them to implement effective rumen inoculation & post-inoculation bug management strategies in order to maximize herd productivity.