Ruminal Bacteria Prevent Leucaena Toxicity in Cattle

Leucaena [Leucaena leucocephala (Lam.) de Wit.] is a tropical arboreal legume currently used as an animal forage in the seasonally droughty areas of the subtropics. Its usefulness is limited, however, mimosine, a toxic nonprotein amino acid which in ruminants is converted to a toxin. Ruminal microorganisms convert mimosine to a potent goitrogen called 3,4-DHP (3-hydroxy-4(1H)-pyridone). Typical signs of toxicity in ruminants include alopecia, anorexia, reduced weight gain or weight loss, excessive salivation, esophageal lesions, enlarged thyroid glands, and low circulating concentrations of thyroid hormones.

Leucaena toxicity has been observed in Australia, the Pacific, and Africa, where the shrub is not naturalized. Australian researchers have attributed the distrubution of the expression of leucaena toxicity to the presence or absence of the ruminal bacteria capable of degrading and detoxifying 3,4-DHP.

The reaction to the leucaena toxin has been controlled in China and Australia with the introduction of DHP degrading ruminal bacteria. Cultures of 3,4-DHP degrading ruminal bacteria were transfered from Hawaii to Australia. Later, Australian cows provided a source of DHP degrading ruminal bacteria for Chinese researchers. In 1987 Dr. Raymond Jones at the SCIRO Davis Laboratory in Townsville, Queensland, Australia has worked with scientists at the Institute of Animal Science, Nanning, in Guanxi Province to transport rumen fluid drawn from resistant animals in Australia to Nanning. After five days no toxin was detected in the urin of innoculated cows.

Another experiment was conducted in which a pure culture of 3,4-DHP-degrading bacteria was inoculated into ruminally fistulated Hereford cattle grazing leucaena. These bacteria successfully colonized the rumen of recipient cattle and persisted through the following winter when there was no leucaena in the diet.

The experiment used cattle at Brooksville, Florida used both Senopol and Hereford cattle. The Senopol cows had microorganisms that could degrade 2,3-DHP, an isomer of 3,4-DHP. These cows were originally from St. Croix, U.S. Virgin Islands where leucaena is an indigenous species. Hereford cattle at Brooksville generally did not degrade either 3,4-DHP or 2,3-DHP. In vitro screening results of the innoculated cows indicated 3,4-DHP degrading ability was not present in any of the heifers at week 0, was present in 1 of the inoculated heifers at week 2, and was present in all heifers by week 6.

Experimental results indicate that viable DHP-degrading bacteria spreads easily to other cows, causing uniniculated cows to become resistant to the toxin. Scientists have speculated this occurs through feces, aerosols and dust, and is spread through food and water contaminated by resistant cows. Cows that aquired resistance to the leucaena toxin from contact with innoculated cows maintained their resistance through the winter when no leucaena was available. The technology needed to carry out the inoculation of the toxin-digesting bacteria will not likely inhibit its application. Before the innoculant can be spread internationally, however, proper precautions will have to be taken for native populations.

For more information contact:

Andrew C. Hammond, PhD
US Department of Agriculture
ARS, Brooksville, FL 34605

and

Jim Mc William
ACIAR
CPO Box 1571
Canberr, ACT 2601
Australia.