LEUCAENA TOXICOSIS AND ITS CONTROL IN RUMINANTS

Leucaena (Leucaena spp., especially L. leucocephala) is an arboreal, t ropical legume that ranges into the cool subtropics and equatorial ele vations up to 1,000 m. One of its uses includes forage for livestock, but introduction of leucaena outside its indigenous range often has le d to acute and chronic toxicosis. The major toxic constituents of leuc aena are the nonprotein free amino acid mimosine and its ruminal degra dation product, 3-hydroxy-4(1H)-pyridone (3,4-dihydroxypyridine; 3,4-D HP). Leucaena also contains appreciable quantities of condensed tannin s. In ruminants, mimosine is a depilatory agent and 3,4-DHP is a poten t goitrogen. In the 1980s, Australian workers demonstrated that the ge ographical limits of leucaena toxicosis were due to the absence of rum inal bacteria capable of degrading 3,4-DHP, and successfully introduce d 3,4-DHP degrading ruminal bacteria from a Hawaiian goat into goats a nd cattle in Australia. Simple in vitro screening methods have been de veloped for detection of 3,4-DHP degraders in ruminal samples and fece s. Also, several strains of 3,4-DHP degrading ruminal bacteria have be en characterized and have been given the genus and species designation , Synergistes jonesii. Ruminal inoculation with ruminal contents from adapted animals, enriched cultures of 3,4-DHP-degrading ruminal bacter ia, and pure cultures of S. jonesii have all been used successfully to establish ruminal populations that are capable of degrading 3,4-DHP a nd preventing leucaena toxicosis. Once established in only a few anima ls, 3,4-DHP degraders easily distribute themselves throughout a herd o f cattle and persist while leucaena is component of the diet. After le ucaena is removed from the diet, 3,4-DHP degraders continue to persist at reduced numbers for several months. With the availability of viabl e approaches to the control of leucaena toxicosis in ruminants, leucae na's potential as a high-quality perennial legume for the tropics and subtropics can be more fully exploited.