Detection of toxicity in ruminants consuming leucaena (Leucaena leucocephala) using a urine colorimetric test

Abstract

Leucaena (Leucaena leucocephala), a productive leguminous shrub for feeding ruminant livestock, contains the toxic amino acid, mimosine, which post ingestion is converted to 3,4-DHP and 2,3-DHP, isomers of dihydroxy-pyridone. While DHP generally does not produce acute toxic symptoms in animals, it has been suggested that it is an appetite suppressant that reduces liveweight gain (Jones 1994). With no observable symptoms, subclinical toxicity is difficult to detect (Phaikaew et al. 2012). In 1982, the DHP-degrading rumen bacterium named Synergistes jonesii was introduced into Australia as a potential solution to DHP toxicity, as it spreads easily throughout cattle herds grazing leucaena (Jones 1994). However, toxicity events reported since the 2003 drought suggest that the toxicity status of herds, previously understood as being protected, may have changed. This may be the result of loss of effective S. jonesii bacteria from the rumen. Widespread subclinical leucaena toxicity has since been confirmed, representing a significant economic threat to the beef industry (Dalzell et al. 2012). At present, testing for toxicity requires a sophisticated chemical analysis of urine samples using high performance liquid chromatography (HPLC). Producers, however, require a robust and reliable means to routinely test for toxicity in their herds. A colorimetric urine test protocol is available, based on the color reaction of mimosine and DHP with FeCl3 solution (Jones 1997). When this simpler colorimetric test was used under a wide range of conditions, false negatives were