A rat bioassay for screening tropical legume forages and seeds for palatability and toxicity

Summary. A rapid evaluation of palatability and apparent toxicity offorage legumes, using a rat bioassay and requiring only 400 g of dried forage or 100 g of seed, is described. Vegetative material is included as 20°/0, or seed as 5%, of a nutritionally complete, 12% protein (casein) diet for rats. Voluntary intake and weight gain of rats on test diets are compared with results from standard diets given ad libitum and at restricted rates. It proved possible to distinguish toxicity from simple unpalatability by comparing the weight gain observed on test diets, with the expected weight gain estimated from the regression of weight gain on intake of the standard diet. The method is satisfactorily reproducible, and discriminated clearly between commercially acceptable forages and others known to be toxic, between young and old or between leaf and stem material of the same plant, and between raw and autoclaved seed. The method was used to screen forage from 340 accessions and seed from 110 accessions held in the Australian Tropical Forages Genetic Resources Centre of the CSIRO Division of Tropical Crops and Pastures. Of 150 species, mostly in the genera Cassia, Crotalaria, Indigofera and Tephrosia, only 40 could be classified as palatable and non-toxic to rats, though there was considerable variation between accessions of some other species. As the rat, which is the common laboratory test animal for plant compounds harmful to poultry, man and other monogastric animals, is likely to be more sensitive to ingested phytotoxins than ruminants in most cases, the technique provides a fast, cheap, conservative, preliminary screen of potential forages.