Abstract 487

Megarrity,-RG; Jones,-RJ

Division of Tropical Crops and Pastures, CSIRO, Davies Lab., Private Mail Bag, Aitkenvale, Qld. 4814, Australia.

Australian-Journal-of-Agricultural-Research. 1983, 34: 6, 791-798; 15 ref.

Daily injections of thyroxine were ineffective in overcoming the toxic effect of feeding Leucaena to goats. The treated goats did not differ from the controls in liveweight changes or body conditions during 15 weeks of feeding Leucaena and both groups developed oesophageal lesions. Treated goats maintained normal serum thyroxine (T4) and triiodothyronine (T3) and did not exhibit the thyroid hyperplasia of the controls. An increase in the serum T4 and T3 of 3 of the controls after 5 weeks of feeding was associated with declining mimosine concentrations in feed and ability of hyperplastic thyroids to produce sufficient T4. Serum analyses suggested zinc deficiency in the controls, associated with their low thyroid status. It was concluded that the goitrogenicity of 3-hydroxy-4(1H)-pyridone (DHP) is only partly responsible for the toxicity of Leucaena to ruminants, and that the low feed intakes and low liveweight gains are related to other effects of DHP. The evidence for extra-ruminal metabolism of DHP is discussed.