Cattle and goats in Australia lack the ability to totally degrade 3-hydroxy-4(1H)-pyridone, also known as 3,4-dihydroxy pyridine (3,4 DHP), the ruminal metabolite of mimosine, a toxic aminoacid present in the leguminous shrub Leucaena leucocephala. Ruminants in Hawaii have this capacity due to the presence of micro-organisms able to rapidly degrade the DHP. A mixed bacterial population capable of rapidly degrading DHP in vitro was isolated from a goat on the island of Maui. Cultures were grown anaerobically, without added sugars, in Medium 98-5 containing DHP. Cultures at a dilution of 10(-12) from the original rumen fluid were introduced into Townsville and further sub-cultured and multiplied in vitro in strict isolation at the Oonoonba Veterinary Laboratory, Townsville. Infusion of the culture into a goat and a steer fed a 100% leucaena diet resulted in cessation of DHP excretion in the urine. After 60 days the serum thyroxine levels and thyroid size were normal and there were no clinical signs of disease. The ability of the rumen fluid to degrade DHP in vitro showed that the bacteria had become established in the rumen. In the absence of any disease in the animals, clearance has been given for the wider use of these cultures in areas where leucaena is grown. The limited evidence suggests that the leucaena toxicity problem can be solved by the use of these introduced bacteria.
To examine the apparent absence of toxicity in ruminants fed on Leucaena leucocephala in Hawaii, U.S.A., goats were offered comparable diets of Leucaena and lucerne chaff (control) in both Australia and Hawaii over a 7-week period. Intake of mimosine, excretion of the ruminal metabolite 3-hydroxy- 4(1H)-pyridone (DHP), and serum concentrations of thyroxine (T4) were measured in both studies. Mean mimosine intakes on the highest Leucaena diets in Australia and Hawaii were similar at about 20 g day-1. In Australia, goats on the all-Leucaena diets became hypothyroid after only 3 weeks of feeding, thyroid glands were enlarged and erosion of the oesophageal mucosa and reticulo-rumen occurred. Excretion of DHP in the urine was related to the daily mirnosine intakes, with recoveries of about 86%. In marked contrast, goats fed on Leucaena in Hawaii exhibited no clinical signs of toxicity and excreted less than 1% of the mirnosine intake as DHP in the urine. No degradation of DHP occurred in vitro with rumen fluid from Australian goats, whereas 71% of the added DHP was degraded after 5 h with rumen fluid from goats in Hawaii. The results support the hypothesis that the differences observed are attributable to a different microbial metabolism of mimosine and DHP in ruminants in Hawaii.
Daily injections of thyroxine were ineffective in overcoming the toxic effects of feeding Leucaena to goats. The treated goats did not differ from the controls in liveweight change or body condition over the 15 weeks of feeding Leucaena and both groups of animals developed oesophageal lesions. Treated animals maintained normal serum thyroxine (T4) and triiodothyronine (T3) levels and did not exhibit the thyroid hyperplasia of the controls. An increase in the serum T4 and T3 levels of three of the control animals after 5 weeks of feeding was associated with declining mimosine concentration in the feed and the ability of the hyperplastic thyroids to produce sufficient T4. Serum analyses suggested zinc deficiency in the control animals, 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.
SummaryThe effect on live-weight gain from dosing cattle with rumen bacteria capable of degrading 3-hydroxy-4(l H)-pyridone (DHP) was measured on cattle grazing leucaena and native grass pastures in south-east Queensland. Dosing increased the growth rate of cattle grazing only leucaena pasture: from 0·52 kg/head per day when not dosed, to 1·03 kg/head per day when dosed, in the period 6·19 weeks after treatment. Dosing did not affect the growth rate of cattle either grazing leucaena with native pasture, or grazing only native pasture. The introduced bacteria spread naturally to untreated cattle after 19 weeks post-dosing.The response to dosing occurred when untreated cattle grazing only leucaena pasture had high urinary concentrations of DHP (maximum 0·28%) and low concentrations of serum thyroxine (< 30 nmol/1). The results show that DHP-induced depressions in growth rate may occur in this environment when cattle graze mainly on leucaena-based pasture. Dosing with DHP-degrading bacteria will overcome this problem.
The effects of age and sulphur fertilization on various sulphur fractions in the legume Stylosanthes humilis were examined. Plant material was obtained in two consecutive seasons from nodulated plants growing in the field and from a glasshouse experiment. A very close relationship between the concentration of total nitrogen (Nt), that of total sulphur (St), and age was found. In young material Nt increased at first then reached a plateau with increases in St (brought about by additions of sulphur). As the plants aged, however, both Nt and St declined and the relation between them became more nearly linear. Nt and St were also closely related in seed from these plants but the St/Nt ratio was much lower than in the vegetative material. A moderate deficiency of sulphur had little effect on the ratio of alcohol-insoluble to total nitrogen. In contrast with published work on other species it was found that sulphur applications increased the ratio of sulphur to nitrogen in the alcohol-insoluble (protein) fraction.
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