AbstractsBiology & Animal Science

Iodine deficiency goitre has been quite common in the human population of Tasmania since the turn of the century, and it has also been found in horses and other farm animals. The recent studies of Clements and associates on schoolchildren, led to the conclusion that the condition was not a simple iodine deficiency, and goitrogenic agents were involved. The first outbreak of congenital goitre in sheep was recorded in 1945. Since then, severe outbreaks have been recorded in 1956, 1964 and 1968. Features of congenital goitre in sheep were that it was sporadic in nature, associated with particular farms, and there were often marked differences in the incidence of the disease between ewe flocks on the one farm. From a survey conducted amongst Department of Agriculture veterinarians we found that congenital goitre of sheep was mainly confined to the Derwent valley and the Northern midland areas of the state. The studies reported here cover the results of a grazing trial established on a property near Bothwell where goitre was endemic, and the results of associated animal house and plant growth trials. The soil type on which pastures were grown was found to be a major factor in the disease, in that ewes grazing pasture grown on a sandy soil had a much higher incidence of goitrous lambs than ewes grazing pastures on clay soil. Analyses of pastures grown on these soil types revealed that the sand soil pastures were lower in iodine than the clay soil pastures. However, neither pasture contained the level of iodine considered by Butler and associates as the minimum level necessary to prevent goitre. An animal house feeding trial using ewes from the grazing trial property indicated that these sheep had similar iodine requirements to those of Butler's. Observations made by Department of Agriculture personnel and ourselves revealed that outbreaks of the disease were associated with conditions of lush pasture growth during the pregnancy period, i.e. that associated with higher than average autumn rains. This correlation could be explained if pasture grown under conditions of liberal water availability had lower iodine levels than pasture grown when moisture limited plant growth. However, experiments with the two main pasture species (perennial ryegrass and subterranean clover) grown under conditions of varied water stress, showed that plant iodine levels were not affected by water availability. Two goitrogenic agents, nitrate and thiocyanate, were shown to be present under field conditions. From animal house experiments and field observations, it was concluded that these substances were not important factors in the etiology of the disease. Similarly, the source of drinking water was found to be unimportant, even though there were marked differences in the iodine content of the two sources available. It seems that between-year variation in goitre incidence and the effect of soil type can best be explained by a varying iodine intake dependent on soil ingestion which varies with pasture…