Utilization of lupin stubbles by merino sheep: studies of animal performance, rates and time of stocking, lupinosis, liver copper and zinc, and circulating plasma enzymes

Abstract

Three experiments were conducted in the West Midland region of Western Australia to evaluate the sheep-carrying capacity of dry narrow-leafed sweet lupin (Lupinus angustifolius cv. Uniharvest) stubbles. In addition, the effect of stocking rate on lupinosis, and the effect of lupinosis on circulating plasma enzymes and liver copper and zinc levels were determined. At low stocking rates (25 or less ha-1) sheep were grazed for up to 80 days with increases in liveweight. However, with higher stocking rates (50 or 75 ha-1) the maximum weights obtained were lower, and reached after a shorter period of time. Sheep grazed at the high stocking rates suffered more liver damage than those grazed at low rates, and it is suggested that outbreaks of clinical lupinosis might be precipitated by grazing sheep at high rates. Sheep that grazed the stubbles and suffered liver damage had higher liver copper levels than those that suffered no damage. Also, there was an indication that the degree of liver damage and the liver copper level were directly related. It is suggested that the toxins of Phomopsis leptostromiformis cause the liver to take up and store copper. A significant negative correlation between the degree of liver damage and the liver zinc level was found. However, it was not determined whether this diminishment of liver zinc was a direct effect of the toxins of P. leptostromiformis or due to displacement by copper which was actively being taken up. Plasma levels of ornithine carbamyl transferase, alkaline phosphatase and aspartate amino transferase, measured during or at the end of the experiments, were not related to the degree of terminal liver injury. They are not considered reliable indicators of the incidence or severity of lupinosis in the field.