Yellow-wood poisoning can be a major cause of lost production, especially in cattle in the eastern areas of Central Queensland. Historically, it was a problem in sheep but very few sheep remain in the major yellow-wood areas.
Yellow-wood (Terminalia oblongata ssp. oblongata) is a bushy, deciduous tree that may grow up to 12 m high but generally only reaches 8 m or less. Its bark is grey, furrowed and somewhat flaky, and its leaves are light green when young, turning yellow with maturity. The leaves are 2.5-5 cm long, 0.6-1.2 cm wide and oblong in shape, with obvious veins. Most leaves are borne in clusters along the stems and drop after heavy frosts.
The plant suckers freely and is usually branched near ground level. It prefers to grow in friable dark grey to brown clay and clay-loam soils, especially on the margins of brigalow scrubs, on drainage lines or around the edges of flooded country. The plant grows in the catchment of the Fitzroy and Burdekin Rivers, and their tributaries.
Yellow-wood leaves contain hydrolysable tannins and condensed tannins, which are thought to be responsible for the liver and the kidney damage caused by the plant.
Conditions favourable to poisoning
Any period of poor normal feed leading to a large intake of yellow-wood leaves is dangerous. The greatest incidence of yellow-wood poisonings is seen during severe droughts, which decrease fodder, and frosts, which cause massive leaf falls from yellow-wood trees. This provides cattle with dead yellow-wood leaves as a major dietary component. In normal seasons, poisoning occurs at the end of winter or early spring when fodder supply is short and leaves are available on the ground.
Pulled country that is heavily suckered is particularly dangerous, as the trees are small and provide stock with fresh green leaves over much of the year.
Two distinct syndromes are noticed in cattle after eating yellow-wood: acute, mainly affecting the liver, and chronic, affecting the kidneys. Cattle surviving the first type of poisoning often progress to develop the second form.
Acute poisoning is caused by the consumption of large quantities of leaves over a short time and may manifest itself within a day of eating the leaves. The tannin damages the liver so that it can no longer effectively deal with waste substances. Affected cattle have signs of abdominal pain (teeth grinding) and are dehydrated.
The hairless skin or skin carrying unpigmented (white) hair, particularly the muzzle and around the eyes, becomes photosensitised (sensitive to the sun). The muzzle cracks; discharge cakes the nostrils; and the beast will continuously lick the area. Consequently, there may be ulceration of the lower surface of the tip of the tongue. The mucous membranes are jaundiced. The conjunctiva may be reddened and discharges may flow from the eyes.
Post-mortem examination reveals a jaundiced carcass with a swollen yellow liver and swollen pale kidneys. There may be ulcerations stained with a green-to-black pigment in the abomasum (fourth stomach) and kidneys may also have a greenish discoloration.
Any cattle that survive the acute phase, or consume only small quantities of leaves over a longer period, may have signs of yellow-wood poisoning due to kidney failure. An affected animal becomes progressively poorer, passes large amounts of urine and may develop soft swellings along the underline and under the lower jaw. Chalky deposits may form on the hairs around the vulva or prepuce. Mildly affected cattle appear rough and underdeveloped. Although some cattle appear to 'recover', they usually fail to progress beyond forward store condition.
Post-mortem examination reveals scarred kidneys with a green pigmentation. The urinary bladder may be thickened and contain a chalky deposit. There may be ulceration of the stomach lining.
Nervous signs are the main manifestation of yellow-wood poisoning in sheep. Affected sheep seem quite normal until disturbed, after which they drop to the ground, extend all four legs and arch backwards. Within a minute, they return to normal. Photosensitisation is rarely seen in sheep. In this form of poisoning, the only fatalities reported are from misadventure. Some sheep fed yellow-wood as drought fodder have developed kidney failure and others have developed photosensitisation.
Sheep recover from the nervous disease if removed from the yellow-wood but there is no cure for poisoning in cattle.
Cattle should be removed from the source of the leaves and put into a good paddock; however, any cattle that survive will never fatten.
The yellow-wood tree is very difficult to control, as it is resistant to most herbicides and tends to sucker readily when pulled. A hot scrub burn will kill the trees but this must be repeated to kill suckers and springbacks that survive pulling operations. Sown pastures provide one method of building up a sufficient fuel load to sustain a hot fire in successive seasons. Removing cattle from affected paddocks during the period of heavy leaf fall is desirable but not always practical.
You must be careful not to overstock areas with high concentrations of yellow-wood trees. During periods of unfavourable seasonal conditions, high protein supplements may be fed to limit consumption of the leaves.