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This article was published in 1966
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Acute Phalaris tuberosa Poisoning of Sheep
E. A. FARLEIGH, B.V.Sc., Veterinary Inspector, Glen Innes
INTRODUCTION
Over many years numerous reports have been received of small losses in sheep, which have at the time been attributed to Phalaris poisoning. Over all, however, these losses have not been particularly heavy, nor has any great significance been placed on them.
However, during the period from May to September, 1965, a number of heavy mortalities took place, which stimulated interest, not only in the poisonous nature of the plant, but also in methods of overcoming the toxicity of Phalaris paddocks, so that the available feed could be utilised. This was a drought period on the Northern Tablelands as well as over most of New South Wales, and the great resistance of Phalaris to lack of rain has been outstanding, so that in most paddocks the only green feed available over the winter period has been Phalaris, which continued to grow, apparently with little or no moisture, also in very low temperatures.
CASE HISTORIES
Case No. 1
Property carrying 5000 wethers, which were rotated every four days through 16 paddocks of approximately 50 acres each. Fine granite country, top-dressed and sown to mixed pastures over a number of years. Sheep were introduced on May 12, 1965, to the next paddock in the rotation, which had been unstocked for 60 days and consisting of mostly Phalaris two inches high, some Lucerne and Rye grass. The following morning over 200 sheep were found dead and during that day the total deaths amounted to 285. The rotation was broken as the next paddock also consisted mostly of Phalaris and the owner was unwilling to risk losing more sheep. The sheep were placed on native pasture country. No further deaths took place.
Following the accepted idea that the plant was more toxic at night or on dull days and after frosts (Moore, 1961), attempts were made from time to time to test the toxicity of the plant and the sheep were placed in Phalaris paddocks from 9 a.m. to 5 p.m. each second day, which still resulted in deaths at various times, up to 44 dying in 24 hours.
The period of time was then reduced to four hours every second day with continued deaths, so daily feeding for four hours was tried, which did not appear to be effective as deaths up to 14 a day took place so a reduction to two hours a day was attempted, which was successful over a period of two weeks from July 14, 1965, to July 28, 1965, with only one death, but the next morning 30 dead sheep were counted.
No feeding of Phalaris paddocks took place until August 15, 1965, when the wethers were ropelled and held on Phalaris paddocks in transit to natural pasture paddocks. Some sheep would have had only two hours on Phalaris while others up to four hours. On August 16, 1965, 360 dead sheep were counted.
On consideration of all facts it was apparent that the sheep were always hungry when introduced on to Phalaris. Mortalities did not appear related to frosts, wet weather or any other constant feature. Owing to extremely dry conditions the original rotation only allowed 14 days' feed and 21 days' starvation before moving to the next paddock. At subsequent attempts to feed, the sheep were held in eaten-out paddocks and fed for limited periods only.
After the last heavy mortality it was decided to adopt the opposite policy and the sheep were fed during the day of August 16, 1965, on a natural grass paddock, which had been saved for some months and late in the evening moved on to Phalaris. No dead sheep were found the next morning, but two deaths took place the following day. The sheep were moved daily or every second day according to the amount of feed available, and since that day to the break of the drought in December, 1965, no further deaths took place.
Although no controlled experiments were conducted it does appear that mortalities would not take place if the sheep were kept full.
Case No. 2
On May 13, 1965, 280 cross bred ewes introduced at 2 p.m. on to Phalaris, which had been spelled for two months. Ten were dead the next morning, two were sick and died in a few hours. Heavy black soil paddock.
Case No. 3
On May 30, 1965, 506 merino ewes entered a paddock of Phalaris at 10 a.m. Twenty-two were found dead next day. Black soil.
Case No. 4
On June 28, 1965, 600 cross bred ewes were introduced to spelled paddock at 2 p.m. Eighty dead in the morning. Black soil.
Case No. 5
On July 17, 1965, 2650 merino wethers, after crutching, were introduced to paddock, which had been spelled for eight weeks. Not seen the following day, but 48 hours later 357 dead sheep were counted. Granite soil.
Case No. 6
On August 14, 1965, 480 12-months-old ewes were unloaded from two semi-trailers during the afternoon, after travelling nearly 300 miles from western New South Wales, on to fresh Phalaris in which were running 20 poor ewes, which were not feeding properly on wheat. Next day 316 were dead or dying-a mortality of 65 per cent. The last death was exactly 24 hours after entering the paddock. None of the 20 poor ewes were affected. Basalt soil.
Three minor losses have been investigated and it is probable that other mortalities took place but were not reported.
SYMPTOMS
These are generally consistent with those described by Gallagher et al. (1964). Varying from acute death, where the animal appears to drop in its tracks through the varying stages of tetanic spasms, legs very rigid, lateral recumbency, excess salivation, to those still walking about with inco-ordination of gait and completely oblivious of man or dogs.
Practically all deaths take place in 24 hours and the recovery rate of the affected sheep would be under 5 per cent.
The appearance on necropsy is not always typical, only patchy congestion of abdominal viscera is to be seen, but petechial haemorrhages of the epicardium are a fairly constant feature and the heart appears to stop in systole. It is not considered that a diagnosis could be made purely on post-mortem appearances.
Laboratory examination of tissues are also inconclusive. Blood levels of calcium and magnesium are normal and G.O.T. and bilirubin estimations show only very slight variations. Brain examinations have shown variations from a severe acute Polioencephalomalacia of the cerebral cortex to apparently normal brain.
Samples of the pasture were negative for nitrate, nitrite and cyanide, but it is now established that at least three tryptamine alkaloids have been isolated from young Phalaris stands, and it is assumed that these alkaloids are responsible for the mortalities. Gallagher (1964) has isolated the alkaloids and produced identical symptoms experimentally by injecting these into sheep.
DISCUSSION
The numerous outbreaks of Phalaris poisoning have been associated with drought conditions when generally the only green pick available has been a leaf growth of Phalaris usually from two to three inches in length.
In all cases sheep have been hungry when introduced on to the pasture and mortalities do not appear to be associated with weather conditions such as rainfall, frosts, cloud cover,etc., or soil types. Although it is suggested that the toxicity of the plant is greater during the night and on cloudy days, this factor did not appear significant in the trials conducted. Feeding for several hours during the afternoon seemed to result in just as many deaths as feeding all day. It does appear that the toxicity of the plant may not vary greatly and the determining factor could simply be the degree of starvation to which the sheep have been submitted prior to feeding on the grass. As Phalaris is one of the main pasture species sown on the Northern Tablelands, it is remarkable that more mortalities do not take place, if the starvation factor is eliminated. As owners in all cases removed the sheep from the paddocks as soon as the mortalities were discovered, it is not known what would have happened if the sheep had remained, but it is possible that deaths would not have continued.
SUMMARY
A number of outbreaks of poisoning due to Phalaris are described and the attempts made to overcome the toxicity of the plant. The significance of hunger in relation to mortalities is suggested.
REFERENCES
1. Moore, R. M., Arnold, G. W., Hutchings, R. J., and Chapman, H. W. (1961) - Aust. J. Sci. 24, 88
2. Gallagher, C. H., Koch, J. H., Moore, R. M. and Steel, J. D. (1964) - Nature 204:542