A cyanogenetic plant is one which contains hydrocyanic or prussic acid or at least the radical CN in the form of a glycoside. In some plants the cyanogenetic glucoside is present only during certain stages of growth, and in the majority of these it is during the young stages that the prussic acid content is at its maximum. A very large number of such plants become atoxic upon reaching the flowering or mature stage and whereas they may cause severe mortality in stock from eating them during the early stage of growth, at a later stage they can be eaten by stock with impunity. As against this, some plants are cyanogenetie at all stages of growth. Native Fuchsia or Eremophila maculata being notable in this connection. Then again, certain plants are only cyanogenetic in certain districts and the HCN content varies considerably, the outstanding example in this connection being the common Milk Weed, Euphorbia Drummondii, which occurs very commonly, growing over wide tracts of country. It is a common weed about the metropolitan area, but a cyanogenetic glucoside has not been demonstrated in specimens from this district, whereas specimens from inland districts are frequently cyanogenetic.
Another plant which varies largely in its cyanogenetic properties is the Native Bird's Foot Trefoil, Lotus australis. In 1937 a profuse growth of this plant occurred in the Walgett district and was responsible for some exceedingly heavy mortalities amongst sheep. A rather unusual feature was the length of time that the plant retained its cyanogenetic content. It is usual for a plant of this type to lose a considerable proportion of its prussic acid content when the seed has reached maturity and the plant commences to wilt; but during the 1937 summer and autumn this did not happen with Lotus australis. The cyanogenetic content remained high throughout all stages of growth.
There is no doubt that climatic influences exert a considerable effect upon the availability of potential cyanogenetic plants as fodder for livestock. A notable instance of this has occurred during the last two summers in which growth promoting rains have fallen during January in both years. The moisture and warmth of the soil at this time of year have proved optimum to the growth of Couch Grass, one species of which, "Blue Couch," or Cynodon incompletus, evolves prussic acid. It is extremely paradoxical that nature should provide adequate conditions for the growth of a grass so attractive to stock and yet when consumed by the animals in sufficient quantity will cause rapid death.
In the majority of cyanogenetic plants the prussic acid content is held in the form of a glucoside and as a result is retained even after the plant is dried. In order to liberate the prussic acid the action of an enzyme is necessary. This may be present in the plant containing the cyanogenetic glucoside, or may be present in some other plant eaten. During the digestive processes the glycoside is split by the action of the enzyme and free prussic acid is liberated. In the majority of the non-cultivated cyanogenetic plants, some of which are trees, the prussic acid is contained as a glucoside; but in the cultivated plants, particularly Sorghum and Sorghum-Sudan Grass hybrids, the prussic acid is in the free state and is given off readily when the plant is cut. Hence, if the plant is cut and allowed to wilt the prussic acid content becomes reduced very considerably and may be eaten by stock without producing harmful effects.
During the summer of 1937-38 which is just passing, the incidence of fodder crop mortalities in dairy stock was much above the average. One cannot state with certainty as to the reason for this phenomenon, but circumstantial evidence closely correlated the destruction and regrowth of these crops with the result of grasshopper invasion. The crops principally involved in these disasters were Sudan Grass, particularly the hybrid forms, Johnston Grass and Ambercane—that is, members of the Sorghum species generally. It has been commonly known that the Sorghum species are most dangerous during the early stages of growth and particularly plants which are making a second growth after cutting. During this summer, however, a number of mortalities in the Gundagai district have occurred as the result of feeding on the mature plants which subsequent tests have shown possessed a high cyanogenetic content. Formerly Sorghum species showed the maximum prussic acid content when stunted in growth, particularly as the result of damage from frost. It must he realised that it does not necessarily follow that mortality will occur from the mere fact that stock have access to cyanogenetic plants. Prussic acid is only toxic at a certain dose rate, that is, so much of the poison is required to kill an animal of a given weight. During the early stages of digestion prussic acid is given off rapidly and absorbed, but if only small amounts are evolved a sufficient concentration of the poison is not attained in the animal's circulation to produce harmful effects, the drug being eliminated almost as rapidly as it is evolved.
The condition of the animal is also a factor in the susceptibility of stock to prussic acid. Travelling stock are in most instances hungry, and when allowed access to cyanogenetic plants, which in most instances are green and attractive, will devour the plant greedily with the rapid onset of toxic effects. In these cases, absorption of the prussic acid is rapid and cannot be excreted at the same rate to counterbalance the poisonous effect. Other stock, not in such a hungry condition, would devour less of the plant and at a slower rate without manifestations of ill-effects, the rate of excretion balancing the rate of absorption.
Before concluding it might be mentioned that very satisfactory results from the point of view of treatment have attended the use of the sodium nitrite-sodium thiosulphate mixture on animals affected with prussic acid poisoning. It is necessary that treatment should be undertaken as early as possible following the onset of symptoms. In conclusion, it might be stated that the following plants have been shown to be cyanogenetic and associated with mortalities in stock in this State—