Chemically Preserved Fence Posts are Harmless to Stock

New Zealand Journal of Agriculture, Volume 98, Issue 3, 16 March 1959, Page 293

Chemically Preserved Fence Posts are Harmless to Stock

By

D. L. HARRISON,

Scientific Officer, Department of Agriculture's Animal Research Station, Wallaceville

TRIALS to assess the danger to livestock of chemically treated fence posts, especially * where arsenic is used in the treatment, have been carried out at the Department of Agriculture's Animal Research Station, Wallaceville, at the suggestion of and in cooperation with officers of the New Zealand Forest Service. Results showed that there was no risk to livestock where the fence posts were preserved with creosote, pentachlorophenol, Boliden 525, Tanalith U, and Tanalith C, which are commercially available in New Zealand. The only danger associated with the use of the preservatives was found to lie in animals having access to stacks of freshly treated timber or to the ground from which such stacks had been removed. Livestock should be rigidly excluded from these areas at all times.

WITH the serious depletion of New Zealand’s indigenous trees and

the need to conserve those remaining, a' greater use is being made of chemically preserved softwoods. To extend their useful life against decay, fungi, and boring insects it is necessary to treat them with preservatives. There is, however, no chemical specifically poisonous to one species; chemicals which protect wood from insects and fungi are also poisonous to animals. The use of treated timber on farms therefore introduces a possible hazard the extent of which will depend on the nature of the preservative and its accessibility.

Most stock owners will have noticed how animals will lick and chew objects either out of sheer curiosity or because they contain something they like. Sheep on a Hawke’s Bay farm chewed so persistently at fence posts that several were almost severed at the base. When the chewed posts were replaced the interference ceased, which strongly suggests that some ingredient in the original preservative was particularly attractive to the sheep. Overseas Experience Deaths of cattle that licked poles protected with arsenic compounds have been reported in Sweden and Switzerland. Access to creosote-treated poles and sleepers has been held responsible for stock losses in many places, though the evidence for creosote poisoning in some instances is not altogether conclusive. It is significant that most of these deaths occurred where freshly

treated wood was left lying on the ground before use. The surface runoff of excess preservative, as well as the area of wood accessible for licking, would be at a maximum in these cases.

Hyperkeratosis (X-disease) caused heavy losses among stock in the United States of America and Germany until the cause was eventually traced, to chloronaphthalenes in some brands of wood preservatives and lubricating oils. It was desirable therefore that the use of chemically treated fence posts, especially where arsenic was used, should be evaluated under New Zealand conditions. before such materials could be confidently recommended as safe for farm use. A summary of the toxicity trials which were carried out at the Wallaceville Animal Research Station is given here.

Preservatives Tested ' The preservatives tested were: Creos ote: A distillation of coal tar, 1. Creosote: A distillation of coal tar, containing varying quantities of tar acids. The creosote used in the trials

was the grade specified in the N.Z. Standard Specification 401 as used by the New Zealand Forest Service. 2. Pentachlorophenol: Essentially chlorinated carbolic acid and used as a 5 per cent solution in petroleum oil. 3. Boliden 525: Copper-zinc-chrome-arsenate. 4. Tanalith U: Fluor-chrome-arsenate with dinitrophenol. 5. Tanalith C: Copper-chrome-arsenate. Both creosote and pentachlorophenol are absorbed into wood relatively unchanged in chemical composition. The last three are multi-salt “fixed”type preservatives with arsenic as the principal toxic ingredient. These are formulated as water solutions which penetrate wood and, during the time allowed for “fixation”, form compounds that are highly resistant to leaching. Though the treating solutions which contain soluble arsenates are unquestionably very poisonous to animals, the fixed compounds, being largely insoluble, should be much less harmful. In practice, the weight of preservative absorbed divided by the total volume of both heartwood and sapwood is termed the loading. As the heartwood is not impregnated, the sapwood will contain a higher concentration of preservative than the overall loading would indicate. In these tests the sapwood only was used. Creosote and pentachlorophenol were used at strengths of three times and Boliden 525 at twice the loadings recommended for fence posts. The Tanalith-treated _ sapwoods were supplied at normal commercial loadings.

The wood for the arsenic preservatives was reduced by milling to a fine powder. With water, this gave a freeflowing suspension suitable for drenching by stomach tube. Creosote and pentachlorophenol were added to ground wood just before dosing. Hoggets and heifer calves of the body weights shown in the table were used as test animals. Toxicity Defined Acute toxicity is defined in these tests as the amount of treated wood in a single dose causing death, and chronic toxicity as the amount of treated wood, given in regular daily doses, causing death from chronic effects in about a month.

TABLE I—TOXICITY OF PRESERVATIVES ON SHEEP AND CALVES

Sheep (66 lb. body weight) Acute Chronic Preservative toxicity toxicity Pentachlorophenol .. 5.7 oz. 2.5 oz. Creosote .. .. 10.6 oz. 1.3 oz. Boliden 525 .. .. 16.0 oz. 2to 4 oz. Calves (220 lb. body weight) Pentachlorophenol .. 1.0 lb. 6.0 oz. Creosote .. . . 3.0 lb. 4.4 oz. Boliden 525 .. .. 3.1 lb. 2to 4 oz.

Hoggets were clinically unaffected by single 1 lb. doses of Tanalith U- or Tanalith C-treated wood, or by 1,2, or 4 oz. doses for 25 consecutive days. Post-mortem examination of sheep on the highest chronic dose rate (4 oz. daily = 6| lb. of wood during the trial) revealed no abnormalities whatever. From these data it is clear that the wood treated with pentachlorophenol had the highest acute toxicity (single dose) in both hoggets and calves but the lowest chronic effect (repeated doses). It was rapidly absorbed and excreted so that cumulative effects were slight. In fact, the equivalent of several acute fatal doses was tolerated over a period with no permanent injury resulting.. Creosote and Boliden 525 were less acutely toxic than pentachlorophenol but showed more marked cumulative effects, and relatively low proportions only of the acute fatal quantity were tolerated daily. The sheep that received the Tanalith-treated woods showed neither clinical nor postmortem changes from normal at the levels drenched. Assessment of the Hazard The chances that some toxicity will occur in practice can now be assessed, using the known toxicological data derived from the trials. The least quantities of wood necessary to cause acute poisoning, 5.7 oz. for sheep and 1.0 lb. for calves (Table 1), are far in excess of what individual animals could obtain at any one time. The amount of wood which ruminants could reasonably be expected to get from a fence post is small. Having front teeth only in the lower jaw, their attack is merely a scraping action aided by licking. Also, chewed timber would not be so finely divided as the ground wood used in the tests so the leaching of the preservatives would be less complete. Posts with a relatively thin sapwood ring could possibly approach our highest test loadings in the outer surface layers, but in general the loadings would be considerably lower than these. The chances of acute poisoning occurring in practice are therefore considered to be negligible with any of the five preservatives.

With the preservative which showed the highest chronic toxicity (in table) the lowest daily intake of preserved wood leading to chronic poisoning is 1.3 oz. for hoggets and 2.0 oz. for calves. These are the least amounts which must he eaten every day for a month. To illustrate these quantities in a more practical way, consider the 3 ft. above ground of a 4g in. diameter softwood post. This would weigh about 12 lb. One hundred sheep chewing wood at 1.3 oz. per sheep per day for 30 days would consume in this time the equivalent of 20 complete fence posts; an extremely unlikely event. With the added consideration pointed out before, it is clear that the chronic hazard is also negligible. As to the arsenic preservatives the trials indicate that even if concentrations of up to four times the usual loadings were attained in sapwood in practice, the chances of stock poisoning would still be extremely unlikely. Commercial timber treatment in New Zealand is governed by a standard code of practice which ensures continued uniformity of composition of the chemicals used and in the methods of treatment. This is particularly important with regard to coal tar creosote which can vary greatly in composition from different sources. The toxicity data for creosote, given above, will not be applicable to other grades which do not conform to the required specification. The only danger associated with preservatives lies in animals having access to stacks of freshly treated timber or to the ground from which such stacks have been removed. Livestock should be rigidly excluded from these areas. Of the preservatives tested only creosote and pentachlorophenol would be involved in re-treat-ment of posts already in use on farms. Where this is carried out the welfare of stock must be the first consideration and particular care should be taken to prevent excessive splashing and surface run-off on to adjacent pasture. It is recommended that animals be moved well away from the site of operations until the chemicals have been fully absorbed into the wood. Conclusions There is no risk to livestock involved in the use of fence posts preserved with creosote, pentachlorophenol, Boliden 525, Tanalith U, and Tanalith C which are commercially available in New Zealand. Injury to animals can arise only through their access to the treating solutions. It is the responsibility of those using preservatives to ensure that this does not occur. Acknowledgment The Tanalith U- and Tanalith C-treated wood was supplied by Hickson’s Timber Impregnation Co. (N.Z.) Ltd., Auckland, and this cooperation is acknowledged.