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This article was published in 1959
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
The Importance of the Frequency of Replenishment in Exhausting Sheep Dips
N. P. H. GRAHAM, B.V.Sc., McMaster Laboratory, C.S.I.R.O., Sydney.
Since the chlorinated hydrocarbon insecticides have been used in sheep dips, attention has been drawn in many scientific papers to the problem caused by the exhaustion of insecticide from the wash, but little has been written on methods of maintaining adequate concentrations in the wash or more specifically, adequate deposits of insecticides on the sheep.
In practice a number of different systems of usage have been devised; consisting of reinforcing the wash from time to time with undiluted concentrate or using very high starting concentrations or using lower starting concentrations and slightly higher replenishing concentrations. However, these systems usually require rather complicated instructions for use which often are difficult to follow. Also, a critical examination of these methods shows that in many cases they leave much to be desired.
Each particular formulation has an inherent exhaustion rate and this is expressed easiest as an Exhaustion Factor; which is the ratio of the amount of insecticide actually removed with a given volume of wash to the amount of insecticide in an equivalent volume at the starting concentration. For non-exhausting dips the Exhaustion Factor is 1, for exhausting dips some figure greater than 1. For slowly exhausting formulations, such as BHC or Dieldrin wettable powders, it is about 2; while for more rapidly exhausting formulations, such as emulsions, about 3.
As the exhaustion rate is constant, it is possible to calculate the theoretical variation in the wash concentration under a variety of usages and to examine the effect of such factors as starting and replenishing concentrations and frequency of replenishments.
For example, if a formulation which has an exhaustion factor of 2 is used to charge a 1000 gal. swim dip at a concentration of 0.02% Active Ingredient. The bath is replenished with fresh wash at the same concentration after every 200 gallons, or 20% of the initial volume, has been used. The wash concentration will fall at a decreasing rate until it becomes stabilised over a range having a maximum of 0.011%, just after replenishing, and a minimum of 0.009% just before replenishing. The mean wash concentration will be 0.01%.
If now the frequency of replenishing is altered and the bath is replenished after every 50 gallons, or 5% of the initial volume, has been used. The range will become narrower; with a maximum of 0.0105% (sic) and a minimum of 0.095%. The mean wash concentration remains the same. 0.01%.
In shower dips it is possible to use a very much higher proportion of the wash volume before replenishment is necessary. For example, it is possible to use as much as 60% of the wash volume before it will be necessary to replenish the sump. At this relatively infrequent rate of replenishment, and using the same concentrations as in the swim dip, the maximum wash concentration would be approximately 0.015% and the minimum 0.006%. The mean of the range still would be 0.01%.
From these examples it will be seen that although the same concentration has been used in all instances for charging and replenishing the wash, the minimum concentration has varied from 0.006 per cent. to 0.0095 per cent.; depending on the frequency with which the bath is replenished. The more frequent the replenishment the lower the maximum and the higher the minimum concentration. The mean wash concentration, however, remains the same. If instead of periodic replenishments, fresh wash at the replenishing concentration is run in continuously to replace that being removed by the sheep, then the wash concentration remains fixed at the main concentration of 0.01 per cent. This system gives the highest minimum bath concentration for any given replenishment concentration.
The initial charging concentration exerts only a passing effect and it is the concentration of replenishments which finally determines the mean concentrations when the wash becomes stabilised and it is the frequency with which replenishments are made which determines the range about this mean value. The starting concentration need only be as high as the maximum of the range, so that the more frequent the replenishment the lower the initial charging concentration which can be used. With a system of continuous replenishment using a formulation having an exhaustion factor of 2, therefore, a bath could be charged at 0.01 per cent. active ingredient and replenished at 0.02 per cent, active ingredient. This would give a fixed wash concentration of 0.01 per cent. active ingredient. There would be no variation and the deposit on the sheep would be higher than the minimum provided by any system of periodic replenishment using the same replenishment concentration.
The mean wash concentration equals the replenishment concentration divided by the exhaustion factor of the formulation. The higher the exhaustion factor or the more rapidly the formulation exhausts, the lower the mean wash concentration.
The average insecticide deposit on the sheep is equal to the average volume of wash removed per sheep multiplied by the mean wash concentration multiplied by the exhaustion factor. Therefore, a given replenishment concentration will give the same average deposit of insecticide per sheep irrespective of the exhaustion rate of the formulation. However, for a given volume of wash removed, the range about the mean value will be wider for the more rapidly exhausting formulation; but actually equal values can be obtained by replenishing the rapidly exhausting dip more frequently.
With continuous replenishment there is no variation about the mean value. With this system, therefore, all sheep dipped get the same deposit of insecticide which is determined by the concentration of the replenishment, and any given concentration or replenishment gives the same deposit irrespective of the exhaustion rate of the formulation. The deposit would be the same as that obtained if the formulation did not exhaust.
With continuous replenishment, therefore, an exhausting formulation behaves the same way as a non-exhausting formulation. The only difference is that the more rapid the exhaustion the lower the mean wash concentration, and so the lower the initial wash concentration. For example, using a system of continuous replenishment equal deposits would be given on all sheep by using: (a) a non-exhausting formulation and charging the bath at 0.02 per cent. A.I. and replenishing at 0.02 per cent. - the bath concentration would be 0.02 per cent. (b) a formulation with an exhaustion factor of 2 and charging the bath at 0.01 per cent. and replenishing at 0.02 per cent. the bath concentration would be 0.01 per cent., or (c) a formulation having an exhaustion factor of 3 and charging the bath at 0.06 per cent, and replenishing at 0.02 per cent. the bath concentration would be 0.0066 per cent. The initial amount of insecticide in the bath can be regarded as a basic working residue, as its equivalent is still in the bath at the conclusion of dipping operations. It should therefore, be kept as small as possible.
Although it is theoretically possible to use a starting concentration lower than the replenishing concentration it may be preferable for the sake of simplicity in instructions to use the same concentration for both charging and replenishing. In practice, however, provided continuous replenishment is used. the one concentration may be used for charging and replenishing the wash irrespective of the exhaustion characteristics of the formulation.
Summarising the discussion, the charging concentration exerts only an ephemeral effect on the wash concentration, and so indirectly on the deposit of insecticide on the sheep. The replenishment concentration determines the mean wash concentration. The frequency of replenishment determines the range about this mean value; the more frequent the replenishments the narrower the range, and so the higher the minimum value. The concentration of the replenishment gives the average deposit of insecticide per sheep, and for any given concentration this deposit is the same irrespective of the exhaustion rate of the formulation. The exhaustion rate of the formulation must be considered with the bath concentration in determining the deposit on the sheep.