A recent trial in the South Otago area on the disposal of spent dip wash to eliminate pesticide residues has highlighted another more serious problem - a high percentage of farmers are simply giving their sheep a wash with dirty water containing very little actual pesticide chemical. While this may make disposal of the dip wash easy there being little or no actual chemical to dispose of it obviously doesn’t do much for lice control.
What is the point of going to all the trouble of dipping sheep if you are only wetting them with dirty water?
The reasons that seven out of nine local farmers who thought they were being careful and doing the job right but in actual fact weren’t go’s back to attitudes inherited from “Dad and Grandad” from the days of Arsenical and Organchlorine (Dieldrin, DDT) type dips.
These chemicals were very very effective and basically you could do a pretty terrible dipping job compared to the standards required today and still achieve good lice control. Because of toxicity and persistence in the environment these chemicals are no longer with us. We have other chemicals to work with which are effective provided certain conditions of use are abided by. This is where the problem arises because the practices inherited from “Dad” appear to work when there are no lice around. It is not until there is a lice challenge that a farmers poor dipping practice is exposed.
The main reason dip concentration falls below the level required to control lice is due to what is termed dip or wash stress. This is caused by organic matter such as dirt, faeces, wool grease etc entering the dip sump. In this polluted or stressed dip wash the chemical is either degraded (broken down) by the organic matter or selectively absorbed and trapped by this matter thus the concentration of dip chemical is progressively lowered until nothing is left but the dirty water itself.
If you follow the guidelines below these will help you avoid this problem.
· Firstly and most importantly the sump of a shower dip or the plunge dip itself must be cleaned out regularly. How regularly depends on the sumps capacity. The rule is - under good (meaning relatively clean) dipping conditions, dip no more than one sheep per two litres of working sump (or plunge dip) volume before draining and cleaning the sump.
For example:- A 10000 litre plunge dip, - clean out after dipping 5000 sheep.
A 900 litre shower sump - clean out after dipping 450 sheep.
In the case of showers, whether you operate the shower as a standard or constant replenishment shower makes no difference to the interval between cleanouts of the sump - the same amount of organic matter enters the sump irrespective of the size of any replenishment tanks.
Unfortunately the majority of showers in use have quite small sumps necessitating frequent clean out which is why farmers often neglect this aspect. The easiest way round this problem is to enlarge the sump. If you commonly dip say, 1000 sheep a day, enlarge the sump to 2000 litres and it will only need cleaned out at the end of the day. The reason why showers have small sumps is historical - the arsenicals etc didn’t suffer from wash stress so sump clean outs were not necessary.
Other factors in minimising wash stress are:-
· Ensure filters and dirt traps are operating effectively and are cleaned regularly.
· Hold sheep in yards overnight prior to dipping so that they empty out.
· After dipping, don’t hold sheep in draining pens - let them straight out. The saving in dip chemical is minimal as chemical is stripped out by the fleece and the draining water therefore contains minimal chemical, but more importantly does contain dirt, faeces etc, and so adds to contamination in the sump.
· Races leading to the dip should be concrete rather than dirt to minimise dirt entering the sump.
· Replenish dip wash at the correct dilution rates on the label. Replenishment rates are usually higher than the initial charge because of the stripping effect.
· Operate all showers as constant replenishment showers - that is continually refill the sump from the supply tanks while dipping rather than let the sump drop and then refill. Maintenance of a full sump reduces wash stress.
· Avoid dipping out - that is the practice of letting the sump volume drop in steps while adding small amounts of chemical but no water. The reduction in sump volume rapidly increases the contamination level of the remaining dip wash and leads to tail enders being dipped in dirty water with minimal chemical. The small saving in dip chemical does not justify the high risk of jeopardising the whole dipping procedure. Dipping out was designed for plunge dips of 10,000L or greater capacity using only unstressed wash.
If these guide lines are followed then farmers will achieve good dip concentration levels for the entire dipping procedure. Obviously to achieve excellent lice control this is but one aspect - other factors such as complete musters, thorough saturation etc are also very important.
John Smart B.V.Sc.
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