Modelling pesticide residues on greasy wool: using organophosphate and synthetic pyrethroid survey data
Authors: Plant J. W. , Horton B. J. , Armstrong R. T. F. , Campbell N. J. 
Published: September 2002

Summary. Several surveys have examined the relationship
between organophosphate and synthetic pyrethroid residues in wool and
associated treatments. These have been combined and summarised using a model
of on-farm survey data. The model estimated the amount of chemical taken up by
the wool at application. This was based on experimental breakdown rates of
these pesticides on wool determined in controlled trials.
For about 10% of survey results the chemical measured on the wool did
not match the chemical the producer said was applied. A further 5% of
results were excluded because the amount of chemical detected on the wool was
inconsistent with the stated time of treatment and shearing. With the
remaining results there was a very high variation in residues resulting from
the same (stated) treatment. It is clear that many producers do not know what
chemicals they have used or how much they applied. The wide variation in
results suggests that some producers may apply excessive amounts of pesticides
while others use too little to have a useful effect.
The model estimated the amount of pesticide taken up by the fleece using the
residue left at shearing and the known breakdown rate for a given method and
chemical group. When organophosphates were applied by dipping, the amount of
chemical taken up by the fleece appeared to increase as the length of the wool
increased. This was generally higher than would be anticipated from label dose
rates but was consistent with the stripping characteristics of these
chemicals. Therefore dipping as soon as possible after shearing left much
lower residues (<10 mg/kg wool) than delayed treatment (often
10–30 mg/kg wool). In contrast the survey results suggest that the
amount retained by sheep as a result of jetting decreased in longer wool.
Jetting treatment rates appear to be lower than recommended, particularly for
sheep with more than 6 months wool. Therefore jetting (as used by producers)
left much lower residues in wool than dipping (with similar length wool) and
was usually only above 10 mg/kg wool if carried out in the last 5 months
before shearing, or if the same sheep received repeated treatments.
The residue of synthetic pyrethroid retained in the fleece after dipping or
long wool backliner application increased as the length of the wool increased
at treatment, and appeared generally consistent with label recommendations.
Current long wool backline products usually left residues of synthetic
pyrethroid above 10 mg/kg on the wool. Short wool dipping left less than
10 mg/kg wool while off-shears backliners usually left average residue
concentrations of about 2 mg/kg wool.
Although the actual on-farm results vary 4-fold above and below the average,
the model can be used to estimate the expected residue concentration and
likely range of results from most standard on-farm organophosphate and
synthetic pyrethroid treatments. This will allow improved provision of advice
so that most producers can meet specified industry standards. It will allow
wool buyers to estimate the risk of purchasing high residue wool based on
producers’ statements about treatments applied.