Further evidence that zinc sulphate compromises the efficacy of dipping treatments using diazinon to control sheep lice (Bovicola ovis)

OBJECTIVE

To compare the wettability and efficacy of diazinon dip wash made with and without the addition of zinc sulphate.

DESIGN

Field experiments using a shower and a plunge dip complemented by in-vitro wettability experiments.

PROCEDURE

A flock of infested sheep was divided into groups and treated in a shower dip with clear or cloudy dam water plus up to 1.5% zinc sulphate. Another infested line of sheep was treated using a plunge dip with nil or 1% zinc sulphate. In both experiments, wetting was assessed after dipping and louse counts were conducted for 9 months after treatment. Five in-vitro experiments compared the wettability of dip wash containing diazinon with up to 1.5% zinc sulphate added.

RESULTS

In the shower dipping experiment, live lice were found at 1 month after dipping in the cloudy water groups with 0.75%, 1.0% and 1.5% zinc sulphate and at 2 months in the 0.75% zinc sulphate group. No lice were found at subsequent inspections or at any time in the groups that were plunge dipped. Zinc sulphate decreased the amount of dip wash retained by wool staples in all in-vitro experiments (P < 0.05).

CONCLUSION

Zinc sulphate should be considered as a risk factor that could cause failure to eradicate a lice infestation. The risk can be overcome by ensuring that all sheep are saturated at dipping and that the dip wash, and any holding tanks, are agitated throughout the dipping event.

The detection of lice (Bovicola ovis) in mobs of sheep: a comparison of fleece parting, the lamp test and the table locks test

Knowledge of the presence or absence of lice in a flock of sheep enables wool growers to make informed decisions as to the need for insecticidal treatments. However, with inapparent infestations, traditional methods of detection are not sufficiently sensitive and, as a consequence, flocks may be left untreated. Conversely, the routine application of insecticide to sheep with no sign of infestation is an unnecessary cost. The sensitivity of 3 procedures for detecting lice was evaluated in 68 mobs of sheep from 50 farms. In 24 mobs of sheep known to be lightly infested, lice were detected in 17% (71%) [corrected] by either parting the fleece of 10 sheep or by the lamp test in which 8g samples of shorn wool from 30 randomly selected fleeces were placed under lamps for 10 min to repel the lice. Twenty of 23 mobs (87%) were found to be infested by the table locks test in which a 30 g sample of locks wool was dissolved in 10% sodium hydroxide and the filtered residue examined with x 40 magnification. A screening test, in which either fleeces on 5 sheep were examined by fleece parting or lice were repelled from 30 shorn fleeces for 5 minutes, detected about 60% of lightly infested mobs. When this was followed by the table locks test 91% of lightly infested mobs were detected. Conducting any one of the tests on more than one mob, and in large mobs testing more frequently, increases the sensitivity of detection of lice within the whole flock.

Grazing a saltbush (Atriplex-Maireana) stand by Merino wethers to fill the autumn feed-gap experienced in the Western Australian wheat belt

Three-year-old Merino wethers were grazed at 4 stocking densities (15, 20, 25 and 30 sheep/ha) in 3 consecutive autumns on saltbush forage. About 50% of edible material was leaf [70% digestible dry matter (DDM), 8.9% crude protein (CP)] and 50% was small stems <5 mm diameter (40% DDM and 4.5% CP). In 1992, 9-month-old wethers also grazed the saltbush stand at 30 sheep/ha. Liveweight initially increased by 1-3 kg during the first 14 days of grazing, then declined at 60-100 g/day until the diminishing forage reserves became critical, by which time the stand had provided for about 450 grazing-days/ha. The performance of the 9-month-old wethers was comparable with the 3-year-old sheep at equivalent grazing pressure. In 1992, the tensile strength of wool staples from the 3-year-old sheep grazed at the higher density of 30 sheep/ha, 17.2 N/ktex, was weaker (P<0.05) than staples from sheep grazed at 15 and 20 sheep/ha, 27.1 N/ktex, and 25 sheep/ha, 24.9 N/ktex. Saltbush plantations can be used to provide a feed source which could partly fill the 'autumn feed-gap' for <9-month-old dry sheep. Grazing at <20 sheep/ha, removal of sheep from the saltbush before the mean liveweight declines below that at the start of grazing, and provision of a reliable supply of water will avoid low staple strength wool. More productive stands of saltbush, grown together with winter-active plant species, are needed to improve the economic viability of this grazing resource.

Settling of insecticide from dip wash mixed with dam water and zinc sulphate and used to control sheep lice (Bovicola ovis)

Insecticidal dipping fluid emulsions, mixed in vitro in dam water containing suspended clay particles and 1% w/v zinc sulphate, were analysed to determine rates of settling of diazinon, cyhalothrin and cypermethrin. Fifteen minutes after mixing, the concentration of the insecticides 5 cm below the surface had declined by 72.5%, 72.8% and 89.4%, respectively. On remixing, the concentration of insecticide in suspension was close to or greater than the initial concentration. In 2 trials, lice were eradicated from sheep showered with dip wash mixed in cloudy dam water to which 1% w/v of zinc sulphate was added. In 12 flock treatments in which 1000 to 2000 sheep were dipped with added zinc sulphate, the concentration of insecticide remained above the minimum lethal for susceptible strains of lice. However, lice were still present 6 months later in 8 of these flocks. When zinc sulphate is added to dip wash, agitation is needed to maintain the insecticide in suspension.

Soil and agronomic factors associated with cadmium accumulations in kidneys of grazing sheep

Mean concentration of cadmium (Cd) in kidneys of hogget sheep from 67 flocks grazing in the Agricultural Region of Western Australia was tested for association with soil, pastoral, climatic and nutritional factors. Hoggets grazing pastures on acidic soils and soils with a sandy-textured surface had higher Cd concentrations in kidneys than hoggets grazing pastures on more alkaline soils or soils with a clay-textured surface. Application of more than 100 kg of phosphatic fertiliser during the past 3 years to loamy soils was also associated with greater Cd concentration in kidneys of the grazing animals.

Efficacy against sheep lice (Bovicola ovis) and fleece wetting of six shower dip preparations

The relative efficacy of 6 shower dip chemicals most frequently used for the treatment of sheep lice (Bovicola ovis) in Western Australia was examined. Groups of 20 sheep infested with lice were treated with products containing either alphamethrin, cyhalothrin, diazinon or diazinon plus piperonyl butoxide and rotenone, formulated as emulsifiable concentrates, and with products containing either coumaphos or magnesium fluorosilicate, formulated as wettable powders. All treatments were applied through a shower dip (Sunbeam model SSD). Inspections for lice were conducted until 9 months after dipping. No lice were found on sheep treated with the 4 emulsifiable concentrate products. In contrast, treatment with the wettable powders, which contained either coumaphos or magnesium fluorosilicate as the active ingredient, did not eradicate the lice infestations. The degree to which the fleece was wetted was assessed 20 minutes after dipping and showed that the wettable powder dips penetrated the fleece less than the emulsifiable concentrate dips. Less fluid was retained by wool staples in an in-vitro test when dip wash was made with the wettable powders. It was concluded that the degree of wetting attained at dipping was an important factor in achieving eradication of sheep lice.

The prevalence of lice (Bovicola ovis) infested sheep flocks in Western Australia (1987-1993)

The proportion of wool bale brands with a positive test for sheep lice in baled wool decreased from 29.5% in 1987/88 to 23.2% in 1990/91 before increasing to 38.2% in 1992/93. Changes in the proportion of wool bale brands with a positive test for lice were highly correlated with changes in the Wool Market Price Indicator. The increase in the proportion of positive lice tests since 1990/91 was associated with an increase in failures to eradicate lice from flocks. These failures were partly a consequence of the reduced use of lousicidal treatments, the development of resistance to synthetic pyrethroid chemicals and an increase in the transmission of lice between flocks.

Cadmium concentrations in kidneys of sheep and cattle in Western Australia. 1. Regional distribution

A sample of 4973 kidneys from sheep stratified by age and shire of origin within the Agricultural Region of Western Australia, was analysed for cadmium (Cd) content during the period August 1989 to April 1991. The geometric mean Cd concentration in the kidney of hogget ewes was 0.9 mg/kg, in 4-tooth ewes 1.47 mg/kg and in adult ewes 3.34 mg/kg on a wet weight basis. The mean Cd concentrations of either ewe or hogget flocks from different Divisions of the Agricultural Region did not differ from each other. The annual increase in Cd concentration of kidney from hogget sheep was estimated to be 0-65 mg/kg. The rate of accumulation of Cd in kidney from cattle and sheep grazing the same properties was similar. Kidneys from a sample of 354 adult cattle from the Kimberley Region and 483 aged sheep from the Pastoral Region, both areas of unimproved rangelands, had geometric mean Cd concentrations of 0.15 mg/kg and 0-31 mg/kg respectively. A higher Cd concentration in flocks from the divisions adjacent to the Agricultural Region may have resulted from the establishment of some volunteer species of winter annual pastures in the rangeland.

Persistence of the sheep body louse, Bovicola ovis, after treatment

Lice were found in samples of baled wool from 198 of 464 flocks treated to eliminate an infestation in the period July 1988 to June 1990. In 287 flocks the insecticide was applied as a backline treatment and in 177 flocks a shower dip was used. Of these flocks, 41.5% and 44.6%, respectively, were found to be infested at the following shearing. After adjusting for the accuracy of the test, it was estimated that infestation in 2 consecutive 12-month periods occurred in 52.6% of flocks with a real incidence rate of 27.4%. Using these estimates, it was calculated that in 34.7% of infested flocks treatment did not kill all lice. There were no differences in the proportion of consecutive infestations between flocks treated with any of the 3 chemicals applied as backline treatments. Among flocks treated in a shower dip, 68.4% using coumaphos, 37.8% using diazinon and 41.5% using cyhalothrin had consecutive infestations (P < 0.05). The detection of lice in 63.2% of flocks that were treated with magnesium fluosilicate was, in part, attributed to the application of this chemical by one operator.

Improving production from September-born Merino lambs with a lupin grain supplement and phomopsis-resistant lupin stubbles

As a means of providing nutrition on dry pasture in the wheatbelt of Western Australia, supplements of lupin grain were fed to late-born Merino lambs and their dams before weaning onto lupin stubble. Each ewe-lamb pair consumed an average 19.5 kg lupin grain over the 46 days before weaning. This produced an extra 3.6 kg (P<0.01) liveweight gain by these lambs compared with their unsupplemented cohorts at weaning. The growth of the weaned lambs grazing phomopsis-resistant lupin stubbles was primarily influenced by the amount of residual lupin grain available to each sheep and liver injury caused by ingestion of phomopsins. In the first 69 days of stubble grazing, sheep stocked at 10ha gained 7.3 kg while those stocked at 20ka gained 2.1 kg (P<0.001). In the first 13 months of life, lambs supplemented with lupin grain before weaning grew more (P<0.05) clean wool by 0.17 kg than the unsupplemented lambs. However, the value of this extra wool would not have covered the cost of the lupin grain eaten. Wool staples produced by sheep grazed at 10/ha on lupin stubble were stronger (P<0.001) than those produced by sheep grazed at 20ha (24.4 v. 17.7 N/ktex). The position of greatest weakness in the staple coincided with weight loss prior to removal from the stubble. Singleton lambs born in September and weaned onto a phomopsis-resistant lupin stubble reached liveweights consistent with survival over the summer and good productivity in their first year of life. The effects of lupinosis and reductions in wool staple strength may have been avoided by monitoring liveweight change and taking the sheep off the stubbles when they were near peak liveweight.

Liver damage and the liveweight changes in Merino wether weaners grazing mixed crops of oats and sweet narrow-leafed lupins

Merino wether weaners were grazed on mixed crops of oats and sweet lupins at 3 stocking rates (15, 30 and 60 sheep/ha), pure lupins (60 sheep/ha) or pastures (7.5 sheep/ha) with and without an ad libitum ration of an oat-lupin grain mix in 2 experiments. Changes in liveweights and liver damage due to lupinosis were measured. In experiment 1, when grazing commenced in late January 1982, the weaners grazed on the mixed crops at 15, 30 and 60 sheep/ha gained 4.8 kg over 91 days, 2.9 kg over 43 days and 1.2 kg over 43 days, respectively, while those on the pure lupins gained 0.7 kg over 14 days. These liveweight gains were all less (P<0.05) than the 10 kg increase over 70 days by the weaners on pasture which received the ad libitum supplement of oat-lupin grain (about 2.2% N). Liver damage developed at a slower rate in the sheep on the mixed crops. In experiment 2, when grazing commenced in late November 1983, the weaners grazed on the mixed crops at 15,30 and 60 sheep/ha gained 1 1.3 kg over 70 days, 6.2 kg over 42 days and 3.5 kg over 28 days, respectively, while those on the pure lupins gained 3.4 kg over 28 days. These liveweight gains compared with a 12.5 kg gain over 84 days by the weaners on pasture which received the ad libitum supplement of oat-lupin grain (about 2.60%N). There was less Phomopsis leptostromiformis on the lupin stems in this experiment and the sheep livers were not as badly damaged as in the previous experiment. The development of liver damage was again delayed in the weaners grazed on the mixed crops. It was concluded that Merino wether weaners can increase liveweights when grazed on mixed crops of oats and sweet lupins. The amount of increase depends on the grain available to the sheep and the degree of lupinosis-associated liver damage suffered by the sheep. Grazing mixed crops reduces the development of lupinosis.

Survey of anthelmintic resistance in Western Australian sheep flocks. 1. Prevalence

Between July 1981 and December 1983, 116 randomly selected sheep farms in the south west of Western Australian were surveyed for resistance to anthelmintics. A faecal worm egg count reduction test was carried out on each farm. Anthelmintics tested were thiabendazole (44 mg/kg) and levamisole (7.5 mg/kg) given by intra-ruminal injection and comparisons were made with an untreated group on each farm. Successful tests were carried out on 84 farms and 68% of these had resistant worms present. The prevalence of thiabendazole resistant populations was for H. contortus 18%; Teladorsagia, 41% and Trichostrongylus, 48% and for levamisole resistant populations H. contortus, 10%; Teladorsagia, 41%; Trichostrongylus, 24%, and Nematodirus, 10%. Multiple resistant populations were found on 17% of farms. Although the distribution of nematode genera varied between the 400 to 750 mm and the greater than 750 mm rainfall zones there was no significant difference in the prevalence of resistance between zones. About one third of resistant populations were severely resistant (less than 60% reduction). It is likely that resistant worms were present on many farms without causing clinical disease and continued anthelmintic selection pressure will result in further development of resistance.

Survey of anthelmintic resistance in Western Australian sheep flocks. 2. Relationship with sheep management and parasite control practices

Owners of 116 farms, whose flocks had been tested for anthelmintic resistance, were interviewed to determine their use of various sheep management and parasite control practices and their knowledge and adoption of recommended procedures for the prevention and control of resistance. Farmers knowledge of current recommendations related mainly to changing drenches and drench groups. Other aspects of the recommended program including reduction of drenching frequency and the use of alternative management strategies were not considered as important by farmers. For most questions a high proportion of farmers (greater than 20%) had no opinion. Associations between various strategies for nematode control and resistance of Trichostrongylus and Teladorsagia to thiabendazole and levamisole were examined. These relationships differed between anthelmintics and nematode genera. A number of factors were related to resistance of one or both nematode genera to one or both anthelmintic groups. These factors included flock size, percentage of ewes in the flock, cattle number, main sheep production activity, grazing strategy, frequency of drenching, changes in the frequency of drenching, number of summer drenches and the method of estimating dose rates. It was concluded that the methods employed to control anthelmintic resistance may vary with the nematode, its resistance status and the anthelmintic to which it is exposed. Modifications to the previously recommended program have been proposed which incorporate selection of the anthelmintic to be used following a test for anthelmintic resistance.

Sandplain lupins (Lupinus cosentinii) as a summer feed for yearling steers

The response of 9-month-old Angus steers, grazing sandplain lupins during summer and autumn and fed oat and oat plus urea supplements, or grazing hay made from the lupins and fed no supplement, was studied in two experiments. In the first experiment there were 3.3 t DM/ha present at the start of grazing. Average daily weight changes over 84 days were -0.548, -0.274 and 0.333 kg/day (P<0.05) for steers fed 0, 1 and 2 kg oats/steer.day respectively. The steers grazing lupin hay averaged -0.417 kg/day. There were 8.4 t DM/ha present at the start of the second experiment and the daily weight changes by steers fed 0, 2 and 6 kg oats plus urea/head.day were - 0.012, 0.524 and 0.952 kg/day (P<0.05), respectively. The steers grazing lupin hay gained 0.215 kg/day. No obvious clinical signs of lupinosis were observed in either experiment. However, in the first experiment the lupins were toxic, liver damage occurred and lupinosis may have contributed to the lower growth rates. In both experiments steers not fed a supplement made compensatory weight gains during winter and spring, when all steers were run together on mixed clover-grass pasture.