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This article was published in 2002
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DRENCH ROTATION / DRENCH RESISTANCE

John Evers District Veterinarian, Young RLPB

In 1987 in response to widespread emergence of clinical drench resistance — ie; clinical parasitism which was not responsive to drench; with BZ (white) or LEV (clear) anthelmintics — DRENCHPLAN was launched in the Young RLPB District.

The main principles on which this "new" approach to control of sheep Internal parasites was based were:

I say "new" because the first 3 had been general recommendation by many D.V.s in southern NSW based on CSIRO research. However, the packaging promotion of DRENCHPLAN by CSIRO / NSW Agriculture / RLPBs was highly effective and brought about a large shift in practices adopted by local producers and advice given by retailers. Control of worms in sheep was now a program and not just a series of ad hoc-p.m. investigations.

At the very start I decided that the best way to validate and support these recommendations was to set up some profile flocks. These flocks would be asked to implement DRENCHPLAN recommendations (Including my local variations on a theme); be subjected to regular monitoring (FEC's and Annual Drenchtests); and this information would be used to validate / modify / promote the program at District level.

This was implemented during 1988 and 3 properties were selected in the higher rainfall (650mm p.a.) improved pasture areas near Young, Harden and Boorowa. One property dropped out due to managerial changes in 1990 and was immediately replaced. Consequently the results I wish to discuss today refer to:

I might say at this point that over recent years 2 of the basic principles on which DPENCHPLAN is based have come under increasing scrutiny ie: summer drenching and annual drench rotation. This can be very confusing for producers and their advisors - every parasitologist has a different theory and modelling to support it. In that sort of environment I take a lot of comfort from 14 years of field observation and monitoring and this gives me the confidence to make my recommendations.

A part picture of the environment I'm working in might help:

(i) CLIMATE - 650mm so - called winter rainfall area, but actual rainfall is almost non-seasonal at about 50mm/month. Eg. Averages for DEC., JAN., FEB. are 53, 51, 43mm respectively. Summers are warm (odd days to 40c) and monitoring of young sheep on pasture country frequently shows significant larval pick-up over summer.

(ii) ENTERPRISES - the area is a mixed-farming type with a variable balance of winter cropping and grazing depending on economics. Merino sheep predominate and up until 1990 Young PLPB ran more sheep than any other (3.5 million) reducing to 2.4 million at present; there has been a swing to terminal 1st cross lamb production over recent years; and most properties ran small herds of breeding cattle. Crop stubbles provide strategic low-worm grazing over summer on many properties.

(iii) WORMS - Osteragia was the predominant parasite throughout the 1980's. It has been interesting to see a swing towards Trichostrongylus dominance through the 1990's in response to pasture improvement programs (perennial species) and increased stocking rates (10-12 DSE/Ha would be common now) the starkest example being cell grazing systems. Haemonchosis is a sporadic problem in wetter summers and autumns in the North East sector.

(iv) DRENCH RESISTANCE - Tony Morton (Gundagai) and myself (Young) saw a rapid emergence of clinical BZ and LEV resistance problems in the mid - 1980's which we felt may have been associated with the mother-of-all droughts for our area (1982). Combination drenches proved quite useful until the mid-90's (I will recommend anthelmintics giving over 80% FECRT on a Trichostrongylus / Ostertagia dominant culture). Since the mid - 90's Combi has only been recommended at 1.25 dose rates after overnight fasting. Rametin mixtures have been used extensively since 1997 and as yet no Mectin resistance has been detected. Not surprisingly, Ostertagia is normally the most resistant species. A very typical drench efficiency profile would be: BZ - 30% LEV - 50% COMBI - 70% RAMETIN/BZ - 90% MECTIN - 100% ie. Many producers are now on 2-way annual Rametin/BZ and Mectin drench rotations.

So what has happened in those 3 profile flocks?

A summary of drench rotations and resistance / efficiency status for mixed scour worm burdens is provided:

ORIGINAL PROFILE

Original profile Property A Property B Property C
BZ
LEV
Combi
89
93
80
100
54
68
94
81
87
1987 BZ BZ -
1988 LEV BZ -
1989 IVOMEC IVOMEC -
1990 COMBI BZ LEV
1991 IVOMEC IVOMEC BZ
1992 BZ LEV COMBI
1993 LEV COMBI LEV
1994 COMBI IVOMEC IVOMEC
1995 IVOMEC IVOMEC IVOMEC
1996 1½ LEV RAM/BZ COMBI
1997 IVOMEC IVOMEC IVOMEC
1998 RAM/BZ COMBI RAM/BZ
1999 IVOMEC IVOMEC IVOMEC
2000 RAM/LEV MECTIN RAM/BZ
2001 MECTIN COMBI/MECTIN MECTIN
2002 RAM/BZ RAM/BZ COMBI
MECTIN USAGE 6/16=37.5% 8/16=50% 5/13.38.5%
CURRENT PROFILE A B C
BZ
LEV
COMBI
RAM/BZ
IVOMEC
½ IVOMEC
35
84
97
97
100
100
25
26
11
73
99
95
94
44
97
94
100
100

I believe this sort of information provides clear evidence that in the Young RLPB District annual drench rotation will delay the onset of Mectin drench resistance. An early FECRT trial with BZ capsules on property A indicated rapid selection was occurring. While it is clear that resistance is beginning to emerge on property B there are factors operating here which make that somewhat predictable:

There is little doubt in my mind that use of Ivermectin capsules is more likely to be associated with rapid emergence of Mectin resistance. On that premise I have very conservative recommendations on capsule use - an early trial with BZ capsules on properly A indicated rapid selection compared to conventional drenching.

As Mectin resistance emerges, we find ourselves exposed to the arguments for and against use of novel Ivermectin / Combinations as opposed to use of Moxidectin. I would have to say that after my experience with LEV/BZ combinations in a similar situation I would have a preference for rotational use of Moxidectin as a first response to low-level Mectin resistance. The use of combinations once resistance to the individual components has been detected has been of limited use on a District basis, eg, LEV/BZ combinations not effective to over 80% reduction in majority of flocks. But the 2 approaches should be assessed by trialling them in these profile flocks as Mectin resistance does emerge - particularly bearing in mind that current Combi activity is much better than average in 2 of these 3 flocks.

Another issue that has been raised recently has been the idea of using different anthelmintics for the 1st and 2nd summer drenches. While this strategy has some appeal it does tend to complicate what has been a very simple message ie. change to product "X" for the 1st summer drench and use that for 12 months. Combined with the fact that many producers drench onto stubbles and don't require a 2nd summer treatment, I haven't yet come to grips with this one for my District.

It is also interesting to speculate on the possibility of manipulating drench resistance profiles by altering grazing practices. Although Ostertagia is the big bogey for us, in my District there is clear evidence that it is not favoured by more intensive grazing systems. However, this would have significant limitations in the management of Merino weaners. Perhaps we'll end up running crossbreds?

It is also worth giving some thought to drench rotation strategy going into a drought - based on the 1980's experiences - it may be possible to further delay meet of Mectin resistance eg. avoiding Mectin rotations or using Moxidectin followed by Rametin/BZ over summer where worm burdens are higher and non-Mectin efficacy is too low. Most of us realise that worm control is a numbers game - if burdens are low-to-moderate then high efficacy treatments are not essential and give some opportunity to delay the onset of further resistance to better treatments (especially with non-Haemonchus burdens in mature sheep).

But, if you had said to me 15 years ago I could swap the drench resistance situation I had then for the one I've got now I'd have happily accepted. On that basis annual drench rotation will continue to be a key element in the DRENCHPLAN program for the Young RLPB District.

 


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