Previous PageTable Of ContentsNext Page


Richard Gibb

“College Creek” Tarcutta

My subject in today’s conference is ‘Drench Resistance in Sheep’ - in particular, when and how resistance was diagnosed, the effect it has had on management and what changes have been necessary as a result.

The area I am talking about is an improved grazing property in low hill country east of Wagga. It has a 27” (680 mm) rainfall, virtually all grazing - only small areas of oats grown for winter feed - so there is no chance of spelling paddocks with a cropping rotation and thus break the worm cycle. The sheep are a self-replacing flock of Bond Corriedales and the stocking is approximately 25% cattle and 75% sheep.

Prior to the start of Drenchplan we used to drench all the sheep 5 or 6 times a year, alternating between clear and white drenches each year. This seemed to be fairly general practice with a lot of farmers at that time. I refer here to a MSD Thibenzole pamphlet of the late 1970s/early 1980s showing two weaner sheep - one drenched monthly and looking like what we all desire, and one undrenched weaner looking scanty and miserable. At that time we followed the policy of “prevention was better than cure”. Any sheep looking ‘daggy’ and not ‘doing’ was drenched. I know of some farmers who routinely drenched weaners every month to 6 weeks or so, not knowing if there were worms present or not.

In the mid 1980s I was asking myself if all this drenching is necessary and what are we drenching for? In the summers of 1987 and 1988 1 felt our weaners were not doing as well as they should, so at the beginning of 1988 I asked Russ Locke (NSW Agriculture and Fisheries Veterinary Officer) to look at them. We did a worm resistance trial and the result was a marked resistance to Benzimadazole (BZ) drenches 50% effective and some resistance to Levaniisole drenches 86% effective.

Our problem is with resistant Ostertagia. We do not seem to have a problem with resistant Trichostrongylus (black scour worm).

Incidentally, we did a post mortem on a weaner and found evidence of a selenium deficiency (white muscle) in the heart.

After discussing our situation with Russ Locke we made some changes:

1. to strictly follow Drenchplan if possible, which we had already started;

2. to monitor our sheep during the next 12 months and to do a proper trial at weaning time - August;

3. to change to Ivomec, which fortunately had just come onto the market, for a period of two years, and then to see if we could go back to a clear drench or a mixture;

4. to wean into paddocks where we had been running wethers instead of, as in the past, ewes and lambs or hoggets (cattle paddock would be even better to wean into).


It is very obvious, as you are probably well aware, that this plays a big part in worm control.

Last year, 1989, was an excellent year for paddock feed, and the following test results show how older sheep, in this case ewes, will throw off worms, which they had picked up while lambing, given good nutrition.

There were not enough worm eggs in the lambs at weaning, August 1989, to do a proper trial.

The older ewes, 4 years to 6 years, at the December test prior to drenching showed virtually no worms (mean of 6, 13 and 19), so Russ Locke and I decided not to drench them as it was pointless, but to test again before the February drench.

I would like to give you the following example on nutrition:

In September 1989 1 bought four young rams and drenched them onto the property with Ivomec. I joined three of these young rams and one older ram to a mob of 220 older ewes at the end of November. They were running in a paddock into which lambs had been weaned. After a month of joining I noticed two of the young rams in particular were rather light in condition - by six weeks they were poor so I pulled them out and replaced them. I suddenly realised that because we had decided not to drench the old ewes these four rams had not been drenched in December. It showed that young sheep under stress, in this case joining, are very susceptible to worm infection. I drenched those young rams and they immediately started to pick up. I am sure that one would otherwise have died. Incidentally, the ewes only had worm egg count mean of 112 at the February monitor test.

Watch for Contamination of Paddocks

At the end of December 1989, 1 had about 10 weaners that had some arthritis running in a couple of small paddocks next to the shearing shed. They had obviously been stressed by the arthritis and had not been drenched with the other weaners at the beginning of the month. They were looking daggy and scouring a little so we took samples. These showed high egg count mean of over 2000.

About three weeks later at jetting (January 14,1990) 1 held the wether weaners in those two small paddocks for a couple of days to eat the grass down. At the monitor test on February 5, 1990 they had a mean worm count of 1280 and were looking in need of a drench.

I mention this to show how paddocks can easily be contaminated and the necessity to drench all sheep.

Today we have far better dry feed available than a few years ago when everyone just used the traditional oats. I refer to the addition of lupins, peas, triticale, etc, either on their own or mixed with oats for feeding to weaners and autumn lambing ewes. We have not had to make drastic changes to management like changing from ewes to wethers as some people have had to do. However, we now wean into paddocks where we have been running wethers or cattle. We have always run our weaner/hogget sheep from 9 months of age (end of March) to winter for the next 6 months in the hill country, where they are lighter stocked, and have a larger area to walk and grow out. There they do not seem to pick up a worm problem as much as they would on the heavier stocked lower country.


The following points are important.

1. Find out if you have a drench resistance problem by a trial at weaning time.

2. Use an effective drench.

3. Weigh your sheep and drench to the heaviest.

4. Test drenching guns for the correct dose.

5. Nutrition obviously plays a big part in worm control. Lower stocking rates might be a solution.

6. If during the year a mob looks wormy, take some samples into the lab. We have found that quite often it is the feed causing the scouring, not worms. The cost for this type of test is $25 if you do not want to know the type of worms, or $35 if you require the type of worms.

7. Wean lambs into the cleanest possible paddocks. This might take 12 months to plan.

Finally, I think it is important that now we have the means of knowing what worms are a problem, and how effective our drenches are, we test. Unfortunately resistance is with us now, and worms cost money. We can delay it, but, resistance is forever.

Previous PageTop Of PageNext Page