If you’ve decided to get serious about breeding worm-resistant sheep, it’s as easy as 1, 2, 3.
1. Decide what WEC ASBV values suit you.
2. Choose studs with suitable WEC ASBV as well as performance and visual criteria important to you.
3. Look only at the rams with WEC ASBV equal or better than your threshold.
Breeding for worm resistance is one part of an integrated approach to worm management. The most effective worm control programs integrate a number of strategies so there is not reliance on just one practice (such as drenching) to deal with worms. Breeding resistant sheep, combined with grazing management can have a very powerful positive impact on worm control.
Worm-resistant sheep take longer to achieve worm burdens that require treatment as many of the infective larvae they ingest do not establish in the sheep. In turn, these sheep deposit fewer worm eggs onto the pastures resulting in less infective worm larvae to re-infect the sheep. This slows the rate of infection and the time before the next drench is needed.
Research conducted near Armidale indicated that paddocks grazed with worm resistant ewes and lambs for a five month period had only one-third the number of barber’s pole worm larvae on pasture.
Each year more Merino and meat sheep studs offer worm-resistant rams for sale. You can easily assess the worm resistance of these rams through their Worm Egg Count Australian Sheep Breeding Values (WEC ASBV).
The more negative the WEC ASBV, the more worm resistance you are getting.
Rams with WEC ASBV between -50 and -70 will make a big impact on your flock, but there will be fewer of these. Those between -30 and -50 will make a moderate impact, and those between -10 and -30 a lower impact, but there will be more to choose from at a wider range of studs. Rams with higher (or more positive) values should not be considered if worm-resistance is important to you. Choose a WEC ASBV threshold that you’d like to stay below.
Worm resistance is just part of a balanced selection strategy. If a stud meets your other performance and visual criteria, check the WEC ASBVs in their sale catalogue. If about one third to one half of the rams have WEC ASBV better than your threshold, then a suitable selection of rams is likely to be available to meet all your needs.
Having decided whether you want to make a lower, moderate or strong impact on the worm resistance of your flock, shortlist only those rams that meet your WEC criteria. Worm resistance is invisible, so a good-looking sheep with poor worm resistance can be tempting, so make up your mind what’s important to your program before you start looking.
It’s my experience from ram sales with ASBV data that prices reflect performance. Therefore, if you want the better rams you should expect more competition at auction.
If the stud you’ve used for years doesn’t offer WEC ASBV, ask them to start. There are plenty of studs that have started to offer this data because their existing clients made the request. Your request might be the one extra they need to make the change.