With high numbers of calves being reared this year, Wormwise vet Mary Bowron answers questions about when these calves will need their first drench.
Number one rule – don’t start drenching until the calves have got worms that need removing.
For your calves to be affected, they must have parasites cycling through them, so they need to have a functional rumen and be eating grass.
It is unlikely they will need drenching until they have been eating a significant amount of grass for three to four weeks.
To decide on when to give their first drench, collect some fresh poo samples and get some faecal egg counts done. If there are no eggs there, then no drench is needed just yet, so retest in a couple of weeks.
If there are eggs present, your vet can help you decide if the egg count is high enough to warrant a drench or to retest in a week. The section on interpreting egg counts on the Wormwise website can help. Interpreting Faecal Egg Counts.
Once the decision is made to drench, consider wisely what drench to use. You want an effective drench, ideally an oral combination product. Don’t forget to calibrate your drench gun and weigh a selection of your calves so you're giving the correct dose.
It is important to check that the drench you are using is actually working. This can be done by carrying out a drench check ten days after drenching. Collect another ten fresh poo samples and get a faecal egg count done. The result should be zero. If not, then you need to seek advice. If drenches used in the past have been ineffective, then newly introduced calves could already be ingesting infective L3 larvae that have developed from resistant parasites which have over-wintered from last seasons’ calves.
Minimising how many parasite larvae calves ingest is important to ensure good growth rates. Eating high levels of larvae each day causes a decreased appetite which means the calves consume less grass. If you are using the same calf paddock as last year and the year before that, then it is likely your calves are exposed to high levels of parasite larvae. Calves are like parasite factories and are the main source of larval contamination to your pastures.
There are a few options to consider for this season if your pastures are at risk of having high parasite burdens:
- Continuing supplements after weaning can help calves with the transition and an added benefit of ‘worm free’ feed.
- Ensuring calves are being rotationally grazed with minimum pasture covers of 1800kg DM/ha. Most parasite larvae sit in the bottom of the sward, so higher pasture covers lowers the risk of ingesting the infective L3 larvae. –
- Providing an alternative area for the calves. Could they graze a different part of the farm, ideally where cows have been? These areas will be less contaminated as adult cows shed less eggs and they are also great vacuum cleaners for parasite larvae.
If you drenched your calves with an effective drench but they are still not looking well, then don’t forget about coccidiosis. Even without the clinical signs of straining, bloody scours or ill-thrifty calves, subclinical disease could still be an issue. Just like parasite eggs, there can be a build-up of coccidia eggs in paddocks that are continually used for calves year on year. Calves probably had access to a coccidiostat while in the shed, so it could be worth continuing this to help with the transition into the paddock.
So, you’ve given their first drench, what next? Previously you may have drenched your calves every 28 days, whether or not they needed it. To decide when they need drenching again, measure faecal egg counts at 28 days after their previous drench. If the egg count is low, then follow the same strategy mentioned above and retest in one to two weeks rather than just drenching.
An egg count at 28 days post drenching is also a great measure of how contaminated their pasture could be. If this count is really high, then it is likely drenching will be beneficial. Strategies mentioned above will help to lower this contamination in the short term.
Watch out for the next article in the e-diary detailing long-term strategies to manage parasites in your calf rearing system.
More information
- Wormwise dairy factsheet (PDF, 1.2 MB)
- When should I start drenching my calves?
- Parasites in calves