Wormwise vet Mary Bowron urges calf rearers to think twice before reaching for the drench gun, as coccidiosis may be the cause of scours in young calves.
I've been asked for advice, so many times, the number is too high to count, so once more I'll repeat, not bowed by defeat, my sermon from high on the mount.
Young calves with scours! We blame upon worms – mostly without diagnosis. I think if we looked, we would probably learn – it's most likely just coccidiosis!
Garth Riddle, Bay of Islands Veterinary Services.
Over-treatment of young calves with worm drench is a common problem on New Zealand calf-rearing blocks whether this is commercial calf rearing operations, or on the 'calf paddocks' of large-scale dairy units.
Dramatic cases of disease caused by drench-resistant worms on young cattle blocks in recent years highlight that this is no longer just a theoretical possibility.
Our common reaction to the appearance of dirty tails and scours in calves in artificial rearing systems is to reach for the drench gun but some more thought into what product is going through that drench gun is warranted.
Baby calves mostly carry low numbers of gut worms until they have a functioning rumen. From one to three weeks of age, a calf is considered to be a single-stomached animal, absorbing all its nutrients from the abomasum. From four to about 10 weeks, the rumen is 'in development'; with a fully functioning rumen not considered to be present until 10-12 weeks of age.
Calves that have had access to grass from birth may sometimes have enough worms to require treatment in the later weeks of the rumen development phase.
However, the parasite that is of greatest challenge to calves at this time is generally coccidiosis. This parasite is different to gut worms and requires different drugs to treat it.
Most calf rearers will be aware that milk powder and meal should have a 'coccidiostat' product in them. These drugs do not kill coccidia outright; they just act as a brake on the parasites' development.
However, in situations of high coccidiosis challenge, this layer of protection is often not enough. A preventive dose of an oral anticoccidial product to calves at around the four-to-five-week age mark can make a huge difference to the appearance and performance
of affected calves, without the need to administer an unnecessary worm drench, or a combination product that may not be optimal for either parasite.
Veterinarians dealing with calf properties where high coccidiosis challenge is an issue, report that products containing Toltrazuril give the best outcome in this situation.
Other keys to keeping calves out of the coccidiosis danger zone include moderate, evenly sized mobs and great feeding (pasture residuals 1800kgDM/ha+ and plenty of meal or other concentrate). As for worms, the closer that calves are made to graze to the ground, the more parasites they will ingest.
So back to the worm question. Most calves will not need drenching until they have been digesting a significant amount of grass for three to four weeks, and some mobs may go a good deal longer than this if worm challenge is low.
To make that first drench decision, collect fresh poo samples from 10–15 calves (keep them separate initially) and get some faecal egg counts (FECs) done. If there are no eggs present, then no worm drench is needed just yet; retest in a couple of weeks; earlier if appearance or performance changes.
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. The section on interpreting FECs on the Wormwise website can help, see Interpreting Faecal Egg Counts. It is really important that calf FECs are interpreted alongside all other information available, especially how the calves are looking and how well they are being fed.
Ensure that you repeat the FEC test 10–14 days after that first drench, to make sure that it actually worked!
In summary, coccidiosis is the biggest cause of dirty tails and poor appearance in calves under about 10-12 weeks old. Not all coccidiosis treatments are created equal. Use faecal egg counts to help you decide when to do your first worm drench; what you are seeing may not be worms.
More resources
Find out more and see our B+LNZ Wormwise programme Hub.