Do yearling heifers need a pre-mating drench?

What’s the evidence for a pre-mating drench?

A worm drench pre-mating is standard practice on many farms. Heifers going to the bull for the first time should be given every chance to cycle early and conceive in their first cycle. But what evidence is there that a worm treatment prior to mating improves this?

Very little, as it turns out.  Most published research is for beef heifers and compares treatment of heifers at various intervals through their growing period, to heifers not treated at all. 

There is little to compare to the New Zealand situation, where we might want to know whether an extra treatment pre-mating has any benefit over and above the regular or semi-regular treatments given to heifer calves in their first year of life.

Luckily, the same factors that drive great reproductive performance in heifers are the same factors that ensure they have developed a good level of immunity to worms by mating.

Immunity to worms in young cattle begins to develop from around six months of age and progressively improves until somewhere between one and two years of age when cattle become quite resistant to re-infection with worms. The majority of well-conditioned, well-fed animals will then carry relatively few worms in their gut and be largely unaffected by the worm larvae that they take in while grazing.

How do I know if my heifers need a drench?

Unfortunately, there is no one test that will give a yes or no answer to this question. Faecal egg counts, if high, indicate that there could be production limiting parasites on board, but if counts are low, it is not a guarantee of low worm burdens. Liveweight, body condition and recent liveweight gain are better guides, and the ‘happy, cheeky heifer’ test with the eye-ometer is also a good piece to add to algorithm!

Fat, shiny individuals who are on great feed, growing well and are at or beyond their weight targets are unlikely to have a worm burden that will affect their mating performance, especially if they were given a drench treatment coming out of winter. In this situation it’s unlikely there has been much re-infection since that early spring treatment.

Drenching of yearling cattle coming out of a winter grass rotation, or after few weeks on grass post-crop or silage wintering, can be good insurance to remove any Ostertagia that have accumulated in the fourth stomach through this time. An injectable combination drench containing a ‘mectin drug is an ideal choice for this treatment.

Repeating a worm drench 4-6 weeks after this, in the name of removing worms pre-mating, is probably a waste of time and money in well-grown, well-fed heifers.

There may be some argument for re-treating individual animals that don’t quite tick the ‘fat and shiny’ box, and/or that are at risk of not hitting their liveweight target. 

Approaching a pre-mating worm treatment in this way allows a useful percentage of the mob to be left untreated and continue to provide some level of refugia in their grazing area. The untreated animals are able to contribute a low number of non-drench-resistant parasites back to the pasture, to help ‘dilute out’ the progeny of any resistant parasites that have been left behind in the treated heifers.