Dairy herd drenching

The question of whether to drench adult dairy cows raises some interesting points due to the conflict between drench sustainability and economic advantage.

From a purely economic viewpoint there are often significant financial returns to be gained from drenching the herd. However, from a drench resistance viewpoint, blanket (whole herd) treatment of adult cows is not recommended.

Drench resistance is developing every time a drench is used, as no drench kills 100% of worms in an animal. The surviving worms, carrying resistant genes, increase as a proportion of the entire population and as they breed with each other more resistant genes develop. At the same time, the challenge to the immune system required to counter parasites can cause production losses in some situations and anthelmintic treatment may lead to improvements in milk production, body condition and reproduction.

One way to address this dilemma is to treat selectively rather than blanket. This way you can target those individuals in the herd that are most likely to give the best financial returns.  The undrenched proportion of the herd then act as the source of refugia which helps to slow the rate of development of drench resistance in a herd. 

The balance of treated versus untreated animals can be shifted either way depending on the importance of drench sustainability versus financial return from treatment.

The individuals in the herd that are most likely to get the most benefit from treatment are the younger cows and low body condition score (BCS) cows. Heifers give the best return through reproductive efficiency. In a New Zealand trial, heifers were found to benefit most reproductively from drenching at calving, with significantly more treated heifers getting in calf at first insemination, bringing the average calving date 12.9 days earlier for treated versus untreated heifers. Low BCS cows give the best return by reducing the amount of energy that is being used to mount an immune response and directing it to putting on condition. 

There are likely to be some herds where blanket treatment is the best decision. These will be farms that have one or a combination of the following factors:

  • Pastures that were dominated by young stock prior to dairy conversion - as they may take several years for the numbers of worm larvae to fall.
  • Farms where calves are grazed on the dairy platform after weaning.
  • Lots of young cows in the herd, especially if under nutritional stress.

Unfortunately, diagnostic tests are of less value in adult stock than in calves. Faecal egg counts and pepsinogen levels are reasonably reliable in younger animals but in adults results are not so easily interpreted.

The other big question with drenching dairy herds is timing. When is the best time to treat? As increased milk production will decide the economics of treatment, it makes sense to treat at calving or early in lactation to capture the greatest benefit.  This would also be good timing to benefit reproductive performance.

Also bear in mind where cows have grazed over the winter – a stint out grazing on a run-off or grazing property that is dominated by young stock for the rest of the year could load cows up with extra worms that could best be removed by a quarantine treatment on arrival back to the milking platform. Conversely, if they are, for example, heading to a crop-feeding situation with the expectation of significant winter weight gain, it may be best to treat them prior to this.

We have a range of competitively priced anthelmintic products available. Most pour-on, nil milk-withhold anthelmintics also have the additional benefit of controlling lice. Contact us if you would like to discuss anthelmintic treatment of your dairy herd in more detail.

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