Assessing the impacts of long-acting drench products

This project investigated the potential effect of long-acting drench products on the rumen microbiome, the effect of parasites on ewe growth, Body Condition Score and fecundity over the summer-autumn period.

This project investigated the following:

  • the potential effect of long-acting drench products on the rumen microbiome.

This was done using 300 ewes in total that were given one of three different treatments (long-acting drench product, controlled release capsule and untreated control). The rumen microbiome of these animals was sampled three times throughout the trial 

  • the effect of parasites on ewe growth, Body Condition Score (BCS) and fecundity over the summer-autumn period.

This was done by assessing 300 ewes (as above) and three measures were taken: 

  • Ewes were weighed and their BCS were assessed at days 0, 36, 78, 182 and 320. 
  • Faecal samples were recovered from a sample of ewes from each treatment group at days 0, 36, 78, 182 and 320 to determine the parasite load, or worm burden. 

Pregnancy status was determined by scanning. During lambing, all dead lambs were collected, and weaning lambs weighed.  


Long-acting drench products can have activity against organisms other than parasites, for example fungi and bacteria. The effects of long-acting drench products on the microbes within the rumen (rumen microbiome) are largely unknown and may influence such things as animal performance and production of green-house gases. 

Long-acting drench products are administered as a pre-lambing treatment of ewes by about 80% of sheep farmers in New Zealand. Pre-lamb drenching is done as farmers expect that by treating ewes, both ewes and their lambs will be heavier at weaning and ewes will be in better condition. In particular, it is widely believed by farmers and veterinarians that poor condition in ewes (low BCS) is caused by parasites and that by focusing drench treatments on these animals, it will result in significant production gains.

Key results

  • Farmers may not be making money from pre-lamb drenching every year.  
  • If resistant parasites are already on your farm, then using long acting anthelmintics will make the issue worse.  
  • To improve profitability, considering ways of improving lamb drop and lamb survival is likely to be more important than drenching.  
  • There was a change in the rumen microbiome of drenched vs undrenched animals. This was the first study to investigate the influence of long-acting anthelmintics on the rumen microbiome and it provided baseline information for future studies to build upon. Future studies are needed to understand what the observed changes in the microbiome of the treated animals in this study may mean for ewe performance as well as methane production.

Benefit to farmers

This study suggested that a financial benefit resulting from the administration of these long-acting drench products pre-mating is unlikely. 

These results are consistent with earlier studies which indicate that 1) parasites are not a major cause of low BCS in ewes on New Zealand farms and 2) studies showing that the production responses to the use of long-acting drench products are, to an extent, temporary and tend to decline or even disappear completely after the products have expired. 

Timeline and investment

This was a 32-month project. B+LNZ investment was $190,000 for the duration of the project.


Popular press

Scientific manuscripts


This work was led by AgResearch and co-funded by AgResearch and B+LNZ with in-kind support from Pāmu.

Lead Scientist: Dave Leathwick.
B+LNZ point of contact: Suzanne Keeling.