Context

elminth (worm) infections are a hugely significant drain on production efficiency in grass-based ruminant systems such as those prevalent in Northern Ireland (NI). At present the control of parasitic helminths relies upon anthelmintics, but their widespread application at whole-herd level is leading to anthelmintic resistance (AR), reducing their effectiveness. 
Problems with AR are well documented globally on sheep farms and increasingly throughout the cattle industry. In Northern Ireland, AR was detected on the majority of sheep farms, while no data is yet available for cattle. A recent study in the Republic of Ireland found poor efficacy on most beef and dairy farms, including all 16 farms on which ivermectin was assessed (with both Cooperia and Ostertagia surviving).
To tackle AR successive EU-funded research projects have investigated new strategies for sustainable worm control, primarily through changes to routine whole-herd / whole-flock treatment protocols . These focus on targeted, selective treatment (TST): 
• Targeting anthelmintics at the right time to maximise epidemiological benefits, and avoid unnecessary treatments; and/or 
• Leaving a proportion of the flock or herd untreated, making use of the fact that most worms are concentrated in a few individuals, and removing them will have large effects on worm transmission and herd health while reducing the number of treatments needed.
A number of research projects have investigated this TST approach in recent years with the conclusion that they could be suitable for use on commercial farms but success will largely depend on its on-farm practicality and wider economic benefit which has not yet been evaluated in Northern Ireland.