To Drench or Not to Drench?

Investigating Pre-Lambing Worm Treatment in Primeline Maternal Ewes

By Emma Kuchel  |  Supervisors: Prof. Michael Friend & Dr Shawn McGrath

Internal parasitism costs the Australian sheep industry an estimated AUD $436 million per year in combined production losses and treatment costs — yet the case for routine pre-lambing drenching in mature ewes remains surprisingly unresolved.

What Is This Research About?

This honours project examines whether administering a pre-lambing anthelmintic drench to mature twin-bearing Primeline Maternal ewes — four weeks before lambing — makes a measurable difference to their worm egg counts and production performance across the lambing period through to weaning.

A cohort of 330 twin-bearing ewes at a commercial property will be split equally into a drenched treatment group and an untreated control group. Faecal egg counts (FEC), live weights, and body condition scores (BCS) will be collected at four key time points: pre-lambing, start of lambing, two weeks into lambing, and four weeks post-lambing.

Why Is This Period So Critical?

Around lambing, ewes undergo what scientists call the periparturient relaxation of immunity (PPRI) — a natural suppression of their immune defences that causes a sharp rise in worm egg shedding. This is particularly pronounced in twin-bearing ewes, who are already under substantial nutritional pressure carrying two foetuses.

When worm burdens go unchecked during this window, the consequences can ripple through an entire flock: reduced ewe body condition, lower milk production, lighter lamb birthweights, and heavy pasture contamination that exposes young lambs to nematode challenge early in life.

Why Does This Research Matter?

Despite the scale of the problem, the evidence base for routine pre-lambing drenching in mature ewes — particularly composite maternal breeds — is surprisingly thin. Most published research focuses on Merino or New Zealand flocks, and no study to date has examined worm-production performance associations specifically in the Primeline Maternal breed. Compounding this, there is growing concern that routine periparturient drenching is one of the most potent drivers of anthelmintic resistance in Australian flocks. Pre-lambing treatments have been shown to accelerate resistance development, yet many producers continue to drench all ewes as a matter of course.

What Are the Expected Benefits for Producers?

The findings of this project are designed to deliver practical, breed-specific guidance for producers managing Primeline Maternal ewes. Specifically, the research aims to answer whether:

• Pre-lambing drenching meaningfully reduces FEC and improves ewe BCS and live weight — or whether ewes in good condition maintain performance without it.

• Lamb birthweights, growth rates, and survival to weaning differ between progeny of treated and untreated ewes.

• A targeted, condition-based approach to pre-lambing treatment could replace routine whole-flock drenching — reducing chemical inputs and slowing the development of anthelmintic resistance.

For producers investing in Primeline genetics and optimising returns from twin-bearing ewes, this evidence could meaningfully reshape how parasite management decisions are made during one of the most production-critical periods of the sheep calendar.

A Note on Resistance

Anthelmintic resistance is already widespread in Australian sheep flocks, with resistance to multiple drug classes documented across the country. Every unnecessary treatment applies selection pressure. This project directly addresses whether producers can confidently withhold pre-lambing treatment in mature ewes in good body condition — a question with significant long-term implications for the sustainability of parasite control in Australian sheep systems.

What Happens Next?

Data collection is underway across the 2026 lambing season. Results will be analysed using repeated measures statistical methods, with findings expected to be available later in the year. A summary of outcomes will be shared with the broader industry once the thesis is complete.