Article Summary: Match rest periods to larval lifespans, leave a taller residual, track faecal egg counts, and drench only the light or high-count animals; layering these pasture-based tactics with climate monitoring and resilient forages keeps worm pressure low, drug efficacy high, and milk or weight gains on target in temperate grazing systems.
Parasite control is part of the routine that keeps dairy and beef herds on track. In temperate regions—where mild temperatures and steady rainfall are the norm internal worms can build up quickly. A single wet month may be enough for larvae to spread from dung pats across an entire paddock, and heavy burdens can reduce growth rates or milk yield before you notice any outward signs. Rather than relying on frequent whole‑mob drenches, you can combine grazing management, monitoring, and strategic treatment to dial worm pressure down to a level that animals can cope with. The following guide shows how each piece of the puzzle fits together on a rotationally grazed farm.
Understanding Pasture Worm Dynamics in Temperate Zones
Gastrointestinal worms such as Ostertagia, Cooperia, and Trichostrongylus have life cycles that mesh neatly with temperate weather patterns. Eggs pass in the dung, hatch, and develop through two larval stages inside the dung pat. The infective third‑stage larvae (L3) migrate onto surrounding leaf material, ready to be eaten.
In a cool, damp spring, an L3 larva can survive for two to three months, sitting within the bottom few centimetres of the sward. Warmth speeds that cycle up—summer larvae may last only four to six weeks—but every spell of rain refreshes their environment. If stock return to the same paddock before most larvae have died, the cycle accelerates and each rotation layers fresh contamination on top of what is already there.
Anticipating high‑risk periods
Keep an eye on local temperature and rainfall records—either through your own weather station or a nearby bureau site. When daytime maxima sit between 10 °C and 25 °C and soil moisture stays above 15 %, conditions are near‑perfect for larval survival. You can then build extra rest days into the rotation or plan to graze older, more resilient cattle while young stock remain on a safer break. A grazing chart that logs climate alongside paddock use becomes a simple forecasting tool over time.
Grazing Interval and Sward Height as Control Tools
Rotational grazing gives you two free levers: how often animals re‑enter a paddock and how short they graze it. Both influence how many larvae are swallowed.
Rest periods that outlast the larvae
Work out a base rest period by dividing current pasture height by the average daily growth for that month. If autumn growth averages 30 kg DM/ha/day and you want to graze at 2,500 kg DM/ha, a rest period of roughly 80 days may be needed. Compare that with local data on larval survival if L3 larvae rarely live beyond eight weeks by May, you will step back onto that paddock when most infective larvae have already died.
During spring flush your pasture may leap ahead 70 kg DM/ha/day, meaning cows come back far sooner. Counter this shorter interval by increasing grazing pressure (higher stocking rate for a shorter occupation) so that animals move before larvae climb higher up the leaf.
Staying above the larval zone
Most L3 larvae live in the lower 5 cm of herbage where humidity remains high. Leaving a residual of 7–8 cm after grazing keeps mouths, tongues, and lower jaws above the danger zone. That extra cover cushions soil moisture and supports regrowth, so the rule serves both parasite control and pasture quality.
Practical tip: If you struggle to judge height by eye, cut a 10 cm length of 32 mm PVC pipe, paint the first 3 cm red, the next 5 cm amber, and the top 2 cm green. Drive it into the grazed sward and aim to leave the green showing.
Targeted Monitoring: Faecal Egg Counts and Pasture Larval Checks
Why numbers matter
Clinical signs scouring, bottle‑jaw, or weight loss only emerge when worm burdens are already high. Faecal egg counts (FEC) act as an early warning, helping you intervene at a lower, animal‑friendly threshold. Laboratories report results as eggs per gram (epg) of dung; a rise above 200 epg in calves or 100 epg in yearlings often warrants closer attention.
Building a routine that fits your rotation
Collect fresh samples from six to ten animals in the same age group every six to eight weeks. Choose paddocks that differ in rest period or moisture to spot problem areas. Graph the results next to paddock history: spikes often trace back to the same “hot” fields.
Pasture larval cultures, though less common, add another layer. By seeding grass clippings in a petri dish, labs count live larvae present on the sward. Including two or three paddocks in late spring and again in autumn indicates whether your rest periods are long enough.
Interpreting results in context
A single high FEC after a drench may signal resistance, whereas a gradual upward trend across two rotations can simply reflect favourable weather. Overlay the data on rainfall and temperature records before deciding on action; this guards against knee‑jerk treatment.
Strategic Treatment Without Over‑Reliance on Drenches
Anthelmintics remain valuable, but worms adapt swiftly when every animal receives the same product at the same time. Integrated management focuses chemical use where it has greatest impact.
Setting action thresholds
Agree on FEC trigger points with your veterinarian—for example, 300 epg in weaned calves or 150 epg in yearling heifers. Combine those figures with growth or milk yield records; treat if egg counts exceed the threshold and average daily gain slips below target. In adult cows, milk recording often shows a dip before clinical signs appear.
Targeted selective treatment (TST)
Rather than yarding an entire mob, draft off the lighter or scouring individuals and drench only them. Leaving 10–20 % of animals untreated maintains a refugia population worms still susceptible to the drench slowing the march toward resistance. Record tag numbers and weights so that the same low‑weight animals are not repeatedly drafted off.
Rotation and combination of activities
Where resistance is already suspected, swap from single‑active drenches to a dual combination (e.g., levamisole + abamectin) or rotate between chemical groups each season. Follow label rates closely: under‑dosing selects for resistant larvae faster than any other error.
Short checklist (one of the few dot‑point sections):
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Check calibration of drench gun at the start of each treatment day.
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Weigh or draft to weight bands; avoid dosing off visual estimate alone.
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Record paddock treated, product, and date in your farm management app.
Future‑Proofing: Breeding and Biodiverse Pastures
Selecting cattle with resilience
Not every animal responds to worm challenge in the same way. Estimated Breeding Values (EBVs) for parasite resistance in beef breeds and Holstein or Jersey dairy indices are gradually becoming available. Choosing sires in the top 20 % for worm resistance can lift a herd’s natural defence over several generations. Keep replacement heifers from dams that required fewer drenches your own records are a storehouse of genetic clues.
Using forages that help the animal help itself
Plants rich in condensed tannins birdsfoot trefoil (Lotus corniculatus), sainfoin, and (to a lesser degree) chicory can lower faecal egg output by 20–40 % when they make up at least one‑third of the diet. Tannins bind some of the protein that larvae rely on to establish in the gut, while chicory’s erect habit encourages cattle to graze higher, skimming above the larval zone.
Adding plantain and white clover widens seasonal growth patterns and improves ground cover, prolonging the interval before cattle graze down to risk height. A more diverse root network also enhances soil drainage, reducing the damp microclimate that larvae favour.
Aligning biodiversity with production goals
Mixed swards change pasture management rhythms. Chicory bolts in its second year and benefits from a tighter spring rotation, whereas birdsfoot trefoil needs lighter grazing to survive. Map out a separate rotation plan for diverse paddocks, noting flowering windows and regrowth rates. Although complexity increases, the reward can be fewer drenches, higher live‑weight gain, and steadier growth through summer slow‑downs.
Pulling the Plan Together
Integrated parasite management works best when monitoring, grazing, and treatment are recorded in the same place. Your pasture wedge or grazing planner already tracks rotation length; add FEC results, drench events, and weather notes. Over a single season you will spot which paddocks, age groups, or weather patterns lift worm risk. After a year or two, those observations turn into clear rules such as “extend rest period to 65 days from mid‑April” or “weigh calves after the third rotation and TST if average daily gain drops below 0.7 kg”—that reduce chemical use and keep performance on target.
Rotational grazing gives you the steering wheel, but steering works only when you look ahead. By watching climate trends, recording grazing intervals, and treating only where numbers justify it, you let cattle graze most days without the hidden tax of parasites. The approach is incremental, yet the benefits compound: healthier animals, slower drench resistance, and a pasture system that looks after itself.
Until we meet again, Happy Biosecurity!
- The Dedicated Team of Pasture.io, 2025-05-13