Recovery of strongylid third-stage larvae from herbage samples: standardisation of a laboratory method and its application in the field

Tracking gastrointestinal nematode risk on cattle farms through pasture contamination mapping

Gastrointestinal nematode (GIN) parasites in grazing cattle are a major cause of production loss and their control is increasingly difficult due to anthelmintic resistance and climate change. Rotational grazing can support control and decrease reliance on chemical intervention, but is often complex due to the need to track grazing periods and infection levels, and the effect of weather on larval availability. In this paper, a simulation model was developed to predict the availability of infective larvae of the bovine GIN, Ostertagia ostertagi, at the level of individual pastures. The model was applied within a complex rotational grazing system and successfully reproduced observed variation in larval density between fields and over time. Four groups of cattle in their second grazing season (n = 44) were followed throughout the temperate grazing season with regular assessment of GIN faecal egg counts, which were dominated by O. ostertagi, animal weight and recording of field rotations. Each group of cattle was rotationally grazed on six group-specific fields throughout the 2019 grazing season. Maps and calendars were produced to illustrate the change in pasture infectivity (density of L3 on herbage) across the 24 separate grazing fields. Simulations predicted differences in pasture contamination levels in relation to the timing of grazing and the return period. A proportion of L3 was predicted to persist on herbage over winter, declining to similar intensities across fields before the start of the following grazing season, irrespective of contamination levels in the previous year. Model predictions showed good agreement with pasture larval counts. The model also simulated differences in seasonal pasture infectivity under rotational grazing in systems that differed in temperature and rainfall profiles. Further application could support individual farm decisions on evasive grazing and refugia management, and improved regional evaluation of optimal grazing strategies for parasite control. The integration of weather and livestock movement is inherent to the model, and facilitates consideration of climate change adaptation through improved disease control.

Assessment of the impact of plant species composition and drought stress on survival of strongylid third-stage larvae in a greenhouse experiment

Grazing livestock is always exposed to infective parasite stages. Depending on the general health status of the animal, the farm management, environmental conditions and pasture exposure, the impact ranges from non-affected to almost moribund animals. The greenhouse experiment was performed to investigate how climatic changes and plant composition influence the occurrence/survival of strongylid third-stage larvae (L3) on pasture. Ten different types of plant species compositions (eight replicates for each) were inoculated with approximately 10,000 Cooperia oncophora L3. The different plant compositions can be assorted to two groups: without legume content and with legume content (52-62% legume content). Half of the replicates were watered adequately, while the other half was hold under drought stress (DS), mimicking longer dry periods. During the DS cycles, the respective containers were not watered until they reached the wilting point. Grass samples were taken 1, 4 and 6 weeks after inoculation, soil samples were taken only once after 6 weeks and all samples were examined for occurrence of L3. After the second DS cycle, the number of L3 present on herbage samples was reduced significantly. The higher the legume content of the pasture composition, the higher is the L3 occurrence on pasture. Independent of the watering scheme, the soil served as the most important reservoir with consistently higher numbers of L3 in the soil compared to herbage.