Authors

NH Henry¹; A Aubry²; A George¹; C McFarland¹; F lively²; K Theodoridou¹; ER Morgan¹;  ¹ Queen's University Belfast, UK;  ² Agri-Food Biosciences Institute, Hillsborough., UK

Discussion

Background and introduction
Gastrointestinal nematodes (GIN) are a significant cause of disease in grazing ruminants causing reduced health and productivity. Anthelmintic resistance is a widespread issue in GIN and a major issue faced by the livestock industry is the necessity to control parasites effectively while reducing chemical anthelmintic use, to ensure continued drug efficacy and to reduce negative environmental consequences. The negative impacts of parasites and the requirement for anthelmintic treatment can be reduced by grazing multi-species swards (MSS), which can lower parasite burdens while enhancing nutrition. Although the effects of individual plant species on parasites, and the impacts of MSS on animal performance have been assessed (e.g., Marley 2003, 2006; Athanasiadou 2004), there is a lack of information on the epidemiological consequences of grazing on MSS and how to design MSS grazing platforms to maximize parasite management benefits.
Materials and Methods
Parasite infections were compared over 5 months in 3 groups of lambs (n=100, 100, 100) rotationally grazing either perennial ryegrass with white clover (PRG), PRG with white clover additionally enriched with red clover, ribwort plantain and chicory (MSS) or grazing alternately on PRG and MSS (50-50). Nematode faecal egg counts (FEC) monthly, pasture larval counts (PLC) (Molento 2016) at three time points, and end-of-season abomasal worm counts were measured. To evaluate the epidemiological consequences of reductions in parasite burdens through decreased onward infection pressure, FEC data were entered into a mechanistic predictive model of nematode population dynamics (Rose 2015), extended to account for sheep movement between fields (McFarland 2022). The same model was used to simulate different rotation intervals under current and projected future climates across the UK, to maximise impacts of reduced FEC and to evaluate the feasibility of optimising both epidemiological benefits and grass / MSS utilisation.
Results
No significant differences in pasture larval count were observed between the PRG, 50-50 and MSS paddocks (p>0.05) but there was a significant difference between the 50-50 group and the PRG group (p<0.05) in FEC on the final sampling date and the end of season abomasal worm counts. As FEC was not reduced in the MSS group, modelling did not predict reduced infection pressure on MSS fields. Simulations showed that rotational grazing did not have significant benefits for GIN infection pressure over set-stocking the same area of land, unless re-grazing intervals were extended beyond realistic limits. Climate change scenarios tended towards more rapid development of infective larvae at pasture, and more rapid decline in infectivity, affording opportunities for greater control through rotational grazing provided residence periods were kept below one week.
Conclusions
Although MSS may have offered benefits for performance of grazing lambs under natural GIN infection, these could not be attributed to antiparasitic activity and might be primarily nutritional. Applying modelling alongside data collection can help predict the seasonal risk of transmission of GINs, and these models can be extended to optimise the implementation of this novel combination of control strategies.