A significant challenge for sheep farmers has always been gut worm infections that damage the digestive systems of lambs, resulting in poor growth and reduced economic and environmental sustainability. The worms have developed resistance to the anthelmintic drugs used to control them, necessitating alternative control strategies. One such strategy is grazing management. Therefore, SEFARI Researchers are investigating if what, and how, sheep graze could influence worm control strategies.
Sheep grazing in one of the set-stocked paddocks (Credit: Andrew Kelloe)
Control of gut worms in sheep is a priority for farmers
One of the biggest challenges faced by sheep farmers is the impact of gut worm infection on the growth of their animals.
Lambs consume worm larvae while grazing, which develop into adults in the sheep’s stomach and intestine, causing damage that prevents sheep digesting and absorbing nutrients effectively. Adult worms produce eggs which are shed in the dung, and then hatch and develop into larvae on the pasture ready to infect a new host animal.
The main method of control of these worms over the past several decades has been chemical anthelmintic drugs (wormers). Unfortunately, worm populations have begun to evolve resistance to these anthelmintics, such that they are no longer as effective as previously. Continued wide-scale use of these drugs will only deteriorate this situation. Alternative strategies for worm control, that do not rely heavily on anthelmintics, are therefore needed.
At Moredun Research Institute, we are conducting a long-term field trial assessing how different grazing strategies may improve worm control and reduce anthelmintic use. This study is part of the Scottish Government RESAS-funded project Improving livestock productivity and sustainability through management and genetics.
Manipulating what animals eat
Traditional pasture is composed largely of perennial rye grass, which is highly palatable, nutritious, and resilient to grazing and disease. However, it also creates a favourable environment for the development of worm larvae and represents a limited diet in terms of micronutrients and trace elements.
“Biodiverse” seed mixes contain a greater variety of plants, including legumes (e.g. red clover, bird’s-foot trefoil) and herbs (e.g. yarrow, ribwort plantain) – see Figure 1.
Figure 1 - The seed mix used in our experiment contains a variety of plant species with different properties, including protein-rich legumes and herbs with potentially anthelmintic effects.
Such a seed mix could offer livestock higher protein and micronutrient content, as well as having potential natural anthelmintic properties. The nutritional benefits of biodiverse pasture, plus their potentially anthelmintic natural compounds, may improve animal nutrition and better equip them to fight the effects of worms. Thus, we might expect animals on biodiverse pasture to have higher growth rates, more effective immune responses, and lower worm burdens and faecal egg counts (FEC). These ideas have, however, never been formally tested.
Furthermore, there may be additional environmental benefits, including a deeper root structure that fixes carbon more effectively, facilitates drought resistance, and improves soil structure, and the biodiverse plant species will likely attract more pollinators.
Manipulating how animals eat
Lambs are traditionally turned out onto pasture and graze the same fields until they are the correct weight for slaughter. Such grazing pressure can deplete the quality and quantity of the pasture, and the fact that sheep remain in the same field means that faeces (and hence worm larvae) will build up through the grazing season.
An alternative to this “set-stocked” strategy is “rotational” grazing, where a field is divided into smaller paddocks, and animals are moved between paddocks every few days. With enough paddocks, this means that lambs are constantly grazing fresh pasture with little parasitic worm larval contamination, and by the time they are rotated back to the first paddock, the pasture will have regrown. Thus, we might expect rotationally grazed animals to be better-nourished and grow faster.
Rotational grazing paddocks at Firth Mains Farm - the sheep have recently moved from the central paddock, where the grass is noticeably shorter, into a new paddock (Credit: Phoebe Beal)
However, what happens to the parasites under rotational grazing is less clear. We know that worms develop from eggs to infective larvae in 10 days under ideal conditions, and although there is high die-off rate on pasture, we know some larvae can survive for a year. So, it is possible that there may be more infective larvae on rotationally grazed pasture compared to set-stocking, due to less grazing in the early stages on larvae maturity. Our experiment will enable us to determine whether this is the case or not.
The experiment
At Moredun's Firth Mains Farm, we are testing different combinations of ‘what and how’ animals eat, to monitor the effects on their growth rates, need for anthelmintic treatment, immune responses, and worm faecal egg counts.
The experiment is being replicated in 2023, 2024, and 2025 so we can see how the different strategies perform across different years. In each year, 120 lambs will be kept in groups of 10 in 12 fields. 3 fields will be rye grass and the animals set-stocked; 3 will be rye grass and rotationally grazed; 3 fields will contain biodiverse pasture and be set-stocked; and 3 will contain biodiverse pasture and be rotationally grazed – see Figure 2.
Figure 2 - The experiment will test the effects of managing animals under 4 different scenarios: lambs are grazed on the same rye grass pasture for the whole season (top left); are rotated round smaller rye grass paddocks (top right); are grazed on the same biodiverse pasture for the whole season (bottom left); or rotated round smaller biodiverse paddocks (bottom right). In the rotational treatment, lambs are moved between paddocks every 3-4 days. Figure created with BioRender.
This approach will enable us to test how advantageous biodiverse pasture and rotational grazing are individually, and whether combining them can have a greater effect than any one of the two strategies alone.
In each year, lambs are being monitored every two weeks. They are blood sampled to measure different markers of immunity and total protein levels as an indication of nutritional state. Lambs are also weighed, and any showing signs of poor growth are given an appropriate anthelmintic treatment. Faecal samples are collected for worm faecal egg counts, nematode species identification and for genetic sequencing to assess intestinal bacterial diversity (the “gut microbiome”). This detailed data set will enable us to learn how the different grazing conditions affect lamb growth and their ability to cope with parasite infection, as well as to look in detail at some of the underlying biological mechanisms.
In the foreground, team members are operating the weigh crate, collecting faecal samples, subsampling for different purposes, and recording information, while in the background a team member is organising blood tubes and needles for the bleeding team (Credit: Andrew Kelloe)
A companion study is being conducted by colleagues at the James Hutton Institute’s Glensaugh Farm, focusing more on the consequences of these grazing strategies for the pasture ecosystem, including plant species diversity and soil health.
Early results and outlook
The first year of data is being analysed, but early indications are that there are differences between the groups regarding their weight gain, immune responses, and gut microbiomes. We expect these differences to become more pronounced with time as the biodiverse pasture establishes.
Repeating the experiment across several years will provide an indication of how long farmers can expect to wait to see a difference with these strategies, as well as showing us how variation between years (e.g. in weather conditions) may affect the effectiveness of these strategies. So please watch this space!
Sampling for the trial is enjoyable but very intensive and involves a large team (Credit: Andrew Kelloe)
Acknowledgements: Thanks to Jade Duncan for co-ordinating the fieldwork at Firth Mains Farm and to everyone who has mucked in with sampling on this project so far, including Leigh Andrews, Phoebe Beal, Camero Cunnea, Eilidh Geddes, Gillian Mitchell, Aiden Petrie, Rebecca Ross, Millie Scott and Sarah Thomson. Thanks also to the Moredun Bioservices team, particularly Stephen Anderson, Steven Fairlie and Callum Wight for their hard work in preparing the farm for this project. Finally, we’d like to thank Nikki Yoxall and Emily Grant for their advice on study design and grazing management techniques.
Written by Adam Hayward, Fiona Kenyon, Philip Skuce (Moredun Research Institute)