By Hannah Davies and Julia Shrubb, BEVA Sustainability Working Group

The sight of bright yellow ragwort flowers evokes strong feelings in many horse- and land-owners, and there is a wide-held belief that ragwort toxicity in horses is very common. Under the United Kingdom 1959 Weeds Act, ragwort is classified as an injurious weed and considered harmful to agricultural practices. Whilst there is no doubt that ragwort can, and does, cause fatal hepatopathy in horses, how commonly does this really occur? Is the narrative, demonising ragwort, fair?

The benefits of ragwort for biodiversity

Common ragwort (Seneci jacobaea) is a native and abundant plant which supports many invertebrates and other wildlife. As a late-flowering, nectar-rich wildflower, it provides an important food source for many insects such as bees, butterflies and moths, with field studies indicating that ragwort supports twice the abundance and diversity of flower-visiting insect species compared with non-injurious species at the same location (Balfour and Ratnieks 2022)

Ragwort provides the sole food source for at least 30 different insect species, 10 of which are sufficiently rare to be on the International Union for Conservation of Nature (IUCN) Red List. These include the clouded knot horn moth, the picture winged fly and the Sussex emerald moth.

Ragwort is also the foodplant of the Cinnabar moth, the distinctive black and yellow caterpillars of which can be seen covering the leaves in the summer months.

Lastly, its long flowering period makes it a major nectar source for many pollinating insects, crucial not only for biodiversity but also for pollination of our orchards and crops.

The risks of ragwort toxicity

However, as the equine profession is more than aware, ragwort poses a risk to horse health through the toxic pyrrolizidine alkaloids it contains.

The true prevalence of ragwort toxicity in horses is very hard to estimate. Surveys often report an apparent high prevalence but most of these are suspected, rather than confirmed, cases. A BEVA member survey in 2014 reported that, of 303 respondents, 124 (41%) BEVA members confirmed that they had seen at least one suspected case in the last year. A large BHS horse owner survey, with 13,963 respondents, reported that 19% (2712) respondents knew of a horse that had been suspected of ragwort poisoning.

Ragwort toxicity is only confirmed by the presence of megalocytocis, the hallmark of pyrrolizidine alkaloids, on histology of liver samples. Durham (2015) reported that, of 865 liver samples examined by 10 pathology services in England and Scotland between 2008 and 2013, 72 (8.3%) were found to have megalocytosis and therefore considered to be ragwort toxicity. From this data, it was estimated that there were typically 57 megalocytosis-positive samples diagnosed in English and Scottish laboratories per year. The total number of cases confirmed on biopsy are clearly an underestimate of the true prevalence of ragwort toxicity as many affected horses will not be biopsied, ante- or post-mortem.

In the BEVA member survey, only 30% of suspected cases were reported to be later confirmed as ragwort toxicity, with 122 confirmed cases reported in the last year. In the BHS horse owner survey, only 21% of the reported cases were confirmed, with 41 confirmed cases reported in the last year from nearly 14,000 respondents.

These numbers of confirmed cases have to be interpreted with caution due to sampling bias, common in surveys, and the factors influencing biopsy submission. The biopsy-confirmed cases are very likely to be underestimates of the true prevalence but may also indicate that the prevalence of ragwort toxicity lower than many horse owners fear.

Most horses will not eat fresh ragwort in extensive grazing systems due to the bitter taste, but with many horses on restricted grazing, due to high stocking densities and/or bodyweight control, the likelihood of ingestion increases. Horses are reported to be unable to detect and avoid ragwort in hay. It was reported 40 years ago that 50-200g of dried ragwort per kg bodyweight can be lethal for horses, but it is unclear over what period of time this needs to be eaten to be deadly (Goeger et al, 1982).

Striking the balance

Can ragwort be safely left on verges and other areas away from grazing animals and land used to make feed and forage? Ragwort seeds can be carried huge distances by the wind, but it is thought that most seeds drop near to the plant, with seeds able to survive in soil for at least 10 years. There is a misconception that ragwort is notifiable. However, acknowledging the benefits of ragwort for wildlife, The Ragwort Control Act of 2003 (an amendment to the Weeds Act 1959) does not seek to eradicate ragwort. Rather, it promotes a strategic approach to control the spread of common ragwort where it poses a threat to the health and welfare of grazing animals/production of feed or forage. The guidance places responsibility on the landowners to control the spread of ragwort if it poses a high risk, within 50 metres, of land used for grazing or forage production. Agri-environmental schemes in the UK are providing landowners with funding to enhance biodiversity and cater for wildlife as part of their ongoing land management in response to the global biodiversity crisis (DEFRA, 2020). Horse owners play an important role in this agricultural system, responsible for the management of 5% of all grazing land across the UK (Office for National Statistics, 2016).

The BHS survey suggests horse owners are often confused about their legal responsibilities regarding the control of ragwort. Almost all respondents (>97%) indicated that more activity is needed for various public policies including monitoring of ragwort by local councils and enforcement of ragwort control regulations.

Can the narrative surrounding ragwort balance the real, but low, risk to horses and other grazing animals with the benefits of the native plant to our ecosystem? It is imperative that horse owners and land owners don’t become complacent to the risks of ragwort toxicity, but at the same time understand the many benefits this beautiful native plant can provide.

References
Durham, A.E., 2015. Surveillance focus: ragwort toxicity in horses in the UK. The Veterinary Record, 176(24), p.620.
Balfour, N.J. and Ratnieks, F.L.W. 2022. The disproportionate value of ‘weeds’ to pollinators and biodiversity. Journal of Applied Ecology. 59 (5), pp.1209-1218
DEFRA. (2020). Biodiversity 2020: A strategy for England’s wildlife and ecosystem services. Department for Environment, Food and Rural Affairs.
Goeger, D.E., Cheeke, P.R., Schmitz, J.A. and Buhler, D.R., 1982. Toxicity of tansy ragwort (Senecio jacobaea) to goats. American Journal of Veterinary Research, 43(2), pp.252-254.
Office for National Statistics (2016) UK Natural Capital: Ecosystem accounts for farmland (Experimental statistics) [online]. Accessed 13/11/2023.
Ragwort policy and legislation Accessed 26/07/2024.

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