Feeding behaviour of sheep grazing on pastures with a high density of tansy ragwort – results of a two-year study on ingested pyrrolizidine alkaloid doses, and the transfer of pyrrolizidine alkaloids into muscle meat and liver tissue

The growth of tansy ragwort (Jacobaea vulgaris) in grasslands is of concern to farmers and authorities because this plant produces pyrrolizidine alkaloids (PAs). Exposure to PAs is associated with adverse effects for humans and animals. However, it has been proven that sheep on species-rich pastures voluntarily ingest large amounts of tansy ragwort and tolerate it [1, 2]. Here, we investigated whether the ingested quantities of tansy ragwort pose a risk to humans due to the potential transfer of PAs into food of animal origin. Methods: The trial was conducted over two years (2020-2021) with 70 sheep on a pasture in northern Germany where the ragwort density (estimated portion of biomass) was between 5 and 70 % in 2020 (mean 26 %, median 15 %) and between 0 and 50 % in 2021 (mean 18 %; median 15 %), depending on the season. Every six weeks (grazing periods), seven animals were slaughtered including of seven animals of two control groups (sheep slaughtered before the onset of grazing in 2020 and 2021, respectively. Tissue samples of both liver and muscle meat (diaphragm pillar) were taken from 34 individuals in 2020 (one died) and 35 animals in 2021 and analysed for their PA content by LC-MS/MS. To estimate the transfer proportion of PAs into the tissues, 182 ragwort plants from a comparable site were collected and analysed for their PA content. Results: The average total content of PAs in dry mass of tansy ragwort was 0.15%. Jacobine, erucifoline, seneciphyl line and senecionine had the highest share (together for about 75 %) of the total PA content. No changes in the qualitative PA composition in tansy ragwort was observed over the grazing season. The daily intake of ragwort per sheep during grazing season was determined as fresh matter and ranged from 0.2 to 4.9 kg/sheep/day. This corresponded to PA doses from 1.6 to 24.3 mg/kg bw/day, assuming an average weight per sheep of 45 kg for the first year and of 60 kg for the second year. At the end of the first grazing season (end of October 2020), the remaining sheep had ingested a total of 361 kg ragwort fresh matter per sheep, or 1.6 kg of ragwort dry matter/kg bw, corresponding to a total PA intake of 2038 mg PA/kg bw. At the end of 2021, even a total PA dose of 2722 mg/kg bw had been ingested. PA doses ingested during the second grazing period in June, corresponded to the maximum biomass of eaten flowering shoots of ragwort in both years, respectively. Only jacoline, which accounted for only 8 % of the total PA amount in ragwort, was detectable in 30 % of liver and muscle meat samples. Jacoline concentrations ranged between the limit of detection (LOD) and 9.0 μg/kg in liver and between LOD and 2.5 μg/kg in muscle meat. Conclusions: Based on the mean jacoline contents, and in combination with the EFSA-derived benchmark dose of 237 μg PA/kg bw, it was calculated at which consumption level of liver and muscle meat a critical margin of exposure (MOE) of 10,000 or more would be reached. For a 60 kg person, a regular daily consumption of 1.1 kg of liver or 5.5 kg of muscle meat per day was determined. These data indicate that although sheep ingested large amounts of ragwort, the exposure to PAs from food of animal origin is low.