Grazing Ecology of Sheep and Its Impact on Vegetation and Animal Health in Pastures Dominated by Common Ragwort (Senecio jacobaea L.)-Part 1: Vegetation.

Abstract

Simple SummaryCommon ragwort (Senecio jacobaea L.) is a worldwide established plant containing toxic pyrrolizidine alkaloids (PA), which can lead to liver cirrhosis in livestock, especially cattle and horses. Controlling ragwort, particularly in conservation areas, is difficult. As sheep seem more resistant to PA, sheep might be a natural defense against the observed spread of ragwort. We tested this in a two-year study on a ragwort-rich pasture with 12 sheep/hectare from May to October 2020–2021. We addressed three questions: (1) To what extent do sheep voluntarily ingest ragwort? (2) In what respect do feeding behavior and nutritional parameters correlate? (3) What impact does grazing have on the yield proportion and number of flowers of dominant plants? We showed that sheep preferred ragwort without any harmful effects. The more ragwort was available and the more it contained sugar, the higher the amount ingested by the sheep. Ragwort accounted for a significantly lower yield proportion of ingested biomass in the second year even though its number of flowers doubled. The yield of biomass of other herbs increased. From the point of view of animal health and nature conservation, sheep grazing on ragwort might be an option to reduce the amount of ragwort in pastures.Species-rich pastures naturally contain potentially toxic plants such as common ragwort (Senecio jacobaea L.), whose pyrrolizidine alkaloids (PA) impose a risk, mainly for cattle and horses. Although in vitro studies showed detoxification capacity of PA in sheep, few field data are available to ascertain whether grazing sheep can both tolerate and reduce ragwort. In a two-year study in a ragwort-rich pasture with a stocking density of 12 sheep/hectare, we documented (1) the extent of voluntarily ingested ragwort, (2) the correlation of nutritional parameters and feeding behavior, and (3) the impact of grazing on the yield proportion and number of flowers of dominant plants. Every six weeks the vegetation underwent a botanical survey and a chemical analysis. Sheep continuously ingested ragwort between 1.2 and 4.9 kg (2020) and 1.0 and 2.2 kg (2021) per individual per day without any impact on animal health. The more biomass ragwort produced, the more it contained sugar (r = 0.59–0.74), and the more sheep ingested it (r = 0.94–0.95). Other herbs increased their yield proportion from 23.3 to 36.5%, while that of ragwort decreased from 26.3 to 18.8% (2020/2021), doubling its flowers. Sheep preferred and tolerated ragwort, making their grazing an option to control ragwort from both an animal health and a nature conservation perspective.