It comes from a Dutch language website. Ragwort hysteria started in the UK but many people have been reading the hysteria from and it has spread.
The website says:
Omdat het jakobskruiskruid zeer giftige stoffen bevat, zoals strychnine, heroïne, cocaïne en morfine, zullen zoogdieren er niet van eten
which translates as
Because the Ragwort contains highly toxic substances such as strychnine, heroin, cocaine and morphine, mammals will not eat it.
It never ceases to amaze me how often people will put stuff out without checking it . This is utter nonsense . None of these substances are found in ragwort.