Pasture management is certainly important and as a very broad generalisation what you say is true Steve. Ragwort germination rates are lower in ‘ideal’ pastures and it is less likely to become established but it will still germinate and if is growing abundantly on nearby ground that makes pasture management all the more challenging. I’ve been doing my own ‘field trials’ over the past few years and contrary to what I had read I actually find more ragwort seedlings in the dense areas of grass than in the bare patches/rabbit scrapes that I’d deliberately left to see what grows.
The majority of seedlings are unlikely to develop to become larger rosettes or even mature plants but they are still toxic and may be eaten ‘hidden’ in amongst longer grasses either fresh or wilted. May be little and often but that is the insidious nature of chronic poisoning. It’s very easy to see flowering ragwort plants, less easy to see the rosettes and even more difficult to spot the early seedlings.
FERA carried out a review of the evidence concerning ragwort impacts, ecology and control options. It is an interesting read highlighting gaps in the data which require further work. Link is here
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CCwQFjACahUKEwjc86HCwd3GAhWGWBQKHQ_PAs4&url=http%3A%2F%2Frandd.defra.gov.uk%2FDocument.aspx%3FDocument%3D12217_Reviewofevidenceconcerningragwortimpactsecologyandcontroloptions.pdf&ei=TYSmVZyZIoaxUY-ei_AM&usg=AFQjCNFACEaNBIAQMjggqQL1fIXsqyZksA&bvm=bv.97653015,d.ZGU
It is worth reading the whole document but (at the risk of being accused of quote mining!) if I can point you to page 42 it says “It has been shown experimentally under greenhouse conditions that 35 days post sowing, 30.8% of seeds will germinate on bare ground as opposed to 14.4% on short pasture and 15.2% on long pasture (Phung and Popay, 1981).” OK so it’s greenhouse/ideal conditions but germination is only reduced by half on long pasture compared to bare ground which in the scheme of things is still a lot of seedlings when you consider that, as a very conservative figure, a single ragwort plant can produce 50000 seeds. If only 1% germinated that’s still a fair number.