I wouldn't worry. They taste nasty (get sap on your hands then accidentally in your mouth!) and are not appetite attractive. Our winter fields have a boundary of huge Douglas Firs. They are perfect shelter (they may as well be in a stable) but the horses never touch them - not to eat/gnaw etc. They rarely even scratch on them for some reason.. they use the other trees (oaks etc) to do the bum scratching..
The probably are poisonous but can't see a horse that isn't absolutely starving trying to eat them more than once!