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!