I'm looking at putting black locust posts around my field, but I've read that black locust is poisonous to sheep, but they're excellent fencing poles as they last forever and need no treatment with chemicals. Will the sheep eat the black locust poles if they can?
Pretty much everything is toxic if you have too much of it. Water, for example, can be toxic to humans. The trick to not going insane in this world is to stop looking at lists on websites that claim things are toxic and just approach the issue with logic and caution instead.
If there is no grass in your paddock, your sheep are starving, and you put up a new fence made from freshly-cut locust, then sure, the sheep will rush the fence and eat what they can as fast as they can because it's new and different and they want to get in before the other sheep eat it all. A few hours later they may get sick and a few days later some may die. The problem is not the species of tree you are using — it's the fact you're starving your animals and you introduced a large amount of alternative 'feed' quite suddenly.
Note: For the purpose of this answer, anything that livestock can physically eat is considered to be 'feed' — not just the stuff that you buy in a sack that comes with a nutrition label. Sheep can't read labels, so the only way for them to find out if something is food or not is to chew on it, eat it, and see if it makes them sick. So grain is feed, hay is feed, fence posts are feed, tool handles are feed, tyres are feed, baling twine is feed, pallets are feed, barn doors are feed...
Sheep (pretty much all livestock) will preferentially eat the feed that tastes the best first. Then they'll eat the feed that tastes ok. They'll avoid eating bad-tasting feed until they get hungry. They'll avoid eating 'toxic' feed — that they think will make them sick — until they are starving.
Introduce new feed in a small, controlled amount. Let them taste it. Let them see if they end up with undesirable side-effects. If nothing bad happens, expose them to a slightly larger amount of the feed a few (three or more) days later. Observe. Monitor. Repeat.
This approach will prevent animals from getting a "massive dose" from the initial — curiosity-driven — exposure. It will also give them a chance to assess the feed's palatability and to decide how hungry they need to be before they resort to eating it. When they see it coming back again and again, the novelty factor will wear off. If the feed isn't palatable then they'll ignore it and go back to eating what they did before — unless you starve them.
You can add blocks of pure arsenic in the middle of your field quite safely if you do it this way. It's not the feed — it's how you introduce it that determines what the outcome will be.
Day 1 — Put a piece of locust firewood, or about a foot worth of post, into their field in an 'obvious place'. Observe. Monitor.
Day 4 — If they've just nibbed at the wood, you haven't spotted any strange-coloured faeces, and none are exhibiting any unusual behaviours, add a second, and perhaps third piece of wood to the pile. Observe. Monitor.
Day 7 — If all seems normal, lean a full post up against some part of the fence. Observe. Monitor.
Day 10 — If all seems normal, lean two or three more posts against the fence. Observe. Monitor.
Day 14 — If all seems normal, lean half-a-dozen more posts against the fence. Observe. Monitor.
Day 21 — If all seems normal, fence in a new paddock entirely with locust. Make sure they have plenty of regular feed (i.e. lots of good grass, maybe some supplementary feed as well) and water — lots of feed options that you know they like more than the fence posts. Move the sheep in. Observe. Monitor.
Day 30 — Build a campfire and burn the printout from the website that claims that black locust is poisonous to sheep.
In nature, wild animals are free to pick and choose what they will eat. Accumulated wisdom (from the herd and the individual) means they don't eat things in quantities that are toxic to them. When you farm animals in a cage or a paddock, you are 100% responsible for the diet of your livestock. If you intend to change their dietary options you need to give your animals the opportunity to develop/update their dietary wisdom. If you don't, and your animals suffer and die, then it's not a random plant's fault — it's yours.
The only thing toxic to farm animals are bad farmers.
- Black locust fence posts will last longer if you strip off the bark.
- In wet and clayey soil, giving the part of the post that goes into the ground (plus about 10cm that sticks out of the ground) a sealing coat is advisable.
- In sandy and free-draining soil, a band of sealant +/-10cm around the ground contact line is all that is advisable.
- A cap on the top will stop water from being drawn down into a post and further extend its life.
- If you debark the posts they'll be really, really hard to staple three months later. You're better off pre-drilling holes for the staples in the comfort of your shed if you leave them that long.
- Black locust posts that have been debarked and left out in the elements for three months are so hard that animals would have a hard time eating them even if they wanted to. No need to worry about anything beyond this point.