Picture the scene:
A seige is underway, despite your best efforts, you can't breach your enemy's walls with your army and you are quickly running low on supplies. In a last desperate effort, you sneak into the town, under cover of darkness, and drop bottles of poison onto their crops. In the morning, you observe the troops on the walls, grey with fatigue and uncapable of fighting.
Now, imagine if you could actually do that.
What I'm proposing is a new type of poison that can be thrown like a splash potion but won't harm a troop if it's thrown at them. Instead, it must be thrown onto a crop (like wheat, carrots or potatoes). It bursts in a 3x3 square and any plants that it hit become poisoned. They look exactly like their non-poisoned counterparts, although they won't stack with them and can be used in all the normal crafting recipes.
However, if they are eaten, or if a foodstuff crafted using them is eaten, the person eating it will be slowed, weakened and lose their food bar at a faster rate, generally reducing their combat effectiveness.
Please take time to consider this and any ideas for changes will be much appreciated.