The Warg-riders use poison on their arrows that is designed to slow their targets when used in small amounts, but a lot can be deadly. A versatile weapon of this potency should not be left in the hands of the enemy...
These barrels are filled with deadly poison used for the Warg-riders' arrows. Taking the poison will make their weapons less effective, and it could be used for another purpose...
Barrels of poison, a favoured weapon of the Warg-riders, are stored around Edsig's Farm.
New pups are being raised as Warg-mounts for riders in the back of Edsig's Farm. The Warg pups have supplies of meat and water that the Orcs placed for them. These would be effective places to put the poison you gathered.
The poison has been placed, and the Orcs in Edsig's Farm will hopefully not realize the source of the problem for some time.