I don't think it will work to add suppression to the fence. The fence charger is designed to support heavily reactive loads by generating pulses every second or two. The pulses are formed by dumping the charge of a capacitor bank through a step up transformer. Any additional reactance will be a very minor load delta.
My experience (several miles of e fence here) shows that interference is usually the result of a poor grounding field for the charger or arcing along the circuit e.g. insulators, buried wire, gates, or lightning arrestors. If it is an old "weed burning" charger, it should be replaced for safety and interference sake.
In general, though, ferrite materials are not suitable for low frequency applications. Look at powdered iron materials like (Micrometals) #26 instead.