This is probably very very boring for most people but the Rat is a very intelligent animal and you need to understand how it lives and feeds for the best chance of finding them and then shooting.
,Rats are Neophobic(wary of new things) so to get artificial bait right is very difficult we find the obvious bait to use is whatever they are eating on the farm, stable or around buildings from corn,barley,horse feed we also find that in some cases yoghurt,cat /dog food can work well especially with young or part grown rats, have a look at the link below that may give you a better understanding.
www.ratbehavior.org/FoodChoices.htm
