Suggestion. What if raiding farmlands also gets you crops as well? For example, a single infantryman can raid and take upto 10 crops? A horsemounted unit can carry 50. And if you also attack with ships, then they double your total unless your ships carrying capacity is less than that. In which case you can only take up to how much the ships can carry as well. Also , seasons. Spring = 1/4, Summer = 2/3, and fall = full value? Winter will yield no crops.

Edit: Actually... numbers don't work. 5 for infantry and 20 for horsemen?