We experimented with herd decay during development, but it caused too many situations where you could end up in a negative feedback loop resulting your character essentially being bricked (among other issues); we can ding your existing herd count directly as we do with Zuds now, but negative herd income isn't something we're looking at doing for that reason.
Haven’t played much as nomads but is there an upkeep for your armies (eating the herd) and sickness for herds?