I suppose that doesn't hurt anything...

I assume this prevents some consumption of rations?

I was wondering why the DM would even need to make a call, but I guess that would be relevant.

Any DM that said no without a reason for a lack of ants would be a bad DM.
- Maybe an ant hunt mini adventure or some minor consequence to this action.
It also might not be enough calories for any kind of sustenance. It could just be the Druid's kink
One of my favorite wizard characters was worried about rations, so I wrote into her backstory that she actually made an original 1st level spell for graduation of Wizard's School.

That went over like a Lead Zeppelin. The spell used a tiny touch of wild magic to "randomly multiply" the ingredients you had available. Just a single berry? You're getting a day's worth of berries. A carrot, a celery stalk, and an onion? Well you're getting all the fixins for a delicious vegetable stew.

The spell created magical food that couldn't be used as the material component for a further cast, and consumed the material components given. It then produced 1 day worth of vegetables, fruit, and berries, per caster level up to 5.

The headmaster and two other professors watched my character demonstrate her spell, at which point the headmaster immediately mind wiped the other two professors, and explained that I could only keep that spell if I swore to NEVER allow anyone else to see me cast it. Apparently it strayed too close to Clerical magic, and could have reignited the Wizard/Cleric War, that never actually happened, and actually turn it into a shooting war this time.

That particular character gave up on creating new spells till she was above 16th level.
Expend a use of Wild Shape to use one ration less?
I mean sure, go ahead.

Well it does hurt the ants tho

They deserve it for their crimes.

My rule as DM is that players can freely expend their normally limited resources if it's done for role playing or fun and doesn't affect the story, combat, etc.

Druid wants to spend all evening as an owl in the rafters of the barn? Cool. Wizard wants to create a pocket dimension in which he can then use Wish to create the universe's best ganja and smoke it in peace? Awesome.

Artificer wants to ... wait, he wants to do WHAT with his pants? Okay fine, but we're cutting to black for the evening. And there may be assassins in the morning now.

Ok so how much exp do I get for killing a hundred ants?

Each ant only gives you 0.0000000001 XP so multiply that by 100.
- Jokes aside
  
  They'd be 0 XP creatures
  
  XP should be awarded for progressing the story, the XP from CR is a helpful guideline for how much XP progressive encounters should award but killing random peasants or forest animals without progressing the story shouldn't award anything.