Rabbits did that to me a couple years ago.

A low, tight fence (hardware cloth), kept them out the rest of the year -- babies can get right through the ordinary, green, welded wire fence.

Dogs, cats, and firearms (if sufficiently rural), are also effective.

Pesky wabbits! I do have 3 brazen bunnies nearby. I think they could be the ones born on my front lawn so they probably view it as "our" garden. Still-a lot easier to deal with than birds or bugs. Can't trust my dogs to do the job, though-caught one devouring a head of lettuce while I was mowing.

I haven't gotten the fence up around my garden yet, so I've been lightly misting the things both deer and rabbits like with Liquid Fence. Gotta stop doing that before harvest, though, unless I want rotten egg with my veggies!

Chuck

I am fighting deer right now in my garden.
they are doing just what you described. I know its deer cause I can see their feet prints in the soil next to the topped purple hulls and corn.

If they ate the first true leaves, I would bet on rabbits or woodchucks.

If it was the cotyledons (seed leaves), it might have been voles or birds.

Jim