1) Love the JS-only angle, beautiful!

2) The combination of cows spreading out and your 3 axis of action (left/right + angle of gun + timing of shots) is really cool.

3) Consider levelling. Every x asteroids, there's a pause (the cows get into the barn to sleep), player rests a moment, and then sun rise and play resumes with faster / more asteroids. Have the cows re-clump after each sunrise and spread out from there.

4) Kind of silly, but if the cows bred more cows infrequently that would be cool.