FLOCK:FERTILISER

From SteamStats Wiki

In order to have a cow make a pile of poo, you have to use the depresser beam on it. If the cow stays still, the poop that's squeezed out of it will pile up into a pile and you will receive 100 points. In order to achieve the achievement, you must make 50 piles of poop that give you 100 points each. Each 100 point pile of poop has to come from a different cow. Note that you can receive 75 points for stepping on a pile of poo, but this adds nothing to the actual achievement.

This has to be done on official maps. Creating a map in the level editer with a hundred cows will not earn you this achievement.

Single player maps with cows on them:
9-6 cows
11-2 cows
12-5 cows
13-1 cows
17-13 cows
25-3 cows
30-3 cows
37-12 cows
42-5 cows
44-3 cows
45-8 cows
48-1 cows
53-15 cows
55-11 cows

Play each of these levels from the level select screen, and approach each cow and try to squeeze out that poop into a 100 point pile, then move on to the next one, then meet the minimum abduction requirement to end the level and save your progress.

(Editer's note: my numbers may be off, as well as include calfs. I didn't confirm if calfs can make 100 point piles, as I got all my piles from adult cows.)