The ranching skill does not level up with use. Currently the only way to "improve" the ranching skill is to spawn with it or spend skill points on the two ranching skills.
Tested by having a dupe do nothing but tend critters for 20 cycles, despite having zero base ranching (+2 from ranching job skill) the dupe gained no levels. Save file attached.
Steps to Reproduce
Have a dupe groom critters, they will never gain any ranching levels.
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.