The general formula for the different resources is :
perTick = (building output * season modifier + job output * upgrades * happiness) * building % effects + automatic effects - consumption * consumption % effects
The relevant part is the first:
perTick = (building output * season modifier + job output * extra....)
You see that the season modifier is only relevant for the catnip fields or any other buildings you later can build (I don't know, but I can't see why not). So it is not more effective to put farmers to work in the spring, as the season has no influence on the farmers.
This is how I handle my catnip successfully right now (32 kittens):
So 32 kittens, 3 are farmers. 76 catnip fields, 32 pastures and 27 aquaducts. Also 11 unicorn pastures. Happiness is 61%, 71% or 81%, depending on the special goods. Also the mineral and iron hoes upgrade (50% and 30% more effective). But the most important part is just 3 kittens as farmer. The skills are 1x proficient and 2x competent. This all results in a overall positive catnip production, so I don't have to change farmers in a bad season. Good luck!