This is a complete WAG, but if I were to take a stab in the dark, I suspect the makeup of Wisconsin's agriculture industry is rather different than California's.
It looks like Wisconsin's industry is geared largely toward livestock and dairy, with fruit and vegetable crops largely composed of things like cranberries, corn, and potatoes:
http://www.aae.wisc.edu/pubs/status/docs/status2012.pdf
California, on the other hand, apparently produces "nearly half of US-grown fruits, nuts and vegetables."
http://www.cdfa.ca.gov/statistics/
It's my understanding that migrant workers in CA are largely employed picking fruits, vegetables, etc.
Again, I'm by no means speaking authoritatively here, but there's certainly more to this than simply comparing the dollar-value of each state's combined agricultural output.