Farrowing on pasture is possible at any time of the year that you can get to the pigs. Better coops would be necessary for winter farrowing in our area. A-frames would be the cheapest coops to build if extra farm labor and rough sawn lumber is available. Seasonal production is by far the lowest cost and highest quality of life for the operator. Southern producers would have to be more concerned about heat.
Pigs can take any kind of temperatures if shade and wading ponds are available.
The lowest stress castration for the pigs and owner is to get the boars when they are one day old. The sow is still slow enough that she won't tear you apart and the pigs are not easily caught after they are 24 hours old. The lowest stress weaning for pigs is to take the sows and leave the pigs. Grazing Corn Late April farrowed pigs are born the ideal time to graze standing corn in my area. I plant about 40 acres of corn for hogging down every year.