In real life the change of seasons, especially weather and daylight, affect animals. This is a natural cycle, which is simulated in the mod.
Even though in many farms today animals are artifically inseminated, we have chosen to follow a natural cycle where animals give birth in spring and summer. In the mod, pigs and sheep give birth to offspring in spring, while cows give birth in summer.
Milk production varies through the year, being highest in spring and lowest in autumn. Sheep produce wool mostly in spring and some in summer, to imitate that shearing is done before the weather gets too warm.
As a result of colder weather in winter, they need more straw as bedding in winter and autumn, than in summer.
It is now only possible to fill the feeding throughs with feed lasting maximum 3 days. If the animals run out of feed, your animals will suffer and die, except if you are playing on Easy difficulty. In winter it is not possible to feed animals grass. This means that it is not possible to reach 100% production in winter. The amount of food has also been changed so compared to vanilla the animals will consume more than what you might be used for.
The cost of keeping animals (daily upkeep) is with the Seasons mod thought to represent for instance veterinary costs. This means daily upkeep also varies through the year and is highest in the seasons when animals give birth.
The soild and liquid manure production is also changed and is with the Seasons mod based on actual real life data for cows and pigs. In the colder seasons, the animals stay more inside so manure is collected and stored for further use. In the warmer seasons the animals stay more outside, thus the reduced manure production then.