Village Blacksmith creates - plough (resources required - wood and cast iron)
Village Blacksmith creates - horseshoes (required resources - cast iron)
Shire Horses - New Animal
Shire Horse Stable - New Building for breeding Shire Horses
Tanner - New Building for making - leather from hides
Saddler - New Building for making - tack, (harness and reigns)
Work Horse - requires plough, horseshoes and tack
New building required - Crop Farm (required resources workers, work horses, hay, oats)
Crop Farm (Lvl 1) should contain stables for the horses.
Crop Farm (Lvl 2) should be able to grow winter crops
Crop Farm (Lvl 3) should be able to house four workers. (requires additional resources same as houses ie food and warmth)
Add feature to allocate animals to crop farms similar to the way workers are allocated to fields.
Result - improved planting and harvesting speeds and increased crop output
I hope this all makes sense.