A change in sow numbers is not the only factor that changes the pork supply on the market. Imports and exports are major factors, along with herd productivity.

In 1930, the United States produced an average of 701 pounds of carcass pork per sow. Last year, that amount was 4,141 pounds of pork per sow. 

The increase comes mostly from three factors: more pigs per litter, more litters per sow per year and heavier slaughter weights.

From 1930 to 1980 the average increase in sow productivity was 2.03 percent per year. Since 1980, the average annual increase in pork production per sow has been 2.79 percent. The faster productivity grows, the faster and further producers will need to reduce sow numbers.