Such a shift would transfer rural China，where almost half a billion pigs are produced each year, about 40% of them on small farm with fewer than 30 sows.

about 40% of them (being) on small farm with fewer than 30 sows.是独立主格结构，介词短语前省略了being. 独立主格结构为伴随情况状语，对where从句作补充说明。