It's worth noting that China is not universally a rice-growing area. If I recall correctly (and I might not be) most of the area of China that historically grew

It's worth noting that China is not universally a rice-growing area. If I recall correctly (and I might not be) most of the area of China that historically grew rice is in the part of that map which is in the moderate selenium territory. So the OP's theory might explain the relatively low amount of cavalry there.