The area with low selenium content is quite large and encompasses much of central China, where Chang'an, the capital of the Han dynasty at the time of the War of the Heavenly Horses (fought over horses) was located. (The modern city of Xi'an in the same place is marked on the selenium map.)
So the theory that selenium deficiency led to difficulty maintaining cavalry without importing horses from elsewhere seems plausible.
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.
Where Egypt allowed seasonal flooding to deposit silt into the Delta passively, my understanding is that China had a more active process for silt harvesting[1]. But in either case, silt only travels so far from the river, even less if carried in a bucket. And although the head waters might be in a mineral rich area, your tributary might not.
I don't remember enough of geography in China to overlay the rivers onto that selenium map, but I'm guessing the horses didn't get fed from the best fields at any rate.
[1] Farmers of Forty Centuries, F. H. King, out of copyright
> I don't remember enough of geography in China to overlay the rivers onto that selenium map
The map includes two black lines corresponding to the Yangtze (ending near Shanghai) and the Yellow River (ending north of Jinan). Of course the rivers changed their course by a lot over the centuries, but I don't think that makes much of a difference in terms of selenium transport.
