Grissom said that fecal tests performed during the relocation effort found virtually no internal parasites in the Yuma-area horses. However, he says, it didn't take long for those wild horses to become infested when briefly confined in corrals that had been used to hold domestic horses. That is one thing that is bred out of the wild horses. The soft-footed horse won't make it. Anything that's been out here several generations has adapted.
Drought often results in a culling of older and infirm horses.
Old mares will not conceive and often perish because they can't take the rigors of constant travel in search of food. Any foals which are born with infirmities often fall by the wayside during these hard times. However, the urge to reproduce often results in fillies being bred as yearlings and giving birth at two when their own bodies are still immature. It cut down on their growth rate and took a heavy toll when their bodies faced the choice of growing or providing for the baby inside. The band's social structure can serve to compound the problems for these young mothers.