Sampling of soil, pasture, and hoof capsules occurred in two periods. The first happened in summer and fall when all foals, between one and six months of age, were still nursing their dams; the second occurred after weaning when foals were nine to 12 months old. Forty-one foals were used in the preweaning period, 28 in the postweaning period.

All foals used in the study were Criollo, a South American breed revered for its tractability, soundness, and stamina under saddle. Mature height tends to be between 14 and 15 hands, and most are considered easy keepers. Foals were born and raised on five farms in Brazil. Mares and foals grazed native pastures consisting primarily of bahiagrass, kaimi clover, blanket grass, dallisgrass, and bermudagrass, though hundreds of other pasture species were likely part of the grazing landscape. They received no concentrate or mineral supplementation.