Just the scientific side of me looking at the other side of the correlation, but what if it is that the larger more successful farms(who most likely have better stallions/mares and better quality foals) all generally start their horses as long yearlings/2yr olds? And then those with less resources and maybe less quality foals, wait longer?

Not arguing the findings, I don't have enough knowledge in the field to do that. Just pointing out the findings are correlational and hence there may be other factors here. What I would like to see is a group of farms each split their herd into 2. One group they start as long yearlings, the others the wait a bit(groups are split randomly) and then see how the horses turn out.

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"Planning to not succeed is stupid. If it doesn't work out, I'll make a new plan then. But I'm not starting out with a plan to not succeed."