Hello,
Please specify what you mean by "the one carrying the lesser weight". As far as I understand, you are referring to a weight that the horse is loaded with, not the horse's own rate, right? In that case it can be either Jockey's Claim (which is not always claimed) or the "Extra equipment" field, which is a string field and sometimes remains empty or contains non-numeric values.
So what is it you want to measure as weight in this case?