Elf On A Shelf
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The issue with the 20% rule is that it doesn't take a horse's type into account and unless people have access to a weighbridge, they're guesstimating at best.
We had a weighbridge at our camp a while back and as a little fundraising thing afterwards I took side on confo type photos of 10 random horses and put them all together in a poster with a little bit of blurb with each horse's height, breed and age. People them paid £2 to guess the weight of the horses and there were prizes for the closest to the total weight. Although it wasn't meant to be, it turned out to be quite an interesting experiment. Generally people over estimated the weight of TB types and underestimated the weight of more cobby types. Many of the horses were around the 550kg mark but they ranged from a 14hh heavy cob to a very slight 16.2hh TB. I know which of those two I'd rather put 110kg on.
Even with my two oldies who are very similar types and weights, (obviously when they were both fit and sound, they're both currently carrying 0kg!) I'd be happier putting more weight on Archie than Monty. Archie's actually the lighter of the two at about 650kg fighting fit to Monty's 670kg but he's much shorter backed, wider shouldered, deeper girthed with nearly an extra inch of bone than Mont who is long backed (and necked) with a huge backside but puny legs. Having said that, there's no way I'd would have put 20 - 21 stone on either of them which is what the 20% rule suggests. I suspect Charlie's a similar weight but I wouldn't put anywhere near as much on him at the moment as he's still a bit gangly and needs to fill out. When he's done though, I think he could carry more than they did - not that he will.
No one can truly guess the weight of the racehorses with much accuracy either. It's quite entertaining to see them try though! We have a weigh bridge that every horse goes on on a Monday so we know what they weigh all season.

