I have heard about this.
If you are blanketing your horse full time, the horse has not just stopped producing heat and is taking it all from the blanket, instead the blanket flattens the hairs on their back that stand up (they produce heat for horses w/out blankets) and don't allow the horse to produce heat in this way.
It is not bad to blanket your horse, but just don't let the horse go w/out a blanket until it is warmer because the hairs become unresponsive when blanket is on for a long period of time
hope this helps!
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