I used to follow that reduce by 5 degrees rule, and then I saw a broody actually raise healthy chicks in the middle of winter.
I'm now much more stingy with the heat lamp, they get it for about the first 10 days, then that's it. They are in my basement which is between 50 and 70 degrees, but they're fine.
I'm now much more stingy with the heat lamp, they get it for about the first 10 days, then that's it. They are in my basement which is between 50 and 70 degrees, but they're fine.