"Research has shown that obesity (condition score 9) is not detrimental to reproductive performance and does not increase foaling difficulty. This degree of body fatness, however, severely limits a horse's performance capability and is considered unhealthy."
Not detrimental?? I was always under the impression that obese mares were more prone to pregnancy complications, mainly premature birth. The source is also what I would consider reliable. It is on the website for the Department of Agriculture for the Government of Alberta and was prepared, at least in part, by a PhD. The article is also fairly recent - last revised 2008.
Since this is contradictory to anything I've ever read, I thought I'd open it up for discussion and see what some other thoughts out there are.