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It also depends quite a bit on location. If you're the only hospital with an ultrasound in 100 miles, and the nearest specialist was 4 hours away, then yeah, you may really have no choice but to either learn some of these procedures or go without it. So it's not uncommon with especially many of the old timer GPs to build a reputation for a procedure that they've really honed, and get referrals from other GPs. Sometimes even when there are some boarded folks around, there's "dr x at practice y who is not boarded but can do cruciate surgery (lateral suture) for $900 if you can't afford the $2000+ TPLO with a boarded surgeon" that a lot of clients find attractive.
And, just as you should (must? it depends on your practice act) offer referral for things that are generally considered beyond the scope of most general practitioners, likewise you have to tell clients that Dr. X is not a ophthalomolgist/surgeon/dentist/neurologist (but is very good at doing procedure Y and you trust him/her). You can't make it seem like someone is a specialist if they are not.