In some cases, and you may want to check with the contract you signed with the rescue, breeding a rescue horse voids the contract and you must return the horse. This is actually a fairly standard and boiler plate clause for most rescues. Now this might not be the case, but it is a legitimate reason.
Why can't I breed a rescue?
The horse doesn't HAVE to be registered, HOWEVER, unregistered horses, tend to have a not so great time in life. And just because it bred with a registered stallion doesn't automatically make it eligible for registration. As an example: If the stallion were a TB, just b/c you bred with a TB doesn't mean you can register the foal with the Jockey Club. If it were a AQHA stallion, it doesn't automatically mean it gets AQHA papers. Typically to be register both parents have to be registered with the same group for thefoal to be eligible. He few registries that require only one parent to be registered are Warmbloods, in which case the foal will have to be inspected before the breed association. OR they are the equivalents of the Continental Kennel Club; and they're just a group trying legitimize horse-puppy-mills and ignorant back-yard-breeders. In these cases they're not worth the papers they're printed on. Horses with papers, generally sell for more money, are more in demand and have an automatic stamp of quality attached to them (even though that last ideal is hogwash). The hard facts are, as much as there are no bad horses, there is NO demand for Oops babies. At least not long term good homes. Even if it DID have papers, that’s not a guarantee either. Slaughter auctions are filled every month with well bred registered horses that still end up in dog food or on dinner plates. There is a great quote from a Benedictine nun, named Sister Joan Chittister "I do not believe that just because you're opposed to abortion, that that makes you pro-life. In fact, I think in many cases, your morality is deeply lacking if all you want is a child born but not a child fed, not a child educated, not a child housed. ... That's not pro-life. That's pro-birth." And to a certain extent that is what the other commentor was talking about. The likely hood of an Oops baby horse suffering at some point is statistcally 99.99%
why does my horse have to be registered? The stallion is registered, I already know that and why can't I breed a rescue?
Should it be when the age is measured in weeks and where a nervous system hasn't developed yet?
Or is it kinder to let it be later in life after being ignored, starved, or beaten, or worse simply sold for food when its owner can't afford to keep it.
Breeding isn't about making making adoable babies, its about directing and perfecting a breed, for a specific purpose.
It is 100% every day and every dollar about investment; Investing of time, money, resources, happiness, health, education, and love.
And if you don't have an excess in every one of those areas, then you need to expect problems because foals are a bank account blank hole and a complete time suck, and if you don't do it right they are dangerous.
I think these are the warnings that the other posted is trying to convey.
I am a breeder and an agent, and as a breeder I can tell you that the reasons my horses come back to me (b/c I will always take one of my horses back no questions to ensure they have a good life, all quality breeders do) and as an agent of Sport Horses, the top three reasons why people get rid of their horses:
1. They don't have the money to keep it.
2. They're getting a divorce and don't have the money, (and usually they don't have money b/c of the horse).
3. They are going off to college and don't have the time to sell it or the money to keep it.
I think other posters are just trying to warn you that, unless you can guarantee
within a sixth sigma of your prediction model that none of these situations can ever happen.
My best advise is to call the vet out and make sure 100% proof positive. And then pay the bill. And if the bill for the farm call, the drugs, the palpation, the ultrasound do not phase you at all and you can still have steak and lobster the rest of the week. Then you might have a chance.
But if you pay that bill and it hurts; like you're going to be eating hamburger helper instead of Applebee's to make up for it ... then call the vet out and get a flush, because a confirmed pregnancy bill is one of the cheaper expenses.
I promise I'm not trying to start a flame war, I'm just trying to be gentle and logical in an area, I am very experienced in.