Aside from what other people mentioned already about nutrition, viability, ease, etc. I have one other thing to add. On such an estate the horses are probably going to be Hunter types and therefore very very expensive considering the times and who would be buying them. Even if they were draft animals it would be super expensive to buy. Have you thought about how if the foal were to die from lack of nutrition or milk it could have some serious repercussions for your MC. An estate that breeds horses would rely on their ability to produce quality animals and improve their own bloodlines. Even one foal, could have years of investment in lineage, money, time. They may have had to pay someone else a stud fee to keep inbreeding at bay and you can't just breed back a mare whose just foaled. So there's lost time and money there. Not to mention the loss of potential. I know from family journals that detail the gossip of the time among other things that costing someone their future bloodlines of their estate of that relies on breeding could mean anything from expulsion from the family service (good luck getting another job because it wasn't just a matter of keeping it off your resume. Gossip travels, particularly among those with the money to afford servants), to jail and/or a fine levied against you. It could also mean death depending on the status of the individual, time period, place. Not just directly but indirectly. A mark like that against you -nobody- will hire you in their home, especially those who aren't well off, if you don't have a job you don't have money for food or a home...
Honestly, even if the MC is in no position to give orders to anyone, other servants on the household will help if only to avoid their employers wrath. Because if the employer comes home to find someone tampered with their animals you can bet everyone will receive some measure of displeasure.
A good horse (and if they have an estate and breeding them) could easily go for the entire year's wages of a hardworking family. Not to mention if the mare panics or gets mad, kicks out, and damages herself. Especially if it's the legs they damage. That would be the death of mare and foal and perhaps generations (horse not human generations) of breeding, if it killed even part of a bloodline then I can almost guarantee that your MC will have a very short life expectancy. A lot of people relied on their animals for survival. Even a hunter estate would be crippled by the loss of even a few mares, perhaps even to the point permanent damage.