It's clear that you definitely know what you want to depict in this piece, it's straightforward, simple and heartfelt. It's obvious you've looked at horses and worked on anatomical issues. While not perfect (the curve of the larger horse's neck is a bit odd) overall, it makes sense and your lighting, coloration and shading of their coats pretty consistent. Good job on that--I personally find horses pretty difficult, they have some complexities about them.
You definitely have made a commitment to detail, and I can definitely appreciate that. The longer hair of the manes and tails are distinct from the short hair of the coats--I see a number of inexperienced artists draw really hairy horses when they really don't look that hairy at a distance. My major critique is that you have perhaps pushed this devotion too far,and I think you may be wasting some of your time with the image on putting in details we as the viewer don't need and in fact take attention away from the things you're doing well. The large horse's fetlocks (I think that's the word, the hair that comes off the leg over the hoof?) seem a little overwrought, and too similar in texture to the grass around them.
Just like you don't see every hair on a horse's hide from a certain distance, we shouldn't see every blade of grass on the field as an individual blade. Because our brains naturally interpret more detail and more contrast as being closer, this flattens out your image. As things move further away from us, they become not only smaller, but also less distinct in shape and color. Anything bright, sharply in focus is going to jump forward. I see you've done that a bit with the trees or bushes in the far background, but you could fudge the grasses a bit too to get a better sense of distance, and tone down some of the flowers just slightly as they retreat into the background as well.