Love the hat, nice touch!
You need to adjust the lighting on the other sheep's faces. You have the ones in the sun shaded, and visa versa.

There are a couple areas for improvement on this one. I'm not sure what you used to cut out the sheep but it looks like they have an outer glow to them. I would suggest looking up masking with the pen tool. Also the depth of field is really shallow on the original source, so I would suggest using lens blur with a gradient channel to make your background match the DOF of the original source. Lastly if the sheep are in the grass, then some of the grass should be in front of them. Just use a mask on them and with a really small brush paint out some grass sticking up. Good luck!