Alrighty! Overall this picture is awesome and I adore the lighting on the fox and the color of the shading for him is aces.
For the grass, I think it'd be best if there was more of it and the blades were thinner. Try to vary in shades of gold rather than having one solid yellow color and blades and then a slightly darker color underneath.
The shading on the grass looks a bit funky right under the tree and fox, most of the grass under them should be dark rather than a few blades sticking up yellow. Also, the shading for the tree should probably be directly behind it and longer since the sun is setting in between the tree and the fox.
The willow, though I'll admit this will totally be a pain, should probably have more defined leaves and shouldn't tamper out once you get near the trunk. You should also probably add some yellow highlights to the edges of the leaves. The black strings in the middle I don't quite get. Anddd be careful to fix the little details like how you can see through the leaves at the top of the picture and the root is sticking up at the bottom.
As for the fox, I think the tree should be casting a shadow on .it annddd I can't quite find where exactly the right back leg is. I think the white on his left cheek should be a little less visible from the angle is is turned at.
From the lighting on the foreground, I think the sky should be a bit brighter and some lining on the clouds.
So yes! very nice picture and I like it a lot!