You may want to render only shadows in your scene and use the alpha channel to composite.

To create a shadow pass in the alpha channel using the Maya Software renderer

1. Select the object with fur (this would be the sphere in the example above) and in the Render Stats section of the Attribute Editor, turn off Primary Visibility.
2. In Hypershade, select Create > Materials > Use Background and assign Use Background to the surface you want to create the shadow pass on.
3. Select Fur > Fur Globals, go to the Advanced section and do the following:
   • Turn off Enable Fur Image Rendering.
   • Turn on Enable Fur Shading/Shadowing (this is the default).
4. Render the scene using the Maya Software renderer.
5. In the Render View toolbar, click the Alpha button to see the alpha of the fur shadow.

To create a shadow pass in the alpha channel using the mental ray for Maya renderer

1. Select the object with fur (this would be the sphere in the example above) and in the Render Stats section of the Attribute Editor, turn off Primary Visibility.
2. Select the fur and in the Render Stats section of the FurFeedbackShape node in the Attribute Editor, turn off Primary Visibility.
3. In Hypershade, select Create > Materials > Use Background and assign Use Background to the surface you want to create the shadow pass on.
4. Render the scene using the mental ray for Maya renderer.
5. In the Render View toolbar, click the Alpha button to see the alpha of the fur shadow.