Hey! I had a problem by using layers for the render.
The landscape is on layer 1 and the (good) rabbit is on the 2nd layer. It does not calculate transparency from the mask of Images as plane.
Here is the results:
I was using different layers 1 for landscape, 1 for the good and 1 for the bad rabbit, to get faster rendering and better control of the scene at compositing.
I thought :
“But if i put the two rabbits on the first layer to solve this issue, it will probably ask too much memory to my computer and the rendering time will be very much longer, or even it may crash my computer ?! and also it will be much harder to get a nice control for compositing 
But i guess don’t have a lot of choices:
-replace images as plane by pure modeling grass to get a nice mask (better control)
OR
-move rabbits on layer 1 (lower control)”
First i thought i couldn’t put both bunnies on layer 1 so i tried to only move the good rabbit on layer 1, rendering at 1920*1080 - 50% , 5 samples, asking a lot of memory:
Is it wonted value when rendering a lot of particles, how much did yours needed ?
Then i tried with both bunnies on layer 1 and with a good surprise it have done it!
But using 4400+Mo of memory, I used GPU Compute for this rendering, my graphic card got 4Go of memory, i don’t get it.
Does blender use memory from Graphics Cards (so did i overclock mine ?) or from the RAM when using GPU ? or both combine ? (i think i should look for a new lesson about it)
Whatever is, here the result at 50% of 1080p 50 samples, done pretty quick (01h:22m:30s):
