I looked at your model and see that the grass is passing along the same amount of energy to any rabbit that eats/collides with it. If you wanted a random amount of energy from the grass, you could change the code in the collision block on the rabbits page. Right now rabbits get exactly 1 rabbit energy from eating a grass. But you could add an equation that would allow a random amount to be transferred. The equation could be something like 'random 10 divided by 10' this would allow a random amount between 0 and .9 to be transferred. The collision in the grass tab does not really do anything since the grass energy isn't really used in any other part of the code. As well, the grass gets deleted according to the rabbit collision block.
Hope that helps but if you have any questions, let me know.
Project GUTS facilitator