Cucumbers can be a fun snack for some rabbits but don't let them eat too much because it can cause digestive problems that lead to diarrhea, bloating, and a poor appetite. This is because rabbits are herbivores that are adapted to eating hay. Their digestive system is built to house good bacteria that ferment the hay and produce many nutrients that the rabbit needs. Problems arise when you feed rabbits things that are too easily fermented because it allows bad bacteria to outgrow the good bacteria. The bad bacteria then produce a lot of gas which makes your rabbit feel uncomfortable and to loose its appetite. This often leads to diarrhea and dehydration. Also cucumbers are not a very nutritionally dense food for rabbits. It lacks many nutrients that they need to stay healthy.
Baby rabbits need a diet that is high in fiber and high in fat (like mamma's milk). Most pellets have too much grain and not enough fiber or fat. However, it is still important to feed pellets because they provide specific vitamins and minerals that they need to grow and be healthy. The chart below shows you the difference between common brands and Sherwood Forest baby rabbit food. Notice the difference in the levels of fiber, starch, and fat.