Why is it cheaper for a farmer to produce a pound of grain than a pound of meat?

To produce grain you just have to grow some plants. Plant some seeds, and water them every day, and eventually ... Grain! But to produce a pound of meat, you have to buy an animal, and then feed it, and then at the same time making sure it is healthy. Think of it this way, what is cheaper and easier to take care of, a pet dog, or a plant in your window?