eating

Question
We have a 20 month old female German Shepherd. We are concerned that she is not eating or eating very little for the past three days. She is a large boned dog who seemed to have this type of issue before. The vet examined her and prescribed Science diet gastrointestinal diet food. She had been eating one can in the morning and one and a half in the late afternoon (now for several months) I've also mixed in two crushed glucosamine tablets, which she ate without a problem. During the week sometimes I would cook some plain ground beef and mix it wuith cooked rice and mix it with a lesser amount of dog food. She ate with great gusto. The next day she would not eat and back away from the food. We are very concerned because it just seems like she doesn't want to eat at all. She ate a little bit from my husband's hand.
Is she just being picky? Do you think she tired of AScience Diet? Should I not give her any human food? We are at a loss?
I would appreciate your opinion!  Thank you!

Answer
Many Shepherds are light eaters especially when they are younger.  The best thing is to stick to a good food and little else.  The Science may be as good as anything.  Just put down what she will eat in 15 minutes and take it up if she doesn't eat it.  That really works better with a dry food.  It is quite rare for a healthy dog not to eat all it needs.   Some say never.  That doesn't mean she will eat what seems right to you or the dog food package says.  It may not even be enough to keep her ribs hidden completely.  But it is healthier for a dog to be lean or even slightly underweight than over weight.  The less a dog eats, the more important it is that its diet is complete and balanced.  The easiest way to make sure of that is to feed a dog food and little else.  Canine nutrition is well known and all but the cheapest foods are carefully formulated to exceed all the requirements.