We had the same aggression from our 6 year old male when we introduced a new puppy to the family. He would guard her food and his. We even put the bowls down in different rooms and he would race between the two to try and eat them. We put him outside for his at one stage but he just sat at the door and went mental when he came in. What we found was the pack order was being shown to the puppy. He was top dog and your dog thinks she is top dog and has rights to dictate who is going to eat what and when and everyone will listen to her regardless of the bowl being empty!! You have established yourself and the other humans as above her....she is now dictating where she stands in the order of things. What we did with our lad was to make him wait. He has to sit and wait for the other dog to go inside on their own and then he is allowed in....message = They have priority over you! I go first, the puppy second then he comes in. For food, I would put hers down first, make him wait in the doorway of the kitchen while she ate hers. Then put his down.....message = she eats before you do! Then I slowly started to let him eat at the same time as she did. He would scoff his and race to hers and snarl her out of the way. She rapidly learnt to snarl back and bark at him. I don't like this behaviour from either dog so as soon as there was any fighting they BOTH had their bowls removed. When they sat down and looked calmer I would replace their bowls....message = you behave you get food, you misbehave you don't! I now have them behaving with manners to each other. The bowls go down always in the order of the puppy first the older dog second....message is still that HE has a place in the order of things but I dictate when he can eat not him, same goes for puppy. I also make sure that from time to time I make the pup wait outside to come in along with us because she has to learn also that she has her place but it is decided by me and my husband. I can now place their bowls within 2 feet of each other. They will wait for the other one to finish and then go and lick each others bowls without the guarding behaviour. Key is to watch them all interact, and pull any dog out that thinks it is the boss of the pack...YOU are so make sure they all know it. Even if it means you take her food away for minutes at a time, do it! See her behaviour as a tantrum throwing child...would you give in to their demands and their fighting over a toy without sending them to their room? I know you send her to her cage but she will eventually see that as her 'punishment cage' instead of somewhere that is to keep her safe. I'd try the 'sit' and fold your arms and don't look at her but watch the others. Pet the others, talk to the others but INSIST she sits there until you give her the command and queue that she has now earned the right to eat and interact with the rest of YOUR pack! Hope that makes sense!!