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The problem with sending a dog off for training is no matter what the trainer might tell you, there is no way for you to know what went on at training. Some trainers use physical punishment as reprimands which can make a dog aggressive in some circumstances. In addition, some of these training facilities will house multiple dogs together when they are not training. If the other dog is dominant, this might cause a dog to start resource guarding which is what you are describing.
I love the training facilities where both the dog and owner go for two weeks. I've found those are fantastic and let me explain why. A dog becomes a little more submissive to the person who it obeys when commands are given even if they are doing it for the reward of a treat. So if someone else is doing the training, your dog becomes more submissive to that person, not you. Of course, when you start giving commands, your dog will at least know what the commands mean and is likely to obey since now they know what you want when that command is given. However, if they do not see you as the ultimate boss, they might not obey when they don't want to do what you ask them to do. In many cases this happens a few weeks after a dog returns home.
A dog does not reprimand the boss. Since she is reprimanding you when she growls it certainly seems like she does not see you as the boss. Now resource guarding can be dangerous if not addressed correctly. I have to tell you that it is best addressed in person with a professional behaviorist. However many owners are able to help their dogs overcome this unwanted behavior with lots of patience and hard work. The following sites go over this in great detail. The last site give many different ideas and techniques to help resolve resource guarding.
If you have taken the time to read the above sites you will notice that the owners gained their dog's trust by not taking things from them unless they gave them something even better. In many cases, food is the cause and since it may be in this case as well, I'll give you information on that as well. In the case of food, I've found that hand feeding gets the dog used to you being around the food. Hand feeding can help in these cases, so I recommend you talk softly to him when hand feeding and you might want to pet his back as well, so he gets used to you touching him when he is eating . I usually progress to putting the food in the bowl and just hold the bowl continuing to talk and pet them. Once the dog is used to this, I will put the empty bowl on the floor and put food in the bowl piece by piece if necessary, so the dog knows that I control the food, not him. Additionally, you might have some really tasty treats in hand and as you get close to the dog start dropping these so the dog is associating your with giving more tasty treats rather than just approaching his food. Once he sees that you are adding food to the bowl and not taking it away, he shouldn't feel the need to growl at you to warn you away from his food. At this point, you want to have an extra tasty treat like hot dog slices and have them in one hand to distract him from his bowl. As he takes the treats, lift a handful of food from his bowl and then put it right back. Be sure he sees you put it back. This teaches him that just because you take the food doesn't mean it isn't coming back.
I start taking food away from puppies and giving it right back when they are just puppies. If you frequently take things your pup enjoys but are dangerous like cooked bones, stinky socks, etc, without giving something better in return, you dog might think that you are planning on taking his food away. You can use the leash to provide some negative reinforcement such as a quick tug and firm low toned "NO" when he growls, so you are doing the right thing there, you just need some positive reinforcement when he is acting the way you want him to. The same technique should be used for toys, but use a "higher value" toy to tempt him away from a toy he has. Higher value toys are ones the dog likes more.
Most dogs would prefer liver or hot dog slices to bones, so those can be a good tempting treat to convince your dog to let go of the bone. Remember that initially you will be praising him for giving up the bone and giving it right back so he sees that non one wants to keep the bone. It will need to be an ongoing training exercise and may take months to stop the undesired behavior.
Teaching the leave it command may help as well. This site has a good method of teaching the leave it command that I have found to work very well.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.