I think she'll just give you a funny look and wonder why you're not intelligent enough to find the clearly uniformed employees that are paid to show you where things are.
The problem with this routine is it comes out of nowhere. It doesn't really have relevance to the situation if she's not currently in the aisle with the dog food. If she's standing in an aisle full of meat and dairy products and you, some random stranger comes up to her asking about dog food and dog brands... she's just going to dismiss you as either creepy, crazy or stupid. The conversation you initiate needs to have relevance to her surroundings, otherwise it's too much of a strech to get her involved in the conversation, let alone you asking her to put her own shopping on pause and follow you to another aisle, just to help you to pick out a dog food brand.
If she is the meat aisle you could ask her about feeding hamburger to your dog, and then go into it from there. Same thing with just about any food in any other aisle. But you first need that anchor to the environment to even begin talking to her.
Another thing to think about, say by some off chance it goes really well; she want's to come back to your place to see the dog. What then?
Lastly, how can you neg her if for not knowing about dog food brands, when you're the very one asking her for advice?
It's a good start, though it needs some work.
~Antithesis