introducing a new dog food

Question
I have a 10 to 11 year old dog that I have been feeding the same dog food to called-MAINSTAY, and have been told,not the most nutricious food. What is your comment on this brand? I recently have introduced him to a new brand of dog food, the first feeding was fine the second day he had the poops.The name of the new food is a brand called- PRESIDENT CHOICE-CHICKEN & BROWN RICE.What is your comment on this brand, and also is it to late to be feeding him something new? can you also give me some advice on how to feed this new dogfood, so my dog doesn't have the poops? (diarreha)Thank you and I look forward to your reply.

Answer
I have no knowledge of either brand (have actually never heard of either of them), but at 10-11 yr old (if it's a large breed), I really don't think what you're feeding him will make much of a different in lifespan.  If it's a smaller breed, that has several years left, it may make a difference.

It sounds like you did a switch cold turkey (stopped feeding one food and started another).  You need to slowly introduce the new food.  That means, buying another bag of the old stuff (can be a smaller bag if you used to buy a very large bag) and mixing the food.  Start with 3/4 old 1/4 new for a couple of days, then 1/2 old 1/2 new for a couple days then, 1/4 old 3/4 new for another couple of days.  It should take about a week to transition from one food to another.  This will slowly get your dog used to the new food so he doesn't get the runs.