2 year old australian shepherd

Question
We love our dog Bo, but just recently he has started biting people that come into our yard. At first I thought he was playing then it progressed to biting and drawing blood.

Answer
The Aussie breed standard reads "strong herding and guarding instinct", and it seems you have a dog that is territorial and guarding your property.  The safest thing is to contain him when guests will arrive, or to put his exercise yard somewhere where people are not tempted to enter.  Unfortunately, with Aussies, it's hard to condition them not to do this, as they are hardwired to be herdy or guardy, and they may not be everyone's friend, even though they are loyal to family and those they know well.  Most will nip and NOT draw blood, but if the person they are trying to influence doesn't move the way they want, the dog ups the ante and may nip harder or bite and let go quickly, usually from behind.  The reason this has started in earnest now is that what he did before was playing, in the sense of it being behavior that prepared him for his adult role.  Now, at 2, he is just reaching the age of social maturity and is ready to try out his skills for real.  In the wild, he would now be ready to defend his territory, hunt for himself and his family, and be suspicious of those not belonging to his own social group.  I don't know how thoroughly you researched this breed before getting one, but the dogs that are still bred for work can still do their jobs, and guarding the farm is one of those jobs.  The way you keep your dog safe is to keep your guests safe, and the way you keep your guests safe is to keep your dog safe.  My advice is to contain him and not give him the opportunity to practice this behavior when it is unwanted.  Also, these dogs need a job, and running in the yard or going for a walk doesn't cut it for them.  Frisbee, agility, flyball and other dog sports can be a good substitute.  So can mental exercise - try the lessons here: www.clickerlessons.com.  Once he understands the clicker, you can probably train him to do just about any task.  (Maybe put the laundry away LOL)