<human>: Hi /r/dogs! 

I was hoping other dog owners might have some ideas for me to try.  Here's my situation...  I have two large dogs, both black lab crosses.  When I let them out to do their business in the morning, it is still dark. The same when I let them outside to do their business in the evenings (7am-ish and 5pm-ish).  There is nothing I can do to alter that, as I leave for work around that time in the morning and I return home at that time in the evenings.  Consequently, when I let my dogs outside (at either time), I am unable to see where they do their business and as such, it's impossible to clean up dog messes until the weekend.  Frequently it snows in the interim and what I'm left with is essentially a huge & disgusting mess in the spring.  

So, I had the idea to not let my dogs out in the backyard anymore, and instead I would take them out on a leash.  That way I can just pick up the mess and it won't be left in my backyard.  This hasn't been a problem with my younger lab.  She pees on command and doesn't take long to do her other business.  My older dog, however, refuses to go when he's on a leash.  If I make him wait until he REALLY has to go, he dawdles when he's outside with me and he takes forever to get his business done.  Since I'm going to be doing this before work, I can't be outside with him for hours on end, waiting for him to wrap it up, if he even goes at all.  I am so tired of the dreaded spring backyard cleanup.  Any ideas as to what I can do to encourage my older dog (he's 13) to get his job done quickly when I take him out on a leash?

Many thanks in advance!

Condense the text into a few key points
<bot>: my 13 year old dog refuses to poop when I take him out on a leash. How can I encourage him to do this so I can just pick up his business right away?