rescued Shih Tzu/Chihuahua

Question
We recently rescued a ShihTzu/Chihuahua mix, approx. 2 years old. We have a large,fenced in back yard and a rescued English Setter (approx 5yrs,adopted 3 months ago). Our problem is the Shi/Chi. She seems to ONLY urinate or poop on walks, NOT in the back yard. For the last couple of days I have resorted to crate training her, taking her out in the back yard, on a leash, to pee/poop. She refuses! Today, it has been 25 hours since she last went (I caved yesterday because of the rain and walked her a few houses so she would go). This seems very unhealthy to me, but she is very stubborn. Do you have any other advise? Is her holding it this long bad for her?

Answer
I myself had a dog that did for about 3 weeks then finally started goingin her yard what often works,is to just to give her a 'mini walk'like walking for a minute on the driveway then to her yard-this is something that is common in rescued dogs,but in time they learn that the yard is an ok place to go,sometimes they used to only going to the bathroom when volunteers walk them,and may have never been in a yard

It takes time but then eventualy get the concept