Heifer long labor

I have a heifer who's water broke about 9 this morning. She has been grazing around all day with only hiding in the trees twice so far. It is 3:30pm now. Ive never had a cow take this long to go into full contraction labor before.
Any ideas? Im okay letting her go a little longer but would hope to see some signs of more active labor.

hope you've gotten her in and checked her by now; if the calf is full breech, often the cow won't go into labor; or if the calf is huge and the uterus is so stretched it may not go into a good labor; or sometimes, she just don't go into a good labor for who knows why but our vet will c-section if you still have a live calf because you can end up with a disaster if you just allow this situation to stay as is.
Cow should be gotten in and palpated to see if the calf is positioned normally.  If it is, then talk to your vet and see what is an allowable amount of time before he will try pull or c-section. 
Let us know how you turned out.

What Pleasant Grove said.  If calf is backwards and legs are tangled, the calf does not go into the pelvis and the cow does not get pressure which will trigger her to start pushing. 
I would definitely get her into a chute to see if the calf is presented correctly.

Augustus how did your heifer end up doing?  <cowboy>