Couple things. Any time you stress calves it may change their stool consistency. If it has an odor and is really foul smelling there is something called Seasonal dissentary. They make a mixture for the water. The other thing is to give them some probiotics (TSC) and added a little beet pulp to their mix to help firm them up. I am NOT A VET nor do I play one on TV. Just things that have worked for us. If it doesn't clear call your vet safe a sample.
Good luck.