Our experience is close to what Dave and 76 describe. This is what works for us:
1. Use trained dogs (border collies) year around to keep the dogs fresh and the cows accustomed to them.
2. If you’re just getting started, wait until after you wean to introduce the dogs. The cows won’t have junior to fret over.
3. “Well trained” means dogs that you can down a respectful distance from the new momma and they’ll stay there. (I’ve occasionally got distracted and looked back to see the dogs watching me from a quarter mile away where I downed them)
4. When moving pairs, use 2 dogs. A momma cow will fuss with a single dog. When she sees the second dog she’ll grab junior and hit the trail.
5. A few times a year somebody will need to get near a newborn calf with a very protective momma. We send a reliable dog to the opposite side of the pair, down him, then walk him up in increments of 3 or 4 steps. When the cow takes after the dog we jump off the horse and deal with the calf while the dog keeps the cow engaged. Just remember to never run to your horse when you’re done. And be sure to clamp down hard on the calf’s muzzle to prevent him from calling momma.
6. For cows that sneakily hide their calves, we ride increasingly big circles around the cow - with a pair of dogs right behind the horse- and watch for a reaction from the cow. Even the most clever cow will have her maternal instincts trump her attempt to be coy.
Our terrain makes the sxs impractical. We use horses. I love to ride to the top of a bluff or knob overlooking the young pairs at first light, watch the cows scurry to find their calves because they see the dogs with me, then ease down and start them to the next pasture.
I hope you find this helpful.