|good fun at a picnic - not good to do with your next difficult calving|
used too early, before correct dilation, or excessively in the case of an oversized calf(needing a c-section or fetotomy if dead)can cause tremendous damage to a cow. it is also very important to see that the calf is correctly positioned(or repositioned)so that the dam is not injured. To quote from a colleague from AABP-L paraphrasing the NRA slogan, 'jacks don't ruin cows, people ruin cows". Don't rush. Simply because she has started calving doesn't mean she's ready to deliver that minute. And jacks aren't a means of getting back to other jobs more quickly. How well calving goes sets the entire tone for her lactation. A damaged birth canal leading to metritis, leading to an LDA isn't a good scenario. And don't forget to use proper lube, liberally and often. A word of caution with J-Lube - it is a very good lubricant, but if there is any chance you will be doing a c-section, do not use it. It can often cause a chemical peritonitis when it enters the abdomen resulting in death of the cow.
Those cautions in mind, pullers/jacks save many calves. Sometimes you're by yourself and have the calf just right and that provides the extra 'oomph' to get the shoulders out. The one I use, a Hercules fetal extractor, is nice in how it works side to side to ease the calf out. It isn't meant to literally 'jack' the calf out - I use it to apply extra traction and 'help' the cow when she is pushing. The downward motion of delivery occurs rather than a straight out pull.
It is of particular importance in a breech delivery(and this was the case last week that brought all this to mind)because once the hips of the calf get to into the pelvis, their is very often an occlusion/closing off of the umbilical artery. Not a problem with a calf with head out, but in posterior breech position the ability of breath isn't there. So a steady, progressing and complete pull once begun is needed and these are the cases where calf jacks can help your cows and calves. Likewise a calf stuck in that position with inability to be delivered not only can die, but the pressure on the cows pelvic nerves may cause calving paralysis.
Use these tools judiciously, and with instruction and it will be of benefit to your herd. Used otherwise, see the quote in second paragraph!