As milking is the main cause of their persistence, that must be done as gently as possible, or even with the teat tube or dilator.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
6. dilator of the neck of the womb, used when conception can not take place owing to a contracted condition of the neck of the womb.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
The opening must be carefully made with the finger, and when that has entered the womb further dilatation may be effected by inserting a sponge tent or by careful stretching with a mechanical dilator.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler