What percentage of your cows are pregnant after insemination?
How many of your cows are not showing heat at 21 days?
What are you vaccinating your cows against?
Is scanning the same as ultrasound examination?
Sometimes vasectomized bulls lose interest. They get no reward for their effort, so to speak.
Have you tried a 21-day heat trial?
Have you considered sychronization by using prostaglandin?
The 21-day trial is a simple aid to improve heat detection. Make a list of all cows expected to be in heat during the next 21 days. As they come in heat, remove them from the list. One third of the cows should come into heat each week. Any cows not seen in heat during the 21-day trial should be examined.
I have no clients that use the coil so my comments are theoretical. Other than GnRH, I don't use any hormones after insemination. Progesterone should be safe, but I still wouldn't use it. I just don't like to do anything which might affect the pregnancy.
If you are certain that the cows are not pregnant, prostaglandin might be your best bet.
I will be away from the computer for a while. Kindly respond and I will reply later today.
You might want to give prostaglandin another thought.
Yes, it can cause abortion if given to a pregnant cow. I do not treat a cow with prostaglandin unless I have cofirmed her open by palpation at 35 days post insemination. If I am not absolutely sure that she is open, I will palpate her a week later. I have not caused any abortions with prostaglandin. If used properly (given to an open cow) it can be a fairly reliable tool to bring a cow into heat.
Another reason for the lack of heats in some (not all) of the cows may be cystic ovaries. Rectal palpation or ultrasound would confirm cystic ovaries.
In response to your question about progesterone inducing heat...it won't.
This has nothing to do with your cows, but I'm very fond of Smithwick's ale.
Have you considered running a bull with the cows?