[HairOfTheDog]

Here's an article by the doctor that introduced it:

Use of a Glass Ball to Suppress Behavioral Estrus in Mares

What I didn't see is how long they leave it in there.... One season, a year, many years?

[Duchess47]

From the article it looks like they were left in for one season. That would give you a year's freedom though from shots and/or implants. My neighbor could have really used this information a few years ago. He finally sold his riding mare because nothing seemed to work.