What you always wanted to know about the subject "extenders" but never dared to ask
An "extender" thins the portion of semen, doesn't it?
That is not correct. The correct answer is that the ejaculation is proportioned into an exact quantity of spermatic cells defined by lawmakers. In order to be able to ship the portion of semen under the most optimal conditions, the semen is filled with a nutrient medium, often erroneously called an "extender". This is done because spermatic cells are only viable outside the body for a maximum of six hours.
People often say that mares react to the "extender" and do not come in foal as a result!
That is not correct either. Mares react to the semen but not to the "extender". The "extender" is only a highly compatible nutrient medium used to maintain optimal viability of the semen for a certain period of time which allows the semen to be shipped.
Do mares that are inseminated with "pure semen" come in foal more readily?
Just the opposite is the case. If too many spermatic cells are added to the straws used for shipping, they hinder each other's motility and "become tired" more quickly. Only a motile spermatic cell will find its goal: the ovum.