Figure 1. Signs of heat.

We have an extensive AI-breeding program with our Dexter herd and we've found that we get the best conception rates during the "longest" days of the year:  mid-June to mid-July.  We keep calendar records on all of our cows (20 cows) and their heat cycles.  We don't use any hormones or drugs to induce their heat cycles or ovulation, so observation is our method.  We do AI-breeding only during a 4 to 6 week long period each summer.

With that background experience, what we've noticed with our Dexters is that we get the best conception rates if we inseminate the cow at the end of her standing heat.  For example, if we first observe her standing to be mounted between 6:00 and 10:00 am, then we call to have the AI tech come to breed her sometime between 6:00 and 10:00 pm. 

In the winter, when the daylight hours are shorter, the cow's heat cycles are also shorter.  If you are synchronizing the cow and using a "timed" AI procedure, follow the AI tech's recommendation on the timing for breeding the first time that you try to breed her.  If the cow doesn't settle after this first attempt, then during her next heat cycle, try to AI breed her 2 to 4 hours sooner than what you did for the first attempt.  It is imperative that she be AI-bred several hours before she ovulates, so it's better to be a little bit early instead of late.

Good Luck!