What triggers the mares transition into anestrus?

Photoperiod.  Is it the amount of darkness or is it the amount of light that determines photoperiodism? Hmmmm.

The pineal gland is what I call the super regulator of reproduction.  It plays a role not only in when the mare cycles but also in regulating the onset of puberty.  The circadian rhythm (an animals internal clock) and a mare's reproductive seasonality is largely determined by the hormones under the control of the pineal gland.  We know that light has an effect on the pineal gland and if we understood that role, then manipulation of said gland becomes possible and subsequent manipulation of her reproductive cycle.

So is it the amount of light that is detected by the pineal or is it the amount of darkness?  Scientist tell us that a substance called melatonin is secreted from the pineal gland and is secreted primarily at night.  In many species melatonin fairly accurately tells how long something is dark.  It has been shown that pulses of light in the middle of the dark period "fool" a mare into thinking that the night has not been as long as it is.  If the night is not as long, then the days must be longer and so it is OK to begin her reproductive cycle again.  

NOTE: While this may help to solve the length of light vs. darkness dilemma, it still does not eliminate the anestrus problem.  Many other factors also are involved in determining a mare's anestrus cycle including nutrition and temperature (degree of coldness).  Extreme cold can override sticking a light bulb in the mare's stall.