Changes in LH pulse frequency and amplitude in intact mares during the transition into the breeding season.

The research focuses on studying how changes in the frequency and amplitude of luteinizing hormone (LH) pulses in mares (female horses) during the transition into breeding season are affected by the length of day, either naturally or artificially altered.

Experiment Setup and Groups

• The experiment included two groups of mares that were subjected to two different conditions: one with an abrupt, artificial increase in daylength and the other with a naturally lengthening day.

Observations

• In both groups, researchers observed that the frequency of LH pulses increased as the year progressed. This increase was inversely correlated with a decrease in pulse amplitude, indicative of a relationship between the two LH pulse characteristics.
• In some cases, a constant (non-pulsatile) pattern of LH secretion was noticed in mares as the day of ovulation approached.
• Mares exposed to an artificial increase in daylight saw their mean LH pulse frequency peak and the onset of breeding season occur earlier than those experiencing natural day length variations.
• There were frequent collections of blood samples during the experiment, and these samples exhibited increased mean serum LH concentrations over time.

Up to Ovulation

• During the 60-day period leading to ovulation, and accompanying an increase in LH pulse frequency, the researchers documented a rise in mean daily serum LH values, but this increase occurred only three days prior to ovulation.
• The intensity of the LH surge around ovulation wasn’t constant through the breeding season; the surge associated with the second ovulation was found to be sharper than the one before the first ovulation.

Conclusions

• The findings of this study reinforce the hypothesis that changes in the frequency and/or amplitude of LH pulses, prompted by variations in photoperiod (the length of day), could play a significant role in initiating the breeding season in mares.