Plasma LH levels in the mare during the oestrous cycle.

The researchers studied the pattern of Luteinizing Hormone (LH) in mares during their oestrous cycle and found that it differs from other species, suggesting it may explain the larger number of second ovulations detected in these cycles.

Methodology

• The study monitored the concentration of LH, a hormone intrinsic to the reproductive process, in the plasma of eleven mares over 10 weeks, covering twenty-six oestrous cycles.
• The LH levels were determined by using a heterologous radioimmunoassay, which employs radioactive isotopes to measure the concentrations of specific substances, in this case – LH.
• The standard used for this assay was highly purified equine LH.

Findings

• The average cycle duration for the mares observed was around 20.5 days, and ovulation typically happened around 4.3 days after heat was first detected.
• It was noted that LH levels stayed low from the 5th to the 16th day of the cycle. They then gradually increased over several days and maintained rising even beyond levels noted immediately prior to ovulation or on the day of ovulation itself.
• A significant dip in LH levels did not occur until the 3rd day post-ovulation, after which the levels began to gradually decline towards dioestrous values (indicating the period between two consecutive oestrus).

Interpretation and Implications

• The study found that the pattern of plasma LH in mares is unique compared to that of other species. Such a difference, particularly the persistence of high concentration of LH, is attributed to the extended half-life of the endogenous LH (LH produced within the body).
• This prolonged presence of high LH concentration might be responsible for the high number of second ovulations observed in many oestrous cycles. This could have implications for breeding schedules and fertility treatments for horses.