Postpartum Breeding
Basic Information 
Definitions
• Foal heat is the first postpartum estrus. Behavioral estrus and ovulation typically accompany foal heat. Expression of behavioral signs of estrus, however, can be absent or subtly demonstrated in individual mares, especially those nursing their first foal.
• Second natural estrus refers to the second postpartum estrus period when no manipulation occurs to hasten the beginning of this estrus (ie, a normal intraestrus period occurs).
• Short cycling after foal heat ovulation is the hastening of the second estrus period by inducing luteolysis. To achieve the shortest parturition-to-conception interval, if the mare is not bred on foal heat, the corpus luteum is typically lysed at 5 to 6 days postovulation with prostaglandin F2α (PGF2α) or one its analogues.
• Delaying foal heat is a breeding strategy in which the foal heat ovulation is delayed by exogenous steroid hormone administration.
Goals
• January 1 is the universal birthday for many breed associations in the Northern Hemisphere. Foals born earlier in the calendar year command superior sales prices to yearlings and 2-year-olds. Thus the goal is to produce healthy foals early in the year.
• Because horses have an average gestation length of 333 to 345 days (range, 320 to 360 days), postpartum mares must become pregnant within 1 month of foaling to maintain a 12-month foaling interval.
Overview
• Begins, on average, 5 to 12 days after parturition.
• Foal heat ovulation occurs, on average, 10 to 16 days after parturition.
• Parturition to foal heat ovulation intervals tends to decrease with increasing day length. According to Loy (1980), 33.3% of mares in January and February, 55.3% in March, 64.8% in April, and 82.9% in May had intervals less than 10 days.
• Should begin approximately 15 to 16 days after foal heat ovulation, with this ovulation occurring 19 to 22 days after the foal heat ovulation.
• First cycle interovulatory intervals less than 14 to 16 days are considered abnormal. Diagnostics to determine causes of early luteolysis should be considered. Potential diagnostics include transrectal palpation and ultrasonography, vaginal examination, endometrial culture, endometrial cytology, and endometrial biopsy.
• Short photoperiods are the primary cause of the higher anestrous periods earlier in the year.
A 5-year retrospective study of a breeding farm composed of 120 to 150 broodmares in southeast Texas had the following anestrous incidence rates: 26.6% (December and January), 23.3% (February), 14.0% (March), 2.6% (April), and 0% (May).• See Adjunct Therapies below for prevention of seasonal anestrus.
Influences of nutrition, body condition, and lactation:
• Mares fed low nutritive rations for the last 3 months of pregnancy and the first month postpartum have lower pregnancy rates and higher embryonic loss rates than mares fed high nutritive rations.
• Mares with thin body condition have longer intervals from parturition to reproductive cyclicity than do mares with good to obese body condition.
• Lactational anestrus in mares is controversial. Season (photoperiod), nutrition, and body condition should be examined when lactational anestrus is suspected.
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