From biosecurity and nutrition to exercise and examinations, Jennifer Hatzel, DVM, MS, DACT, of Colorado State University’s Equine Reproduction Laboratory, explained what veterinarians should know about managing pregnant mares and foaling prep during a Burst session at the 2024 American Association of Equine Practitioners Convention.
First Trimester
In the first trimester, the foal grows from the size of a pea to a chipmunk. In terms of biosecurity, ensure all new horses brought onto the property are vaccinated, and keep pregnant mares away from any new horses. (This should be the standard throughout gestation.)
“That includes horses that are going to shows,” said Hatzel. “That does mean they are leaving the property and coming back and might have been exposed to something.”
Veterinarians can perform reproductive exams on the following days:
- Day 12-14 after ovulation to confirm pregnancy.
- Day 14-16 to ensure it is a singleton. Now’s the time to crush twins/triplets.
- Day 25 to detect the heartbeat.
- Day 35, around the time of endometrial cup formation, to reconfirm pregnancy.
- Day 55-90 to examine the genital tubercle for a “gender reveal party.”
In terms of appropriate exercise for broodmares, “We definitely want to keep them moving, keep them feeling good,” Hatzel said. “But we don’t encourage [owners] to add onto what they’ve already been doing.”
No nutritional changes are required during the first trimester. Owners should deworm based on fecal egg counts, and mares should be at least 60 days pregnant before receiving any necessary vaccines.
Second Trimester
In the second trimester (Days 120 to 240), the foal grows from the size of a kitten to a hardy raccoon. Owners don’t need to implement additional biosecurity changes at this time, and they can maintain the mare’s current level of exercise. Hatzel advised against strenuous exercise, especially in hot climates.
Reproductive examinations will vary markedly based on client and breeding operation.
“We have clients that like to kick their mares out to the back 40 after they have determined they are pregnant and not see them again until they waddle up in the spring,” said Hatzel. “However, I do advise that at least a fall pregnancy exam prior to the initiation of pregnancy vaccines is warranted, so that way they are not wasting their money on vaccines for a mare that may not be pregnant.”
Third Trimester
In the third trimester (Days 240-340), the foal grows from the size of a beagle to its size at birth. This is the period of maximal growth, so the mare’s nutritional demands will change and increase. Be certain to remove mares from tall fescue fields.
Veterinarians can perform transrectal or transabdominal ultrasound examinations during this period. Transrectal exams can locate the orbit to ensure correct orientation of the foal for parturition. Ultrasound exams can measure the combined thickness of the uteroplacental unit (CTUP) and try to identify signs of ascending placentitis, as well as confirm fetal viability.
For high-risk mares, reevaluate every 14 to 30 days.
Most mares should have their exercise reduced during the third trimester, but it’s important to keep them active enough to support lymphatic drainage.
At the eleventh hour, two to four weeks prior to foaling, it’s time to go through your “foaling checklist,” which includes the following:
- Removal of a Caslick’s, if present
- Submitting a blood sample from the mare (within two weeks of parturition) to test her antibodies toward neonatal isoerythrolysis.
- Utilizing mammary secretions to determine her readiness for foaling.
- Using some form of alert system to determine when the mare is in active labor.
- Utilizing a camera system to monitor her when away from the farm (if possible).
“I think ultrasound evaluations have a lot of value throughout the entire length of gestation, not just at the forefront, so keep checking those pregnant mares,” said Hatzel.
Related Reading
- Factors Affecting Pregnancy Rates in Mares Bred With Frozen Semen
- Disease Du Jour: Equine Neonatal Field Care
- How To Manage the Late-Gestation/High-Risk Mare in the Field
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