Breeders routinely produce equine embryos in vitro (in the lab) by maturing oocytes and fertilizing them using intracytoplasmic sperm injection (ICSI), a process in which a single sperm is injected directly into the egg. However, historically, researchers have conducted little work examining how systemic therapies administered to mares affect oocyte quality and subsequent blastocyst development in vitro. Kristen Loncar, DVM, MS, Dipl. ACT, of Equine Medical Services, in Columbia, Missouri, addressed this question during her presentation at the 2025 American Association of Equine Practitioners Convention, held Dec. 6-10, in Denver, Colorado.

How NSAIDs Affect Mare Reproduction

Many mares need daily nonsteroidal anti-inflammatory (NSAID) administration to maintain adequate quality of life. However, phenylbutazone (Bute) can block important locally acting signaling molecules called prostaglandins, which might cause an increased incidence of hemorrhagic anovulatory follicles (follicles that grow but fail to release an egg) and decreased uterine clearance after breeding. “Follicles will get bigger and bigger, but (the mares) won’t ovulate,” said Loncar.

Normally, luteinizing hormone (LH) triggers ovulation and the formation of the corpus luteum, which then produces progesterone to support early pregnancy. The LH surge also leads to an increase in prostaglandins within the follicular fluid. These prostaglandins play a role in forming the follicular wall and eventual follicle rupture. “If we are blocking those prostaglandins, we aren’t going to see that final maturation, and we’re not going to see that ovulation,” she added.

After breeding, prostaglandins help the uterus contract to expel any dead sperm, excess fluid, and inflammatory debris. Loncar describes this as a “very normal, very necessary processes.” Blocking prostaglandins causes mares to retain fluid for multiple days, so they might need additional therapies.

How NSAIDs Affect Equine Oocytes

Loncar’s team sought to investigate whether Bute affects egg quality or embryo development. In the first study they used 14 Quarter Horse-type mares ranging in age from 7 to 15 years divided into Group A and Group B. Group A received liquid Bute orally at a rate of 4.4 milligrams/kilogram once daily for 10 days. Group B served as a control. On Day 10 all mares underwent transvaginal aspiration (TVA) with subsequent in vitro oocyte maturation and ICSI. The team used frozen-thawed semen from a Quarter Horse stallion that had previously yielded good ICSI outcomes, said Loncar.

They recorded oocyte recovery and maturation rates, cleavage rate (speed of cell division) after fertilization, and blastocyte rates (percentage of fertilized eggs that develop into an embryo) after ICSI. After the cycles were finished the researchers rotated groups, so each mare served as her own control.

Results showed no significant differences in any measured categories and no statistical difference between groups for day of blastocyst development.

In a second study Loncar and her colleagues included different formulations of Bute. This time they divided 15 mares aged 7 to 11 into three groups. Group C received liquid Bute intravenously; Group D received paste Bute orally; and group E served as an untreated control. On Day 10 all mares underwent TVA with subsequent ICSI. The team recorded oocyte recovery and maturation rates, cleavage rates, and blastocyst rates. Once again they rotated mares so each group received each treatment.

The researchers saw no statistical differences in any categories, but they noticed the blastocyst rate for mares treated with paste was lower than others. Loncar theorized there might be something in the paste that could potentially influence oocyte and blastocyst formation, but more research is needed. In addition, they saw no statistical difference in what day the embryos developed.

Take-Home Message

“Bute did not cause a significant decrease in blastocyst production when given to healthy mares for a 10-day period,” said Loncar. These results are similar to those in other species, but contradictory to a previous study in horses.

She said she found the results encouraging: “We feel very comfortable keeping mares on Bute if they require it to maintain an adequate quality of life, so we are not going to change how we’re handling these mares.”