Running AI and ET Programs in Southeast Summer: What Heat Actually Does

Running AI or ET programs in Florida and the Southeast from June through September is not the same work as running them in October. The biology is harder, the conception rates are lower, and the gap between a well-run fall program and a well-run summer program is real. It is also consistently underestimated — producers compare seasons without accounting for weather, and when summer numbers come back lower, the protocol or the technician gets the scrutiny instead of the calendar.

What heat stress does to the reproductive biology

Heat stress — prolonged exposure to the temperatures and humidity that define a Southeast summer — disrupts reproductive function at several points. In cows undergoing synchronization, heat stress during the periovulatory period impairs oocyte quality, follicular development, and GnRH response. In early pregnancy, elevated core body temperature is directly embryotoxic in the first few weeks post-conception, before the embryo has had time to signal maternal recognition of pregnancy.

For AI programs, both problems are active simultaneously. The oocyte developing in a heat-stressed cow, fertilized in a heat-stressed uterine environment, trying to establish a pregnancy while the cow is still heat-stressed. Every stage is working against you at once.

For ET programs using frozen embryos, the picture is different. The embryo was collected from a donor that was not in a Southeast summer, at a time when oocyte and early embryo development happened under better conditions. The heat stress problem for ET programs is concentrated on the recipient's uterine environment and luteal function — not on the embryo itself. That is a meaningful and practical difference.

This is one of the practical arguments for ET over AI in summer programs in the Southeast. You are not asking a heat-stressed oocyte to become a viable pregnancy. The embryo already survived the hardest part elsewhere.

What the conception rates actually look like

Summer AI in Florida is genuinely difficult. In my experience, summer AI conception rates run in the 15 to 20 percent range — a fraction of what a fall program on the same operation produces. That is not a protocol failure. That is what bovine reproductive biology produces in sustained heat and humidity. Producers who have only run fall programs and try their first summer AI are often caught off guard by how stark the difference is.

Summer ET programs with quality frozen embryos hold up considerably better. I averaged around 60 percent conception on ET programs this season, though the range by group was wide. The variables that drive that variation are worth understanding: some donors are reliably more fertile than others regardless of embryo grade — operators who track results across seasons notice this pattern. IVF and conventional embryos behave differently post-transfer. The lab the embryos came from matters in ways that are not always obvious from the grade sheet. And the weather during the first 35 days after transfer — when early pregnancy establishment is most vulnerable to heat stress — can swing results in ways that are completely outside your control on transfer day.

Management adjustments that move the needle

Timing handling to early morning is the most consistent improvement available. Cattle that are moved, sorted, and worked before 8 AM spend less time heat-stressed during the procedure itself. Shade access in holding areas matters even for short waits — 30 minutes standing in direct sun in a crowded lot before going through the chute adds to the cumulative heat load that affects GnRH response.

Water access during a sync program is obvious but frequently underestimated in terms of volume. Cows under heat stress drink substantially more than maintenance, and a holding facility whose water supply does not keep pace during a full program produces a subtly water-stressed group that does not respond to hormonal protocols the same way well-hydrated cattle do.

On GnRH injection days — the doses that drive ovulation timing — I work as early as practical. The hormonal response is more reliable before the peak heat load of the day. This is not always possible to fit around producer schedules, but when it is, it helps.

When to delay rather than push

There are seasons where waiting is the right answer. A recipient pool that has been under heat stress all summer, thin going into August, being synced for an early-fall transfer window is a program built on a shaky foundation. A program started in October — when temperatures have dropped, cattle have recovered, and are on improving nutrition — will materially outperform the August version.

If the embryos or the breeding window are not time-sensitive, the calendar conversation is worth having before committing to a summer program. The biology will be what it is regardless of when you start the CIDR.