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. In my experience it is also consistently underestimated — producers compare seasons without accounting for weather, and when the 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 temperatures above 80 degrees Fahrenheit combined with the humidity that defines 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 two 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 by semen 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 the same time.

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.

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 with demand 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 specifically — 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 accommodate around producer schedules, but when it is, it helps.

What to expect from summer conception rates

My fall and winter AI programs in Florida, run on well-synchronized cows in good condition, produce conception rates in the 55 to 65% range as a typical target. Summer programs on the same producers with the same protocols run 40 to 55%. That 10 to 15 percentage point drop is normal for this climate. It is not a protocol failure. It is the biology of bovine reproduction in a hot, humid environment, and the expectation going in needs to reflect it.

Summer ET programs with quality embryos and well-managed recipients should still achieve 55 to 65% conception rates, assuming the recipients are not themselves severely heat-stressed and are in adequate body condition. The advantage over summer AI is real and consistent in my experience.

When to delay rather than push

There are seasons and situations where waiting is the right answer. A recipient pool that has been under heat stress all summer, that is thin going into August, and that the producer is hoping to sync 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 from summer, and are on improving nutrition going into the breeding season — 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.