Heat in June. Litters in September.

A litter born in late September was conceived in late July. A litter conceived in late July was carried by a stud whose semen quality was set, almost entirely, by what happened to him this week. That timeline — the one most owners and quite a few breeders don't think about — is the reason a heatwave in June is, quietly, a fertility event.

This is the fourth piece in our heat-week series and the one we'd argue is the most marketplace-relevant of the four. The first three were about the animal in the heat: the signs (Monday), the prevention (Tuesday), the first aid (Wednesday). This one is about what the heat is also doing — to the sire's reproductive cycle, to the bitch's earliest embryonic days, and to the litters that will be born two and three months from now.

It is a less dramatic piece because the harm is invisible at the time. But the bills arrive on schedule, six to ten weeks later. Worth knowing about, especially this week.

The sixty-day window most breeders only half-know

A dog's spermatogenesis cycle — the journey from stem cell to ejaculated sperm — takes about sixty to sixty-five days in canids. A stallion's is slightly shorter, around fifty-seven. What this means in practice is that today's ejaculate is not made today. The sperm a dog releases on Friday represents a cohort of cells that started their journey at the end of April. The sperm he releases on a particularly hot day next week represents cells that started their journey at the beginning of May.

And, crucially, the sperm he will release in late August represents cells that are being produced this week — during the heatwave.

That offset is the whole story of this piece. Heat does not damage today's semen quality in any meaningful way. The cells already in late maturation are largely past the heat-vulnerable window. What heat damages is the production line: the cohort of dividing germ cells that, sixty days from now, will be the ejaculate of late August. By the time the breeder ships that semen or covers a bitch with that dog, the heatwave is a memory and the damage has been done. The connection between the two events is not obvious unless you are looking for it.

What the heat actually does

A dog's or stallion's testicles sit outside the body specifically because spermatogenesis requires a tissue temperature about three to five degrees Celsius cooler than core body temperature. The scrotum is, in a literal sense, a temperature-regulating organ. The vascular plumbing — the pampiniform plexus, the cremaster muscle — exists for one purpose: to maintain that gradient.

In a heatwave, the gradient gets harder to hold. Ambient air temperature in the high twenties or low thirties, particularly when combined with humidity, raises scrotal surface temperature above the operating tolerance of the dividing cells inside. The dividing cells — spermatogonia and early spermatocytes — are the most temperature-sensitive of the lot. Heat exposure of even a few hours over several consecutive days can produce measurable disruption: increased rates of cell death in the germ epithelium, abnormal mitotic division, faulty chromosome segregation. The cells that survive but were disrupted go on to mature into sperm that look, six to eight weeks later, like sperm with reduced motility, abnormal morphology, or reduced count.

The well-cited studies — Threlfall's work from the eighties, England's review papers, the work coming out of Davis on stallion summer subfertility — all converge on the same number. Four to eight weeks after a heat event is when the ejaculate quality dips. It then recovers, on a curve, over the following two months as fresh, unstressed cells mature through the production line.

That is the line. Heat hits the upstream end of it. The downstream end shows the bill a quarter later.

The bitch's side, which gets discussed less

Bitches and mares are not exempt — they are simply subjected to a different mechanism. The most welfare-relevant timing for a bitch is the first three weeks of pregnancy, when embryos are implanting and the placenta is establishing. Heat stress in those three weeks — fever from any cause, but also environmental heat exposure in a non-cooled bitch — is associated with elevated rates of early embryonic loss. The owner does not see this as a "lost litter"; they see it as a smaller litter than the dam usually carries, or a pregnancy that didn't quite stick. Many do not connect the small numbers to a hot week two months earlier, because nothing dramatic happened at the time.

Mares experience this too, and the equine veterinary literature is more developed on it than the canine: the term "summer subfertility" is well-established. The mechanism is similar — environmental heat raising core temperature during the most temperature-sensitive window of embryonic development. The result is the same: a covering that doesn't take, or a pregnancy that resorbs in the early weeks, neither of which is necessarily attributed to the heat by the breeder unless they have been keeping records over years.

The honest thing to say is that the bitch's contribution to heat-related fertility loss is harder to quantify than the dog's, because the dog's side is measurable on a single semen evaluation while the bitch's side requires repeated breeding over multiple cycles. But the effect is consistent enough across well-run kennels and studs to be worth taking seriously.

What this means for stud bookings this summer

This is the practical paragraph, and the one breeders we know will appreciate most.

If you are a breeder who has booked a cover for late July or early August on the assumption that the stud will perform to his usual standard — particularly if the stud is in the south of the country, where the heat is harshest — the honest position is that he may not. The four-to-eight-week window after a heat event covers exactly the period when the stud's semen will look poorer than his profile suggests. A cover taken in that window may produce fewer puppies than the breeder is paying for, or fail to take at all.

There is no shame in either side flagging this. A welfare-aligned stud owner who has just had his dog through a hot week will tell his bookings that the next month is not the right window for a cover — and offer to defer, or to refund. A welfare-aligned bitch owner with a planned cover in late July will phone the stud owner and ask what the dog's conditions have been like this week. The conversation, in both directions, is what keeps litters from being smaller than they should be and bills from being paid for puppies that don't exist.

Frozen semen, banked at full quality before the summer, is the obvious hedge. Many of the breeders we talk to who use frozen routinely do exactly this: they collect and bank in spring, then ship from store through summer, and only collect fresh again in autumn. It is more expensive in the short term and substantially cheaper in the long term, particularly for in-demand stallions whose summer ejaculates would otherwise serve fewer mares than they could.

What good breeders do during a heatwave

A short list, in case the timing matters to you this week.

They hold off on covering bitches during a heatwave week, even if the cycle timing is otherwise correct. Better to miss the cycle and try the next one in autumn than to take a cover during a heat-elevated window and risk small numbers.

They cool the breeding shed and the stud's accommodation. The stud who spends his days in a 22°C kennel and his nights in a 19°C house comes through a 30°C week with substantially less testicular insult than the stud kennelled outside in shade.

They time semen collections for early morning, when scrotal temperature is at its daily lowest, rather than afternoon.

They keep records. The breeders who have been doing this for twenty years can tell you, by year, which summers cost them litters and which didn't. The pattern is real enough that it shows up in their own data, even when the wider profession is still arguing about whether the effect exists.

They are honest with their buyers. A puppy advertised in October for delivery in December is a puppy whose father was working through the heatwave you are reading about now. The breeder who tells you that, and shows you the semen evaluation done after the heat, is the breeder you want. The one who doesn't volunteer the information may not be hiding anything — many are simply not aware of the offset — but the better operators do know, and do say.

A note for buyers reading this in spring

For prospective puppy or foal buyers who happen to be reading this piece outside of a heatwave week: the implication for you is just one question, asked of any breeder before a litter is conceived. Was the sire heat-exposed in the eight weeks before the cover? The answer should be specific. A breeder who says "no, he was in the air-conditioned barn" is telling you the relevant truth. A breeder who says "I don't know" is also telling you the relevant truth — that they aren't tracking it. Both are useful answers; the breeder you don't want is the one who is vague.

A semen evaluation done in the week of the cover answers the question directly. It is a cheap piece of due diligence. We mention it because we encourage breeders selling on GenoVaq to do it routinely, and we encourage buyers to ask for the report. The marketplace itself does not, today, surface a stud's heat-exposure history — it surfaces the lifetime testing record. The week-by-week exposure data is the conversation the buyer has with the breeder, and worth having.

The point of the piece

Heat in late June is not, in this country, a once-a-decade story any more. The summers are warmer; the heatwaves are longer; the consequences for breeding animals are more frequent than they were when many of the current standard practices were established. A welfare framework that doesn't account for the sixty-day offset between a heatwave and its visible bill is a welfare framework that is missing the most predictable kind of loss in the calendar.

The thing to remember is the offset. The dog whose semen was banked in May is fine. The dog who has spent this week in the sun is producing the August batch. The litter that does or does not happen in September was decided this week, not next week.

It is the kind of thing that, once you see it, is hard to unsee. Worth telling your stud owner, your veterinarian, and your buyer about — because the dogs and mares cannot tell you themselves, and the bill, when it arrives, will look like simple bad luck unless someone bothered to keep the dates.

— Rene