On the surface, freezing canine semen seems like a perfect solution for preserving valuable genetics for the future. Indeed, it’s estimated that—if properly handled— frozen semen may remain viable for upwards of 10,000 years. That’s a great deal of peace of mind for breeders who want to maintain the pipeline of genetic contribution from a stud dog; especially ones that, in many cases, represent the culmination of their life’s work.

For decades, freezing canine semen has been billed as both a means of dipping back into a bygone gene pool, as well as an insurance policy against the untimely loss of a promising young prospect. We’ve all heard stories of freak accidents cutting short the lives of exemplary dogs before having the opportunity to optimize their full genetic contribution and better their respective breeds. Other times, a pedigree takes an unintended tangent, and many breeders consider it helpful to revisit past genetics to steer their breeding program back on track. In both these scenarios, the forethought to accumulate a bank of quality frozen semen can be a lifesaver for breeders.

In practice, however, frozen semen has at best a mixed reputation. As a veterinarian who devotes much of his clinic time to canine reproduction, I’ve discovered in my career that very few clients understand what’s involved in the freezing process. More notably, fewer still are provided with reasonable expectations for achieving pregnancies when the time comes to thaw and utilize the semen they’ve bought or paid to freeze and store—in many cases for years. Other times, clients seek to breed their female using frozen semen and lack a general familiarity with the costs, efforts, and expected outcomes of this endeavor.

My intent is not only to thoroughly explain what’s involved in the process of freezing, but more importantly, to convey to interested breeders the pros and cons of this process; both in the near term and in the future when the semen is utilized. In my experience, some veterinarians, and many private semen banks, fail to paint an objective portrait of frozen semen. This is a disservice to breeders, that can generate a great deal of frustration and unanticipated expense along the way.

What’s Involved with Freezing Semen?

First, let me dispel a commonly held myth: Frozen semen is not simply fresh semen stored at cold temperatures. Semen is a living matrix of moving spermatozoa (sperm), often upwards of a billion of them in a typical sample from a young, healthy hunting dog. The sperm swim freely in supportive fluids produced by the prostate gland, and they are generally resistant to subtle swings in temperature. However, placing a fresh sample directly in the freezer would certainly be a death sentence for the sperm, as the DNA packed tightly in their microscopic heads would swell with the formation of ice crystals and readily burst below 32° F—like a beer chilled too long in the ice box.

The process of freezing semen is laborious and time intensive, but by no means overly complicated. The sperm are carefully separated from their natural fluids and bathed in a mixture of osmotically balanced nutrients formulated to protect against freezing damage. The ingredients in this cocktail, referred to as cryoprotectants, vary widely. There are many commercial products available, and even more homebrewed or proprietary formulas with, lamentably, very little standardization across semen banks with regards to recipes or technique.

Once the sperm acclimate to their new substrate, the sample is gradually cooled to refrigerator temperatures over a period of hours. It’s then loaded into 0.5 mL straws or distributed into pellets before finalizing the freezing process.

The actual deep freezing involves ridiculously cold temperatures. We’re aiming to pause nearly all metabolic processes here, not just protect against spoilage. Neither a commercial freezer nor dry ice can come close to providing enough chill, which is why semen freezing involves liquid nitrogen. At roughly -290° F, liquid nitrogen provides a relatively stable environment to achieve this depth of freezing. This commercially available liquid can be safely stored in vacuum-walled metal tanks, along with the frozen samples, more or less indefinitely, with virtually no further degradation of the semen over many lifetimes.

Smaller, modified versions of these storage vessels can be used to transfer samples and transport them around the country or globe. In fact, frozen semen is often the preferred sample type for shipping overseas, as these portable tanks are designed to preserve samples far longer than those intended for fresh, chilled semen samples. If you’ve ever had a timely and perishable package held up in customs, or delayed by a winter storm, then you understand some breeders’ strong preference for shipping frozen semen across the pond.

When ready to inseminate, the straws or pellets are brought to body temperature in a water bath and loaded into a syringe for breeding. Once warmed, the sperm shake off the chill of the liquid nitrogen and resume swimming in pursuit of an egg to fertilize with as much enthusiasm as they showed days—or decades—ago.

What is the Downside to Breeding with Frozen Semen?

My largest gripe as a reproductive veterinarian lies in the fact that semen banks don’t discuss the cons of this process loudly enough, or often enough. While there are many reputable semen storage facilities across the globe, there are many that seem eager to take your money to freeze and store, with very little concern for a breeder’s success when that sample is utilized down the road. Freezing semen successfully is but one component to the process of bringing to life those valuable genetics.

First, breeders must understand that frozen semen is inherently compromised. If a bath in foreign chemicals, followed by a plunge in liquid nitrogen, sounds to you like harsh treatment for a biological sample, you’re well on your way to understanding just how much the freezing process affects the quality of semen. It’s not uncommon for a fresh sample, with better than 90 percent motility, to thaw at half that quality or less. Reproductive veterinarians get excited when we thaw a rare sample for breeding that exceeds 75 percent motility.

Freezing can cause visible damage to the shape and structure of the sperm—called morphology—that’s obvious under a microscope. This damage often inhibits a sperm’s ability to swim normally, or otherwise fully participate in fertilization. Other times, the damage to a thawed sample is subtle but nonetheless equally detrimental to its overall fertility in ways that don’t show up under the lens. As stated previously, there are many variables—of both material and technique—in the freezing process, and there are countless more individual factors that could affect the way each stud dog’s sample responds to this type of harsh treatment.

Secondly, frozen semen has an incredibly short window of viability once thawed. This fact is particularly disheartening given that canine semen is perhaps the heartiest in all the animal kingdom. Where a fresh canine sample could sit on the shelf at room temperature for several days and still achieve a pregnancy, even the highest quality canine samples offer fewer than 12 hours of viability after being frozen then thawed.

With that in mind, its use in breeding must be strategic and timely. Toss aside old wives’ tales and forget about breeding blindly on some arbitrary day in the cycle. Ovulation timing, through progesterone blood tests in the female, is critical. Success is further optimized by incorporating even more hormone assays, such as the luteinizing hormone, which will help dial in the ovulation window more tightly.

Furthermore, frozen semen requires the use of advanced assisted breeding techniques provided by a licensed veterinarian, such as transcervical or surgical insemination. Vet bills could easily reach $2,000 or more, in ovulation timing and the breeding procedures. These extra costs are incurred in addition to stud fees, and the funds sunk into long term storage of a frozen sample. Unfortunately, these expenses are rarely discussed when breeders agree to freeze and store a stud dog. Nonetheless, they contribute to many instances of sticker shock when it’s time to thaw and use a few straws.

Adding to the complexity, it’s disturbingly common for the paperwork accompanying a shipped frozen sample to not reflect the reality of what we’ve just thawed and intend to use. Motility assessments conducted by certain semen banks routinely exaggerate the quality of the final product. In my experience, at least one-in-five samples that we receive thaw at just a fraction of the motility that was advertised in its accompanying paperwork. This news lands poorly on breeders who have invested in blood tests, only to discover on breeding day that our sample of swimmers is too lean or weak to get the job done.

Lastly, but certainly not least importantly, even with great ovulation timing and a passable post-thaw motility, pregnancy rates with frozen semen pale in comparison to those achieved with fresh samples. Sometimes the reason is obvious at the time of breeding, other times breeders and veterinarians are left scratching our heads following a miss that, on paper, checks all the boxes needed for a pregnancy. By all means, if a stud is still alive and producing litters, always prioritize breeding to a fresh sample whenever possible. I cringe when I hear breeders request frozen over fresh, so they can simply receive the sample early for convenience’ sake, to use when their female is ready. While I understand the motivation, in these instances the benefit of convenience is far outweighed by the hefty hit to pregnancy rates achieved by this method.

Strategies for Success when Breeding with Frozen Semen

The adage, “freeze early, freeze often,” exists for good reason. The very best frozen samples begin as excellent fresh contributions from young, healthy stud dogs. As soon as a young prospect demonstrates his genetic value to a breeding program, begin the process of collecting him. Far too many breeders wait until (or beyond) the twilight of a stud’s most prolific breeding years to begin freezing; often when semen quality has begun to decline. With the degradation seen during the freezing process, shortcomings in semen quality from an older male will be amplified. About ten percent of the requests I get to freeze come from breeders who are faced with terminal illness or debilitating injury in their stud dog. By then, it’s far too late to store anything of value.

Keep a diverse inventory of samples in storage. Breeders should accumulate samples from multiple collections at different times of year. And don’t hesitate to use a few breeding doses periodically in highly fertile females along the way to demonstrate the potency of what’s been stored. As a veterinarian, I feel highly optimistic about using frozen semen if straws from that same sample have already been used successfully.

Taking multiple collections also offers protection against loss or damage to stored samples. While I mentioned that frozen semen can last up to 10,000 years, I can assure you that human error, or acts of God, seem to strike with far greater frequency. Flood, fire, tank failures, and disgruntled employees have all been blamed for the untimely thawing of frozen samples. I’d also argue that if you haven’t utilized a sample in 20 years, it’s probably no longer worth paying to store it.

Most importantly, set yourself up for success. Breeding to frozen semen is a strategic move to utilize past genetics, not a Hail Mary. With so many variables affecting its viability, increase your odds of success with good ovulation timing in a young, fertile female. If a pup from that frozen semen litter embodies what his sire represented, hang on to him as a genetic touchstone that can more efficiently move that valuable DNA forward in the future.

In my opinion, there’s far too much nostalgia wrapped up in frozen semen. It’s common for the genetics of a bygone stud dog to hold more merit than those that comprise the gene pool today. A solid breeding program should always have an eye towards the future, with a mission statement to create better ambassadors of the breed. If breeders need to dip back into their past too often, it may suggest those goals aren’t being met.