The decision if or not to spay or neuter your service dog candidate and when to do it depends on many factors:
the breed or mix your dog is
sex of the dog
if or not you can live with an intact animal in your environment
if or not your dog is frequently displaying a few reproduction-related behaviors
if or not the certification you do requires it
Spay or Not and At What Age?
You’ll hear many things about whether or not and what age to alter a dog. You need to do your research before you decide what is appropriate for you, your dog and your situation. This is an especially important consideration for service dogs since certification depends on the behavioral and physical abilities of the dog. Spaying and neutering too early can result in health and behavioral issues in many dogs. One thing across the board is to avoid altering puppies when they are under 6 months of age. A few breeders such as doodle breeders may require it in their contract or may even have the pups already done before you take them home at 8 weeks.
Why & When Is Altering Done?
Spaying and neutering is typically done as a prevention for population explosion/unwanted dogs, to prevent health issues such as cancers (experts are now question the validity of this belief.) and to reduce sexual-related behaviors of roaming, humping and urine markingin males dog. (The last two behaviors may be caused by other factors as well so may alss be unaffected by neutering). A common practice in some regions has been to alter the puppies as young as 8 weeks before they go to their new homes; this is seen most commonly in dogs from shelters and rescue organizations and some breeders. Recent long-term studies have shown juvenile altering especially for some breeds (golden retrievers, labradors, German shepherd dogs) is not a good idea.
What is Done to the Dogs?
Spaying and neutering a dog removes the sex organs and hormones associated with them. In females the uterus is removed (as in human hysterectomy) and the ovaries. In neutering (also known as castration) the male’s testicles are removed.
Long term Effects of Spaying Too Early
Dogs spayed or neutered as juveniles (less than 6 mos old) show many undesirable long-term effects. What occurs is that the hormones normally emitted by the sex glands are not present and this affects both the temperament and physical development of the dogs in question. In females, fearfulness, overly long leg bones, low bone density issues, hip dysplasia, ACL tears and increased risks of cancer have been identified. In males, all of the above except fearful nature is replaced by aggression.
Two long-term studies of a large number of dogs show behavioral and physical effects are a real possibility.
*In 1998 and 1999, 1444 Golden Retrievers by the Golden Retriever Club of America
*German Shepherd Dogs
Overall Summary of Studies done on animals altered at a juvenile age.
Article: Early Spay-Neuter Considerations for the Canine Athlete
What Age Is ideal?
A 2020 study by University of California Davis looked at 35 common dog breeds and suggests some guidelines for when and if to spay and neuter the sexes and at what age would be least detrimental.
In general, if you are going to spay/neuter your service dog, a minimum age is just at the time the dog reaches physical maturity. This way, physical development (especially the bone plates which are among the last to mature) has been completed. The ideal age may also be affected by sex. (Im HH, Yeon SC, Houpt KA, et al. Effects of ovariohysterectomy on reactivity in German shepherd dogs. Vet J 2006;172(1):154-159.)
Guide Dog programs typically spay females after their first heat and males at about 8 months of age. Could this partly explain the high failure rate of dogs due to behavioural issues (some as high as 50%)?
Slight Risk for Increased Fear or Aggression When Neutered
Two studies suggested that dogs that are spayed/neutered there may be a slight increased risk of becoming more fearful or aggressive towards strangers. This may be breed-specific.
Is Spaying/Neutering Necessary?
Do you need to alter your animal at all? That depends on the laws of your region, the breeder, the program you belong to and the individual dog in question. In British Columbia for example, dogs need to be altered to be certified as a service dog.
Does altering males actually decrease or prevent aggression issues? Studies show that if the altering is done at the time of puberty, it decreases the hormonal levels and usually results in calmer behaviour. If the altering is done after puberty, there may be no behavioral improvement.
Here is a link to a 2007 summary of studies on spaying and neutering risks and benefits of dogs at all ages.
Article: Long-Term Health Risks and Benefits Associated with Spay / Neuter in Dogs
If your situation allows you to choose not to spay or neuter you dog, be a responsible owner and do not allow your animal to reproduce, unless you are knowledgeable and experienced in the area of breeding.
One way to do this without spaying or neutering is ask your vet to do a vasectomy on your male dog or perform a tubal ligation in your female dog. This stops all possibility of reproducing without altering the natural hormone levels in the dog. Do be warned, though, these operations, while actually easier to perform, are not common and the vets may not want to do them. You may need to educate your vet or find one who is willing to do it. Only you can decide if the benefits are worth the extra effort.
Of course the common sense method of preventing your female from breeding is to protect her from male dogs (with solid fences, etc) when in heat and keep your male dog with you at all times.
Article: Behavioral and Physical Effects of Spaying and Neutering Domestic Dogs
Here is a link to a list of studies on specific topics related to spay and neuter: