The scores
Kirkland Nature's Domain Puppy: B (79/100) — Chicken, chicken meal, peas, garbanzo beans, lentils. Grain-free. Salmon oil DHA, supplemental taurine, five-strain probiotic blend.
Kirkland Signature Adult: B (78/100) — Chicken, chicken meal, brown rice, white rice, cracked pearled barley. Grain-inclusive. Fish meal, chicken fat, and a broader grain base for sustained maintenance energy.
How the ingredients compare
Puppy top 5: Chicken, Chicken Meal, Pea, Garbanzo Bean, Lentil
Adult top 5: Chicken, Chicken Meal, Brown Rice, White Rice, Cracked Pearled Barley
Both formulas lead with chicken and chicken meal — identical protein anchor from the same Diamond Pet Foods facility. The divergence starts at position three: Puppy goes grain-free with peas + garbanzo beans + lentils + fava beans + sweet potato, while Adult stays grain-inclusive with brown rice + white rice + cracked pearled barley. The puppy-vs-adult decision is largely about whether your dog benefits from grain-free (sensitivities, owner preference) or grain-inclusive (proven digestibility, DCM-breed safety margin).
Where the Puppy formula pulls ahead
Salmon oil for DHA: Puppy specifically includes salmon oil (position 13) for brain and retinal development during the 0-12 month neurological window. Adult Kirkland Signature relies on fish meal for general omega-3 contribution without the dedicated DHA emphasis. For growing puppies, the salmon oil inclusion is directly developmentally supportive.
Five-strain probiotic blend: Puppy lists Lactobacillus plantarum, Bacillus subtilis, L. acidophilus, Enterococcus faecium, and Bifidobacterium animalis — five live probiotic strains supporting microbiome colonization during the critical 0-6 month gut development window. Adult Kirkland Signature's probiotic strain count is narrower.
Supplemental taurine: Puppy lists supplemental taurine to mitigate DCM-investigation concerns associated with legume-heavy grain-free diets. Adult doesn't need the same mitigation because it is grain-inclusive, but for puppies on grain-free formulations this is responsible formulation.
No corn, wheat, or soy: Neither formula uses these, but the Puppy grain-free approach is a full step further from the corn-and-wheat budget puppy pattern. Owners worried about grain tolerance get a cleaner option in Puppy than in Adult. Shop on Amazon →
Where the Adult formula holds its own
Grain-inclusive for DCM-susceptible breeds: The FDA's 2019-ongoing DCM investigation has focused on grain-free legume-heavy diets and their potential link to taurine metabolism disruption in susceptible breeds (goldens, cockers, Dobermans, Great Danes, Newfoundlands). For those breeds, the grain-inclusive Kirkland Signature Adult is the safer long-term choice — rice and barley don't carry the investigation's concern mechanism.
Proven digestibility for sensitive GI: White rice (position four) is one of the most digestible carbohydrates in veterinary nutrition — commonly used in bland diets for GI recovery. For dogs with sensitive stomachs, the rice-heavy carb base of Kirkland Signature Adult is easier on the gut than the legume-heavy Puppy formula.
Sustained availability: Kirkland Signature Adult is the flagship SKU at every Costco warehouse nationally. Kirkland Nature's Domain Puppy has narrower warehouse distribution and may not be stocked at every location. If availability matters, Adult is more reliable. Shop on Amazon →
The bottom line
For puppies 0-12 months (0-18 months for large/giant breeds — those owners should verify calcium levels with their vet regardless), Kirkland Nature's Domain Puppy is the right choice — salmon oil DHA, supplemental taurine, and the five-strain probiotic blend are developmentally tuned. For adult dogs (12+ months), Kirkland Signature Adult delivers the same Diamond Pet Foods manufacturing quality at adult-appropriate caloric density with grain-inclusive digestibility. DCM-susceptible breed owners feeding Puppy should discuss long-term grain-free feeding with their vet — consider transitioning to the grain-inclusive Adult formula at the 12-month mark rather than continuing on Nature's Domain. Both deliver warehouse-club pricing on premium-tier ingredients.