Dog food and horse hay are not interchangeable: dog food should be formulated to meet a dog’s needs at its life stage, while hay is forage that commonly forms the foundation of a horse’s diet. The difference reflects how each species gets nutrients and how its overall diet is built.
Why dogs and horses need different kinds of food
Dogs need a diet that supplies nutrients in forms and amounts appropriate for dogs. For dogs, check the food label’s nutritional adequacy statement: it identifies whether the product is complete and balanced and which life stage it is intended to support. A product labeled for intermittent or supplemental feeding is not presented as a complete diet. Merck Veterinary Manual guidance on dog and cat foods explains how to interpret these statements.
Horses, by contrast, are adapted to use forage. Microbial fermentation in the cecum and large colon helps make energy from forage available, which is why pasture and hay can supply a major share of equine nutrition. Hay is therefore horse forage, not a substitute for dog food.
How forage fits into a horse’s diet
Merck Veterinary Manual’s February 2026 review says forage should account for at least 50% of a horse’s total intake. Its separate nutritional-requirements review, updated in August 2025, describes a daily forage amount of at least 1.5–2% of body weight on a dry-matter basis. These are distinct measures, not alternate ways to express the same percentage.
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Merck’s horse-feeding guidance says most healthy horses should have access to hay and/or pasture forage, salt, and fresh water. Many horses receiving good-quality forage need little or no concentrate, but forage composition and the horse’s individual needs matter. Workload, health, body condition, dentition, and the quality of available forage can all affect feeding decisions. A hay type or feeding amount suitable for one horse is not automatically suitable for another.
When forage quality or the horse’s needs indicate that additional nutrients are required, an equine ration balancer or other feed may be considered. These products are conditional additions, not requirements for every horse. Forage-based substitutes such as hay cubes may also be part of a horse’s forage plan when appropriate.
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Dog nutrient needs depend on life stage
Dog food should be selected for the dog’s life stage, rather than judged against the nutrient profile of horse hay. Merck’s September 2024 update to its small-animal nutritional requirements review reports AAFCO minimum protein guidance of 18% on a dry-matter basis or 45 g per 1,000 kcal of metabolizable energy for adult dogs. For growing puppies and reproduction, the corresponding guidance is 22.5% dry matter or 56.3 g per 1,000 kcal.
Those figures describe canine guidance and are not stand-alone feeding targets. They cannot be compared directly with a hay analysis as if both described equivalent diets: species, energy intake, life stage, and how the food is used all differ. Choose a complete and balanced dog food labeled for the dog’s life stage, and consult a veterinarian if the dog’s age or health makes feeding choices less straightforward. Merck Veterinary Manual’s small-animal requirements guidance provides the underlying life-stage context.
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Change a horse’s diet gradually
Because the horse’s hindgut depends on a microbial population that helps digest forage, abrupt diet changes can disrupt digestion. Merck describes a transition period of approximately 10–14 days for dietary changes to help minimize disruption of microbial balance and digestive upset. When changing forage or feed, make the transition gradually and seek veterinary guidance if the horse has health concerns or a specialized diet.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to choose for each animal
- For a dog: use a complete and balanced food intended for the dog’s life stage; do not use horse hay as its diet.
- For a horse: build feeding around suitable forage, with salt and fresh water available; decide whether additional feed is needed based on the horse and forage.
- For either animal: avoid treating a product intended for one species as a nutritional substitute for the other.
For more detail on equine forage and feeding practices, see the Merck Veterinary Manual’s horse-feeding guidance and its review of equine nutritional requirements.
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