WildAgain Wildlife Rehabilitation, Inc.

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Mammal Nutrition

Good Wildlife Nutrition is Critical

Providing the proper nutrition to wildlife in a rehabilitation setting is a critical element to an animal's ultimate recovery and/or growth and development leading to a successful release.  Unfortunately, selection of the right substitute milk formula for infant and juvenile aged mammals can be a daunting task, for both novice and veteran wildlife rehabilitators.

Selection of proper diets is a complex subject with many variables which may, at times, involve using elaborate mathematical equations to calculate nutritional components and energy values. Rehabilitators attempting to achieve the best nutrition for the wild animals in their care find this subject both challenging and controversial as well as filled with many uncertainties and questions. Rehabilitators are strongly encouraged to seek out understanding of nutrition from a variety of sources.

WildAgain's Nutrition Calculator™

The WildAgain Nutrition Calculator™ contained on this website is provided as a tool for rehabilitators to calculate the nutritional composition and kcal value for substitute milk formulas. It is not designed for use by a member of the general public or rescuer who wants to keep and raise a wild animal. Members of the public should not be providing rehabilitative care for wildlife, but rather taking wildlife to a permitted wildlife rehabilitator 

Additionally, the WildAgain Nutrition Calculator™ does not suggest or endorse a specific or mix of product(s) for individual species; specific concentrations or dilutions or amounts of formula to be fed; or specific feeding frequencies. Those decisions must be made based on many other factors.

That said, have you ever wondered about the nutritional composition and the kcal value for the substitute mammal milk formula you use?  But you haven't had the inclination or the time to do the math?  A tool is now provided on this website to help with that task.  It's very user-friendly and easy to use.

The WildAgain Nutrition Calculator™ allows a wildlife rehabilitator to enter the desired ratio of dry milk replacer product(s) to water with the calculated result providing the key nutritional components and kcal values. 

The tool also allows the user to put in their own weights for products and water, depending on how each individual may measure out dry products or water.  Overriding the default values that are supplied requires that the rehabilitator have a precise gram scale that will yield measurements with accuracy to two decimal places.  A suggested way to determine an override weight is to measure the product 10 times and take the average of the ten weights.  If a different vessel is used to weigh the product than the red spoon supplied with the PetAg® products, then the weight of the water should also be weighed in the new vessel.

Cautionary Note

Calculating these numbers should be viewed as just the starting point for determining the best formula to give a particular species.  Many factors must be considered, including the age, weight and condition of the animal, as well as its observed tolerance for a given formula mix and any other ingredients, concentration of the formula, and feeding amounts and frequency. Don't stop at just "running the numbers!"

Beware of "Cookbooks"

Some of the existing wildlife rehabilitation literature makes suggestions as to which formulas to use with certain species, even recommending very specific recipes.  Manufacturers of some of the milk replacer products also offer specific mixing instructions for their products for use with certain species, as well as frequency and amount of feeding.  Since many factors influence the relative success of one formula over another, these "cookbook" recommendations should be viewed only as a starting point when determining the best formula to use for a specific species.

Matching Mother's Milk

It appears the goal of many of these substitute formula recommendations is to closely match the nutritional components and energy content of the mother's milk of the particular species as determined by past studies.  Some of the species have had 2 to 3 separate studies performed to determine the composition of the mother's milk.  Unfortunately, the differences in values between these studies for a single species range as high as over 100% for certain nutrients and over 50% for the energy content.  Where a species has had only one study performed, this same level of error or uncertainty may likely be present in those reported values as well.  These reported values, from studies performed 20 – 40 years ago, should best be viewed as an approximation of what may truly exist in nature.

Another misconception of just matching the chemical composition of a substitute milk formula with mother's milk in the wild deals with digestibility. For purposes of this discussion, digestibility refers to disappearance of food from the gastrointestinal (GI) tract, which includes preparation of food for absorption (true digestibility) and subsequent absorption in the GI tract. Just matching the numbers does not take into account the fact that marked differences exist in various species regarding ability to digest certain foods.  It also fails to take into account that digestibility is variable, determined by such factors as level of food intake, digestive disturbances, nutrient deficiencies, ingredients, and feeding frequency.  Additionally, even if digested, nutrient utilization can be affected by a number of factors including species, age, feeding amount and frequency, disease, parasites and level of stress.

Errors Inherent in the Math

Mathematical equations exist to precisely calculate the nutritional components and energy content of any given formula. However, error can be introduced at different points in performing the calculation. 

When calculating the nutritional composition of a formula, error can be caused by using either an incorrect or less precise analysis from the manufacturer. Many of the commercially available milk replacer products carry labeling on the container that provides an approximate analysis of the contents, often called the "Guaranteed Analysis".  This labeling is usually required by law and provides component guarantees for minimums for protein and fat, and maximums for carbohydrates, crude fiber, ash and moisture.  This analysis is also referred to as a "proximate analysis".  While the percentage values provided are directionally correct, they do not provide the degree of accuracy needed to calculate more precise nutritional components of a substitute milk formula.

The manufacturer of a milk replacer product, upon request, can normally provide a much more detailed and accurate chemical analysis of the product.  This description of the product, normally referred to as a "Typical Nutritional Analysis", provides a detailed listing of the components of the product.  This listing often provides details for protein, amino acids, fat, cholesterol, fatty acids, fiber, starch and sugars, minerals, vitamins, and ash. It also generally provides gross energy and metabolizable energy values.  Since this analysis is more precise and detailed, this is generally the preferred set of data to use when calculating nutritional and energy values for mammal formulas.

Error can also be introduced in the way the manufacturer arrives at the value for metabolizable energy (ME) for a product, usually expressed in kcals per gram.  The most precise way to calculate ME is to take gross energy (GE) of the food fed minus products of elimination (feces, urine and gaseous products of digestion, mostly methane).  GE is the energy value of complete oxidation of the food. This is a very complex and time consuming test and will vary among species for the same food product.  Some manufacturers simply estimate ME using factors for protein, fat and carbohydrates contained in the product, but generally indicate this can vary +/- up to 5%.  To the extent a formula is mixed from multiple products, this degree of error is multiplicative.

Another source of error can be introduced by the way the rehabilitator measures the dry and wet contents of a formula prior to mixing.  The value for metabolizable energy contained in the typical nutritional analysis provided by the manufacturer is based on a precise weight for a given volume of the product.  This accurate weight of the product for a given unit of volume measure plus the accurate weight of water for the same unit of measure are critical elements to determining a value for metabolizable energy for a formula.  To the extent the rehabilitator measures or packs a dry product differently than the manufacturer, the resultant calculation will be incorrect. 

Slight changes in the manufacturing process of milk replacer products can render previously reported calculations obsolete.  This may be the case for some of the dry powder products produced by PetAg®, since many of the nutrient component percentages and kcal values are no longer correct in some of the wildlife rehabilitation literature.  Since the authors of those publications do not indicate the typical nutritional analysis data used at the time the calculations were performed, and only provide the results of the calculations, it is unclear why the reported values are incorrect.

The current typical nutritional analysis for various powdered dry milk replacers are shown in the calculator.  Using these typical nutritional analysis, the calculator provides the correct compostion and energy values for the ratios desired for various products when mixed with water.

Feeding Regimen Also Introduces Uncertainty

Once a rehabilitator chooses a milk replacer product or combination of products to use, the proper concentration of the formula must be determined as well as the feeding frequency.  Some of the previously published suggested ratios of product to water yield a formula that is too thick for some species to handle, such as tree squirrels.  Some rehabilitators have reported that a too thick, pancake batter-like formula has caused diarrhea, dehydration and other problems, necessitating dilution of the formula with more water.  In many cases, further dilution with water requires the formula to be given more frequently to insure the proper amount of nutrients are provided to the animal in a 24 hour period.  The rehabilitator must work diligently to get an effective combination of product (or products), formula concentration, and feeding frequency to produce appropriate elimination, proper hydration, and healthy and sustained growth and development (physical and behavioral).


-  Nutrition is an absolutely critical component of effective rehabilitation!

-  Beware of the "Cookbooks" that make formula selection look too easy.  It's not. Some of these publications are out-of-date as to technical data on certain milk replacer products.  They can also be misleading in not providing appropriate cautions to the user as to the many other factors that need to be considered.

-  Discuss the product specifications with the manufacturer.  Don't just rely on the label on the product (Guaranteed Analysis). Ask them for the much more complete Typical Nutritional Analysis.

-  Develop and tap into your rehabilitation network.  Talk with rehabilitators from around the country that work with the same species.  Use the NWRA or IWRC Membership Directories to help with contacting rehabilitators.

-  Don't just "Run the Numbers".  There is much more to determining the potential success or failure of a formula.

-  Use a feeding regimen that works.  This is the right combination of selection of product(s); concentration (strength or dilution); and feeding frequency.  Adjust as necessary to find the right combination.

-  Watch your animals closely.  Develop keen observation skills.  Observe regularly, keeping periodic notes or records as necessary on each animal.  Assess success by seeing healthy, normal elimination, proper hydration and sustained and regular development (weight gain, coat development, alertness, behavior, etc.).

-  As a community, we need more research on species specific nutrition and on feeding regimens that work best and produce consistent and successful results. Those of us caring for wildife need to significantly increase  our understanding of nutrition as a major component of effective wildlife rehabilitation and encourage development of resources that are up-to-date, accurate, and complete.


Church, D., and W. Pond.  1982.  Basic Animal Nutrition and Feeding – Second Edition. John Wiley & Sons, NY.

Fowler, M. 1984. "Nutrition and Feeding or Orphaned and Injured Wild Animals", in Wildlife Rehabilitation Volume 2. National Wildlife Rehabilitators Association, St. Cloud, MN.

Marcum, D. 1997. Rehabilitation of North American Wild Mammals – Feeding and Nutrition. (self-published.)

Moore, A. and S. Joosten.  1997.  NWRA Principles of Wildlife Rehabilitation – The Essential Guide for Novice and Experienced Rehabilitators.  National Wildlife Rehabilitators Association, St. Cloud, MN.

Pet-Ag, Inc. , 1993. Zoologic® Nutritional Components Milk Matrix Formulation and Mixing Guide.  Hampshire, IL.

Pet-Ag, Inc.  Typical nutritional analysis data sheets for Esbilac®, MultiMilk™, KMR® and Foal-Lac®.

White, J. 1993.  Basic Wildlife Rehabilitation 1AB. International Wildlife Rehabilitation Council, Suisin, CA.


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