Most nutritionists formulate with some level of safety when it comes to nutrient requirements. This is why nutrient requirements and commercial dietary specifications are not equivalent terms.
The root of the problem stems from the unavoidable fact that all requirement studies are conducted under experimental conditions that allow animals to respond to the tested nutrient, without interferences from outside stress factors, such as health, density, management, facilities, etc. To account for these highly variable and largely uncharted stress factors, all too common under commercial conditions, field nutritionists add a margin of safety over published nutrient requirement estimates.
The following is a non-exclusive list of cases that call for margins of safety:
Label guarantees. By definition, in least-cost formulations, the probability for any nutrient to be below target is always 50 percent. Thus, formulas may be slightly over-fortified to meet declared concentrations in feed labels as required by law.
Nutrient variability. Certain ingredients can be highly variable in terms of nutrient composition. Agro-industrial products of unknown quality and origin can be highly problematic if they are not assayed frequently, especially if they make up a substantial part of a diet.
Formulation basis. Diets formulated on total rather than digestible nutrient basis are often over-fortified to account for unknown variability in nutrient digestibility, especially in low-quality and novel ingredients.
Nutrient losses. Several nutrients (for example, amino acids and vitamins) are susceptible to destruction during thermal processing (pelleting, expanding, flaking), and also during prolonged storage, where significant losses of bioavailability may occur.
The “average” animal. Published nutrient requirements usually refer to the “average” animal, which in reality does not exist. This definition of convenience leaves half animals in a group undernourished while the other half is overfed. Production systems that place emphasis on rapid turnover require high rates of growth that can only be sustained by diet overfortification to allow for proper nutrition of most animals.
Typical margins of safety range from 5 to 30 percent, depending on the gravity of the factors taken into consideration. As a rule of thumb, a 5 to 10 percent safety margin over requirements is adequate when a quality control program closely monitors incoming ingredients and finished products. Higher safety margins are needed when significant variations in nutrient composition are expected due to ingredient selection, processing conditions, storage and animal performance.
Minimizing unknown factors and monitoring quality in ingredients and finished diets obviously reduces the need for excessive safety margins. If animal performance (weight gain and feed intake) is known from records, then a feeding program with minimal safety margins that closely meets requirements can be easily designed by a qualified nutritionist.
Nevertheless, it is always beneficial to challenge dietary specifications in any animal production system that manages to monitor performance with any degree of success. To this end, margins may be gradually increased (or decreased) in 5 to 10 percent increments as animal performance continues to be evaluated. This exercise should cover a long-term period and a wide range of conditions.