Submitted to: Journal of Dairy Science
Publication Type: Literature Review
Publication Acceptance Date: 10/4/2022
Publication Date: 1/27/2023
Citation: Hall, M. 2023. Invited review: Corrected milk: Reconsideration of common equations and milk energy estimates. Journal of Dairy Science. 106(4):2230-2246. https://doi.org/10.3168/jds.2022-22219.
Interpretive Summary: Corrected milk equations are used to standardize milks that have different percentages of butterfat, milk protein, and lactose to a common energy basis so they can be compared more fairly. Corrected milk values on a weight basis have been used widely for evaluating milk output of countries, comparing performance among cows, and experimental treatments. The energy equations used to develop the corrected milk equations and particularly the energy values used for milk protein and butterfat still may have room to improve their accuracy. Milk protein can vary in the proportions of casein, whey protein, and nonprotein nitrogen it contains, which will affect its energy value. For butterfat, the profile of different fatty acids that make up the fat can affect its energy content. Using the specific energy values of individual fatty acids in triglycerides could address the fat energy issue, but needs to be investigated further. The corrected milk equations that use all three milk components are most accurate to use because they will account for all major energy-containing compounds in milk. For research, the corrected milk equations do not add clarity to describing differences, so using the milk energy values without converting them to a corrected basis is advised. Use of the most accurate corrected milk or energy values will provide the best values for fair comparison.
Technical Abstract: Corrected milk equations were developed to standardize milks of differing compositions to a common energy basis, yet still allow them to be expressed as the weights familiar on farm and in commerce. All corrected milk equations are derived from equations relating the energy content of milk to its composition. First, an energy equation is used to calculate the energy value of the composition of milk to correct to, and then that value is divided into the energy equation to give the corrected milk equation. As straightforward as this seems, there has been confusion as different equations purported to correct to the same milk composition have used different energy values as divisors or different milk energy equations. To know exactly what corrected milk data represents, the corrected milk equation, citation for the energy equation used, composition of the corrected milk, and energy content of milk used as the divisor must be described. The accuracy of corrected milk equations depends on the accuracy of the energy equations used to create them. The “energy” predicted by these equations is the heat of combustion or gross energy (GE) of milk. Energy equations evolved over time as different milk component analyses became more available. Inclusion of fat, protein, and lactose gives the most accurate prediction of energy content. Omission of a component requires the assumption that it is constant, or that it is firmly correlated with an included component. Neither of these assumptions are true. Accuracy of energy equations is affected by the accuracy of GE values of individual components given how much of the energy they account for, and their uniformity in composition. Lactose is a chemically distinct molecule with consistent GE values. Milk protein can vary in composition related to casein, whey proteins, and nonprotein nitrogen which could affect its GE values. With no usual measurement of these fractions, the GE of casein has been used for milk true protein. The GE of fat is affected by the profile of fatty acids with different chain lengths, saturation, and branching that give different GE values. The GE of fat has potential to be calculated using the fatty acid profile, published GE of fatty acids, and predicted values based on their chemical structure. Although useful in the field, use of divisors and rounding of coefficients in corrected milk equations do not add clarity and may add error to research data. Reporting uncorrected milk energy values may be most appropriate for research. If more refined and accurate assessment of milk energy is desired, further work needs to focus on how to accurately calculate GE of fat and protein and evaluate whether it gives appreciable improvement over the current values.