Milk weights and component percentages alone do not allow for producers to appropriately assess the status of their operation or make economic-based decisions. With the advent of corrected milk equations, attempts have been made to standardize milk weights and composition of milk as corrected to the energy content of a specified level. In short, energy corrected milk (ECM) equations strive to put cows on an equal basis to compare over time and are derived from equations relating milk gross energy to milk composition. ECM typically achieves this by adjusting to 3.5 percent fat and 3.2 percent protein with the following equation: ECM = 0.327 x milk pounds + 12.95 x fat pounds + 7.2 x protein pounds. Is this the most appropriate equation for your farm?
Over the past 100 years, many equations have been postulated and evolved over time as different milk analyses have become available. A large variety of equations exist that use differing numbers of milk components and their respective coefficients. The accuracy of ECM equations is largely dependent on the accuracy of the energy values used to create them. Additionally, inclusion of multiple components provides more accurate predictions than by simply using fat content alone. If components are omitted, this implies or assumes that said component content is constant or highly correlated with an included component – neither of these are true. Accuracy of energy equations is affected by the accuracy of gross energy values of the individual components and how milk composition varies. Lactose is one component with consistent reported gross energy values. Current application of these equations does not disclose what they represent, therefore attempts to challenge existing methodologies and how equations were derived should be made.
One of the earliest ECM equations widely used was the 4% fat-corrected milk (FCM) equation of Walter Lee Gaines and Frederick Alexander Davidson in 1923 (FCM = 0.4 x milk kg + 15 x fat kg). Digging in, this equation was developed based on milk energy, milk fat, and solids-not-fat (SNF) composition values from a popular press article. No direct calorimetric measurements were reported to be made on the milk samples used to produce these values. This FCM equation was progress but wasn’t promised to put milk weights on a comparable energy basis. Years later, Tyrrell and Reid (1965) dug in and compared milk gross energy data from their study to the energy predictions of the 4% FCM from 1923. They found good agreement between 4% FCM and actual energy content of milk between 3-4% fat, but at lower fat percentages it underestimated the energy content of milk.
Several fat and protein-corrected milk (FPCM) equations used in the field have been derived from work done by Tyrrell and Reid; however, the coefficients may vary depending on which energy value of milk is used and if crude protein is converted to true protein. Furthermore, milk energy equations are typically developed from individual sets of milk analyses which creates difficulty in gauging their accuracy. The Tyrrell and Reid equations have accurately predicted the energy content of the average of the milk data used to generate them, and the equation shared earlier in this article is one of them. Additionally, equations that most closely predict milk energy are ones that incorporate fat, protein, and lactose, and that use the energy values of the components as their coefficients.
This brief article merely scratches the surface on the work done in the field of ECM equations, and its intent is to spark consideration of current equations being used and to monitor their appropriateness for your farm. There are many other individuals that have dedicated efforts to this area of research, and Mary Beth Hall recently did a fantastic and thorough review of these equations and energy estimates. For further reading, I would direct you to her article in The Journal of Dairy Science.
