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What is RFI?
Residual feed intake (RFI) is a measure of feed efficiency that is calculated as the difference between an animal’s actual feed intake and its expected feed intake. RFI is independent of body weight and size. Differences in the trait cannot be detected by physically looking at an animal, it requires accurate and simultaneous measurement of an animal’s feed intake and body weight. Animals with low RFI values are more efficient than those with high RFI values.
Is RFI the same as feed conversion ratio?
Although they are both measures of feed efficiency, RFI and FCR are not the same. RFI is independent of body weight and growth rate, while FCR is highly dependent on an animal’s body weight and growth rate. FCR represents a gross measure of feed intake while RFI is able to differentiate between an animal’s maintenance and growth requirements. In most cases, improvements in RFI also result in improvements in FCR, however, the two values are not always perfectly correlated. Animals with low FCR values do not necessarily have lower than expected feed intakes, especially if they have high growth rates, while animals with low RFI values always have lower than expected feed intakes.
Do low RFI animals result in reduced performance?
No, selection for RFI does not result in cattle with reduced performance and ADG. Efficient and inefficient cattle can have both high and low ADGs. To prevent a decline in performance, it is important to select for low RFI animals with sufficient ADG. GrowSafe does not recommend single trait selection.
How is RFI calculated?
RFI is calculated as the difference between an animal’s actual feed intake and their expected feed intake. An animal’s expected feed intake depends on it’s body size and growth rate, it is calculated using dry matter intake, mid-test metabolic body weight and average daily gain data from the entire contemporary group – dry matter intake is regressed on mid-test metabolic body weight and average daily gain to produce coefficients for average daily gain and metabolic mid-test weight which are used in the following formula to calculate each animal’s predicted feed intake:
- Y = β0 + β1 X1 + β2 X2 + ε
- Y = predicted feed intake
- β0 = equation intercept
- β1 = coefficient for mid-test metabolic body weight
- X1 = mid-test metabolic body weight (of the individual animal)
- β2 = coefficient for average daily gain
- X2 = average daily gain (of the individual animal)
- ε = residual term
From there, subtract an animal’s expected intake from its actual intake to come up with it’s RFI. All feed intakes are ‘dry matter’ intakes, not ‘as-fed’ intakes. Because the formula is dependent on data from the entire contemporary group, adding or removing animals from that group can affect the coefficients in the formula, which can affect an animal’s expected feed intake and therefore affect their RFI.
- Animal A: Expected Intake =8.5, Actual Intake =10 RFI = 10 – 8.5 = 1.5
- Animal B: Expected Intake =10, Actual Intake =9.5 RFI = 9.5 – 10 = -0.5
- Animal B is more efficient because it has a lower RFI value and because it eats less than expected for its size and level of production.
Should I be using RFI or FCR?
RFI is the trait that should be used when selecting for feed efficiency. Using FCR to select for feed efficiency inadvertently results in larger cattle with greater maintenance requirements, this is because FCR is highly correlated with animal growth rate. Additionally, genetic selection for ratio traits like FCR often comes with unpredictable results as emphasis is unequally assigned to the ratio’s component traits. In the case of FCR, greater emphasis is applied to the gain component than the feed component, inadvertently resulting in selection for larger and faster growing animals.
Contemporary groups and how do they affect RFI?
Contemporary groups are groups of similar animals used to remove biases from genetic and RFI evaluations. Animals within a contemporary group should be of the same sex, similar breed, and be within approximately 90 days in age of one another. A contemporary group’s expected feed intake is calculated using the feed intake, body weight and average daily gain data collected from the entire contemporary group. Therefore, all RFI scores are relative to one another and the addition or removal of animals from the contemporary group results in changes to the expected feed intake and thus changes to each individual animal’s RFI.