Math
Feed Efficiency Pays Big Dividends
Residual Feed Intake (RFI) is a calculation showing the actual feed intake of an individual minus its expected feed intake.
WHAT IS ACTUAL FEED INTAKE? The actual intake portion of the calculation comes from a precise measurement of how much an individual eats over a 56 day trial. This measurement is done using GrowSafe equipment which measures the weight of feed disappearance from individual feed bunks which are mounted on scales. Electronic Identification Tags identify which animal is eating in each single feeding station as it occurs.
WHAT IS THE EXPECTED INTAKE? The expected intake portion of the RFI calculation is derived from a comparison to pen-mates in the same test. The expected intake of each animal is found by comparing its consumption to pen-mates with similar body weight and average daily gain. In essence, actual feed conversion is used here, but only in comparing like animals.
WHY NOT USE FEED CONVERSION TO COMPARE BULLS? Different weight bulls have different maintenance requirements and individual bulls gain at different rates. To make simple feed conversion a useful genetic selection tool, these two things need to be accounted for. If all bulls tested in each test pen were of the same body weight and gained the same, then feed conversion could be used make genetic distinctions. To structure a test where all animals are very close in weight would be difficult and until animals are tested we do not know their gain to group them by similar gain. The RFI calculation also allows us to test animals of any age and get a feed efficiency measure that is at least comparable in nature to other test groups of different age and gain rate. Of course it is optimal and standard for all responsible test stations to test for RFI in groups that are similar in age and weight that have been through a warm up feeding period together.
WHAT DO THE DIFFERENT RFI VALUES REALLY MEAN IN FEED CONSUMPTION? An RFI value of zero calculated on an animal means that it was calculated as exactly average within its group of test mates for feed efficiency. Within the same test group an animal with an RFI of -3 required three pounds less feed per day to gain at the same rate and weight of the animal with an RFI of zero. By the same calculations, and animal in the same test group with an RFI of +3 required 3 pounds more feed per day to gain at the same level as the zero RFI animal. Currently, all RFI numbers measured for individuals come from only their own test group and are not tied in their calculation of RFI numbers to other test groups. While you cannot make a direct comparison from one test pen to the next at the same test station, one years data to the next at one test station, or even one test station to the next in the same year, you can make safely make general comparisons of individuals RFI across test groups, test stations and years.
WHAT DOES RFI MEAN IN TERMS OF FEED COST? For an animal with a -3 RFI compared to an animal with a +3 RFI there would be a total of a 6 pound difference consumed per day by each animal. Six pounds per day difference spread out over just 200 days with a ration cost of $200 per ton would mean a feed cost swing of $120 between the two animals to get the same weight gain.
WHAT CAN YOU EXPECT PROGENY COST DIFFERENCES TO BE? Heritability of RFI is 35 to 40%, which makes it moderately heritable. If you replace a +3 RFI AI Sire with with a -3 RFI AI Sire, and feed out 100 progeny with a $120 cost swing that was realized genetically by 40% this computes to a $4800 savings.
WHEN DID RFI COME INTO BEING AND WHATS COMING NEXT? RFI was developed as a measure of feed efficiency by researchers in 1963. Heritability of RFI is 35 to 40%, which makes it moderately heritable. Carcass Traits are considered moderately heritable and great strides have been made utilizing carcass epds. So the genetic differences in RFI suggest that substantial genetic progress can be made. As genetic progress is made and populations being tested for efficiency become more efficient test groups should be more efficient than current animals. For comparison to past data this progress will have to be taken into account when making genetic decisions. This may be done with a standardization of test stations which computes this into RFI numbers, or EPD's will become the selection tool for Feed Efficiency and utilize test station RFI data.