Specifically FCR is the mass of the food eaten divided by the body mass gain, all over a specified period of time. FCR is dimensionless, i.e. there are no measurement units associated with FCR. Animals that have a low FCR are considered efficient users of feed. Sheep and cattle need more than 8 kg of feed to put on 1 kg of live weight. The U.S. pork industry claims to have an FCR of 3.4-3.6 . Farm raised Atlantic salmon apparently have a very good FCR, about 1.2.
Nonetheless this value is an underestimation of the FCR, for it does not take in account that more than often the feed is in kg of “dry” weight and the live weight is in kg of “wet” weight. When both factors are transformed to either dry or wet weight the FCR increases to between 4 and 5, depending on the feeding practice. Presently, technological advances in feed research are enabling salmon farmers to lower the “true” FCR to 4 and under (i.e. by substituting fish meal and fish oil with plant products such as soy and derivatives) which in consequence lowers the Salmon’s average trophic level.
It is important to understand that the above discussion of dry weight vs. wet weight for FCRs in Salmon also applies to other animals, such as poultry, cattle, and beef. Those FCR's are also a conversion of dry feed to wet animal weight, so really the FCRs are still to the same proportions.