[8][9] The outputs portion may be calculated based on weight gained, on the whole animal at sale, or on the dressed product; with milk it may be normalized for fat and protein content.
However, comparisons of FCR among different species may be of little significance unless the feeds involved are of similar quality and suitability.
[8] The dairy industry traditionally didn't use FCR but in response to increasing concentration in the dairy industry and other livestock operations, the EPA updated its regulations in 2003 controlling manure and other waste releases produced by livestock operators.
[13][14] In the US, the price of milk is based on the protein and fat content, so the FCR is often calculated to take that into account.
[16] As of 2011[update], pigs used commercially in the UK and Europe had an FCR, calculated using weight gain, of about 1 as piglets and ending about 3 at time of slaughter.
[18] Because FCR calculated on the basis of weight gained gets worse after pigs mature, as it takes more and more feed to drive growth, countries that have a culture of slaughtering pigs at very high weights, like Japan and Korea, have poor FCRs.
[23] On a diet of straw, which has a low metabolizable energy concentration, FCR of lambs may be as high as 40.
[21] As of 2011[update] in the US, broiler chickens has an FCR of 1.6 based on body weight gain, and mature in 39 days.
)[30] As of 2015[update] farm-raised Atlantic salmon had a commodified feed supply with four main suppliers, and an FCR of around 1.
[33] For herbivorous and omnivorous fish like Chinese carp and tilapia, the plant-based feed yields much lower FCR compared to carnivorous kept on a partially fish-based diet, despite a decrease in overall resource use.
[36][37] This means that ruminants make a positive net contribution to the supply of edible protein for humans at global level.
[34] Although there are few studies of the feed conversion ratios of edible insects, the house cricket (Acheta domesticus) has been shown to have a FCR of 0.9 - 1.1 depending on diet composition.
Reasons contributing to such a low FCR include the whole body being used for food, the lack of internal temperature control (insects are poikilothermic), high fecundity and rate of maturation.
[34] Although cultured meat has a potentially much lower land footprint required, its FCR is closer to poultry at around 4 (2-8).