However, not all proteins have the same value in human diets and range of systems has been developed to adjust the protein for their biological value in human diets. One such system is based on feeding experiments to produce a biological value for proteins. This adjustment for protein nutritional value has been used to calculate water efficiencies in terms of protein biological value for a range of productions and countries based on water efficiency data. Although on average proteins from livestock adjusted for biological value requires more water (67,637 litres/kg) than protein from plants (25,593 litres/kg), there are some animal proteins sources that are statistically the same as those produced by plants in the terms of water efficiency. There was no plant protein source considered in this study that was significantly more water efficient than protein from eggs; only soybean protein was a more water efficient source of protein than milk, goat meat and chicken meat when protein production was compared on the basis of biological value. Sheep and pork protein, although more water efficient than beef, used significantly more virtual water than plant protein sources to produce a kilogram of protein adjusted for biological value.
Beef production has the highest demand for water of the livestock industries investigated. This is in part likely to be due to the common usage of irrigated fodder, longer turnover times and the use of higher rainfall grazing areas for beef production compared to small ruminants. The longer turnover times are a function of a longer gestation period, lower reproductive rates and longer growing time to market than other livestock species. In poverty prone regions of the world ruminants are grazed or feed mostly on crop residues, used for crop production and probably requiring much less water than those in the developed world where grain or irrigated forage are commonly as livestock feed. The water efficiencies for these countries were not included in the study above.
Overall humans only require 75 grams of protein per day with consumption beyond this amount being converted to energy. Thus, the water used to meet the first 75 gram of dietary protein can be obtained efficiently from animal products. The role of livestock proteins and other livestock nutrients for health as well as livestock utility for crop production have not been included into calculations of water productivity but should not be overlooked in any evaluation of the role of livestock in human food production.