The efficiency of biomass transfer tells us what percentage of the biomass at one trophic level ends up in the next level. The calculation is: efficiency (%) = biomass transferred to the next level ÷ biomass available at the previous level × 100. For example, if cows eat grass containing 2000 kg of biomass and gain 300 kg of biomass, the efficiency is 300 ÷ 2000 × 100 = 15%. Transfers are often around 10%.
Efficiency is low because biomass is lost at every level through egestion, excretion and respiration, and because some parts of organisms are not eaten. This is why food chains are short. After four or five trophic levels there is too little biomass left to support any more consumers, so food chains rarely have more than five trophic levels. The top consumers are also fewer in number than the animals below them.
This also affects how we feed people. Eating plants such as wheat directly puts humans at the second trophic level. Eating meat adds another step, and more biomass is lost at that step. So growing wheat can feed more people from the same area of land than rearing animals for meat.