We all know that not everything we eat is transformed into endogenous substances. In other words: imagine you would increase your weight by 0,5 lbs/200g by eating 0,5 lbs/200 g of noodles.
The reason why this is not so is that our body metabolizes the larger part of what we eat to maintain vital processes, which need energy, among them breathing, the beating of our heart and even speaking and thinking.
The human body consumes on average 1,800 kcal a day for these passive processes, which is called its basic consumption.
According to the kind of bodily activity further calories are consumed to supply the energy for the respective activity. This is called the performance-based amount, which is about 500 – 1,000 kcal.
When you eat more calories than your body consumes by the end of the day, your organism has a surplus, which is transformed into mass (muscles, fat, connective tissue, bones).
This principle is also true for other living beings. Thus for a livestock animal to gain weight as quickly as possible it needs to eat more calories than it consumes.
It depends on the individual animal how much feed it needs to build up one kilogram of usable meat, and it depends on the composition of the feed and on the kind of husbandry.
Hens and pigs need less feed than cattle, which need up to 16 kg.
The average rate of a cow is 12 kg of feed to build up 1 kg of usable meat.