Meeting the world demand growth in meat, milk and eggs requires increasing quantities of food crops to feed livestock. Feed/output ratios are known at local scales but not at national levels where heterogeneous breeding systems are coexisting. To fill this gap, we estimate over 47 years (1961-2007) with millions of FAO data how many calories and proteins of plant food products (PFP, mainly cereals and oilcakes, imported or locally produced) were used by countries for their animal food production (AFP). The empirical findings served to document and discuss the declining average productivities of PFP in AFP over the years, and to parameterize a simple model of livestock production that well simulate past evolutions in seven world regions. Results are also used to explore the need for food crops in 2050 according to five hypothetical scenarios of human diets ranging from "full veganism" to "full westernization". Simulations show that plant food production should increase from 4 to 131 compared to 2007 while the population increases by 36%.