The total feed demand for the livestock herd is dependent on the weight of the livestock herd and the per weight unit feed requirements. The per unit feed requirements increase with GDP per capita as populations move from meat sources such as chickens to more feed intensive ones such as pork and especially beef. But they also are reduced by change in the efficiency of converting feed to animal weight.
Some of the food requirements of livestock are met by grazing, thereby reducing the feed requirements. The feed equivalent of grazing depends on the amount of grazing land, the productivity of that land (computed in the initial year and highly variable across countries), and grazing intensity (which increases with crop prices).
Finally, the feed demand can be modified directly by the same exogenous crop demand parameter that modifies industrial crop demand.