Most proprietary blocks and loose mixes use urea as the major source of nitrogen. The risk of urea poisoning is greatest after rain because dissolved urea can concentrate in pools of water. Poisoning occurs when stock drink this water or lick excess of the softened supplement. Most problems can be avoided by drilling drain holes and occasionally moving supplements and turning blocks over. Providing safe limits for urea content of supplements is difficult.
Cattle can tolerate more urea than they can utilise if they are built up to it slowly.
About 35g of urea per day is sufficient for substantial production response from breeders weighing around 400kg. Up to 20g per head per day is sufficient for weaners. However, providing a fixed amount of supplement per head per day is not possible in extensive situations so these intake quantities can only be used as guides. The relative merits of the various commercially available blocks depend on these factors. Many proprietary blocks use salt as both an attractant and an intake regulator.