Stocking Rate in Animal Units

Stocking rate is animals per unit of land, expressed in animal units so that a herd of mixed sizes and classes can be added up into one figure. The animal unit is conventionally a 1,000 lb (454 kg) beef cow with or without a calf at foot; a 1,400 lb cow is 1.4 AU, a yearling around 0.7, a ewe with lambs about 0.2. Converting to animal units before dividing is what makes a rate from one farm comparable with a rate from another.

The distinction that matters most on this page is between stocking rate and carrying capacity, and they are confused constantly because they share units and appear in the same sentence. Stocking rate is a DECISION: it is what the manager did, how many animals were turned out on how much land. Carrying capacity is a MEASUREMENT: how many that land can actually feed, for the period wanted, without degrading. Only one of the two is under the manager's control, and grazing goes wrong precisely when the first is set above the second.

Which area to divide by is its own quiet source of error. The number on the title deed includes bush, sloughs, rock, road allowance, steep ground and everything beyond the distance stock will walk from water, and none of it grows grazed forage. Cattle also distribute themselves unevenly across what remains, congregating near water and shade and on gentle slopes, so a paddock at a comfortable average stocking rate can be severely overgrazed near the trough and barely touched half a kilometre away. Water placement, fencing and salt placement are the tools that fix this, and they change the effective carrying capacity without changing a single hectare of the title.

Stocking rate also has to be read alongside the grazing system it belongs to. The same number of animals on the same land continuously all season is a completely different pressure from the same animals rotated through paddocks with rest periods between, because what damages a sward is not being grazed but being re-grazed before it has recovered. A high instantaneous stocking density for one day followed by forty days of rest is normal in a rotational system and would be catastrophic as a season-long average.