I've found recently that a number of cattlemen and other livestock producers really don't understand the difference between stocking rate and stock density. Some tend to use the terms interchangeably, though they're actually two very different concepts, with very different effects on the pasture-livestock system.
Simply put, stocking rate is the basic relationship between livestock and the forage resource. It's the number of animals on the entire grazing unit for the entire grazing season. Think of stocking rate in terms of either animals per acre or liveweight per acre. If we have 100 acres and 100 yearling steers weighing 700 lbs., the stocking rate is one steer/acre or 700 lbs. liveweight/acre.
Using an appropriate stocking rate for your forage resource is the first step in managing a sustainable pasture or range operation. An acre of land will only produce so much forage and support only so many livestock. Stocking rate affects the pasture's overall productivity, species composition, likelihood of soil compaction occurring, individual animal performance, and a host of other factors in the pasture-livestock relationship.
Stocking rate is often used interchangeably with carrying capacity, which is also incorrect. Stocking rate is the number of animals on the pasture, while carrying capacity is the number of animals the pasture will support year after year while achieving an acceptable level of performance.
Yes, we can increase carrying capacity via grazing management that more efficiently utilizes the forage resource. But we can't automatically double or triple carrying capacity by embracing the newest fad in grazing management.
Stock density explained
Meanwhile, stock density is a term that came into common use with the advent of short-duration, time-controlled grazing management or management-intensive grazing (MiG). Stock density is the number of animals or animal liveweight on a specific area of the pasture for a specific period of time. Stock density is essentially animal concentration, and is a tool we use to accomplish many management goals in MiG.
Stocking rate, carrying capacity and stock density are expressed as number of animals or animal liveweight/acre. The difference is in the time factor and the impact of animal concentration for short periods.
Regarding the 100 steers on 100 acres scenario, if the pasture is continuously grazed with animals potentially present on every acre every day, stocking rate and stock density are the same: one steer/acre or 700 lbs. liveweight/acre. You can't effectively manage animal impact with continuous grazing.
If the pasture is divided into 10 paddocks of 10 acres each, and all the animals placed on one paddock, the stocking rate doesn't change but stock density increases 10-fold. Stock density is now 7,000 lbs./acre.
We're delivering 10 times the animal impact on an acre as before, but staying there for less time. Now, every acre is being rested 90% of the time and grazed 10% of the time.
If we further divide the pasture into 40, 2½-acre paddocks and put the animals on one paddock, we've increased grazing pressure by 40-fold (or to a stock density of 28,000 lbs./acre). The impact on the pasture is very different from either of the previous two scenarios, but the time factor is also very different. A particular area of the pasture is only being grazed 2.5% of the time and rested 97.5% of the time.
In each of these scenarios, the stocking rate hasn't changed but animal impact has changed profoundly. High stock density is a tool to achieve more uniform grazing with less selective grazing, a high utilization rate on stockpiled forage, manipulate species composition of a pasture, accelerate the nutrient cycling process, and a host of other management factors. It's a very useful and powerful tool.
I've heard stock density described as a hammer. A hammer can be used to pound a spike into tough wood, or extract an old finish nail from a fine piece of furniture. The same tool can have many uses when the craftsman knows how to use it.
Any tool also can be used to smash a fine cabinet to pieces in the hands of someone ignorant of its intended use. A big part of the art of grazing management is knowing how to use the stock-density hammer.
Jim Gerrish is a grazing management consultant based in May, ID, and former lead pasture researcher at the University of Missouri's Forage Systems Research Center in Linneus. Reach him at 208/876-4067, email@example.com, or visit http://americangrazinglands.com. For more information on his grazing schools, see the ad on page 49 of this issue.