The attached articles (hopefully in chronological order) were published over the past ~ year in Graze magazine (www.grazeonline.com). Happy reading!
… and I should also mention that there are other archived articles that have appeared in publications like OnPasture here on the forum that can be found by searching older posts.
These are good questions as I think many graziers (including myself) tend to get confused over the concepts of density vs. stocking, etc.
Density is independent of time. It's simply the amount of animals (expressed in # of head, pounds or standardized conversion like "animal unit") per acre. The greater the density, the more frequent the rotation to keep them fed. As an example, if I put a herd that weighs 50,000 lbs on a half-acre, I've created 100,000 lbs of density. At that density, I'd likely have to move the animals at least a couple times/day, depending on just how much food was there to begin with.
Stocking (rate) is density for a period of time, regardless of whether or not the site can actually support that number of animals. Stocking capacity is how many animals an area can support for a unit of time. Examples: a strategy used by some grazing operations is to adjust the stocking rate over the course of the season to take advantages of surplus grass. One local friend does this by custom grazing our heifers from early May till early August before they come back to the farm to be bred. During this period, he has a higher stocking rate on his pastures. The stocking capacity of his farm, however, doesn't change much from one year to the next. He can support a certain number of pounds of animals on his farm each year, with some variation for weather.
The stocking capacity of newly-established silvopastures is usually quite low. But any grazing system under good management should see a gradual increase in stocking capacity over time until it matures, at which point the annual increase in capacity will level off.
For calculation purposes, different classes of ruminant livestock are normally converted to Animal Units (1000 pound units), though small differences may exist between classes depending on the type and quality of feed. In other words, in a brushy young silvopasture, an AU of goats or sheep may be more efficient (potentially higher stocking rate) than an AU of cattle. Something to keep in mind if trying to really fine-tune your stocking rate, but there are numerous variables that can affect the math.
Multi-species grazing in a single group can distort the calculations as the large animals may utilize forages & browse not preferred by the small animals, and vice-versa. You can find plenty of good articles on-line about the benefits of combining species (as well as the challenges and downsides).
Density (how much), frequency (how often), duration (how long), timing (when, in terms of point in the growing season or stage of maturity of the plants) as well as other variables like ground conditions and type of livestock combine to express the "grazing impact". This is the bottom-line consideration when attempting to manage the understory vegetation in silvopastures. If you don't take all of them in to account, you may not get sufficient impact to promote the good plants and limit the bad ones. Animal impact is about the only practical impact that we have to manage vegetation in silvopastures as the trees, shrubs and other obstacles prevent the use of tools commonly used in open pastures (mowing, spraying, tillage)