The animal unit, and what it is doing here
An animal unit is conventionally a 1,000 lb animal, and dry matter intake for grazing livestock is usually taken at roughly 2.5 to 3 percent of body weight per day. So one animal unit eats somewhere near 26 lb of dry matter a day, or around 780 lb a month, which is where the animal unit month comes from. Converting a mixed herd to animal units is what makes the arithmetic possible at all: a 1,400 lb cow is 1.4 AU, a 900 lb heifer is 0.9, and five 150 lb ewes together are 0.75.
Dry matter is the point of the definition. Standing grass is mostly water, so a figure in lb of green forage means nothing without the moisture content. Every yield, intake and demand number on this page is dry matter, and mixing a fresh-weight figure into it will overstate your pasture by a factor of three or four.
Yield is the input that decides everything
Change the herd size and the answer moves proportionally. Change the forage yield and the answer moves proportionally too — but yield is the input you are least likely to know and most likely to guess high. Annual dry matter production per acre spans a range that is genuinely enormous: improved, fertilised, irrigated pasture in a good year against short-grass rangeland in a dry one is not a difference of percentages, it is a difference of an order of magnitude or more. Two adjacent fields on different soils differ measurably. The same field differs between a wet spring and a drought summer by more than most people would believe until they have watched it happen.
The place to get a local figure is your county extension office, which will have production data for your area and your forage species, and the NRCS soil survey, which gives forage production estimates by soil map unit for favourable, normal and unfavourable years. Those three-year ranges are the useful part. Plan on the normal year and have a written plan for the unfavourable one.
Harvest efficiency, and why half the grass disappears
Forage that grows is not forage that gets eaten. Animals trample it, foul it with manure and urine, bed on it, and leave residual that has to stay for the plant to regrow from. The share actually consumed is the harvest efficiency, and it is the difference between a pasture that supports a herd and one that runs out in August.
| Management | Efficiency commonly quoted | What happens |
|---|---|---|
| Continuous, set stocked | Around 30% | Animals regraze favourite patches, ignore others, waste a lot |
| Simple rotation, few paddocks | 40-50% | Better spread, longer stays still allow regrazing |
| Managed rotation, short stays | 50-70% | Dense grazing, quick moves, adequate rest |
Pushing efficiency higher is not free. Grazing the sward closer takes more of the plant’s reserves, lengthens the rest period the paddock needs, and reduces what the field produces over the whole season. Very high efficiency figures on paper often describe a pasture being mined rather than managed.
How the paddock count comes out
The paddock arithmetic is simple and worth understanding rather than accepting. If you want a paddock rested for 30 days and animals stay 3 days in each, you need 30 divided by 3 paddocks resting, plus the one being grazed: eleven. Shorter stays need more paddocks for the same rest, which is why intensive systems end up with many small divisions and a lot of temporary fence. Longer rest in a dry spell needs more paddocks too, or the same paddocks used for longer, which is where a drought plan does its work.
Water and lanes are the constraint that catches people. Every paddock needs stock water, and running animals a long way back to one trough undoes much of the benefit by concentrating traffic and manure on the lane. Plan water access at the same time as the divisions, not after.
What the numbers cannot see
This calculates forage against demand. It does not know your forage species, whether the stand is legume-rich or a tired grass monoculture, what the growth curve does across your season, or whether a summer slump means the total annual yield arrives in a shape you can use. It does not know your animals’ condition, their stage of production, or what supplement they are receiving. It has nothing to say about parasites, mineral nutrition, or toxic plants in the stand, all of which are questions for a veterinarian and an extension livestock agent.
For the fencing that makes the divisions real, the livestock fence calculator works out posts, wire and braces per division. When the pasture runs out, the hay and feed storage calculator is the other half of the year, and it is worth running both together before deciding on herd size. Pasture fertility sits with the fertilizer application calculator, though the rate belongs on a soil test rather than a guess.
Questions people ask
How many acres does one cow need?
Anywhere from well under one to more than fifty, and anybody who gives you a single number without asking where you are is guessing. The figure is set almost entirely by forage production, which depends on rainfall, soils, forage species, fertility and the particular year. Improved irrigated pasture in a humid region and short-grass rangeland in an arid one differ by more than an order of magnitude, and the same field varies by a factor of two or more between a wet year and a dry one. Get the yield figure for your soils from the NRCS soil survey and your county extension office, put it into this calculator, and the acres per animal unit falls out. That is the right order to do it in.
What exactly is an animal unit?
Conventionally a 1,000 lb animal, consuming dry matter at roughly 2.5 to 3 percent of its body weight per day, which comes to around 26 lb of dry matter daily or about 780 lb a month. An animal unit month is one animal unit for one month, and it is the currency grazing leases and public land allotments are usually written in. The point of the unit is that it lets a mixed herd be added up: a 1,400 lb cow is 1.4 AU, a 600 lb yearling 0.6, a group of ewes converted the same way by weight. A cow with a calf at side is often counted somewhat above 1.0 because the pair eats more than the cow alone, and different agencies use slightly different conversion tables, so check which convention applies if a lease depends on it.
How many paddocks do I need for rotational grazing?
Divide the rest period you want by the number of days animals stay in each paddock, then add one for the paddock currently being grazed. Thirty days rest with three-day stays is eleven paddocks. The rest period is the variable that matters and it is not a constant: forage that regrows in 21 days in a wet spring may need 60 or more in midsummer, so a system built for the spring number runs out of rest in July. Practical systems either add temporary divisions when growth slows, or accept longer stays and lower efficiency, or take animals off pasture and feed stored forage. Water access at every paddock is usually the real limiting factor rather than fence.
Why is harvest efficiency only 50 percent?
Because grazing animals are not harvesting machines. They trample forage, lie on it, foul it with manure and urine and then refuse to eat near it, walk lanes into it, and select the plants they prefer while leaving others to go rank. On top of all that a residual has to be left standing for the plant to regrow from, and grazing below that residual costs you the next cycle. Continuous set stocking is commonly put near 30 percent for these reasons; managed rotation with short stays reaches 50 to 70 percent because animals are dense enough to graze uniformly and are moved before they start regrazing regrowth. Efficiency figures well above that generally mean the sward is being grazed harder than it can recover from.
What do I do when the pasture runs short?
Decide in advance rather than in August. The workable options are the same everywhere and they all cost something: reduce numbers, feed stored forage, rent additional grazing, or wean early to cut demand. What does not work is waiting to see if it rains, because by the time the shortage is visible in the sward it is also visible in the animals, and pasture grazed below its residual through a drought takes a season or more to recover afterwards. Write a trigger into the plan — a date, a rainfall figure, a measured forage height — at which you act. Your extension livestock agent will have drought planning material for your region, and a veterinarian should be part of any decision about animal condition and nutrition.