The main factors you would need to consider when calculating paddock sizes according to Sara E. Place, assistant professor with the Sustainable Beef Cattle Systems of Oklahoma State University’s Department of Animal Science, are:
1. How many animals are you grazing?
2. What is the dry matter intake of the animals?
3. How long are you allowing the cows and/or heifers to graze the paddock at one time (12 hours, 24 hours, and etc.)?
4. How much forage is available in the paddock?
5. What is the forage quality?
“The goal of a good grazing program is to have the cows and heifers harvest high-quality forage, yet not overgraze, which is why rotational grazing programs are popular,” Place said.
“The producer is trying to harvest the forage at its optimum quality and then leave enough so that the forage can recover and be ready for grazing again in 12 -28 days (typically, 4 inches in height left). More variability is added into the mix when you consider factors such as the weather, season, and the type of forages you have in your paddock (warm versus cool season grasses).”
Stacey Hamilton, extension state dairy specialist and instructor for the Department of Animal Science and the University of Missouri Extension Dairy program, offers the following sample calculation.
Example herd of 100 cows:
• The example goal is to offer 30-35 pounds of dry matter (DM) from the pasture per cow per day, so we need 3,000 – 3,500 pounds of DM per day.
• We have a preferred minimum of 1,000 pounds of DM/acre available in the pad before cows are turned in. For example, if they graze down to 2-2.5 inches that would be a residual of 1,250-1,500 pounds of DM/acre, in order to have 1,000 pounds available we need 2,250-2,500 pounds of DM/acre total starting off (probably 5-8 inches depending on stand density). Grazing to 2-2.5 inches in forages such as tall fescue or perennial ryegrass ensures that adequate carbohydrate stores (energy) are available for rapid forage regrowth. Other forages may require a different post-grazing residual.
• So a 100-cow dairy would need to have approximately 3-3.5 acres per day if we are offering 30-35 pounds per cow per day and have 1,000 to 1,200 pounds of dry matter available.
• Now the producer can configure actual paddock size. The producer can have a bunch of small 3-acre paddocks or they could make the paddocks 6-9 acres and then break these down daily (3-24 hour grazings) or every 12 hours (6-12 hour grazings) for a 9-acre paddock. It is not recommended to go more than 3 days in one paddock; this is to prevent cows from back grazing and slowing re-growth down. Going to larger paddocks gives flexibility in allocating pasture for the needs of the cows as well as is easier to bring in equipment to mow, spray or fertilize. It does require a bit more labor to set up temporary fences but this allows for the flexibility to allocate exactly what you want to adequately feed the cows.