Pastures: Grazing Management
Animal units of various species and classes of livestock
|1,000-lb dry cow||1.0|
|1,300-lb dry cow||1.3|
|1,000-lb lactating cow (first 4 months after calving)||1.4|
|1,300-lb lactating cow (first 4 months after calving)||1.6|
|2,000-lb mature bull||1.7|
|550-lb growing-finishing heifer (1.0 lb/day gain)||1.0|
|550-lb growing-finishing steer (2.0 lb/day gain)||1.23|
|110-lb brood ewe||0.15|
|132-lb brood ewe||0.17|
|154-lb brood ewe||0.18|
|175-lb brood ewe||0.20|
|300-lb mature ram||0.40|
|110- to 132-lb replacement ewe, lambs, and yearlings||0.22|
|220-lb replacement ram, lambs, and yearlings||0.42|
|1,000-lb dairy cow (maintenance)||1.0|
|800-lb dairy cow (last 2 months of gestation)||1.0|
|1,000-lb dairy cow (last 2 months of gestation)||1.2|
|1,300-lb dairy cow (last 2 months of gestation)||1.5|
|1,500-lb mature dairy bull||1.4|
|2,000-lb mature dairy bull||1.9|
|550-lb growing dairy heifer||1.0|
Grazing Pressure and Stocking Rate
The closeness to which a pasture is grazed is defined as grazing pressure. Grazing pressure is affected by both the stocking rate (animals per acre) and the available forage (pounds of forage per acre).
High grazing pressure causes close grazing and short pasture stubble. Animal performance is reduced because animals are forced to consume all portions of the pasture, including the poorer quality forage (Figure: "Effect of grazing pressure on production per animal and per acre"). Excessively high grazing pressure results in overgrazing.
The effect of grazing pressure on production per animal and per acre
Reference: Figure 1.8-2, The Agronomy Guide
Low grazing pressure refers to selective grazing, with unlimited forage available to animals (Figure: "Effect of grazing pressure on production per animal and per acre"). Animal production rates (gain per head per day, etc.) usually are high at low grazing pressure. Very low grazing pressure, or undergrazing, results in wasted forage and consequent poor animal production per acre.
An optimum grazing pressure is one in which available forage matches animal needs; this is analogous to the quantity of forage offered daily to animals in confinement when rations are balanced. To maintain an optimum grazing pressure, it is important to adjust the stocking rate so that available forage is maintained within an optimum range.
Available forage not only influences the stocking rate, but also can affect dry matter intake. As the amount of forage decreases, the per-animal intake decreases. Reducing available forage from 1,000 to 500 pounds per acre has been shown to reduce average intake by almost 40 percent. For optimum grazing, maintain available pasture at 1,000 to 1,500 pounds of dry matter per acre. On average, a 3- to 4-inch bluegrass/white clover sward has about 1,000 pounds of dry matter per acre; in tall grasses and legumes, 6 to 8 inches of growth will contain this amount.
Unfortunately, not all available forage in a pasture is used. Some of the forage is trampled or fouled by urine or manure and remains uneaten. The amount of available forage that is consumed is called utilized forage. At high grazing pressures, animals are forced to consume more of the available forage, resulting in a relatively high (greater than 80 percent) utilization rate. At low grazing pressures, animals will refuse to eat portions of the forage, thus decreasing the utilization rate.
Suggested paddock sizes (A/animal unit) based on grazing period and available pasture in intensive rotational grazing system
Note, as the length of grazing period increases (lower grazing pressure), the percentage of utilization decreases.
|Grazing period (days)||Pasture utilization1 (percent)||Available pasture (lbs dry matter/A)|
|--- A/animal unit2 ---|
Grazing pressure affects not only animal performance but also the plant species in a pasture. Long periods of low grazing pressure commonly cause a loss of legumes from the stand. At the other extreme, long periods of high grazing pressure can result in a temporary or long-term decrease in pasture production. High to excessive grazing pressure for long periods with little or no opportunity for selective grazing also causes low animal intake and low gain rates.
Short periods of high grazing pressure can serve useful purposes. One to two weeks at high grazing pressure, three or four times through the grazing season, can help maintain legumes in the stand and utilize forage that might otherwise be wasted. It is important not to expose lactating cows or growing animals (calves, yearlings, lambs) to long periods of high grazing pressure. Beef cows and ewes can tolerate high grazing pressure after breeding.
How animals are managed on pastures can have strong effects on forage production and persistence, as well as on animal production. Selection of grazing management practices is a key decision in designing a pasture management program. Some important grazing practices are listed below.
The entire pasture area is grazed continuously for periods from 6 weeks long to the whole growing season. This method appears simple because it requires few fences and water sources, and less monitoring of the pasture condition. Uncontrolled continuous grazing uses a fixed stocking rate on a fixed land area and often leads to under- or overgrazing and poor pasture utilization.
Controlled continuous grazing methods try to maintain an optimum grazing pressure. Grazing pressure can be increased by increasing the stocking rate (adding animals) on a pasture or by temporarily fencing out part of a pasture for hay harvest. High grazing pressure may be relieved by removing some animals from the pasture, by allowing access to other pastures or hayfields, or by feeding harvested feed to the animals on pasture. For Kentucky bluegrass and perennial ryegrass, maintain a grazing height of 2 to 3 inches. For tall grasses, maintain a grazing height of 4 to 5 inches.
Close continuous grazing can lower pasture production and decrease persistence of many grasses and legumes. Therefore, it is important to use forage species adapted to continuous grazing.
Clipping pastures with a rotary mower is a good practice if pastures are undergrazed and growth gets ahead of the grazing animals. Clipping removes this tall, rank vegetation and encourages new growth of higher-quality forage.
Since animals tend to congregate at water, soil nutrients from their wastes become concentrated in one area of the pasture. Providing more than one source of water helps distribute nutrients more widely across the pasture.
To maintain a productive pasture in a continuously grazed system, it is important to avoid overgrazing. During periods of dry weather, remove the animals and give the pasture a rest.
The pasture area is divided into two or more subpastures, also called “paddocks,” with each subpasture being grazed and “rested” in turn. Putting the whole herd in a small paddock results in rapid use of available forage. When a paddock is grazed off, animals are moved to a fresh one, allowing each paddock to be grazed clean and to have a rest period. The rest period is critical for the continued productivity of some forage species.
Time that animals are on a paddock is referred to as the grazing period; time off the paddock is the rest period. Rotational grazing systems vary in design and intensity. Typical rotational grazing systems usually have less than seven paddocks with a grazing period of one to two weeks. Animals have a high degree of selectivity during the first part of the grazing period. With these lax systems, plants grazed at the beginning of the grazing period are likely to have regrowth that is eaten before the grazing period ends, resulting in lower productivity.
Intensive rotational grazing systems usually have more than seven paddocks with grazing periods of less than one week to as little as one-half day. There is more control of what the animal eats, and typically, better growth and persistence of pasture plants. When managing an intensive rotational grazing system, it is important to remember that the stocking rate is very high within one paddock and that overgrazing can occur very rapidly. Animals must be moved when the available pasture falls below its optimum range. Allow grazing to begin when tall grasses reach 7 to 8 inches, and remove animals when grass heights fall below 3 inches. Be- gin grazing short grasses such as Kentucky bluegrass or perennial ryegrass when they reach 5 to 6 inches; remove animals when grass heights are 1 to 2 inches. To maintain milk production levels, you may need to remove highly productive dairy animals from paddocks before the plants reach these heights.
Typical paddock rest periods of rotational grazing systems
Rest periods should be based on the growth rate of the pasture and will vary depending on the season and weather conditions. Growth rate is affected by soil productivity and fertility levels. In most pasture systems, areas vary in soil productivity, and growth rate also varies. Therefore, rest periods may vary even within a pasture system. The best way to manage this situation is not to use a set rotational scheme, but to move animals to those paddocks that have reached their optimum available pasture. Keep animals off a particular paddock until it reaches this level.
|Season||Weather conditions||Growth rate||Rest period|
|Spring||Cool, moist||Fast||10–14 days|
|Spring||Warm, dry||Medium||14–20 days|
|Summer||Hot, moist||Slow||30–35 days|
|Summer||Hot, dry||Very slow||40–60 days|
Paddock size depends on the number and class of animals being grazed, the amount of available pasture at the beginning of grazing, and the desired grazing period. Table: "Suggested paddock sizes (A/animal unit) based on grazing period and available pasture in intensive rotational grazing system"provides some suggested paddock sizes for intensive rotational grazing. Adjustments may be necessary if the stocking rate changes. For flexibility, the use of temporary interior fences is recommended.
The number of subdivisions needed for a rotational grazing system depends on the number of days the animals graze in a paddock and the maximum summer rest period needed.
Paddock number = (maximum summer rest period ÷ grazing period) + one
Example: (40 days rest ÷ 3 grazing days) + 1 = 14 paddocks
Note: The species and class of grazing animal may determine the grazing period. Lactating dairy cows need consistent-quality forage; therefore, the grazing period may be anywhere from 1⁄2 to 2 days. Beef cows, brood ewes, and most other ruminants do not require as consistent a quality of forage, and a grazing period of 3 or more days may suffice.
Stocking rate and available pasture also are important to managing a rotational grazing system. Spring management usually involves diverting some of the pasture for hay or silage. For more information about designing a pasture system, refer to Agronomy Facts 43: Four Steps to Rotational Grazing, available from your local extension office.
Excluding mature animals while allowing young calves and lambs access to high-quality, highly palatable forage is known as creep grazing. This can be achieved by raising the electric fence high enough to allow young animals to walk under but low enough to restrict the movements of larger, more mature animals. A special creep gate also can be used to allow the young animals to graze the “creep” pasture and to return for suckling, much like a creep grain feeder. In rotational grazing systems, the “creep” gate can allow young animals to graze the next fresh paddock before mature cows or ewes.
Grazing of pastures by two or more species of animals (beef and sheep, sheep and goats, etc.) has advantages. Animal species differ in the types of forage they prefer. Beef cattle prefer legumes over grasses, while sheep prefer immature grasses and weeds over legumes. Goats consume shrubby vegetation, in addition to forage plants. Sheep, goats, and cattle tend to be complementary in their grazing habits; thus, two or more animal species together can better utilize the forage.
Goats also help control weedy vegetation. Co-grazing does not increase the overall animal-equivalent carrying capacity of a pasture. For purposes of stocking, a cow and calf may be considered equivalent to six average-sized ewes and lambs or nannies and kids. Co-grazing systems with sheep or goats should use rotational grazing (14 days or less per pasture) to minimize parasite larval survival and reinfection.