ONA REPORT

published in

THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL


September 2010

Grazing management of warm-season grasses

Dr. Joao Vendramini and Dr. Lynn Sollenberger
University of Florida/IFAS


For questions or comments regarding this publication contact: Joao Vendramini


Grazing management is defined as the manipulation of livestock grazing to accomplish a desired result. The desired result depends upon the enterprise, but for most producers economic goals are of primary importance. When deciding what grazing management strategies to use producers should consider the characteristics of the forage being grazed, animal requirements, input costs associated with adopting a particular system, and the probability of return on investment.

In the southeastern USA, warm-season perennial grasses are the basis for cow-calf operations. When managed properly, warm-season grasses have excellent production and acceptable nutritive value during the spring, summer, and early fall.

The most critical grazing management decision to optimize forage quantity and nutritive value is stocking rate. Stocking rate is defined as the amount of land allotted to each animal during the grazing season. The stocking rate adopted can affect how much of a plantís leaf area is remaining after grazing and how many growing points are available to provide regrowth. The most accurate procedure to determine stocking rate involves matching the existing forage mass with adequate cattle liveweight exposed to the pasture. Research shows that for most warm-season grasses during the growing season, 1 lb of forage DM is necessary to maintain 1 lb of animal liveweight exposed to the pasture. While many producers have access to measure or estimate animal liveweight, accurate forage mass determination may be challenging. When forage mass cannot be accurately determined, stocking rate decisions can be made based on forage stubble height. Generally, tall-growing, bunch grasses that elevate their leaves and growing points should be grazed to a taller stubble height than low-growing grasses, like bahiagrass. The recommended grazing stubble height for common warm-season grass species in Florida is described in Table 1. The low-growing grasses typically have leaves and growing points at or very close to the soil surface to protect them from being overgrazed. For the animal, closer grazing forces them to eat more stem. Because stem is less nutritious than leaf, close grazing will result in lower weight gain or milk production per animal. Undergrazing allows animals to selectively graze the nutritious leaf portion of plants and does not stress the plant. However, it results in poor utilization of the pasture resource. Although meat or milk production per animal may be high when pastures are undergrazed, production per acre will be low.

The use of rotational or continuous grazing is another important decision to be made in grazing systems. After a plant has been grazed, its ability to regrow depends on how much leaf is left. The carbohydrate reserves in those remaining leaves will supply the energy for plant growth. Rotational grazing with the correct stocking rate will maintain adequate stubble height and carbohydrate reserves after grazing to maximize forage regrowth. In addition to increased regrowth rates, rotational grazing at the proper stocking rate also improves pasture persistence by allowing better stubble height control. Controlling target stubble height on rotationally grazed pastures is important because overgrazing can result in a loss of desirable species and an increase in weeds. While continuous grazing may require less producer input, it does not provide pastures with a period of rest. As a result, continuously grazed pastures generally have a greater proportion of forage that is trampled, soiled, and rejected by the animals compared to rotationally grazed pastures.

In general, grazing method does not affect the average daily gain of beef cattle grazing warm-season grasses. However, the greater forage production and utilization associated with rotational grazing does allow for greater stocking rates, which typically results in greater liveweight gain per unit of land.

To summarize:

  1. Stocking rate decisions should be made based on existing forage mass on the pasture and controlled based on the recommended stubble height.
  2. Rotational grazing with adequate stocking rate potentially facilitates stubble height control, increases forage production and persistence, and may result and greater liveweight gain per unit of land.
  3. Overgrazing is detrimental to forage production, pasture persistence, and animal performance.

    Table 1. Target stubble height for persistence of warm-season grasses in Florida.
    Species
    Target stubble height (inches)
    Bahiagrass
    2
    Bermudagrass
    3 - 4
    Stargrass
    6 - 8
    Limpograss
    8 -10


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