August 2020 Articles

Archives by Months

Livestock and Forage Interactions

Livestock and Forage Interactions Source: M. W. Alison and E. K. Twidwell, LSU AgCenter
Beef cattle livestock systems tend to be pasture (forage) based and as such it is important to understand both pasture (plant) and animal requirements to effectively manage a grazing enterprise. In grazing management there are choices that have to be made and each decision can impact the plant and/or the animal in a positive or negative manner. At times one must decide whether to favor the plant or the animal.
Pasture stocking rate is defined as number of animals per unit area during a defined period of time. This measure alone is not necessarily useful unless the amount of forage in the area is known. Grazing pressure is a term relating animals to forage mass and would be more useful when making decisions about grazing. For example, grazing pressure can be high when forage growth is slow and low when forage growth is at a peak even at the same stocking rate. Therefore, one must consider seasonal changes in production when managing pastures and determining stocking rate.
Determining the proper stocking rate is the most profound factor affecting the success of any grazing management strategy. The amount of forage produced will vary throughout the year. While it is rather difficult and time consuming to accurately measure forage production, producers can visually estimate forage growth. With experience, producers can develop estimates of forage growth during the year that can be useful in developing an optimum stocking rate for their operation. The proper stocking rate can vary depending on the production goals and length of the grazing period. As a general rule gain per animal is optimized with low stocking rate (grazing pressure) and gain per unit area is optimized with relatively high stocking rates (grazing pressure). Since gain per animal declines as stocking rate increases it is inherent that individual animal gain and gain per unit area cannot be maximized at the same time. So there will need to be some consideration as to the goals when determining the stocking rate. The key to good grazing management is matching the stocking rate to the grass growth rate.
Some things to be considered are what class of livestock is grazing. Growing animals and early lactation cows require a higher plane of nutrition so stocking rate (grazing pressure) should not be at a level to restrict intake of quantity or quality forage. Dry mature cows have only a maintenance requirement so could be grazed at a relatively high stocking rate without a detrimental effect on the animals. Another consideration is the importance of individual animal performance vs. output per unit area. When animals are purchased for grazing, performance per animal may be of more importance than output per unit area but when custom grazing for pay per weight gain output per unit area would be the important factor. Another factor to consider is what is being grazed. If an annual forage crop such as annual ryegrass is being used then stocking rate can be high enough to utilize the forage produced without regard of long term effect on the plant. But if a perennial forage such as bermudagrass is being used there needs to be consideration given to long term survival.
Improvement in livestock production is primarily dependent on improvement in quantity and/or quality of forage consumed over time or space. So it is important to consider how grazing impacts forage production. Plants, like other organisms, grow based on an energy balance between demand and production. Plants produce energy compounds (sugars/carbohydrates) through photosynthesis which primarily occurs in leaves. The first use for these carbohydrates is for leaf and tiller growth. When carbohydrate production exceeds plant growth demands the excess carbohydrates can be stored in plant tissue (stem bases, rhizomes, or stolons) and utilized during periods of deficit energy balance.
Grazing inherently removes parts of plants and this changes the dynamics of plant growth. Plants must re-grow leaves or initiate new tillers following grazing. The most efficient method for re-growth is to depend on carbohydrates from continued photosynthesis. So to enhance efficiency of plant growth it is beneficial to leave enough leaf material to allow for adequate carbohydrate development from photosynthesis. All plants do not exhibit the same growth habit (upright vs. prostrate) so beneficial grazing management can differ depending plant species. In other words, adequate leaf material can be left on some plants with closer grazing than grazing height for other plant species. For example, bermudagrass has prostrate growth and when it is grazed, the animal is mainly removing leaf material. The plants can be grazed to a fairly low height (2”) and they will regrow relatively quickly. Summer annual forages, such as pearl millet, have a more upright growth habit, and a higher grazing height (6”) is needed for optimum regrowth. This can certainly be an important point to remember when planning a grazing system.
Grazing frequency and intensity and season of the year are three important factors affecting plant response to grazing. General concepts for grazing a plant are to use high frequency with low intensity grazing or low frequency with high intensity grazing. These management practices allow the plant to maintain a positive/equal energy balance or allow time for recovery growth and carbohydrate storage. Use of these practices as described should promote sustained productivity of the pasture resource.
Season of the year is an important factor for two reasons. First pasture growth rate is not linear over time so allowance has to be made in stocking rate (density) based on plant growth at any time in the season to adequately maintain photosynthesis. Secondly, throughout Louisiana, perennial plants have a dormant period during which survival is dependent on stored carbohydrates. So the pasture should not be grazed intensively immediately prior to dormancy since the buildup of storage carbohydrates is necessary to maintain plant vigor.
Grazing systems are management tools designed to enhance and stabilize livestock production over time. The optimum grazing system is certainly dependent on production goals and can change based on animal type, forage species, climatic conditions and many other situations. So it should be reasonable to assume that utilization of multiple stocking methods instead of a single system could be appropriate for achieving management goals.