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 Pasture & Range: August 1998 Other Pasture & Range Articles |
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by Matt Mattox |
Several months ago I wrote an article about how to get more out of your grass, or how to get more of the grass that you grow into your animals. In that article I briefly talked about several things such as increasing forage production, improving nutrient conversion, understanding animal behavior, improving managerial efficiency, and increasing harvest efficiency. This article will focus on increasing harvest efficiency on rangelands or native grass communities. Harvest efficiency is the percentage of forage grown in a year or growing season that is actually consumed by livestock (or other targeted animals). Harvest efficiency is also referred to as "utilization" or "defoliation efficiency." No matter what term you use, it still boils down to getting more out of your grass.
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Stock density is a tool that can be used to increase harvest efficiency, and therefore get more of the grass that you grow into your animals. Stock density is the number of animals grazing a given unit of land at any one point in time. Stock density is not to be confused with stocking rate, which is the number of animals grazing on a unit of land over a specified time period such as a growing season or year. For instance, if you have 100 acres of property that carries 10 animal units for a year, then your stocking rate would be 10 acres/animal unit (100/10 = 10). If today your 10 cow-calf pairs are grazing in a 25-acre pasture, then the stock density for that pasture would be 2.5 acres/animal unit. An animal unit is the equivalent of a mature cow, weighing approximately 1,000 pounds, either dry or with a calf up to six months of age.
Rangelands are native plant communities, and the quality and quantity of the forages that are produced by these lands is largely dependent on current, as well as past, grazing management. Normally the land that supports these "native grass" communities is still in native grass because of some limitation(s) such as shallow soil, slope, erosion, brush, or low rainfall.
The vegetation on rangelands to a large degree has to be managed to be selfsustaining. In other words, it is generally not economical or practical to implement cultural practices such as irrigation and fertilization on rangelands. With this in mind, many successful operations have developed stocking rates on rangelands by following the "take half-leave half" rule. This rule implies that you will leave half of the forage that is grown during the growing season to be used by the range plants so that they can remain healthy and continue to provide enough forage to sustain the number of animals that you are managing. The stocking rate is then set using the remaining "half" of the forage.
However there are other "forces" at work besides livestock that also need to be considered when setting stocking rates on rangeland. Grazing by wildlife, defoliation by insects such as grasshoppers, trampling, natural weathering, and manure and urine fouling can account for up to 25% of the available forage on rangelands. Therefore, to avoid overgrazing rangelands we normally only have 25% of the grass to devote to livestock. Stocking rangelands so that livestock only consume 25% of what is actually grown can seem very inefficient. But in many cases if you graze these areas much harder, the productivity of the vegetation can be reduced sharply. And generally, once rangelands begin to degrade, it becomes exceedingly difficult to economically recapture lost productivity due to the limitations of the land.
The understanding and manipulation of stock densities can and has been successfully used to increase the utilization of range vegetation by livestock without overgrazing the resource. If you are set up to rotationally graze, you can use stock density along with proper rest periods to reduce the effects of the "other" forces listed above. For example, using grazing pressure to utilize growing forage before it "turns over" into the litter component reduces weathering of the vegetation. The faster that a plant grows, the faster that it turns over. This process is largely driven by rainfall. So to make this work for you, it is essential that you are aware of rainfall amounts and distribution throughout the growing season. As the amount of precipitation increases, so does the potential for capturing this turn over.
The chart illustrates the use of stock density to potentially increase the utilization (defoliation efficiency) of rangeland vegetation.
This chart projects four lines, each representing rangeland with the following annual precipitation levels: very high (36+ in.), high (24-36 in.), moderate (15-24 in.), and low (‹15 in.).
For illustration purposes, if you will follow the line with the squares (very high rainfall) from right to left you will see that utilization (defoliation efficiency) remains at 25% until stock densities go below 2.5 acres per animal unit. This is essentially due to lack of enough grazing pressure to keep up with the grass before it turns over into the litter component. Below 2.5 acres/ animal unit, the potential to increase the amount of grass that you can get through your livestock increases sharply. Again, this is due to the potential to graze the grass before it "turns over."
Stock density can be expressed in different units such as acres/animal unit (AC/AU), animal units/section, or liveweight/acre. For those of you who relate better to liveweight/acre, (i.e. 700 lb. liveweight/AC ), you can convert the information in the chart from AC/AU to liveweight/AC by simply dividing the weight in an animal unit (1000 lbs.) by the acres or percentage of an acre in the corresponding stock density. For example, a stock density of .50 AC/AU is the same as 2000 lb. liveweight/AC.
Will this work for you? Let's use an example of native grass country that is in a 35-inch rainfall belt (high rainfall) and grass growth is sufficient to support a stocking rate of 8 acres/animal unit on a year-long basis. This particular producer has 800 acres of this kind of land; therefore he can carry 100 animal units.
His place is cut into four pastures of approximately 200 acres each, and he rotates all of his animals in one herd. His average stock density is 2 acres/animal unit or 500 lbs. liveweight/acre. Look back at the chart and find the point at which a stock density of 2 acres/animal unit intersects the high rainfall line, and follow it over to the left margin. In this example he should be getting about 29% utilization.
If he manipulates his stock density to go from 2 to 1 acre/animal unit, he can potentially increase utilization to 33%. Since he's carrying 100 head, to get the stock density to 1 acre/animal unit, he would need to reduce his pastures to half of their current size (200 acres to 100 acres). On an 800-acre piece of land this would require four additional fences that total approximately 2.3 miles of fence (this also assumes a common water point). Using one wire high tensile electric fence, the total cost including labor would fall in the range of $3150 - $4000. By manipulating stocking density, this producer could potentially carry 14 additional cow-calf pairs by increasing utilization from 29% to 33%.
Interest and depreciation for 10 years on this system would cost $50/cow/year for the 14 additional cows. This would be cheaper than the normal $80-$100/cow grass lease for rangeland of this quality. This method of increasing utilization of rangeland vegetation is generally more economical and less risky in higher rainfall areas. This is because higher rainfall areas have more potential for increasing utilization; therefore, returns on fencing and/or water developments are more likely. These areas also ordinarily recover from grazing faster.
Stock densities can be manipulated by simply adding or subtracting animal numbers on a pasture; however, if enough pastures aren't available this method can result in some pastures getting overgrazed and some undergrazed.
Another means of using this method would be to fence the property so that stock densities necessary to reach target utilization rates were met, but only increasing numbers during high growth periods (i.e. stockers during spring and early summer). The increased number of paddocks created would add flexibility to the operation and allow for more rapid range improvement, and provide the ability to rest pastures longer during droughts.
Note on the graph on page 3, that as precipitation level declines, the lines become flatter — less rain means less growth, and therefore less potential to increase utilization by livestock. Also notice that on rangelands that are in low precipitation zones, stock densities generally need to be less than 7.5 acres/animal unit before any increase in utilization over 25% is captured. The potential to increase utilization tops out at just over 32%; again, lack of water and slow plant recovery are the limiting factors.
Probably the most long-term successful use of manipulating stocking rates on rangelands would be to use the method to increase utilization to the 30-45% range and not push utilization to 50%. You stand a better chance of not overgrazing your rangelands and reducing productivity this way. I would encourage you to increase your knowledge of rangeland vegetation, soils, and weather patterns before diving in too deep, as there are many variables to manage for on native grasslands.