- Does a high stocking rate pay?
|Last update: November 24, 2010 08:30:45 AM|
Does a high stocking rate pay?
Dr W Botha & Ms W Mellet
Grootfontein Agricultural Development Institute, Private Bag X 520, Middelburg, Eastern Cape, 5900
Grazing capacity, or the prediction of grazing capacity, is one of those grey elusive concepts. Therefore it is not strange that everyone of us has an own viewpoint. Grazing capacity is the result of climate over centuries, local environmental factors (such as the origin of the soil), as well as the influence of mankind. On a close examination, grazing capacity - the potential of rangeland to produce feed for the grazing animals is the result of the evolution of the rangeland under the present climate. The dominant factor however, is the long term rainfall. This rainfall determines the amount of water that might be available for plant growth. The lower the rainfall is the more dominant this factor become. The amount of water available, determines the number of plants (density), as well as the botanical species composition. With the amount of water available, the season of rainfall does play an important role. The difference between the species composition, as well as the production rate between winter and summer rainfall, is enormous.
On a local scale, environmental factors such as soil depth, origin (dolerite, shale), slope and aspect play a decisive role. The vegetation on dolerite soil tends to be more grassy and sweeter than the vegetation on a shale soil. However, mankind plays an enormous role on this scale because we can determine the vegetation by means of the animal. Fence line effects is indicative of this influence and therefore, a common scene.
Stocking rate must be investigated from two sides. The first, and most common side is from a production perspective. This perspective looks at what and how much is produced. The second aspect or side is that of the resource and the ecological sustainable use of this resource. This side looks at what is happening to the vegetation. At the end of the day neither of these are correct. The only way to look at is a combination of the two perspectives. No one can produce by means of animal products without taking the environment into account.
From a production perspective at least two kinds of approaches are applicable. In the first instance production can be expressed as the amount of animal products per head of animal. This is a little bit confusing, because different animals differs on a large stock basis. Therefore, if production per animal is used, the rate of production should be expressed in terms of a large stock unit. With this unit in mind, the production of different types of animals can be compared. On the other hand, the rate of production can be expressed
on a hectare basis. In this case the amount of products produced is expressed in products per hectare. To determine if a high stocking rate pays, both these two methods will be discussed. It is furthermore very important to look at production over a period of time, the longer the period, the better the conclusion will be.
Production per animal
In Figure 1, the long term animal production in a stocking rate trial at Grootfontein Agricultural Development Institute is displayed. In this trial three different stocking rates (7, 13 & 20 ha/LSU) are tested with three different breeds. This graph displays the total production (body weight) data of Dorper wethers over a 14 year period. The animals were weighed every fortnight and were replaced on a yearly basis. In this graph, only the increase in body weight over this period is displayed.
From this graph, it is very clear that there are huge differences in the total production over the period of 14 years. In the first instance, the animals of the
heavy stocking rate produced just more than half of that of the moderate stocking rate. This lower production were accentuated during the period from 90 to 150 weeks weighing periods. This deviation was triggered by an extreme dry period and animals from the heavy stocking rate were, at that stage, withdrawn from the trial.
Although the stocking rate ratio between the moderate and light stocking rate is also 2:1 as in the case of the heavy and moderate stocking rate, the increase in production over time were much less than the difference between the heavy and moderate rates. In theory an animal can eat not more than his capacity and therefore cannot produce more than a certain amount irrespective the amount of surplus food available.
This phenomenon was described by Jones and Sandland (1974). With the above production data the Jones and Sandland model was applied for the production per head. The results are shown in Figure 2.
According to Jones & Sandland (1974) production per head increases as the stocking rate is getting lower. Due to the fact that this relationship is linearly, they also proposed that only two rates are necessary to predict the change in animal production with a change in stocking rate. This fact is confirmed by the results of this trial. They also postulated that there is a point where a reduction of stocking rate, no longer resulted an increase in production. This fact is also demonstrated by the small increase in production from the moderate to the light stocking rate.
Production per hectare
The second perspective is that of production expressed as production per hectare. Jones & Sandi and used the relationship in Figure 2 to compute the production per hectare. This production, computed from the trial at Grootfontein, is illustrated in Figure 3.
In Figure 3, it is noted that according to Figure 2, the animal production per head decreases with an increase in the stocking rate. Because of the increase
in animal numbers, the production per hectare increases. At some stage, the decrease in production per head results a decrease in production per hectare, because the reduction in production is more than the increase of animal numbers. Optimum stocking rate and the highest production per hectare is reached at the maximum as indicated in Figure 3. From a production point on a farm level, this is the level that production should be kept. In this case the optimum stocking rate over the period is 0.13 LSU/ha or 8 Ha/LSU. Similar results were obtained at Carnarvon Experimental Station.
The second perspective is the sustainability of the production process, in other words, what is happening with the vegetation at the different stocking rates? To evaluate the sustain ability, analysis of recent plant surveys at Carnarvon Experimental Station (February 2002) were done. This surveys are used, because four different stocking rates against the three of Grootfontein could be evaluated. The results from the surveys are displayed in Table 1. The data of the three camps per stocking rate were summed together. The following elements are used: Canopy spread cover, predicted veld condition index (Botha, 1998) and veld condition.
From Table 1, it can be noted that there is a decline in the canopy spread cover with an increase in stocking rate. This implies a decrease in the canopy spread and/or a decrease in the number of plants. The difference between the moderate and light stocking rate is marginal, but extensively between the very heavy and heavy rates. According to Botha (1998), veld condition index (Vci) can be predicted from the mean annual rainfall and grass component. The grass component on this experimental site, because of the origin of the soil (shale) is virtually absent. From the results of the surveys the Vci(s of the heavy stocking rates are below the predicted value. Therefore the veld condition is lower. In this context, veld condition is an indication of the health status of the vegetation. According to the values of the heavy stocking rate, the health status of the vegetation is poor. In the case of the moderate and light stocking rate, the Vci's are close to the predicted scores. However, the prediction by Botha (1998) is for vegetation in a fair condition. This means, that although the stocking rates were moderate to light, the health status of the vegetation is only moderate.
During the same survey an estimation of the number of plants per hectare was made. The results were:
With the veld condition indexes of Table 1 in mind this results confirms the theory that, with heavy stocking rates not only the plants decreases in size (lower canopy spread cover), but also in numbers.
What must be kept in mind is that at the beginning of the trial the health status of all the treatments was the same. Therefore it is clear that the health status of the vegetation under heavy stocking rates had deteriorated, while the status in the moderate and light stocking rate were maintained.
Total production as expressed by production per hectare increases up to a point with an increase in stocking rate. At the peak of the curve, maximum production is reached. An increase of the number of animals at this point leads to a decrease in production. Therefore the animal numbers or stocking rate should be kept equal to that of the peak.
From the point of the health status of the vegetation, the health status decreases if the stocking rate is too high. Therefore, for ecological sustainable use of the vegetation, stocking rates should be moderate at a specific environment.
According to the trend from the two perspectives, a compromise between the two aspects is better than either aspect. A golden rule is that with a decrease of approximately 10 to 15 % in maximum animal production per hectare, the health status of the vegetation can be maintained or sometimes improved. Therefore at this point, production is sustainable. High stocking rates do not pay.
BOTHA, WvD. 1998. Grazing capacity studies in the Karoo. University of the Orange Free State, Bloemfontein. PhD Thesis
JOHNES, RJ. & SANDLAND, RL., 1974. The relation between animal gain and stoking rate. J. Agric. Sci. 83: 335 - 342.