- Elements of the trampling factor in stock
|Last update: April 2, 2012 03:17:15 PM|
ELEMENTS OF THE TRAMPLING FACTOR IN STOCK
MECHANICAL action on the ground surface through trampling, is an inevitable attribute of stock. It is usually acknowledged and accepted that trampling of the ground surface has a detrimental effect on the soil.
The main effects of trampling are the formation of footpaths, stirring up of dust compaction of soil, dislodging of stones and clods on slopes and banks. The loosening and pulverisation of the soil surface can lead to serious soil fertility erosion through the action of wind. The puddling of the ground surface under raindrop action is also a constant and serious threat on a loosened surface; in this case a thin and relatively impervious layer is often created which causes high run-off rates, poor infiltration of water, and development of bare patches.
Trampling can also cause the breaking up of plants, the disruption of the progress of vegetative reproduction and the disturbance or destruction of seedlings and young plants. All three results of trampling aggravate the action of the main erosion agents, wind and water.
On the other hand the positive effects of trampling are the breaking up of ground surface crust (chipping) to improve aeration and water infiltration, loosening of soil to provide a more favourable seedbed, trampling in of seed, the breaking up and fragmentation of organic material such as dry leaves, woody twigs and litter, and the pulverisation and mixing of dung pellets and other types of animal droppings with the soil. It is suspected that trampling may even have a controlling effect on some veld insect pests.
It is not possible to eliminate the detrimental effects of trampling entirely, and encourage the favourable effects only. It appears, however, that under practical management it is possible to minimize the detrimental influences and promote the positive effects. Generally, however, the detrimental effects of trampling by domestic stock overshadow the positive effects.
MAIN ELEMENTS OF TRAMPLING
In its daily natural course of grazing, the animal has to traverse a certain minimum distance on the veld in order to satisfy the requirements for physiological processes, energy and production.
The degree of trampling on a specific area is closely governed by stocking density, soil type, and duration of the period of trampling, the availability of fodder as determined by vegetation composition, quality and production. This latter aspect has direct bearings on carrying capacity.
The daily walking distance of the grazing animal is primarily governed by the availability of palatable and utilisable fodder plants. When the availability of fodder is at a minimum, the walking distance tends to a maximum, and conversely, when the availability of fodder is at a maximum, the walking distance tends to a minimum (Roux, 1976). Factors also influencing walking distance are climate, season, topography, occurrence of physical barriers and other natural or artificial structures obstructing or deflecting movement, the location of stock watering points, lick and feed troughs, the size and shape of grazing camps, and distribution and concentration of palatable and edible plants.
Additional factors which playa direct role in trampling are breed of stock, age, sex, physiological conditions, weight size and conformation. The latter has direct bearing on hoofprint area and mechanical impact (pressure) on the ground surface.
The inherent characteristics of a grazing animal further influences grazing behaviour and selective grazing habits. Flock or herd instincts, such as gregariousness or solitary habits further influence movement.
TRAMPLING POTENTIAL AND RELATIVE TRAMPLING
Trampling potential can be defined as the inherent capacity of a grazing animal to exert a mechanical force, through walking, on its environment. The ground surface is most directly affected.
The primary attributes of trampling potential of stock breeds are number of hoofprints per unit distance, total hoof area, pressure per unit area exerted per hoof, and distance covered over time. Pressure per hoof is half the weight of the body as only two hooves are on the ground surface at any one time while walking is in progress. Allowance for extra pressure is not possible.
In order to obtain values on the main elements of trampling, namely body weight, hoof area, hoofprints over a measured distance and walking distance, the following determinations were made:
The distance walked by Merino wethers per day was determined by means of a rangemeter (England, 1954) (Fig. 1). The rangemeter, with adaptations, functioned satisfactorily on Karoo veld. It is doubtful whether the rangemeter can be used on boulder-strewn or very uneven ground.
Strides counted per 100m
The number of strides (or hoofprints) per 100m was determined by tagging the right front leg of the animal and counting the number of strides over a marked distance of 100m. The experimental animal, during the counting procedure, was allowed to walk comfortably along with a small flock. Stride length was calculated.
Stride length calculated
Stride length can be approximated by measuring the distance between the right front hoof tip and the hind right hoof tip. The animal should stand in a comfortable and normal position. This measurement, multiplied by 1,2 for sheep and cows, and 1,5 for goats gives a close approximation of stride length. The number of steps per 100m can be calculated; this value is usually slightly less than the stride length calculated from strides counted.
These preliminary factors, 1,2 and 1,5, appear to be promising. Further research is however required to determine more accurate factors and the necessary correlations and relationships.
In the determination of hoofprint area all four hooves were impressed on a thin layer of plasticine on hardboard. The imprint outlines were traced (Fig. 2) and the area determined planimetrically.
Live body weight was determined by straightforward weighing on a suitable platform scale.
RESULTS AND DISCUSSION
Table 1 shows the results of various measurements
Relative-trampling factor (RT-factor)
From the results a basic trampling factor was devised and conveniently standardised on a "standard" sheep (See Table 1). The calculation of a RT -factor was based on the formula:
Where W = live weight in kg
A = total area cm2 (4 hooves)
H = Hoofprints per 100m (counted)
The last term in the formula, the reciprocal of the square root of hoof area, has bearing on the reduced mechanical effect of trampling with an increase in hoof area; the larger the hoof area the less the mechanical impact on the ground surface. A very small hoof would tend to have, what is popularly known as the stiletto-heel effect. This effect should, however be studied separately in order to determine its formulation accurately. The value of the standard Rt -factors was expressed relative to this standard value (Table 1). The RT-factor can be multiplied by distance walked in order to determine relative trampling effect differences.
In Table 1 it can be seen that cows have the lowest RT -factor and the Angora doe the highest. The figures are however by no means conclusive, but do give an indication of relative trampling order. Many more determinations are required to obtain reliable relative trampling sequences.
From a total of 28 measurements from four Merino wethers fitted with rangemeters, an average walking distance of 9025m was obtained. The maximum distance recorded was 12 400m and the minimum 5 500m. The wethers were run on a mixed karoobush-grassveld dominated by Eriocephalus glaber (Kapokbush). The presence of this shrub concentrates or deflects stock movement to the open spaces between the shrubs (interspace grazing).
Stride, length and walking distance
Walking distance is a very important element of trampling. A grazing animal exerts a greater influence on its environment the further it walks. From the results obtained on stride length from ten replicates of stride counts of five Merino wethers, it was found that highly significant differences (P = 0,05) existed between the wethers. The difference between the highest and lowest average stride lengths were 78,42cm (± 0,64) and 70,00cm (± 0,85) respectively. These results indicate that reliable standard stride-length tables (or strides per 100m) could be drawn up for various stock breeds. Walking distance, having a large variability is a less reliable basis for comparing stock. However from the results obtained by Louw et al (1948) on walking distance by various breeds of sheep, it is clear that each breed has a characteristic walking distance under similar conditions. An interesting study would be to correlate stride length with walking distance.
These results obtained, show that walking distances, and consequently trampling, varies considerably from day to day. For instance, the distance for a Merino wether, in mid-winter, varied from 5,9 to 12,5km in 7 days with an average of 9,08km over 10 days.
The average number of hoofprints per wether per 24h, over the distance of 9 025m, is 48 825 which gives a hoofprint area coverage of 78,92, or a possible complete coverage of 1 ha in 127 days.
However, trampling can never be spread uniformly over a grazing area and various degrees of the superimposition of hoofprints are inevitable as a result of the formation of footpaths, trampling around watering points, shelters etc. According to Van den Berg (1977), where half of the area under grazing is obstructed to trampling, approximately 50% of the ground surface has no hoofprints, 21 % covered by one, 12% covered by two prints superimposed, 7,5% with three, 4% with four, and 2,0% with five and 3,5% with 6 or more. It is evident, considering the factors influencing walking, that considerable departure from this theoretical superimposition of hoofprints can be expected. It does however, provide a possible basis for comparisons.
It is clear that a considerable variety of variable factors are directly and indirectly involved in trampling by grazing animals.
Body weight, walking distance, stride length, and hoofprint area are the most important elements involved in trampling.
It is clear that stock breeds differ in their capacity to trample soil. The formula for relative trampling (RT -factor) provides a basis from which trampling can be ranked for stock in general.
Relative trampling is further of particular importance where the carrying capacity of veld is concerned. For example, where the large stock unit equals 6 small stock units, it can be accepted that the combined effects of six small stock units would be greater than that of one large stock unit.
Trampling is a most important ecological factor coupled with the grazing animal, and one that requires more detailed study especially in respect of its mechanical effect on vegetation, as distinct from the actual grazing of vegetation.
Thanks are expressed to Mr P.A. Schlebush, Technician, Animal Husbandry Section, for his contribution on stride length measurements and counts. Also to Messrs A. Mulder and L. Nel for their indispensable assistance in determining ranging distance, hoof area, weight and stride length measurements and Miss J. Viljoen for the drawing of Figure 1
ENGLAND, G.J., 1954. Observation of the grazing behaviour of different breeds of sheep at Pantyrhuad Farm, Carmarthenshire Brit. J. Anim. Behav. 2(2), 56-60.
LOUW, D.J., HAVENGA, C.M. & HAMERSMA J., 1948. The walking habits of sheep. Fmg. S.A. 23,753-755.
ROUX, P.W., 1976. Laws of the veld. Eastern Cape Regional News Letter. Eastern Cape Region, Dept. Agric. Tech. Services.
VAN DEN BERG, J.A., 1977. Ontblaring en vertrapping as komponente van beweiding. Glen Agric. 6, 1 April.
Karoo Agric 1 (2), 9-12