agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 9.4 1999
PERIODS OF STAY, ABSENCE AND THE GRAZING CYCLE
D M Gammon
Agric Foods (Pvt) Ltd, P.O. Box 559, Bulawayo, Zimbabwe
Controlled
Selective Grazing
Non-Selective Grazing
INTRODUCTION
Animals graze selectively on preferred areas and preferred species. If allowed continual access to an area (continuous grazing), the animals regraze these preferred areas and species when they reach a suitable height. The result is that the preferred grasses are grazed frequently and are grazed low, which causes a loss in vigour and production, and may result in eventual death. These species are gradually replaced by less palatable and/or less productive grasses, and animal production declines.
To prevent this deterioration, or to improve the composition, vigour and production of veld, it is necessary to introduce some control of the frequency and severity of grazing, and the intervals between grazings. In addition to controlling the stocking rate, this is done by dividing the grazing area into camps which are grazed and rested in rotation (refer to Production Guideline 9.2 in this series). It is necessary to discuss the concepts of periods of stay and absence, and grazing cycle, in the context of the different philosophies associated with rotational grazing systems. Two major forms of rotational grazing have evolved, differing in regard to the approach to selective grazing, and the intensity to which species in different palatability categories should be grazed in each grazing cycle.
The object of controlled selective grazing (CSG), or high production/performance grazing (HPG), is to apply the ideal treatment to the desirable grasses in order to maximise their production and favour their competitive ability. At the same time, good animal performance is provided for. Unpalatable grasses are underutilized, which may reduce their competitive ability and cause them to become moribund.
Important principles in this regard, with the emphasis on the palatable grasses, include the following.
- The number of grazing periods occurring during the growing season should be limited, so as to maintain a high level of vigour and production. Expressed another way, the interval between grazings should be long enough to ensure high production.
- The severity or height of grazing should be controlled, so that re-growth is rapid.
- The animal should be allowed sufficient scope for selection to ensure good performance, and the interval between grazings should not be so long that the poor quality of the herbage presented to the animals depresses animal performance.
In any grazing system, the variables which affect the application of the above principles are:
- the number of camps in the system;
- the period of stay (the uninterrupted time during which a group of animals occupies a camp) or the period of occupation (the total time that a camp is grazed by one or more groups of animals);
- the period of absence (the time that a camp remains ungrazed between successive periods of stay); and
- the length of the grazing cycle (the time taken to complete one rotation through all the camps of a system).
Although these are discussed as separate variables they are all linked, and any change in one will automatically change the others.
The longer a group of animals stays in a camp, 1) the greater is the probability that individual grass tufts will be grazed, 2) the lower will be the height to which grasses are grazed, and 3) the more restricted will be the animal's scope for selection. This will lead to reduced vigour of, and production by, the more palatable grasses, and poorer animal performance. The length of the period of occupation should be controlled, so that regrazing andPeriod of stay or occupation (period in)
low grazing are reduced to a minimum, and so that the animal's scope for selection does not become restricted to the extent that its performance is depressed.
The ideal period of occupation will vary according to several factors.
Stocking density. This refers to the stocking rate in any one camp during the period of occupation, and depends on the herd and camp size. The more, and hence, smaller, the camps in a system, the sooner will the palatable grasses be grazed; the sooner will regrazing and low grazing occur; and the sooner will the animal's scope for selection become restricted. Therefore, the more and smaller the camps, the shorter the period of occupation should be.
Veld type. The lower the proportion of palatable grasses and the production of palatable forage, the sooner will regrazing and low grazing occur, and the sooner will the scope for selection be restricted. Therefore the period of occupation should be relatively short in sourveld, and it should be longer in sweetveld at equivalent levels of forage availability.
Quantity and height of grass available. When there is relatively little grass available, as is the case in spring, the tendency to regraze and graze low is high, and animals rapidly run out of grass, to the detriment of their performance. Therefore, at the start of the grazing season, the period of occupation should be reduced. This could also be beneficial at the end of the season, when growth has stopped and the quantity and height of palatable grass becomes limiting.
Rate of grass growth. The more rapid the growth rate, the sooner regrazing is able to occur, and therefore the shorter should be the period of occupation. Conversely, when growth slows down towards the end of the season, the period of occupation should, in theory, be lengthened in order to provide a longer period of absence for regrowth. However, this conflicts with the previous paragraph, in that it will increase the intensity of grazing at a time when growth rate is slow. The end result must be a compromise between these two objectives.
Period of absence (period out) and grazing cycle
Grass grows rapidly if it has been lightly grazed. If it has been heavily grazed, it initially grows slowly before entering a phase of rapid growth. After this phase, growth slows down and reserves are replenished. As grass grows out, its quality deteriorates rapidly in sourveld, and more slowly in sweetveld. In sourveld, the period of regrowth should be long enough to allow reasonably high production, but not so long that herbage quality declines to unacceptable levels. This period is usually not long enough to replenish reserves and maintain vigour, and therefore the provision of specific rest periods is necessary for this purpose (refer to Production Guideline 9.3 in this series). In sweetveld, since the seasonal deterioration in quality is less marked, this period may be made long enough to at least partially replenish reserves. Suitable intervals between grazings (estimated by the grazing cycle) are approximately 40 days for sourveld, 55 days for mixed veld, and 80 days or longer for sweetveld.The interval between grazings in successive cycles is governed by the length of the periods of occupation and absence. When the period of occupation is short, the interval between successive grazing impacts on individual plants will be approximately equal to the length of the cycle, since individual plants are likely to be grazed only once during any period of occupation. With longer periods of occupation, grasses may be regrazed within a period of occupation, so that some grasses may be regrazed at an interval close to the length of the period of absence.
Ideally, the rotation should be flexible, with periods of occupation adjusted so that the palatable species are moderately grazed only once during such a period, and with periods of absence long enough to allow adequate regrowth to occur. However, this requires skill and frequent inspection of the camps. A guide to suitable periods of occupation is obtained by dividing the desired interval between grazing, or the grazing cycle, by the number of camps being used in the rotation. As the number of camps becomes smaller, the figure so derived becomes greater. With fewer, larger camps a longer period of occupation can be tolerated, but the tendency for regrazing will increase, and some grass will be grazed again at an interval close to the length of the period of absence. Lengthening the period of occupation may not improve the situation, because, although the period of absence will be increased, more regrazing and low grazing will occur within the period of occupation, and animal performance will be depressed.
Table 1 provides a guide to suitable periods of occupation, periods of absence and grazing cycles for mid-summer in different veld types, with varying numbers of camps used per group. This excludes camps in the system which are being rested. These figures apply to camps of average size and grazing capacity. For camps which differ markedly from the average, proportional adjustments should be made to the period of occupation, using the following formula:
Days in = (Grazing cycle x Camp size) / Total area
The objective of non-selective grazing (NSG), or high utilisation grazing (HUG), is to force animals to graze unpalatable grasses, with the intention of reducing their competitive ability. High total animal production, resulting from increased utilization of herbage with relatively high stocking rates, is intended to compensate for poorer individual performance.
Table 1. Guide to suitable periods of occupation (IN), periods of absence (OUT), and grazing cycles (GC) (in days) for different Bioresource Groups in mid-summer.
|
BRG |
Number of camps per group of animals1 |
|||||||||||
|
3 |
4 |
5 |
6 |
|||||||||
|
In |
Out |
GC |
In |
Out |
GC |
In |
Out |
GC |
In |
Out |
GC |
|
|
1 to 8 9, 11, 12 and 15 13, 14, 16 to 20 21 and 23 22 |
10 14 18 23 28 |
20 28 36 46 56 |
30 42 54 69 84 |
8 10 14 17 21 |
24 30 42 51 63 |
32 40 56 68 84 |
6 8 11 14 17 |
24 32 44 56 68 |
30 40 55 70 85 |
5 7 9 12 14 |
25 35 45 60 70 |
30 42 54 72 84 |
1
Excludes camps being rested
This approach may be appropriate in veld dominated by unpalatable grasses, particularly if they are Increaser I species (refer to Production Guidelines 6.1, 6.2 and 6.3 in this series). However, selective grazing continues to occur even under intense use, and by the time that a camp appears to be evenly utilised, the most-palatable grasses
will have been grazed more frequently, and more intensely, than the less-palatable grasses. Great care is therefore necessary to avoid damaging the palatable grasses through over-utilisation. The periods of occupation and absence, and the cycle length, are relatively long and very variable, depending on the condition of the veld and the time of year. Regular inspection of camps is essential in deciding on the timing of moves.