agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 7.5 1999
FIRE AND GRASS
M B Hardy
Western Cape Department of Agriculture
Plant Adaptations
Burning as a Management Tool
INTRODUCTION
Much of the vegetation of KwaZulu-Natal is maintained as grassland by regular burning. Most grasslands would become dominated by scrub thickets or forest if fire were excluded from the system. Lightning and rock falls are thought to have been the major cause of fires before the arrival of mankind, and these sources still start many fires in, for example, the KwaZulu-Natal Drakensberg each year. The early inhabitants of KwaZulu-Natal used fire as an aid to hunting and to encourage the growth of geophytes (plants with bulbs and tubers which were used as food). With the advent of pastoralism, fire was used particularly in the sourveld areas to improve the quality of veld grazing.
Grass burns most readily when dry. In KwaZulu-Natal, the grasses generally become dormant and dry towards the end of May or early June. Regrowth normally starts in September. Fires caused by lightning would have occurred mainly during August and September, whilst rockfalls may also have been the cause of fires during the dry winter months. Early hunters are also likely to have set fires at these times. The grassland vegetation of KwaZulu-Natal has apparently evolved under a system of regular burning and is therefore well adapted to regular burning when dormant.
Many grasses are well adapted to fire, and indeed, unless they are regularly and completely defoliated, they become moribund and die. Briefly, the basic unit of the grass plant, the tiller, develops through the growing season, producing leaves and eventually an inflorescence. At the same time, new tillers are initiated at the base of the plant. After one or two seasons of growth, the main tiller dies off and the newly-initiated lateral tillers will develop the following season. Light is required for tiller development to occur, and therefore if the old, dead material is not removed, the growth of new tillers during the next season will be retarded. If no defoliation occurs over a few years, then the whole grass tuft smothers itself (i.e. becomes moribund) and dies. Burning during the dormant season removes the dead plant parts. The newly initiated tillers are protected from damage by fire as they lie within the base of the plant and, in some species, are underground. Once soil moisture and temperature conditions are suitable for growth in the spring, these tillers will begin to grow.
The adaptations of grasses to complete defoliation (i.e. newly initiated tillers remaining at or below ground level during the dormant season) do not apply if they are burned during the growing season (October to May). In this period the tillers of many species, for example Themeda triandra (Redgrass), are elevated well above ground level and defoliation by burning will destroy the new growth. If the practice of burning during the growing season is continued, those grass species which do not have tillers below ground at this time of year will die out. For these reasons the grass communities of KwaZulu-Natal are not adapted to burning during the growing season (refer to Production Guideline 7.2 in this series for more details of growth patterns in grasses).
Many trees and shrubs commonly found in the drier grassland areas (e.g. Bioresource Groups 13, 14, 16 and 18) are also adapted to fire. Although they are susceptible to fire in the seedling and early growth stages, they are not killed by fire once they have passed these initial stages of growth and development (refer to Production Guidelines 7.3 and 7.6 in this series for greater detail on the effect of fire on woody plants).
Introduction
Fire is a useful and indispensable aid to veld management and must form an integral part of the veld management strategy practiced on the farm. The most important considerations relating to burning grassveld are why burn, when to burn, how often to burn, what management to apply following a burn and how to burn.The five major reasons commonly given for using fire in grassveld are:Why burn?
- to burn off unpalatable growth left over from previous seasons;
- to control the encroachment of undesirable plants in the veld;
- to protect the veld (and farm) from wildfires and accidental fires;
- to stimulate growth during those months of the year when there is little young forage available, and thus provide green feed when it does not occur naturally; and
- to destroy parasites.
Of these the first three are considered valid reasons, while other two constitute bad management practices. Each is discussed in more detail below.
Burning to remove old, dead overburden of grass
To remove all the grass that accumulates during a season by grazing alone is difficult. In practice, unpalatable, low-quality herbage accumulates during a grazing season, particularly on the less-acceptable plants. This leads to an overall lower quality of forage on offer to the animals, with the new season's growth being diluted with low-quality residual herbage from past seasons. In addition, accumulated dead growth from the previous season results in tillers within the tuft becoming smothered and as a result they might die. There is no replacement of these tillers because basal buds die with the parent tillers or are smothered by dead herbage. This moribund herbage must therefore be removed by fire.Research indicates that the new growth following a burn is more nutritious than new growth in unburned veld (although the difference disappears as the season progresses). Many farmers, therefore, tend to burn annually (particularly sheep farmers) in order to provide animals with the highly-nutritious herbage which accumulates after a burn, and to remove old low-quality herbage left over from the previous growing season. The farmer must, however, consider the negative effects of annual burning on forage production from veld. This aspect is discussed in the section: "How often should the veld be burnt?".
Burn to control invader species and bush encroachment
Invasion by undesirable species occurs in both under- and over-utilized veld. A weakening of the competitive ability of the grass component permits an encroachment of invader species. Fire can assist by destroying the seedlings of invader species and by increasing the competitive ability of the grass sward. Conditions must, however, be favourable and it is necessary to burn when conditions will ensure a hot fire (see below).Fire may be used effectively to control at least some encroaching plants. In sourveld areas where Leucosidea sericea (Ouhout) and other similar shrubs and bushes begin to encroach on grassveld, fire can be used, initially at two-year intervals and later at longer intervals (every four years), to control the trees. The veld should be rested for a period prior to the burn to ensure the accumulation of sufficient fuel loads. The timing of the burn is also important, as a very hot fire is required to cause die-back of shrubs and trees. (Note that fire alone will not kill the trees, it will merely kill off top growth. Follow-up fires are required to keep the trees short and limit their competitive effects on grass growth). Research indicates that the conditions required for a fire to be sufficiently hot to cause die-back of trees are: a fuel load (comprising mainly of grass) of more than three tons of dry matter per hectare; an air temperature above 25EC; a relative humidity of 30% or less; and a head-fire (i.e. burn with the wind) must be applied.
The same approach should be used to control bush in the mixed and sweet grassveld areas. It is true, nonetheless, that many shrubs and trees coppice after burning. Here effective control of shrub and tree growth can be achieved only if browsers can be introduced to use the coppice growth and thereafter to reduce the tree's potential to recover after being burnt (refer to Production Guideline 7.6 in this series for more information on the effect of fire on woody plants).
Burning as a security measure
The development of a firebreak system is essential in most areas if veld fires are to be safely contained. In fact, many farmers burn their veld late (after the commencement of plant growth) merely for the reason that "it was too dangerous to burn at the recommended times". This is usually the result of an inadequate firebreak system. A well-planned system of firebreaks is an invaluable aid in any veld burning programme. A few internal breaks will ease the task of burning off selected areas of veld within the farm boundaries. With such a system, veld burning may be carried out efficiently, safely and effectively. Furthermore, firebreaks on farm boundaries are required by law to protect adjacent properties from runaway fires.Burning to stimulate out-of-season growth
The use of fire to produce out-of-season green feed is practised by many farmers. The flush of green growth in early August (after a July burn) or in April or May (after a March burn) is a very valuable source of fodder when no other good-quality fodder reserves are available on the farm. In the short term it does save the farmer the cost of buying or producing feed for these times of the year. In the long term, however, out-of-season burning does great damage to the veld. There is a loss of overall grass production, a reduction in basal cover and therefore increased erosion, and a change in species composition (the grass species which survive are not generally those which are preferred by the grazing animal).
- Early Burning
This was a widespread practice in earlier days when farmers burned in June and July to provide early-season fodder particularly for sheep. It is less prevalent now, mainly because of adequate fodder planning. New growth does have a higher protein, TDN and mineral content compared to unburnt grass in spring. This difference disappears during the season. Reduced production does, however, limit intake of nutrients.
Early burning stimulates tufts to grow, drawing on reserves held in the crown and root system of the plant for spring growth. This early growth is grazed or frosted, causing an additional draw on stored reserves as the plant regrows following grazing or frosting, seriously affecting the vigour of the plant. Palatable species, in particular, are selectively grazed and thus damaged.
A burn in the dry conditions which prevail during winter, destroys surface mulch (organic matter). The mulch is responsible for improving rain infiltration, retaining moisture and reducing erosion. This is particularly so in humic soils. The loose organic herbage is, in addition, washed away by early rains. Removal of the mulch thus reduces the effective rainfall of the area.
After burning in June or July the soil surface is exposed for a long period. Wind removes burnt herbage, surface mulch and soil. The soil surface is compacted by the impact of raindrops in early spring storms and then sun-baked, causing a crust. This situation results in high runoff and reduced infiltration. The soil surface is also exposed to the sun and wind, causing it to dry out. The dry profile results in an increase in species such as Microchloa caffra. Contrary to popular opinion, firebreaks (which are burnt in late autumn and early winter) do not produce ‘the best veld’. (Note, as a point of interest, that the effects of early burning on soil moisture can be clearly demonstrated by auguring in a firebreak and in the adjacent veld. The firebreak - which is burned in May/June - will have a much drier soil than the adjacent un-burnt veld).
- Late burning
Farmers often burn after the growing season has commenced for a number of very practical reasons. For example the area may have just gone through a drought and if the farmer burns his veld in early spring and the ‘spring’ rains are late, he may have no roughage at all for the stock. There may, therefore, appear to be quite rational reasons why the farmer will delay burning until late in spring or early summer. However, once grass growth has commenced, burning results in a very serious set-back to the grass plant. Many of the new-season's tillers might be destroyed, necessitating the development of an entirely new set of tillers. This delays growth considerably. Close grazing thereafter results in further loss in vigour of the plant. Also, stored reserves, used to initiate growth in spring are depleted resulting in low vigour potential of the plant to regrow following a late burn. Redgrass (Themeda triandra) is particularly susceptible to late burning and will eventually die out in veld which is regularly burned after spring growth has been initiated.
- Autumn or Summer Burn
Late summer or autumn burning is practised, mainly by sheep farmers, to fill a fodder flow gap for ewes and their lambs during autumn and winter. Because autumn is the time when the plant stores the reserves which are required to maintain it over winter and to have the energy to produce new growth in spring, grazing of out-of-season growth in the autumn prevents these essential survival processes from occurring. The vigour of the plant is thus drastically reduced as is the livestock production potential of the veld.
Out-of-season burning cannot be justified on economic or biological grounds. This is because of the reduction in forage production potential, the deterioration in veld condition, plant vigour and basal cover, and the soil loss which follows such a practice.
Burning to control parasites
Parasites, such as ticks, are not effectively controlled by fire. For example, to escape fire, ticks fall to the ground and enter cracks in the soil. Most fires do not influence soil temperatures and the ticks are not harmed. The use of fire therefore cannot be justified for this purpose. Dipping remains the only effective measure to control ticks.When to burn?
Ideally, veld should be burned at a time which will ensure that the leaf canopy will recover in the shortest possible time after the burn. Research has shown that this ‘ideal’ time is in early spring just after first good ‘spring rains’ (15 mm within 24 hours). At this time soil temperatures have started to rise and moisture becomes available for plant growth. In most areas (except on moist sites, such as wetlands), veld which is burned too early remains bare until after the first good rains. Such early baring of the soil, while it may have no appreciable short-term effect on plant recovery when the spring rains arrive, may be expected to have an adverse long-term effect, as the chances of erosion by both wind and water are increased. In addition, veld which is burned early (June or July) does not recover to a stage acceptable for grazing any earlier than veld which is burned in August or early September.In practice it is difficult to identify the ‘ideal’ burning time, especially if the first effective spring rains are preceded by smaller rainfall events which, together with increasing temperatures, cause some plant growth. Hence, when the first effective rains do fall the grass may already be growing. The farmer also needs to plan in advance, and it is often impossible to base the timing of his programme on an unpredictable event such as the ‘first spring rains’.
The following points may be used as a guide and form the basis of the Veld Burning Guidelines for KwaZulu-Natal.
- Earliest date
Burning veld at a stage when the leaf canopy will recover as rapidly as possible will ensure the greatest raindrop interception, retention of moisture and therefore rapid growth. The earliest date after which burning should be considered is therefore based on long-term climatic records which are used to indicate when the temperatures rise and spring rains are most likely to occur. These criteria have been used to determine the earliest date for burning for each Bioresource Group in KwaZulu-Natal.
- Last date
Growth in veld where the area is inherently moist (i.e. moisture in not limiting to plant growth) is generally initiated by increasing temperatures. In such areas it is often necessary to burn before the first spring rains have fallen. A ‘last date’ for burning is therefore suggested for all Bioresource Groups. This is done to limit the negative effects of late burning (see above). It is always advisable to burn as soon as possible after the first date given in the Veld Burning Guidelines for KwaZulu-Natal.
- Burning of rested veld
Veld rested the previous season, or latter half of the previous season, builds up vigour and has an extended translocation period. Stored reserves should therefore be high and spring growth is usually more vigorous and initiates at an earlier date than in unrested veld. The blanket of ungrazed herbage also stores more moisture in the soil profile. Veld which has been rested for the whole of the previous season could therefore be burnt earlier than veld which had not been rested.
- Burning in the dry Thornveld areas
Because the veld in these areas is sweetveld, grass should remain acceptable to animals through the winter and ungrazed herbage can be removed by the animal instead of by fire. Certain bushveld areas do, however, tend to become moribund after a season of high rainfall. The excess, moribund herbage can only be removed by burning. Also, burning can be used for bush encroachment control purposes. (Note that the permission of the Executive Officer of the Department of Agriculture is required before burning in Bushveld areas - see the Veld Burning Guidelines for KwaZulu-Natal in Production Guideline 2.8 for further information).
How often should veld be burned?
Ideally, one third to one quarter of the veld should be rested each year. The remaining two thirds (or three quarters) should be managed to achieve good utilization of the grass by the end of the grazing season. The rested veld could be utilized with the aid of a suitable lick during autumn and winter. Any herbage remaining in the rested area should then be burned at the beginning of the following season, and a new area put up to rest. This means that veld should be burned every three to four years. In practice, this ‘ideal’ situation does not work because (1) there are usually insufficient camps available per herd; (2) uniform utilization of the grass may not be achieved; (3) above-average grass production during good seasons leads to dry matter accumulation; and (4) animals will graze certain grass species only at certain times of the season. Old, low-quality grass will always accumulate in sourveld and mixed veld areas. The farmer must strive towards the ‘ideal’ by subdividing existing camps and following a flexible rotational grazing system. Where the problems outlined above occur, the following guidelines should be used:
- never burn more than half of the veld on the farm each year;
- do not burn more frequently than once every 2 years; and
- do not burn if less than one third of the total seasonal production is left from the previous season.
As discussed earlier, some farmers tend to burn the veld annually to provide their animals with the highly nutritious re-growth which occurs after a burn. The problem with annual burning, though, is that it leads to a drastic reduction in dry matter production. Burned veld is not as productive as unburned veld, and by the end of summer burned veld would have produced approximately one third less dry matter than unburned veld. For example, if the production potential of the veld is 3 300 kg of dry matter per hectare per annum, only 2 200 kg will be produced in the season after the burn. Can the farmer afford to lose 1 100 kg of potential feed per hectare each year?
What management should be applied after a burn?
Post-burning management is critical and should be aimed at maintaining a vigorous, competitive sward. The most important effect of grazing is the removal of the grass plant's ‘factory’ (its leaves). This ‘factory’ provides the plant with energy for growth. After a burn, therefore, the plant must have sufficient time to produce leaves before grazing may commence. Research has shown that repeated defoliation during the growing season reduces potential grass production. It has also been shown that early defoliation following a burn is related to the depression of dry matter production. Up to 60% of the total season's dry matter production could be lost as a result of grazing too soon after a burn.In terms of veld management, then, veld should not be grazed too soon after a burn. A rule of thumb is that the grass plant may be grazed when the first leaves have reached from 10 to 15 cm. At this stage there should be sufficient leaf herbage left for the plant to continue growth after the animal has taken a bite. If the veld has been burned just after good spring rains (e.g. mid-August), it should be ready for grazing approximately eight weeks after the burn (e.g mid-October). This period might be shorter or longer, depending on how favourable conditions are for plant growth after the burn, and on the vigour of the veld.
Resent research has, however, suggested that for good sheep production the sheep must graze as soon after the burn as possible. To limit the negative effects of this practice the veld must be rested for the whole of the following growing season. The full season's rest should provide the plants with sufficient vigour to recover from severe grazing by sheep in the previous season.
How to burn
The burning method a farmer uses is critical to grass production. If the crown of the plant is moist, very little damage is done to the emerging tillers, resulting in rapid regrowth after a spring burn. A head fire, that is burning with the wind, throws the heat upwards, removing topgrowth without burning into the grown of the tuft. With the heat being distributed well above the grass canopy a head fire is also effective for bush control purposes. Head burns are therefore advised in almost all circumstances.A back burn (burning against the wind) subjects the ground surface and crown of grass tufts to excessive heat and thus damage. Burning against the wind is therefore discouraged. Time of the day and air temperature are also important. When burning to remove old, moribund grass, the ideal conditions are considered to be when air temperatures are below 20EC and the relative humidity is above 50%. These conditions are most likely to occur before 11h00 and after 15h30.
CONCLUSION
Fire is an invaluable tool for use in veld management. It is essential for maintaining vigorous veld and for ensuring long-term animal production from the veld. However, its misuse can have detrimental consequences. An understanding of the principles relating to burning is essential for its effective employment.
Note: Veld burning guidelines for KwaZulu-Natal are given in Production Guideline 2.8 of this series.