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6. OPPORTUNITIES FOR IMPROVEMENT OF FODDER
RESOURCES
Fertilization
The poor growth of natural pasture in winter, its medium to low quality,
and deficiencies in phosphorus as well as nitrogen in the great majority
of soils, has led to the introduction of nitrogen to the ecosystem by
the application of inorganic fertilizers or through legumes and fertilization
with phosphorus to facilitate their establishment and production. Legume
introduction with phosphorus fertilization prevents campos degradation
and improves in a sustainable manner this natural resource without destroying
the sward. This also allows strategically developed improved areas to
be used by efficient ruminant classes, enhancing global productivity at
the farm level. The fertilization of the campos with phosphorus alone
has low impact on the botanical composition and forage production increase,
less than 15 percent, due to the low frequency of native legumes.
In contrast
to sown pastures that are introduced to restore soil properties and to
make the cropping phase of the intensive pasture sustainable - cropping
rotations after removing of native vegetation - campos improvement aims
to capitalize on and improve the good soil and plant conditions already
existing. The use of relatively low doses of N and P2O5 (90 kgN/ha/year;
44 kg P2O5 ha/year) favour the increase of soil fertility level, especially
if this fertilizer is applied in split doses, one at the beginning of
autumn and other at the end of winter. This strategy can be used on pasture
with good winter perennial grasses in a relative frequency over 20 percent.
Autumn application favours the regrowth and growth of winter grasses and
the extension of the period of growth of summer grasses up to the beginning
of autumn. On the other hand, fertilizer application at the end of winter
continues favouring the growth of the winter grasses as well as earlier
regrowth of the summer grasses. The earlier regrowth of C3 and C4 species
as well as the decrease in the resting period of C4 species tends to reduce
the period of scarce winter growth.
So long as
the fertility level of the system is increasing, forage production of
fertilized campo stabilizes at a value which is 60 percent above that
of campo without N+P. The seasons when fertilization can have a greater
influence from the point of view of grazing management are autumn and
winter. The daily growth rate (DGR) in autumn is greater in fertilized
campo. In winter fertilized campo DGR is nearly 100 percent superior to
unfertilized campo.
Forage N
and P content is always higher in fertilized campo. In natural pasture
the highest N and P values are registered in winter and spring and the
lowest in summer, when the forage is ripe and generally there are water
deficits. In winter fertilized forage N content reaches 2.3 percent, while
without fertilizer it is 1.7 percent. In spring the values are 2.8 and
1.9 percent respectively. In summer values drop to 1.4 and 1.1 percent,
respectively. Taking as an example in winter, natural campo produces approximately
38 kg/ha of crude protein (CP), whereas fertilized campo produces around
95 kg/ha of CP. The P content (mgP/gDM) in winter and spring is about
2.3 mgP/gDM with fertilization and 1.8 mgP/gDM without fertilizer. In
summer, these values are 1.9 and 1.5 mgP/gDM, respectively, and in autumn
1.5 and 2.2 mgP/gDM.
Throughout
the year winter species relative frequency is higher in fertilized campo
than in non fertilized. The increase of C3 grasses is related to the addition
of nutrients which raise soil fertility. The stimulus to perennial winter
grasses through fertilization is a way to change sward botanical composition,
improving winter production.
Productive
winter species such as Stipa neesiana, Piptochaetium stipoides, Poa lanigera
and Adesmia bicolor tend to increase their presence with fertilization.
Good summer grasses such as Paspalum notatum and Paspalum dilatatum also
increase their frequency. Coarse grasses such as Bothriochloa laguroides
and Andropogon ternatus are less frequent and Schizachyrium spicatum is
even less frequent with fertilization since it is a poor environment species,
showing the same behaviour in improved campos, whereas as fertility increases
its frequency decreases until it disappears. Paspalum plicatulum also
decreases with fertilization, although this decrement can be related to
a palatability increase since its leaves remain green longer than in unfertilized
campo. Native legumes increase their relative frequency to values close
to 5 percent. Weeds are of little importance and do not increase with
fertilization (In: Berretta, 1998).
In situations
where the campo has a high proportion of summer species, while winter
ones are annuals, as on granitic soils, results are very different. Fertilization
at the beginning of winter favours the presence of winter annual grasses,
Vulpia australis, Gaudinia fragilis, with a limited productive potential
near the end of the season; the disappearance of these species when ending
their cycles leaves spaces that can be colonised by undesirable plants.
Spring fertilization increases growth at the end of summer, when summer
grasses bloom and seed. The organic matter digestibility (OMD) of fertilized
forage is higher than the non treated sward. Fertilization with N noticeably
increases spring and summer production, but has a reduced effect in winter;
nutrient stimulates the increase of annuals to the detriment of perennials.
Improvement
by introducing legumes
The need to improve the primary production of natural pastures, as well
as their quality, has led to legume introduction through over-seeding,
sod-seeding, zero-tillage or direct drilling, as a way to increase secondary
production. In this process the correction of low soil phosphorus levels
is essential.
The study
of factors controlled by man allows an understanding of the different
aspects of the induced vegetal succession, contributing to the success
of the application of this technology.
In campo
improvement we must take into account:
- the vegetation, species that compose it which indicate the quality of
the campo; this can be related to their productive and vegetative types
and growth season;
- the soil type, topography, stoniness, erosion, drought risks, drainage,
etc.;
- livestock system use that is intended to be given the improved paddock,
i.e. cattle, sheep, breeding, fattening, weaning, etc.
These factors
condition the choice of the species to be introduced, as well as the way
in which their seeds will be in contact with the soil, to have water and
nutritive elements, regularly and efficiently.
The establishment
productivity and persistence of introduced forages in natural pasture
depend, mainly, on the way in which the reduction of the competition of
the existing sward has been made and is also closely related to seedbed
quality as well as the adaptation to the environment of the introduced
forages.
Sowing. In general it is necessary to graze beforehand with cattle to
reduce the tall grasses and dead material; the stocking rate will be based
on the available forage at the end of spring and summer growth, but it
will have to be high. If the summer is very rainy the growth of the campo
will be high and towards its end, a certain amount of dead material and
culms will remain, so the stocking rate would have to be increased to
eliminate this. In the final stages of preparation of the sward it is
necessary to use sheep to reduce the height to two centimetres. Grazing
can be continuous, although it is advisable to do it in such a way as
to reduce the height, to allow regrowth and then grazing again, therefore
reducing the reserves of the plants to favour germination, emergence and
establishment of the introduced forages. According to the growth of the
grass, grazing will have to be every 30 to 45 days. When combining grazing
with rest periods, the instantaneous stocking rate has to be much higher
than with continuous stocking. This sward conditioning preparation aims
to obtain safe sites for good contact of the seeds with the soil. Generally
it is very difficult to reduce the vegetative cover below 50 percent,
although the height of the sward is reduced; the height at the sowing
is important to protect seeds from climatic adversities.
Sowing is
done with a fertilizer spreader, mixing inoculated seed with the fertilizer,
and spreading immediately before sowing in a short time for the fertilizer
not to affect the seed and rhizobium. It is possible to use a grain-drill,
with light tension on its discs, leaving shallow rows in the soil. During
the 60s, at the start of the campos improvement technology with legume
introduction, a sod-seeder was used; with a chisel which opened a row
6 cm wide and 4 cm deep, in which the seed and fertilizer were placed.
When vegetation is too dense or there is a large layer of dead material,
an off-set disc (opened) is used to produce small drills. In recent years,
direct drilling has become important, including the use of non-selective
herbicides. If canopy conditioning is properly done by controlled grazing,
there are no important differences with the results obtained after herbicide
application.
Chemical
agents must be used very carefully. It is preferable to use non selective
contact herbicides, so as not to affect the growth capacity of native
plants negatively. When systemic herbicides are used, the dose must be
low to preserve native species of good quality.
Suitable
sowing time is autumn, April, when there is humidity in the soil and the
temperature is still adequate for fast germination and plant emergence.
In some circumstances sowing can extended until May, although low temperatures
can reduce germination and seedling development. Legumes should have 4
to 5 leaves when the temperatures begin to fall. Most failures of over
seeding are due to water shortage during establishment, because of the
reduction of rain in winter and at the beginning of spring.
Fertilization.
Fertilization with P is significant as much for the establishment and
for forage yield in the first year, as for the maintenance of a high production
level; it is a necessary condition (although not the only factor) for
the persistence of the legume fraction. Fertilizer doses should not be
lower than 40 kg/ha of P2O5 and in the case of white
clover (Trifolium repens), it is advisable to use at least 60 kg/ha.
For annual maintenance, levels greater than 30 kg/ha of P2O5
can be used for species of Lotus, but for white clover, the maintenance
fertilizer dose has to be higher than 40 kg/ha of P2O5.
Legumes for improvement.
Several evaluations of diverse
genera and species of legumes have been made through the years, looking
for those adapted to different soil types and so particular conditions
for sowing on the natural vegetation. The most recent studies include
diverse species of Trifolium, Lotus, Medicago, Ornithopus, Desmanthus
and Vicia. From this set, in medium and deep soils, the white clovers
cv ‘Zapicán’ and ‘Bayucuá’, Lotus
corniculatus, Lotus pedunculatus cv ‘Maku’ continue to stand
out. Lotus subbiflorus (= L. hispidus) cv ‘El Rincón’
is a highly utilised annual legume in the improvement of campos all over
the country, but particularly on granitic soils (agro-ecozones 4A and
4B) in which it is the most adapted.
Seeding rate
of the main species and cultivars utilized in campos improvements are
shown in Table 12.
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Table
12. Main species and cultivars and their seeding rate, for campos
improvement. |
|
Species |
Seed
rate (kg/ha) |
|
Trifolium
repens (cv. ‘Zapican’, ‘Bayucuá’) |
4
- 5 |
|
Lotus
corniculatus ( cv. ‘SanGabriel’,
‘INIA Draco’) |
10
– 12 |
|
Lotus
pedunculatus (cv. ‘Maku’) |
2.5
- 3.5 |
|
Lotus
subbiflorus (cv. ‘El
Rincón’) |
4 – 5 |
|
Trifolium
pratense
(cv.’Estanzuela 116’, ‘INIA Mizar’) |
6 –8 |
|
Ornithopus
compresus
(cv. ‘INIA Encantada’) |
20 |
These rates are for the sowing of the pure species; when
mixtures are sown, the individual rate is lowered, i.e. for Trifolium
repens + Lotus corniculatus = 2 + 10 kg/ha. Seeds are inoculated with
specific rhizobium for each legume; afterward a sticker is added to be
finally pelleted with calcium carbonate. Legumes generally used for improvement
have no specific scarification requirements.
This technology of campos improvement requires low inputs,
is environment friendly, promotes the continuous development of the native
vegetation and takes it to a more productive level, accelerating the processes
of fattening in different zones of the country to obtain a better individual
performance and a higher carrying capacity (Table 13). These results were
obtained in grazing conditions with rotational stocking, 5 to 8 paddocks,
7 to 12 days of grazing period and 30 to 40 days of rest, in a grazing
season of approximately 300 days.
|
|
| Soil
type |
Stocking
rate (AU/ha)
|
Liveweight
gain (kg/ha)
|
Productivity
(kg LV/ha) |
| Granitic
(4A – 4B) |
1.55 |
533 |
406 |
| |
1.85 |
680 |
485 |
| Leached,
low fertility (East) |
1.53* |
700 |
473 |
|
*
Includes mixed with wethers, 2:1 ratio. LV = Liveweight. AU = Animal
unit. |
Effect of the introduction of legumes on the botanical composition of
natural pastures. Once the introduced species are established and with
time, one of the most important changes observed in the vegetation is
the increase of winter species (C3). In other basaltic region of similar
vegetation, summer species (C4) frequency is always higher than winter
ones. Winter species relative frequency is about 75 percent, with similar
values for native grasses and the introduced Trifolium repens.
To ensure that introduced species persist in the pasture it is necessary
that they flower and seed so as to assure their next autumn regeneration,
spending the summer partly as plants and partly as seed. Introduced legumes
flower and set seed, as do winter native species (such as Poa lanigera,
Stipa neesiana, Piptochaetium stipoides and Adesmia bicolor).
Therefore, the conservation of these species in natural pastures is related
to rest periods which allow them to flower and fructify and to a soil
fertility level increase as well. In various improved campos there is
an increase in the frequency of the acclimated exotic grass Lolium
multiflorum that in many cases is introduced by animals and prospers
with the increase in soil fertility.
In more degraded vegetation types, composed of unproductive and poorly
palatable coarse grasses and forbs, legume introduction also induces positive
changes. The relative frequency of legumes (Trifolium repens, Lotus
corniculatus) is about 60 percent; the productive winter native grasses,
Stipa neesiana and Piptochaetium stipoides and acclimatized
ones such as Lolium multiflorum, increase their frequency, and
unproductive ordinary grasses and forbs are reduced. Annual Lotus subbiflorus
and perennial legume (Trifolium repens, Lotus corniculatus,
L. pedunculatus) overseeding of pastures composed almost exclusively
by C4 species, with an annual production of 3,400 kgDM/ha, takes forage
production up to values of 8,600 kgDM/ha. When legumes are introduced
into campos where ordinary grasses and forbs prevail, the resulting secondary
production is similar to those pastures with higher frequency of native
fine grasses (Berretta and Risso, 1995; Risso and Berretta, 1997; Risso
and Berretta, 2001).
The addition of P would help to return to natural campo some of what
has been removed by centuries of grazing, since the introduction of cattle
at the beginning of the seventeenth century, besides contributing to natural
pasture animal and plant biodiversity maintenance. We have to conserve
our natural resources, without degradation, thinking of a sustainable
development in economical, ecological and social terms.
Accumulated research results have contributed to applying better pasture
management practices, which result in long term biological and economical
benefits for the farming community and the whole society, with special
care for animal and plant community biodiversity and water conservation
for human and animal use. Both animals and plants will be, for a long
time, the main world food and fibre source, hence conditioning our actions
and behaviour in the way that we preserve our natural resources for future
generations.
Futures challenges to animal and pasture research will be primarily:
- To release continuously new plant material for extensive and intensive
production systems, for grazing and forage conservation purposes;
- To evaluate the impact of irrigation on cultivated pastures and improved
campos, particularly in periods when water deficit occurs;
- To produce better quality and safer products to satisfy local and overseas
consumer demands;
- To integrate the knowledge generated for plant and animal components
at the production system level, offering easy tools to farmers to facilitate
the adoption of the new technologies available;
- To evaluated the impact of the technologies proposed on our natural
resources, particularly with the use of fertilizers, herbicides, insecticides
and grazing intensity.
- To take into consideration the requirements of the different components
of each industrial sector, particularly the consumers, at both national
and international level;
- To consider the influence of social, ecological and economic aspects
on the decision making unit at farm level in the definition of future
research and technology transfer strategies;
- To define and establish a proper scheme for the technology adoption
processes, considering the cultural, economic and ecological particularities
of each production system at farmer and regional level.
Prof. Bernardo Rosengurtt, the pioneer of natural grassland studies in
Uruguay, in 1943 wrote: “We must protect our prairie heritage with
great intensity, both nationally and privately, so as to hand it down
unharmed to future generations.”
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