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.
by introducing legumes
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.
improvement we must take into account:
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.
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
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.
Seeding rate of the main species and cultivars utilized in campos improvements are shown in Table 12.
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.
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.
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.”
- Universidad de la República. Facultad de Agronomía. www.fagro.edu.uy
- Facultad de Veterinaria. www.fvet.edu.uy
- Secretariado Uruguayo de la Lana (SUL). www.wool.com.uy
- Instituto Plan Agropecuario (IPA). www.planagro.com.uy
- Instituto Nacional de Carnes (INAC). www.inac.gub.uy
was prepared by Dr Elbio J. Berretta.
Acknowledgements: the author would like to thank his colleagues: G. Ferreira, C. Mas, F. Montossi, and D.F. Risso, for providing comments.
was completed in August 2003 by the author and was edited by J.M. Suttie
and S.G. Reynolds in August and September 2003 and slightly modified by
S.G. Reynolds in May 2006].