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 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.

Table 12. Main species and cultivars and their seeding rate, for campos improvement.


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’)


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.

Table 13. Annual animal productivity of improved campos in grazing conditions with rotational stocking of steers in different zones of Uruguay.

Soil type

Stocking rate (AU/ha)

Liveweight gain (kg/ha)

Productivity (kg LV/ha)

Granitic  (4A – 4B)




Medium-deep basaltic    (1 - 6A)




Leached, low fertility (East)




* 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.”


- Instituto Nacional de Investigación Agropecuaria (INIA). (National Agriculture Research Institute) www.inia.org.uy

- 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

Development and Extension:
- Ministerio Ganadería Agricultura y Pesca:
OPYPA (Oficina Planificacion y Politica Agropecuaria), www.mgap.gub.uy/opypa
DIEA (Direccion de Estadisticas Agropecuarias).www.mgap.gub.uy/diea
DICOSE (Direccion de contralor de Semovientes). www.mgap.gub.uy/dgsg/dicose

- Instituto Plan Agropecuario (IPA). www.planagro.com.uy

- Instituto Nacional de Carnes (INAC). www.inac.gub.uy

Farmers Organizations:
- Asociacion Rural del Uruguay.www.aru.org.uy
- Federacion Rural del Uruguay.
- Federacion Uruguaya de Grupos CREA.




Acosta, Yamandu

Dairy production


Altier, Nora

Plant pathology


Alzugaray, Rosario



Ayala, Walter

Pasture management


Bemhaja, María



Bermúdez, Raúl

Plant evaluation


Blanco, Pedro

Leader Rice Program


Boggiano, Pablo

Pasture management


Brito, Gustavo

Meat sciencce


Carámbula, Milton

Professor Pasture physiology


Dalla Rizza, Marco

Plant biotecnology


de Mattos, Daniel

Animal genetics


Díaz, Roberto

Head Crop production


Durán, Henry

Head Animal production


Ferreira, Gustavo

Agro economy


Formoso, Daniel

Pasture management


Formoso, Francisco

Physiology and seed production


García, Jaime

Plant breeder


Lavecchia, Andrés

Rice physiology


Mas, Carlos

Pasture management


Mieres, Juan

Animal nutrition


Millot, Juan C.

Professor Forage


Molfino, Juan M.

Soil classification


Montossi, Fabio

Leader Sheep and Goat Program


Morón, Alejandro

Soil fertility


Olmos, Fernando

Pasture ecology


Pigurina, Guillermo

Manager Agro- industry Meat Chain


Pittaluga, Oscar
Beef production

Real, Daniel

Plant breeder


Rebuffo, Mónica

Plant breeder


Ríos, Amalia

Weed management


Risso, Diego F.

Leader Forage Plant Program


Zanoniani, Ramiro

Pasture management


Zorrilla, Gonzalo

Seed production




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This profile was prepared by Dr Elbio J. Berretta.
Senior Researcher: Native pastures ecology and grazing management.
Since October 1999, Regional Executive Director, Northern Experimental Station
INIA Tacuarembo.

Ing. Agr., Dr. Ing. Elbio J. Berretta
Director Regional
INIA Tacuarembó
Ruta 5, km 386
45000 Tacuarembó - Uruguay
Tel: ++ 598 63 24560 - 24561
Fax: ++598 63 23969
e-mail: berretta@inia.org.uy

Acknowledgements: the author would like to thank his colleagues: G. Ferreira, C. Mas, F. Montossi, and D.F. Risso, for providing comments.

[This profile 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].