Pasture reclamation humid tropic | [Recuperacion de pastos en el tropico humedo]

In order to develop a low input technology for degraded pasture reclamation in the humid tropics, activities in this section during 1990 were concentrated in three initiatives: (1) degraded native pastures reclamation (Torourco) without mechanization; (2) contribution of the Desmodium ovalifolium N to an association with Brachiaria decumbens, and (3) reclamation of a degraded pasture of B. decumbens by the introduction of Centrosema macrocarpum. All works are carried out within a collaborative agreement between INIAA-IVITA and CIAT at the Pucallpa region in the Amazon of Peru. Considering that the low availability and the high cost of mechanization are the main restrictions to reclaim for the vast extension of degraded pastures in the humid tropics, it is very important to seek economic alternatives. The relative success of any alternative to mechanization is supported, among other factors, by an adequate information about the soil physical properties. Compaction is the main expression of the soil deterioration. This, when is measured with a penetrometer, is named mechanic impedance (MI). The values of MI measured in a degraded Torourco at 0,10, 0.20, 0.30 and 0.40 m depth oscillated around 0.87 MPAa, registering 2 maximum at 0.10 and 0.40 m depth. Values of MI at 0.10 m depth (1.02 MPAa) seem to be marginal. One of the restrictions to establish improved pastures directly on a Torourco pasture is the competition from the native vegetation. The prediction of changes in the botanical composition of the Torourco pasture as a reaction to different operations would be useful to help in the designing effective weed control measurements. In this context, a trial in 3 farms of Pucallpa with 3 Torourco types were carried out, with predominance of (1) Axonopus compressus + Mimosa pudica; (2) A. compressus + Paspalum vignum, and (3) Homolepis aturensis. The trial consisted of the application of ploughing, fertilization, systemic herbicide, contact herbicide, grazing spells, 1-year of pasture spell and burning. Preliminary results suggest that the most marked effects as biomass control was the application of the systemic herbicide and the use of machinery. These two treatments reduced total biomass coverage and produced a change in the botanical composition towards the predominance of broad leaf weeds. In the Torourco pasture with predominance of H. aturensis this effect was more marked, even with the use of a contact herbicide. In order to reach a successful establishment on direct planting on Torourco pastures, it is important that the species included have a high ability to compete with the native species. Assuming that the improved pastures have a greater response to the increased nutrients availability, the fertilization effect on several improved species was studied. Preliminary results suggest that the use of even small amounts of fertilizers may contribute to the aggressiveness of promissing species such as Arachis pintoi, Centrosema macrocarpum and Stylosanthes guianensis planted on Torourco without mechanization. Similarly, the competition capacity of these 3 species was increased when the number of plants, either by a higher planting rather or an increased planting density was used. The control of the native vegetation is one of the problems associated with the establishment of improved pastures on Torourco, without the use of machinery. The alternatives to the mechanized control are the use of herbicides, intensive grazing, resting and burning. These alternatives were tested during the establishment of Brachiaria dictyoneura, A. Pintoi, C. pubescens and S. guianensis plots. The mechanized control produced a larger soil cover by the 4 species than the one obtained with the use of intensive grazing. However it was not larger than that obtained with the use of systemic herbicide. The resting and burning treatment will be planted the next year. In a trial established in 1985 it is being evaluated the effect of the inclusion of Desmodium ovalifolium in a Brachiaria decumbens pasture in comparison with a B. decumbens pasture alone fertilized with 0, 150 and 300 kg N/ha/year with regard to biomass productivity and animal productivity. Preliminary data show that neither the N application, nor the inclusion of D. ovalifolium increased significatively the biomass productivity during the 140-days evaluation. Similarly, the N fertilization or the inclusion of the legume did not have any effect in the animal productivity. The D. ovalifolium content is stabilized around 50 percent. Additional information is still to be processed, especially on the biomass N concentration and animal selectivity to interpret the data. A degraded B. decumbens pasture was reclaimed planting C. macrocarpum in strips. This association is being evaluated under grazing for two years in comparison with B. decumbens pasture alone. The data indicate that the inclusion of C. macrocarpum did not increase the biomass productivity of the pasture, or improved forage digestibility, nor the N concentration of the selected forage. The low 1egume content in the pasture could partially explain the absence of a positive effect. After 24-months of grazing, the C. macrocarpum is stabilized between 15 and 20 percent of the total biomass.