Resumen

TORRES, René et al. Effect of the methods of grazing on modulated savannahs. II. Protein and minerals composition in plant and soil. Zootecnia Trop. [online]. 2003, vol.21, n.4, pp.449-466. ISSN 0798-7269.

Summary The research was carried out in the Experimental Module of Mantecal, Apure state, Venezuela (7°35' N and 69°10' W), during the period 1984-1998, with the objective of evaluate the interaction between the primary and secondary production on hyperstational savannas. The dominant vegetation in the Bancos (high physiographic zone) was constituted by the grasses Panicum laxum, Paspalum chaffanjonii, Leersia hexandra and Axonopus compressus and more than other 80 species. In the Bajíos (medium physiographic zone) the vegetation was conformed by the grasses P. laxum, L. hexandra, P. chaffanjonii and Hymenachne amplexicaulis, plus another 50 species, while in the swamps or Esteros (low physiographic zone) the annual flood rhythm induces the dominance of L. hexandra and H. amplexicaulis grasses and other 15 hydrophyte species. Two hundred ha were assigned with similar proportions of high, medium and low physiographic zones to three different grazing methods: continuous (CG), deferred (DG) grazing as the use of higher areas during the peak of the rainy season, the medium areas in the transition rainy-dry and dry-rainy period, and the low areas during the dry period, and deferred-rotary Grazing (DRG), with similar grazing management than DG, but with three subdivisions in their medium and six on low physiographic zones, respectively. The production system in the three treatments was a cow-steer, with breeding season during the dry period, stocked at of 0.50 AU/ha/year. The DM protein concentration of the pastures were highly deficient only under CG, during the dry period (CG=5.8; DG=6.6; DRG=7.3%), with an increase at the beginning of the rainy season on: high (14.1%) < medium (16.3%) < low areas (16.5%); and for treatment on: CG (13.6%) < DG (16.0%) < DRG (17.3%). The levels of phosphorus were widespread deficient (0.15%) and for calcium was deficient especially under CG (CG=0.14; DG=0.21; DRG=0.20%), to approach minimum levels of those recommended at the beginning of the rainy period, in medium and low areas with deferred uses (CG=0.16; DG =0.24; DRG=0.24%). Similar pattern showed magnesium during the dry (0.15%) and rainy seasons (0.15%), with emphasis in medium and low areas, and for sodium in high (0.09%) and medium areas (0.09%). Potassium showed deficiency during the drought season, mainly under DRG (CG=0.56; DG=0.40; DRG=0.25%); similarly, copper for low areas (CG=26; DG=13; DRG=9 ppm), and zinc for medium and low areas (26 ppm). In this last area, sulphur showed deficit in all treatments (CG=0.10; DG=0.11; DRG=0.10%). These last three elements elevated their readiness at the beginning of the rainy season. During the dry season, iron increased up to 1.000 ppm in medium and low areas, and manganese up to 300 ppm in high and medium areas of the treatments, with significant reductions at the beginning of the rainy season. L. hexandra grass was positively correlated with the levels of DM protein concentration and calcium, as well, H. amplexicaulis grass with iron and cooper, and P. laxum grass with manganese, potassium, zinc and cooper.

Palabras clave : Savanna; grazing method; protein; minerals.