Resumen

TORRES, René et al. Effects of the methods of grazing on modulated savannahs. I. Succession of the pasture. Zootecnia Trop. [online]. 2003, vol.21, n.4, pp.425-448. ISSN 0798-7269.

Summary The research was developed in the Experimental Module of Mantecal, Apure State, Venezuela (7°35' N and 69°10' W), during the period 1984-1998, on hyperstational savannas, characterized as affected by five months of deficit hydric and five months of excess of humidity. The dominant vegetation in Bancos (high physiography) was constituted by Panicum laxum, Paspalum chaffanjonii, Leersia hexandra and Axonopus compressus grasses and more than other 80 species; in Bajíos (medium physiography), the vegetation was conformed by P. laxum, L. hexandra, P. chaffanjonii and Hymenachne amplexicaulis grasses, plus another 50 species; in the swamps or Esteros (low physiography), the annual flood rithm induces the dominance of L. hexandra and H. amplexicaulis grasses and other 15 hydrophyte species. 200 ha were assigned with similar proportions of high, medium and low physiographies to three different grazing methods: Continuous Grazing (PC), Deferred Grazing (PD) or 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 (PDR), with similar grazing management than PD, but with three subdivisions in their medium and six on low physiographies, respectively. The production system in the three treatments was a cow-steer and stocked at of 0.50 AU/ha/year. The treatments effect on the forage of the higher areas, under PC produced a decrease of the General Aerial Coverture (CGA), of L. hexandra and P. laxum grasses and an increment of weeds and species of low acceptability. Under PD a bigger stability of the CGA existed and of the most desirable species too; whereas, under PDR there was an increase of the main forage species (L. hexandra). The values of CGA were 46.7; 54.2 and 57.5% to P.C.; P.D. and P.D.R. In the medium areas there was little effect of treatments in the CGA (PC =48.1; PD =62.3 and PDR =54.7%), with a greater contribution of the least desirable species Panicum versicolor grass under PC, while in PD and PDR 4 and 6 times more contribution of L. hexandra, was detected when comparing against PC. In the low areas PD and PDR contributed more as twice as coverture of the desirable species of Esteros that PC. The CGA were 29.6; 54.1 and 48.6% to PC, P.D. and P.D.R. Similar tendencies were found in the Desirable Dry Matter (MSD) yield for areas and treatments, with clear definitions in the high (PC =1014; PD =2418; PDR =2004 kg/ha) and low areas (PC =843; PD =2057; PDR =1609 kg/ha), but not so on medium areas (PC =1933; PD =2153; PDR =2087 kg/ha). The vegetation with PC showed slow recovery at the beginning of the rainy season, and  had in Bancos 135 days and in Esteros 225 days with yields below 1.000 M.S.D. kg/ha. Thus, a regressive succession existed under PC well defined in the low and high physiographies, and an smaller progress in this medium. While on PD the maintenance of the resources of the vegetation existed, with improvement in low physiography; and under PDR a progressive secondary succession occurred. These processes justify the poor condition of the pasture under PC, on behalf of a good condition for PD and a tendency to the excellent condition for PDR.

Palabras clave : Savanna; grazing method; forage.