Efeitos do pastoreio alternado ovino-caprino sobre a composição florística da vegetação herbácea de uma caatinga raleada

Abstract

The experiment was conducted on the Crioula farm, at the Centro Nacional de Pesquisa de Caprino, CNPC-EMBRAPA in Sobral, Ceará, with the objective of evaluating the effects of the alternate grazing by sheep and goats on the phytossociological parameters of the herbaceous cover of a thinned caatinga, subjected to different stocking rates, during several periods. The experimental area covered by woody vegetation on initial stages of secondary sucession, was subjected to selective thinning, with the removal of the undesirable brush species, specifically marmeleiro (Croton sonderianus Muell. Arg.), by sucessive cuttings of the regrowth, during the rainy season. The experimental animaIs were Morada Nova sheep and SRD (with indefined breed) goats females. The animals, grouped by ten, were distributed in 4,0 6,0 and 8,0 hectares paddocks, corresponding to the three stocking rates, that is, 0,4 ha/animals 0,6 ha/animals, and 0,8 ha/animals. The alternate sheep and goats grazing system was divided into two phases: the first corresponded to exclusive grazing by sheep, that stocked the paddocks from January 1988 to December 1991; the second phase consisted of exclusive grazing by goats and it lasted from February 1992 to December 1994. The evaluated parameters were the phytomass availability and the frequency of grasses and forbs. Nineteen samplings were made along the months of January, February, March, June, Setember and November, in the years of 1988 to 1994, constituting the 19 "environments'' or periods. On each paddock and on each period, 100 samples for frequency and 20 samples for phytomass availability of the herbaceons floristic components were collected. At each sampling date, composed samples were taken for evaluation of dry mater (at 65°C), crude protein and neutral detergent fiber. The experiment followed a randomized complete block design with two replications and distributed in a crossed factorial of three stocking rates and 19 environments. The sheep grazing affected the botanical composition of the herbaceous cover, by lowering the frequency of the high nutritive value grasses and by increasing the frequency of low nutritive value grasses and of the forbs; while the goat grazing made possible the restabilishing of the more important grasses, and the stabilization of the forb composition. The similarity among the periods did not change for the 0,4 ha/animal stocking rate paddocks, it showed an increasing the 0,6 ha/animal parcels and it decreased in the 0,8 ha/animal. The similarity among the stocking rates increased during the ocupation by sheep period, and decreased in that by goats, being stronger in the 0,4 to 0,6 ha/animal, and in the 0,6 to 0,8 ha/animal stocking rate. The standing phytomass availability, measured as kg/ha and kg/animal, increased in the rainy season environments, and decreased is those of the dry season, with the higher values found in the 0,4 and 0,8 ha/animal stocking rates. The effect in the florisric composition was shown by the reduction of the grass percentage, and by the increasing in the forb participation, during the sheep grazing; the inverse was true for the goat grazing. The grass availability (kg/ha and kg/animal) decreased, and that of forbs increased in the environments of sheep grazing, the inverse being true for the environments of goats grazing. The crude protein content of the grass and forb as higher in the rainy season, lowering gradually in the dry season. The neutral detergent fiber percentage of the two groups of species showed different behaviour, that is, the grasses presented the higher value in the rainy season, while the forbs showed their higher value in the dry season. The results indicated that the alternate sheep and goat grazing on the thinned caatinga conditions may stabilize the botanical composition of the herbaceous cover, and it may possibly be the most adequate option for the sustainable grazing utilization of this floristic component.