Morphostructural and population variations of Andropogon gayanus and their relation to the adventitious vegetation under intensive grazing


Rey L. Machado Castro

Estación Experimental de Pastos y Forrajes Indio Hatuey Central España Republicana 44280, Matanzas, Cuba

 

ABSTRACT
In an established pastureland of A. gayanus CIAT-621, subject to an intensive rational management system (instantaneous stocking rate of 210 LAU/ha –average of three years), the variations of some morphostructural indicators, their interrelations and quantitative expression on the stability of the pastureland, were analyzed. In addition, the effect caused by seasonality and number of rotations on these indicators was determined; and the changes and trends with which the vegetation components evolved, and what type of relations existed between these indicators of the plant cover and the above-mentioned ones, were quantified. After 21 rotations as average, the live shoot (LS) production rate was practically 6 times higher than the dead shoot (DS) production rate; the bunch diameter (D) and number of bunches (Nb) increased; unlike the leaf length (Ll) and leaf width (Wl), which underwent a slight decrease. The total morphostructural variability was high (89,1 %) and the stability indicators had a higher bearing on it (42,9 %) than the senescence components (24,1 %). A significant (P<0,05) season x year interaction was found for all these indicators and their relations, except for the density of dead shoots (DDS) and D. The maximum value of LS was found in the dry season of the second year and in the rainy season of the third year; a similar pattern was found in LS/D. The highest value of DS/D was detected in the dry season of the three years: The DDS was higher in the dry season than in the rainy season. D, in the third year, significantly differed (P<0,05) from the two previous years; while the number of bunches (Nb) was higher in the rainy season of the third year. Ll and Wl were significantly higher (P<0,05) in the rainy season. The production of LS, total shoots (TS) and their densities, were significantly higher (P<0,05) when rotating on 21 to 22 occasions, as compared with 18-19 and 14-16. A similar pattern was found in the LS/DS ratio. The area populated by A. gayanus slightly decreased until the dry season of the second year, recovering later. The total number of invasive species was high (53) and it varied seasonally in a narrow range for the perennial types (23 to 26) and a little wider for the annual, biennial and short-life perennial types (6 to 13). The number and area covered by the grasses and perennial legumes were extremely higher than those of the species from other families with this life cycle. All the short-cycle species, independently from the family, remarkably decreased in number, covered area and dispersion. Between 27,4 and 53,7 % of the area was populated with species that can be selected by the animals and only from 0,8 to 4,9 % was colonized by species which can be occasionally selected or not selected at all. The invasion by aggressive woody plants or other damaging species was practically null. The legume population interacted slightly and negatively with the dead shoots; while the latter interacted strongly and positively with depopulation and leaf dimensions. Negative correspondence was found between the invasive grasses and the number of bunches of A. gayanus, as well as between the live shoots and depopulation; while the species from other families had nothing or very little to do with the relations and changes that occurred in vegetation. The obtained results allow to arrive at the following recommendations: To utilize, as general strategy in management, rational grazing with high instantaneous rates and resting times according to the optimum point, in order to maintain the morphostructural and vegetation composition stability; to consider, as management alternative, the rotation of the highest possible number of paddocks in optimum point during the rainy season to enhance shoot production in the dry season, as well as to make a better utilization of the pasture that arrives at the reproductive phenophase in the latter season; to include among the indicators to be measured in this type of studies, those related to the morphostructural components and vegetation composition, which allow to characterize and evaluate the stability and diversity of the pastureland; to take into consideration the methodological aspects and the obtained results in the elaboration of didactic materials for undergraduate and postgraduate teaching.