Estratégias de manejo do pastejo para Panicum maximum cvs. Quênia e Tamani

Abstract

The objective of this study was to evaluate grazing management strategies and physiological characteristics for Panicum maximum cvs. Quênia and Tamani under different intensities of grazing and often defined by 95% light interception by the canopy. The experiment was carried out in a randomized complete block design in a factorial arrangement (2x2), with two cultivars: Tamani and Quênia (Panicum maximum cvs BRS Tamani and Quênia) and two grazing intensities: high and low, Which defined a post-grazing height of 15 and 25 cm for weed-grass; And 20 cm and 35 cm for Quênia, with three replicates, totaling 12 experimental units (120 m2

each). In each cycle samples were taken for quantification of forage accumulation and morphological separation, post and pre-grazing. Measurements of LI, LAI and leaf angle were performed weekly in all regrowth cycles. The rates of liquid photosynthesis, stomatal conductance and individual leaf transpiration rate were measured in the same cycles in which IAF and IL were characterized. The evaluations were done in four seasons (autumn/spring/summer/15, fall/16). The individual leaf photosynthetic rate showed cultivar interaction x grazing intensity x seasons of the year Tamani, in autumn/15, when handled under low grazing intensity showed the lowest foliar photosynthesis rate. The other treatments at this station were similar. The highest values of leaf photosynthesis occurred in spring/15 and the lowest in autumn/16. The canopy photosynthesis rate was higher in the spring/15 season and similar among the other seasons. Stomatal conductance was lower in autumn/16, and higher in spring/15. The intrinsic efficiency of water use and the rate of transpiration were similar for Quênia and Tamani. However, both presented differences between the seasons. The intrinsic efficiency of water use was higher in autumn/16 and was similar in other seasons. The perspiration ratio was similar in the seasons of summer/15 and autumn/16, where the highest values were obtained. Leaf transpiration rate did not differ between cultivars. Regardless of grazing intensity or cultivar, annual forage accumulation was 22 Mg MS.ha-1 .an-1 . The accumulation of forage evaluated by season showed marked effects, although there was no effect of cultivar or grazing intensity. In summer/15 the highest production occurred, followed by autumn/15, spring/15 and autumn/16. In winter/15 the lowest forage accumulation was obtained. The highest leaf proportions were obtained when

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management under low grazing intensity, independently of the cultivar, was carried out. Different proportions of the stem component, among the grazing intensities, were verified only for Quênia, where it was lower under low grazing intensity. For Tamani there was no difference between grazing intensities for stem accumulation. Regarding the accumulation of dead material, the highest proportions were obtained when managed under high grazing intensity. Between the two grasses, Tamani presented a higher proportion of dead material.

Quênia and Tamani grasses, managed according to a frequency criterion of 95% IL for pre- grazing, were 55 cm and 35 cm high, respectively. It should be discontinued when the height

of the canopy for Quênia and Tamani reach 25-30 and 20-25 cm, respectively. These recommendations guarantee adequate forage production and allow higher values of canopy photosynthesis throughout the regrowth cycle. In the Amazonian biome, Quênia and Tamani present physiological responses markedly affected by climatic variations, which shows physiological changes in the forage plant under conditions of adverse abiotic factors. However, they are promising cultivars of Panicum maximum for use as fodder in pasture systems in the Amazon biome.