Produção de forragem de capins do gênero Panicum e modelagem de respostas produtivas e morfofisiológicas em função de variáveis climáticas. / Forage yield of Panicum grasses and modeling yield, morphological and physiological traits in response to climatic variables.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

The brazilian livestock industry is based on the use of tropical pastures with high production potential which benefits from favorable environmental conditions over the year. However, the seasonal pattern of climatic conditions, mainly temperature and daylength, concentrates most of the annual forage accumulation during the spring and summer. Thus, knowing the yield, morphological and physiological responses of forages to temperature and daylength, in the absence of moisture constraints, may allow for the development of prediction models, which can be useful in planning forage-livestock systems. This study was carried out at the Departamento de Zootecnia of ESALQ-USP in Piracicaba, SP, from December 2002 through April 2004, with the purpose of generating a comprehensive dataset on the productive, morphological and physiological characteristics of Panicum spp. and, from this dataset, develop and evaluate models that describe forage accumulation and sward characteristics as related to photothermal units (a climatic variable which combines the effects of temperature and daylength). Five cultivars of Panicum spp. were used in the study: Atlas, Massai, Mombaça, Tanzânia and Tobiatã, harvested either every 28 (Atlas, Massai and Mombaça) or 35 (Tanzânia and Tobiatã) days during the ‘summer’ (from 21 September to 22 March) and every 49 (Atlas, Massai and Mombaça) or 63 (Tanzânia and Tobiatã) days during the ‘winter’ (from 23 March to 20 September), leaving a stubble of 35 cm for Atlas, Mombaça, Tanzânia and Tobiatã and 15 cm for Massai. Plots (10 Η 4 m) were irrigated to avoid water deficit and fertilized with 250 kg N ha-1 during the ‘summer’, resulting in total annual rates of approximately 450 kg N ha-1. The experimental design was a randomized complete block with four replications. On each harvest, forage accumulation was measured on every plot and, from these data, annual and seasonal production for ‘summer’ and ‘winter’ were calculated. Additionally, two ‘summer’ regrowths and a ‘winter’ regrowth, were characterized every week or every two weeks, respectively, by measuring residual mass, forage accumulation, sward height, leaf area index (LAI), mean foliage tilt angle, light interception (LI) and leaf photosynthesis, from which canopy photosynthesis was calculated. Massai was the most productive grass, with total annual yield of 30 Mg DM ha-1, followed by Mombaça, Tanzânia and Tobiatã, which yielded 20 Mg DM ha-1yr-1 on average, and by Atlas with annual yield of 12 Mg DM ha-1. During the ‘summer’ Massai and Mombaça had similar yields of 18 Mg DM ha-1, followed by Tanzânia and Tobiatã which yielded a mean 14 Mg DM ha-1, and by Atlas which yielded 8,5 Mg DM ha-1. During the ‘winter’ Massai, Tanzânia and Tobiatã yielded 7,5 Mg DM ha-1 while Atlas and Mombaça yielded 3,7 Mg DM ha-1, resulting in a more pronounced seasonal growth pattern of Mombaça, which concentrated 80% of its production in the ‘summer’ compared to the other grasses where this value was 70%. Despite being the highest yielding grass, Massai had the lowest canopy photosynthesis rates (1,08 mg CO2 m-2 s-1) and this may be related to the higher leaf proportion in the forage mass. The mean LAI of Massai swards (2.0) was lower than that of the other grasses (2.25 to 2.68) and the residual forage mass on Massai plots (4 Mg DM ha-1) was about half of that of the other grasses (7 Mg DM ha-1 on average). This generated less of a dilution effect of the leaf mass on the total forage mass, and most of total forage mass of Massai came from leaves (photosynthesizing organs). The models that describe the evolution of forage mass and leaf mass above the stubble, as well as LAI, LI and sward height differed among cultivars and between seasons. The forage accumulation models provided yield estimates that were consistent with observed values, although under and overestimations occurred, mainly for Atlas and Tanzânia during the ‘winter’ and early ‘summer’. These models, however, allowed for the identification of the point of maximum average growth rate during the regrowth, as well as the characterization of the canopy morphological and physiological characteristics at that point in time, for both ‘summer’ and ‘winter’ regrowths. The use of models based on the photothermal unit proved effective for the prediction of yield and yield-related responses of Panicum grasses. In order for these models to become useful tools for systems planning and management, further study is needed in addition to bigger and richer data libraries and a wider range of environments and management practices.

ASSUNTO(S)

plant physiology forragem climatology capins grasses crop production morfologia vegetal climatologia fisiologia vegetal plant morphology produção vegetal

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