Forage yield, chemical composition and morphogenesis of Brachiaria brizantha cv. Piatã under regrowth

With the objective to evaluate the effects of regrowth period (14, 21, 28, 35 and 42 days) on green dry matter yield (GDMY), chemical composition and morphogenetic and structural characteristics of Brachiaria brizantha cv. Piatã, was carried out an experiment under greenhouse with natural conditions of light and temperature. GDMY yields and regrowth, leaf blade length, and leaf lifespan rate increased consistently (P<.05) with regrowth period, however the nitrogen, phosphorus, magnesium and potassium contents decreased as regrowth period, while calcium contents were not affected by regrowth period. Maximum GDMY, leaf appearance and elongation rate, and leaf blade length were obtained with regrowth periods at 38.2; 41.1; 31.3 and 38.9 days, respectively. These data suggest that cutting at 35 to 42 regrowth days were optimal for obtain maximum yields and regrowth of rich forage and pasture persistence. naturales de luz y temperatura. El aumento del período de rebrote resultó en mayores rendimientos de forraje (P<0,05) y vigor de rebrote, sin embargo, implicó disminuciones significativas en los contenidos de nitrógeno, fósforo, magnesio y potasio, mientras que los niveles de calcio no se vieron afectados. Las tasas de senescencia fueron directamente proporcionales a los períodos de rebrote, ocurriendo lo inverso en cuanto a la tasa de aparición de hojas. Los mayores rendimientos de forraje, tasas de aparición y de expansión de las hojas y y el promedio de las hojas se obtuvieron, respectivamente, en 38,2; 41,1; y días de rebrote, respectivamente. El período de descanso más adecuado para pasturas de B. brizantha cv. Piatã, con el objetivo de conciliar la producción, el vigor del rebrote y la calidad del forraje, se sitúa entre 35 y 42 días de rebrote. et al. (2011), evaluating B. brizantha cv. Marandu in field conditions under different intervals between cuts, estimated average values of 0.083 leaves/tiller/ day; 1.61 cm/day/tiller and 18.59 cm for LBL.


Introduction
The experiment was conducted with the objective of evaluating the effect of rest periods on the productive performance of tropical pastures in soils under forest vegetation from Amazon. In the tropical regions, pastures cultivated represent the most economical source for feeding cattle. However, given the climatic oscillations, fodder production during the year has seasonal fluctuations, abundance during the rainy season (October to May) and deficit in the dry season (June to September), which negatively affects the productivity indexes Animal (Costa et al., 2009).
The use of appropriate management practices is an alternative to reduce the effects of seasonality in forage production. The growth stage at which the plants was harvested directly affects the yield, chemical composition, regrowth capacity and persistence (Dim et al., 2015;Paiva et al., 2019). Cuts or less frequent grazing provide greater forage yields, however, concomitantly decreases occur accented in their chemical composition, with greater deposition of fibrous material, decrease in leaf/stem ratio (Costa et al., 2007;Pereira, 2013;Nascimento et al., 2019). Therefore, one must seek the balance between yield and forage quality, to ensure the nutritional requirements of animals while securing the persistence and productivity of pastures.
The productivity of forage grasses stems from the continuous emission of leaves and tillers, important process for the restoration of leaf area after cutting or grazing and ensuring its sustainability. The formation and leaf development are essential for plant growth, given the role of leaves in photosynthesis, starting point for the formation of new tissues (Paiva et al., 2019). Grass morphogenesis during vegetative growth can be characterized by three factors: Res., Soc. Dev. 2020;v. 9, n 1, e133911801, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1, 1801 the rate of appearance, elongation rate and longevity of the leaves. The appearance rate and longevity of the sheets will determine the number of living leaves/tiller (Nabinger & Carvalho, 2009;Taiz et al., 2017;Avelino et al., 2019). These characteristics are determined genetically and can be affected by environmental factors and management practices.
This study evaluated the effects of rest period on the forage production, regrowth, chemical composition and morphogenetic and structural characteristics of Brachiaria brizantha cv. Piatã.

Methodology
The research was performed under controlled conditions using the quantitative method.
As there are still gaps about the effect of the rest period on the productive performance of tropical pastures, it was decided to use the hypothetical-deductive method (Pereira et al., 2018).
The experiment was conducted in a greenhouse using a Yellow Latosol, clay texture, forest phase, which had the following chemical characteristics: pH = 5.1; Al = 1.1 cmolc/dm 3 ; Ca + Mg = 2.3 cmolc/dm 3 ; P = 3 mg/kg and K = 87 mg/kg. The experimental design was in randomized blocks with three replications. The treatments consisted of five rest periods (14, 21, 28, 35 and 42 days). The establishment fertilization consisted of 40 and 44 mg/dm 3 of phosphorus and nitrogen in the form of urea and triple superphosphate, respectively. The grass standardization cut was performed 35 days after thinning the plants at a height of 10 cm above the ground.
The parameters evaluated were green dry matter yield (GDMY), leaf appearance rate (LAR), leaf expansion rate (LER), leaf senescence rate (LSF) and leaf blade length (LBL). The LER and LAR were calculated by dividing accumulated leaf length and total number of leaves in tillers, respectively, by regrowth period. The LBL was determined by dividing the total leaf elongation of the tillers by the number of leaves. The grass regrowth vigor was evaluated through the production of GDMY at 21 days after the first cut.
The LSR was obtained by dividing the length of the leaf that was yellowish or necrotic by the regrowth period. N contents were analyzed according to the procedures described by Silva & Queiroz (2002); while the contents of P, Ca, Mg and K were determined according to the methodology described by Silva (2009). P and K contents were quantified after nitroperchloric digestion. P was determined by colorimetry; K by flame photometry and Ca and Mg contents by atomic absorption spectrophotometry. Res., Soc. Dev. 2020;v. 9, n 1, e133911801, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1, 1801 5 Data were subjected to analysis of variance and regression considering the significance level of 5% probability, using the Sisvar statistical analysis program (Ferreira, 2011). To estimate the response of the evaluated parameters, as a function of the rest periods, the choice of regression models was based on the significance of the linear and quadratic coefficients by Student's t-test at 5% probability.   The regrowth vigor was significantly (P<0.05) affected by the regrowth period, with higher GDMY yields obtained with cuts at 28, 35 and 42 days, which did not differ (P>0.05).

Results and Discussion
The effect of regrowth plant age was adjusted by quadratic regression model and described by the equation Y = -3.25 + 0.4317 X -0.0054664 X 2 (R 2 = 0.96) and the maximum GDMY of regrowth estimated at 33.8 days. Costa et al. (2007) noted that the maximum regrowth vigor of Brachiaria humidicola occurred between 28 and 35 days after cutting the grass plants. Res., Soc. Dev. 2020;v. 9, n 1, e133911801, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1, 1801 Regrowth speed is highly correlated with the preservation of apical meristems, because with their preservation the formation of photosynthetic tissues occurs through the expansion of new leaves, while with the removal of apical meristems the new growth depends on the development of new buds, notably of basal origin, for leaf production (Difante et al., 2011;Cunha et al. 2012). The calcium contents were not affected (P>0.05) by the regrowth period, while the N, P, Mg and K decreased with the advance of the grass growing stage, evidencing a dilution effect of its contents with the increase of the forage yield. The effect of regrowth period was linear and negative, being described by the equation y = 27.11 -0.3539 x (r 2 = 0.98); y = 1.69 -0.005285 x (r 2 = 0.97); y = 5.12 -0.01014 x (r 2 = 0.98); y = 4.20 -0.03486 x (r 2 = 0.96) and y = 19.32 -0.117 x (r 2 = 0.94), respectively to the levels of N, P, Ca, Mg and K.
In general, higher concentrations were recorded with regrowth periods between 14 and 28 days (Table 1). Table 2 reports the effects of rest periods on the morphogenetic characteristics of the forage grass. The relationship between regrowth periods and the LAR, LER and LBL was adjusted to quadratic regression model, defined respectively by the equations: Y = 0.27 -0.005153089 X + 0.0000626826 X 2 (R 2 = 0.98); Y = 1.11 X + 0.093937 X -0.001501429 X 2 (R 2 = 0.96) and Y = 4.38 + 0.4510157 X -0.005797727 X 2 (R 2 = 0.97). The maximum values for LAR, LER and LBL were obtained at 41.1; 31.3 and 38.9 days, respectively (Table 2).