Sistemas de cultivo dos capins Marandu e Mombaça baseado em adubação nitrogenada e irrigação e modelagem de carboidratos de reserva do capim Marandu usando o CROPGRO-PFM

Abstract

With nitrogen (N) fertilization in tropical grasses, there is an increase in productivity, as long as other environmental and nutritional factors are not limiting. The objective of this study was to evaluate the effects of two doses of N and the use or not of irrigation (I) in Urochloa brizantha (Hoschst. Ex A. Rich.) R.D. Webster (Marandu palisadegrass) and Megathyrsus maximus (Jacq.) B.K. Simon & Jacobs (Mombasa grass) on agronomic, bromatological and structural characteristics in two seasons (rainy and dry) and the use of indirect measures and the mobilization of non-structural carbohydrates (NSC - % DM) in roots in the Marandu palisadegrass. The design was in randomized blocks in a factorial scheme (production systems - PS: Rainfed - R and I × N fertilization: 0 and 550 kg of N ha−1 year−1) with three replications, with 12 plots of 7 × 4 m for each grass. The experiment started in October 2017 and ended in March 2019, 16 regrowth cycles were taken (28 days in summer and 42 days in agrostological winter) in the city of Lavras, Minas Gerais, Brazil. Herbage accumulation (HA - kg DM ha-1) was measured in each cycle, harvesting the forage enclosed in rectangular metal frames (0.5 × 1.0 m) at 15 cm from ground level for the Marandu palisadegrass and 40 cm from ground level for the Mombasa grass (two points per plot). The bromatological analyzes were of crude protein (CP - % DM), neutral detergent a fiber (NDF - % DM) and in vitro dry matter digestibility (IVDMD - % DM). Measurements of canopy height (CH - cm), light interception (LI -%), leaf area index (LAI), leaf angles (°) and specific leaf area (SLA - cm2 g-1) were taken every 14 days in the summer and 21 days in the winter. Mixed linear models were used with N fertilization, PS and treatment interactions as fixed effects, and blocks as random effects. The seasons were: agrostological summer 1 (October 2017 to April 2018), agrostological winter 1 (May to September 2018) and agrostological summer 2 (October 2018 to March 2019). The season effect was included an repeated measure. For the Marandu palisadegrass in two cycle, the SPAD values and the chlorophyll contents (μg cm-2) were measured on days: 4, 9, 17 and 28 and the HA. The chlorophyll concentration was estimated with atLEAF CHL PLUS (10 leaves in the sampling rectangle). The measurements of Rising Plate Meter (RPM), CH, LI and LAI were performed on days: 0, 4, 9, 17 and 28 of each cycle. Two regrowth cycles were considered as representative of the summer and two of the agrostological winter, measuring the herbage mass (HM) in the residue, below 15 cm. During the two characteristic summer regrowth cycles, the plots were accompanied by evaluations on days 0, 4, 9, 17 and 28 and on the two winter on days 0, 4, 9, 17, 28 and 42. In the CSM-CROPGRO-PFM the general average of the three replicates of the experimental units was used. The contents of NSC, aboveground biomass (AGB - kg DM ha-1), HA, leaf production (LP - kg DM ha-1), stem (SP - kg DM ha-1), leaf (% AGB) were evaluated. N fertilization promoted an increase in the productive and structural characteristics, chemical composition of the Marandu palisadegrass and Mombasa grass. The use of N fertilization associated with I in the R period of the year (winter) was effective in the productive and structural characteristics, chemical composition and indirect measurements (RPM and SPAD value) can be used to estimate the agronomic and chemical characteristics of the Marandu palisadegrass. In summer, the reestablishment of NSC contents occurs around 9 days and in winter it occurs at 17 days of regrowth, with or without N fertilization, under R and I conditions. The CSM-CROPGRO-PFM model works well for R conditions with or without N fertilization throughout the year and for the I condition it is necessary to change the parameters related to agronomic characteristics.