Maintenance of the Economic Performance of Eucalyptus in Competition With Weeds

The aim of this study was to evaluate the growth of the hybrid clone Eucalyptus grandis × Eucalyptus urophylla submitted to competition with weeds and determine the period prior to economic loss (PPEL). The experiment was carried out in Ulianópolis, Pará State, Brazil, and consisted of the treatments continuous control and no control of weeds. Weed management was carried out by integrating both mechanical and chemical control. Stem base diameter, height, canopy diameter of eucalyptus plants were measured on the 16th month after planting. Wood volume was estimated by means of a volumetric equation. The treatments continuous control and no control of weeds were compared using the Hotelling’s T test and the parameters for PPEL calculation were defined from volume estimation. The treatment continuous control promoted significant gains in eucalyptus growth. PPEL tended to be reduced by 3.8287 and 0.2393 units with the increase of a unit in the price of wood and the increase of planting yield, respectively. For different eucalyptus wood prices, the calculated PPEL ranged from 40 to 161 days of coexistence with weeds.


Introduction
The establishment of planted forests has gained increasing prominence in the forest sector, following the trend of sustainable development of wood production.Wood production from commercial reforestation generates environmental benefits by reducing pressure on native forests and their biodiversity, promotes economic gains by generating income, and fulfills a social function by creating direct and indirect jobs (Gabriel et al., 2013;Vechi & Magalhães Júnior, 2018).
Regarding the ecological interactions, Pereira, Barroso, Albrecht, and Alves (2014) cite that the term interference encompasses the direct and indirect effects on a crop that are the result of the presence of weeds.Among the direct interferences, the most common is intra-and interspecific competition, which tends to occur from the moment that two or more individuals develop in the same space and are dependent on the same limited resources for their survival, such as water, nutrients, light, and space in the case of plants (Odum, 2004;Pereira et al., 2014).This phenomenon is one of the challenges for the success of commercial reforestation since weed occurrence damages planting productivity and leads to complications in operational activities (Pitelli, 1987;Londero, Schumacher, Ramos, Ramiro, & Szymczak, 2012).
Weed communities can be considered as an unfavorable biotic factor of universal occurrence since they affect crops of any species, generating high management costs.An example of this is that in Brazil, in 2014, herbicide use totaled 476,860 tons, representing more than half of the amount of plant protection products marketed that year, reaching US$ 3.90 billion (Ferreira & Vegro, 2015).In the management of eucalyptus plantations, weed control is a factor of considerable relevance for wood productivity and final net income, representing around 20-25% of the total cost in a 7-year cycle (Rodigheri, Pinto, & Dhlson, 2001;Queiroz & Silva, 2016).
Especially for eucalyptus, competition for water is highly damaging because it can cause water stress in young plants, being the most important limiting factor during the seedling establishment stage (Garau, Lemcoff, Ghersa, & Beadle, 2008).The study carried out by Toledo, Vitória Filho, Pitelli, Alves, and Lopes (2000) shows the effect of the competition on Eucalyptus urograndis, in which the individuals maintained in coexistence with weeds for 364 days suffered reductions of 70.43 and 68.56% in diameter and height, respectively, in relation to those free from weed infestation.
In eucalyptus areas, weed management is mainly performed by chemical control with glyphosate-based herbicide (Viana et al., 2010) or, less frequently, with formulations based on carfentrazone-ethyl, fluazifop-P-butyl, flumioxazin, glyphosate potassium salt, glufosinate ammonium salt, isoxaflutole, oxyfluorfen, and sulfentrazone, which are herbicides registered in the Brazilian Ministry of Agriculture, Livestock and Supply for use in the Brazilian territory (MAPA, 2018).
Because glyphosate is a nonselective herbicide, it is applied in a directed way, avoiding reaching eucalyptus plants (Machado et al., 2010).Even with precautions in the application, Tuffi Santos, Meira, Ferreira, Sant'Anna-Santos and Ferreira (2007) reported that glyphosate phytointoxications have been verified in reforestation with eucalyptus.The adequate and rational planning of weed management, besides reducing phytointoxications in plants and minimizing the heterogeneity of planting, also allow maximizing gains on productivity of the forest planting.Some studies have already showed that during the first year of eucalyptus development there is an ideal period for weed control aiming at the best growth of the forest stand, which varies according to the clone and region of study (Toledo et al., 2000;Londero et al., 2012;Tarouco et al., 2009).Therefore, taking into account the lack of information on the management and relationship between weed and eucalyptus in the Amazon, this study aimed to evaluate the growth of the hybrid clone Eucalyptus grandis × Eucalyptus urophylla submitted to competition with weeds and determine the period prior to economic loss (PPEL) to the conditions of the Amazon region.

Material and Methods
The experiment was carried out in Ulianópolis, located in the southeastern mesoregion of the Pará State, in a plantation established in February 2013.The climate predominant in the region is Awi according to Köppen classification, with an annual average temperature around 26.3 °C and annual precipitation above 1700 mm (Bastos, Pachêco, Figueirêdo, & Silva, 2005).
The soil of the experimental area is classified as medium textured Oxisol (Latossolo Amarelo Distrófico, Brazilian Soil Classification System) (EMBRAPA, 2013).Mowing, stump removal, and subsoiling operations at 60 cm depth were performed mechanically.Initially, the fertilization was carried out with 450 kg ha -1 of natural reactive phosphate applied during the subsoiling at pre-planting and, together with the planting operation, the equivalent of 150 kg ha -1 of NPK with the formula 06-30-06 + micronutrients (0.5% B + 0.3% Zn + 0.3% Cu) was applied in lateral furrows next to the plants.In addition, 1.2 t ha -1 of limestone was applied mechanically after seedling planting.
Experimental plots of 24 × 24 m (576 m 2 ) were used with a planting spacing of 3 × 3 m.Two planting rows were eliminated to form the border, resulting in a useful area of 144 m 2 (16 plants per plot).The experiment consisted of the treatments continuous control (CC) and no control (NC) of weeds, distributed in a randomized block design with four replications.In the treatment NC, the eucalyptus were kept in competition with the weeds until the tenth month after planting, from which the weed control was performed with the same procedure of the treatment CC.
Weed control was performed in an integrated manner, including crowning (manual weeding) of eucalyptus plants with a radius of approximately 0.5 m and application of 1008 g a.e.ha -1 of glyphosate (Scout) using backpack sprayer equipped with an anti-drift device and set for a 200 L ha -1 of spray solution volume.
Eucalyptus growth was evaluated on the 16th month after planting, in which the stem base circumference was measured with a measuring tape (converted to diameter), the total height was measured with the Haglöf clinometer, and the canopy diameter was measured with a measuring tape.Tree wood volume was estimated by means of the Husch Equation (1): LnV = -9.7262+ 2.6417LnD (1) where, LnV and LnD correspond to the neperian logarithm of the volume and diameter of the stem base, respectively.
For treatment comparison, the Hotelling's T 2 multivariate test was performed at a 5% significance level, in which the variables stem diameter, height, and canopy diameter were considered.In order to identify which variables were significantly influenced by treatments, individual confidence intervals for the Student t distribution with Bonferroni protection at 5% significance were used.
PPEL (2) was calculated based on the methodology of Vidal, Fleck, and Merotto Jr. (2005): where, TC is the total cost of weed control, including the fixed and variable costs, PL the daily percentage loss in eucalyptus growth due to competition, and Y is the eucalyptus yield, given by the product between the price of the m 3 of wood and productivity.
It was considered only one procedure of weed control for and model parameterization.TC was defined from the data obtained in this study, while eucalyptus wood price was obtained from the literature.The parameter PL was obtained by the difference between the productivity in wood volume of the treatment continuous control and the treatment no control (3).
where, V cc corresponds to the productivity in volume of the treatment continuous control, V nc is the productivity in volume of the treatment no control, and the value 480 refers to the growth period until the evaluation.The analyses were carried out in the software R by means of the packages stats and ggplot2 (R Core Team, 2018).

Results
From the Hotelling's T 2 test for treatment comparison, a significant difference was observed between the continuous control (CC) and no control (NC) of weeds.Knowing the difference between treatments, individual confidence intervals were calculated to identify in which variables a significant difference was observed between the treatments CC and NC.All the variables presented a significant difference between treatments since any of the intervals had the value zero (Table 1).In addition, the competition with weeds resulted in reductions in stem diameter, height, and eucalyptus canopy diameter by 56, 56, and 31%, respectively.Note. (1)Intervals with 95% confidence; (2) Treatment means; (3) Significant at 5%.
From the research records, the average total cost (TC) with plant control was defined in approximately 51.15 US$ ha -1 for a glyphosate application with a manual backpack sprayer.The price of m 3 of eucalyptus wood was obtained by the average values of 2018 up to June, which varied from 9.09 to 39.75 US$ m -3 depending on the product to which the wood is used.
With the wood price and considering a productivity of 16 m 3 ha -1 up to the tenth month of eucalyptus growth, the calculated yield (Y) was of 158.36 to 635.99 US$ ha -1 .Using the information from TC, Y, and the calculated daily percentage loss (PL) of 0.2%, different values were determined for the period prior to economic loss (PPEL) of eucalyptus (Table 2). jas.ccsenet.

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Table 1
. Upper and lower Student's t individual confidence limits with Bonferroni protection of the variables base diameter (D), height (H), and canopy diameter (CD) of the hybrid E. grandis × E. urophylla for the difference between the treatments continuous control (CC) and no control (NC)