Yield of Pennisetum glaucum L . Under Phosphate Source Doses

Phosphorus deficiency has been a limiting factor in crop yields due to the low availability of this nutrient in the soil. Thus, the objective of this work was to evaluate the growth and P concentrations in plant tissue of two cultivars of forage millet grown submitted at doses of phosphorus (P). The experiment was set in greenhouse conditions in a Hapludox. The experiment was carried out in a completely randomized design in a 4 × 2 factorial scheme, with four replications, where the factors were the control treatment (without fertilization with P) and three levels of P (50, 100 and 200 kg ha) as simple superphosphate and two cultivars of pearl millet (BN2 and ADR500). The following were evaluated: plant height, stem diameter, dry matter of the aerial part (DMAP) and roots (DRM), phosphorus content in the leaf, in the stem and in the roots. The interaction between doses of P with the cultivars did not significantly affect the studied variables. Cultivar ADR500 provided greater height, stem diameter and dry matter of the aerial part. The best doses were 166 kg ha, 173 kg, 203 kg ha and 165 kg of phosphorus ha to height, stem diameter, DMAP, DMR and phosphorus content in the leaf, respectively. The increase of phosphorus rate increased content of P in the dry matter of the stem and roots on evaluated cultivars.


Introduction
Tropical humid soils are characterized by the high weathering degree and the low content of available P to plants (Rocha et al., 2005).In such soils, P is the nutrient that most limits forage production, mainly due to its low mobility and availability, restricting the growth of plants (Costa et al., 2009).
Among the tropical forage, pearl millet (Pennisetum glaucum) is of great importance and represents a high potential of production associated with high quality forage in a short period.Moreover, it is a forage indicated to successive summer crops in Brazilian subtropical and tropical regions as well (Negreiros Neto et al., 2010).The cultivars BN2 and ADR500 are adapted to the production systems of forage and biomass, presenting late cycle and habit of erect growth (EMBRAPA, 2003).
Pearl millet has bromatological composition similar to corn (Gomes et al., 2008), which is an economic alternative due to low production cost during the inter-harvest season.Thus, it is verified that intensification of use of this forage has been increasingly frequent, requiring fertilization in training and maintenance as well (Patês et al., 2007).As for the production potential of this forage, pearl millet can achieve up to 60 t ha -1 of green mass and 20 tons of dry matter per hectare when grown in early spring (Pedrico et al., 2010).
Soil fertility plays an important role in plant growth, in its productivity and in the nutrient contents in plant tissue (Braz et al., 2004).Therefore, fertilization is an essential practice in the structure and stability of forage plants (Bastos et al., 2008), mainly phosphorus fertilization, whose soils have low availability in this nutrient.
The production potential of a forage plant is genetically determined, however, for achieving such potential, environmental conditions and management must be observed.Among these conditions, the low availability of nutrients such as P is one of the main factors that affect productivity and the nutritive value of forages.Therefore, when nutrients are not in satisfactory quantities or in some conditions that makes them little available, their deficiency in the cells promotes alterations in the plant metabolism (Taiz et al., 2017), reducing their growth as consequence.
In this sense, knowledge of the amount of P due to the low soil supply capacity and requirements of the crops for this nutrient is essential for allowing the recommendation of the nutrient at the appropriate dose for the growth of plants, according to their requirements.Thus, the objective of this work was to evaluate the growth potential of pearl millet forage cultivars and P concentrations in plant tissue, grown under phosphate source doses.

Characterization of Experimental Area
The experiment was carried out from August to November 2012, in a greenhouse at the Institute of Agricultural Sciences (Instituto de Ciências Agrárias) at Federal Rural University of Amazon (UFRA), in Belém, state of Pará.The experimental units were composed of pots with a capacity of 5 dm³ of soil, collected in the topsoil 0-20 cm of a dystrophic Yellow Oxisol (EMBRAPA, 2013), in a remaining forest area of Embrapa Amazônia Oriental (01°41′10″ S and 48°32′24″ W).According to classification of Köppen (1948), the climate in the area is the Afi, with an average annual temperature of 26 °C with high rainfall, with an average of 2,754.4mm per year, where rainy season is from December to May and a less rainy season is from June to November (Nechet, 1993).

Experimental Design
The experimental design was complete randomized in a 4 × 2 factor scheme, where the factors were the application of varying rate of P at 0, 50, 100 and 200 kg ha -1 , corresponding to 0.0, 0.291, 0.581 and 1.163 g of simple superphosphate plot -1 and two pear millet cultivars (BN2 and ADR500), with four replications, totaling 32 experimental plots.
Fifteen seeds were sown per pot, keeping two plants after thinning, held 10 days after emergence (DAE).During the experiment the humidity was maintained at 60% of the total pore volume.

Data Collection
To evaluate the vegetative cycle of pear millet, biometric data were collected 70 days after sowing, using the following variables: stem diameter (using digital calipers), plant height (measured with a tape measure from the plant neck to the final end of the last leaf), panicle length and green mass and dry mass of roots, stems, leaves and panicles.
At 70 DAS, when the grains were in milky soft -1 dough stage, pearl millet plants were harvest with division in panicle, stem and leaf.Plant material of the aerial part and roots of the pearl millet were packed in paper bags, weighed to obtain fresh mass and then dried in an oven with forced air circulation at 65-70 °C for 72 h, until the constant weight.The P content in the leaf, stem and root was extracted by nitric-perchloric digestion and determined by flame spectrophotometer according to the methodology described by Silva (1999).

Data Analysis
The results were submitted to analysis of variance when significant by the F test.The cultivars were compared by Tukey test (p < 0.05) and the effect of P doses were analyzed by regression, adjusting the equations to adequately express the behavior of the variables.The values for maximum agronomic efficiency (MAE) of pearl millet cultivars were determined using the first derivative of the functions corresponding to each growth variable, equating them to zero.Furthermore, for the production of dry matter, maximum economic efficiency (MEE) was calculated which was 90% of MAE (Cruz et al., 2008).jas.ccsenet.