Introduction

In Mexico, maize (Zea mays L.) is the most important value for their plantings and production area; in the year of 2013 reported 7.487 million hectares (^{SIAP, 2014}). Performance and generation of biomass of this culture, are closely related and depend on the amount of photosynthetically active radiation ^{(Peil et al., 2005)} as well as moisture and nitrogen availability in soil (^{Wojcik, 2001}). Why in Northern Mexico soils with low organic matter content (<2%) and high carbon-nitrogen ratios (>10), causing a decrease in dry matter production by poor crop development, Foliage short duration and decrease in photosynthetic efficiency of the leaf area (^{Cueto et al., 2006}).

Different alternatives have been implemented in order to increase the efficiency of nitrogen fertilization, one is the use of nitrification inhibitors (^{Weiske et al., 2001} and ^{Barth et al., 2001}) that granular fertilizers are added soluble or liquid. These inhibitors contain the molecule 3,4 - dimetilpirazolfosfato (DMPP), temporarily disable the action of bacteria Nitrosomonas, so that delays the oxidation of NH4+ to NO2, and precludes the transformation nitrate, allowing availability of NH4+, important in plant nitrogen nutrition (^{Zerulla et al., 2001}), reducing losses by leaching source (^{Irigoyen et al., 2003}) and considering release (^{Carrasco-Martin, 2002}). Adequate availability of nitrogen sources, radiation and moisture contribute to increased plant photosynthetic rate as they are able to synthesize carbohydrates are subsequently distributed between different organs; This allows the vegetative growth in the early stages of cultivation, then in bloom and finally fruit filling, which is reflected in crop yield (^{Maddonni and Otegui, 2006}).

A tool that allows assessment of crop development is the growth analysis (^{Sedano et al., 2005}). This technique is based on the ratio of dry weights of the plants and foliar part of its area over time (different stages of cultivation). This allows to determine the growth rate of the culture (^{Jarma et al, 2010}; ^{Orozco et al., 2011}) and assimilation rate of photoassimilates (^{Ramirez- Seañez et al., 2012}), By estimating the efficiency indices as crop growth rate (TCC) and net assimilation rate (NAR). Also, calculate the components related to photosynthetic apparatus such as leaf area ratio (RAF), leaf weight ratio (FLR), leaf area index (IAF) and specific leaf area (SLA) (^{Escalante and Kohashi, 1993}; ^{Hunt, 2003}). In this regard, ^{Sedano et al. (2005)} indicates that indexes and components allows to study relationships between nutrient supply and demand for them.

When considering the above variation likely sources of nitrogen in the maize generate a positive impact on photosynthetic efficiency and allocation of photoassimilates, which can be identified by analysis of growth. Therefore, the aim of this study was to identify the effect of two sources of nitrogen in the cultivation of forage maize by evaluating the distribution of biomass and its growth dynamics to determine their production efficiency.

Materials and methods

The work was done in Torreon, Coahuila, Mexico (25° 32' north latitude, 103° 14' to 1 120 m). The soil of the experimental area was a háplico calcisol (^{INEGI, 2000}), the surface layer has a clay loam textural class, its pH is 8.33, CE 1.6 mS cm-1, a PSI 1.36, 0.089% total nitrogen and with 1.70% organic matter. The hybrid used was HT9019Y (single cross, forage, yellow grain and early cycle). The culture was established in the spring-summer of 2014, the topological arrangement of plants was 0.76 m spacing between rows and 15 cm between plants, resulting in a population density of 90 000 plants per hectare.

Two treatments were distributed in a randomized block design with eight replications, where the experimental plot consisted of six rows of 12 meters in length and 0.76 m between rows. It was fertilized with the dose of 160-80-00 (N-P₂O₅-K₂O) from two nitrogen sources; traditional first (T), which consisted of the application of urea with a concentration of 46% nitrogen and second by applying a nitrogen product with 26% of a nitrification inhibitor (3,4-DMPP) of slow release ENTEC^® (Ecology Nitrogen Technology). These two sources of nitrogen monoammonium phosphate (MAP: 11-52) was added as a source of phosphorus to complete the dose. Applying splitting 50% + 100% nitrogen phosphorus at planting and 50% nitrogen to the first auxiliary irrigation.

Four irrigations were applied by surface irrigation, applying a full sheet of 0.90 m; one of sowing and other assistance to the 40, 60 and 80 days after sowing (DAS), and that the best yields and harvest index of corn are obtained with four irrigations (^{Reta et al., 2000}). Pests presented were red spider which biologically controlled release Chrysoperla carnea in 10 000 lacewings per hectare, and armyworm for which Dimethoate and Cypermethrin EC 400 was applied at 0.25 L ha^-1. The presence of weeds are controlled by applying the herbicide Faena^®.

The record production of the dynamics of dry matter (MS), was performed by destructive sampling at 20, 40, 60 and 80 days after sowing (das). Each sample consisted of the collection of two plants with complete competence, the sample was taken from the two central rows in each experimental plot under the methodology proposed by ^{Radford (1967)}, and Kohashi ^{Escalante (1993)} and ^{Hunt (2003)}. Each plant was separated vegetative organs (leaves and stems) and reproductive. Subsequently, each was placed in paper bags and subjected to drying in a muffle furnace at constant temperature (65 °C) for 72 hours to obtain the weight of the MS. The sum of these weights represents the weight of the total DM per plant and biological yield (^{Escalante and Kohashi, 1993}). The leaf area was determined with an area meter LI-COR Model L 1-3100. With the values of dry matter of the leaf blades, total dry matter, leaf area and the time between sampling rates following growth (^{Sedano et al., 2005}) were calculated:

Where= TCC is the crop growth rate (g/m²/d); P₁ initial weight of the dry matter; P₂ final weight of the dry matter; A is the area where the dry weight was recorded; initial time t₁; end time t₂. Through this function increasing biomass is obtained per unit time.

Net assimilation rate:

Where is SO= net assimilation rate (g/m² / d); P₁ initial weight of the dry matter (g); P₂ final weight of the dry matter (g); AF₁ initial leaf area (m²), AF₂ end leaf area (m²); lnAF₁ natural logarithm of the initial leaf area (m²); lnAF₂ natural logarithm of the final leaf area (m²); initial time t₁; end t₂ when sampling (days), used to estimate the efficiency of the plant photosynthetic time.

Leaf area ratio:

Where= RAF: is the ratio of leaf area (cm²/g); AF: leaf area (cm²); PS: weight of the total dry matter (g), this value is an indicator of the size of the photosynthetic apparatus of the plant being the product of the values of specific leaf area and leaf weight ratio.

Specific leaf area:

Where= AFE: specific leaf area (cm²/g); AF: the leaf area (cm²); PS_AF: weight of dry matter of leaf area (g); indicates the thickness of the sheet representing the leaf area per gram of sheet.

Leaf weight ratio:

Where= RPF: leaf weight ratio (g/g); PS_AF, weight of dry matter of leaf area (g); PS is the weight of the total dry matter (g), with the use of this equation for producing assimilated leaves and foliage of the plant material is estimated.

Where= IAF: leaf area index (m²/m²); AFT is the total leaf area (m²); S is the area of occupied land (m²), with this equation is estimated leaf area per unit area of land.

The estimates were made by plant and for a square meter (9 plants m^-2). Submitting the results of an analysis of variance using the SAS statistical package View 9.1 (1999) and mean separation test (Tukey p≤ 0.05).

Results and discussion

Production and distribution of biomass. The dynamics of accumulation of biomass per square meter for the two sources of nitrogen, showed statistically significant differences (p≤ 0.05) in the final stages of growth (Table 1). By using the nitrogen source Entec differences 262.4 g m^-2 were found. This behavior had a slower rate during the first 20 dds, which was gradually increased in the following days, with the maximum accumulation of biomass at 80 dds. While the 60 DAS, the accumulated crop greater percentage of their total dry weight of the plant organs (97.4%) compared with the other nitrogen source (urea). In this measurement, the dry weight accumulated in vegetative organs with Entec exceeded by 51.8 g m^-2 at accumulated with traditional source; so that in the final phase of growth, growing 30.7% accumulated photoassimilate in the reproductive organs, while the traditional source had only 26.8% (Table 1). And can be attributed as indicated by Zerulla et al. (2001) and Irigoyen (2003), who mentioned that the source of slow release nitrogen (Entec) greater stability and availability of nitrogen occurs throughout the cycle, with the benefit of a tax loss for very low leaching.

This biomass accumulation throughout the crop cycle using nitrogen source Entec coincides with the highest LAI values as the total dry matter production is the result of the efficiency of the foliage in the interception and use of solar radiation available during the growing season (^{Díaz et al., 2010}).

The largest allocation of photoassimilates to the leaves allowed further growth of reproductive organs when the nitrogen source was used Entec, which represent greater IAF, impacting this in crop yield. This behavior is similar to that reported by ^{Noriega et al. (2011)}, who they indicated that the greater the number of sheets increases performance. Also ^{Azofeifa and Moreira (2004)} mention that as the number and size of leaves increases the IAF and light absorption rate and dry matter production.

Growth rates. The analysis of variance of growth rates (TCC, and LAI TAN) between sources, showed statistically significant differences (p≤ 0.05) among 60-80 dds (Table 2). Although 60 dds with both nitrogen sources, the culture reaches its maximum IAF, was with Entec source with which presented the highest values (3.01 m² m^-2).

The metabolic processes of the crop to use the source Entec were faster, since their values and TCC were so superior to those presented by the other source of nitrogen (Table 2).

The results obtained from both sources in the early stages of growth coincide with its low IAF (Table 2). The IAF crop is low in early stages of development due to incomplete coverage and low percentage of light (^{Brown, 1984}; ^{Baracaldo, 2010}). However, at 60 dds fertilization with Entec to submit as IAF, he kept larger to capture solar radiation and leaf structure carbohydrate production. The crop growth rate is directly proportional to the intercepted light, provided by the leaf area index (^{Saleem et al., 2010}).

It also generated the highest values of SO, indicating that higher photosynthetic efficiency of plants. An increase in the final stages indicates intraspecific variation and relationship between leaf area and total biomass (^{Mora et al., 2006}).

In relation to the components of the relative size of the photosynthetic apparatus to the RAF and AFE in the last phase of growth, statistically significant differences between the nitrogen sources were detected, unable to maintain the same relationship between the magnitude of its leaf area and its MS (Table 3), showing an ADR Entec and AFE (13.81 and 113.80 cm² g^-1) than the traditional source, with differences of 4.46 and 12.76 cm² g^-1, respectively.

In the nitrogen sources higher values of RAF, AFE and RPF were recorded in the first phase of plant growth, and gradually decreased as advanced culture age, maintaining similar values RPF with both nitrogen sources (Table 3).

Higher values of AFE, RAF and the RPF, early crop development are because plants use their photoassimilates greater extent for the development and growth of the photosynthetic apparatus, generating energy costs resulting in a lower weight (^{Carranza et al., 2009}). Because regardless of nitrogen source for the final stages of growth, the plants accumulated the same amount of photoassimilates in their leaf blades. However, higher values of RAF in the final growth phase with Entec source, attributed to the incorporation of nitrification inhibitors generate better nitrogen utilization by the corn to remain stable in the form of NH4+ longer on the floor (^{Gardiazabal et al., 2007}).

The process of decline of these indices from the early stages of growth in treatments and is considered normal in the early stages cultivation invests most of photoassimilates in their vegetative structures and the development of their photosynthetic apparatus; and end of the cycle these photoassimilates be used in the growing number and size of fruit (^{Gaytan et al., 2004}).

Plants grown under the influence of Entec source AFE had high values indicating an increase in leaf area per gram dry weight, presenting larger but thinner sheets. These values imply that invests less leaf biomass per unit area, which is strongly correlated with a variety of physiological parameters (^{Porter, 2002}). Which is contrary to the traditional treatment, possessing lower AFE values, so it could be assumed to have a higher content of cell wall components, especially lignin generating more hard and thick leaves, whereas the leaf thickness determines the availability of chloroplasts space to accommodate so thick sheets have vacant spaces along mesophyll cells that are not occupied by chloroplasts and therefore low rates of photosynthetic activity (^{Oguchi et al. (2003)}.

The RPF demonstrated similar values of the plant, regardless of nitrogen source, regulates and distributes evenly between photoassimilates that produces the vegetative organs (^{Orozco et al., 2011}).

Conclusions

The source of slow release nitrogen (Entec), influenced the rate of metabolic processes in plants and photosynthetically was more efficient, having an effect on the distribution of biomass and growth dynamics in presenting the highest values and TCC SO. In addition, it increased leaf area per gram of dry weight, presenting larger but thinner sheets. This implies that invests less leaf biomass per unit area, which is strongly correlated with a variety of physiological parameters in its favor.