The first Lot 1 had a low intensive agricultural use and maize has been grown every 4 to 5 years with long fallow periods. In the second Lot 2 corn has been grown every year with intensive use of chemical fertilizers. Both Lots have contrasting chemical characteristics such as salinity, electrical conductivity and phosphorus contents.

The soil attributes, analyzed by Phytomonitor (2018) [11] , were compared with reference data from Nom-021-Semarnat-2000 [12] . Even when the pH’s are neutral, the electrical conductivity of Lot 1 is lower (EC = 0.66 mS/cm) than Lot 2 (1.53 mS/cm). Sodium (Na) is higher in Lot 2 (330 vs. 165 ppm). The organic matter (OM) is satisfactory in both lots, but it is higher in Lot 1 (2.78% vs. 2.11%).

According to the official Mexican standards [12] Phosphorus (P) in Lot 1 is in the optimal range (17 ppm) however, in Lot 2, with more intensive use, P is in excess with 80 ppm. Potassium (K) is in excess in both Lots with more than 1000 parts per million (ppm).

Eight treatments were studied in experimental units of 5 m × 4 m (20 m²) with four rows of 1 m wide and 5 m long. The corn population density was estimated in 50,000 plants ha⁻¹ (Figure 1 and Figure 2). The treatments resulted when the two varieties (Figure 3 and Figure 4) named: Chichen Itza (Chich) and Sac Beh (Sac) were combined with the following four levels of fertilization: 1) chemical (Chem 1) fertilization (N-P₂O₅-K₂O): (60-80-00), 2) chemical fertilization (Chem 1) with biofertilizers (Bio): (60-80-00 + Mycorrhizae + Azospirillum), 3) second dose of chemical fertilization (Chem 2) with biofertilizers (Bio): (30-80-00 + Mycorrzas + Azospirillum) and 4) The control (00-00-00). The treatments were distributed in a completely randomized block design with three repetitions in each Lot and were identified according to Table 1.

Six plants with complete competence were selected, and at the end of the physiological cycle the Total Dry Biomass (TDB) production (stems, leaves, husk and grain) were measured and converted into t∙ha⁻¹. The stems, leaves, and husk were considered as the Partial Biomass (PB) weighed under field conditions (Figure 5) and considered as the main variable influencing Grain Yield (GY) production. With the TDB and GY, all in dry base, the HARVEST INDEX (HI) was calculated with the formula: GY/TDB. An Analysis of Variance (ANOVA) with their Coefficients of Variation (CV) were performed to all variables.

The seeds were inoculated with a mixture (1:1 ratio) of both: 1) INIFAP^TM brand biofertilizer with Rhizophagus intraradices (Mycorrhizae fungus) at a concentration of ≥60 spores and 2) Azospirillum brasilense (Bacterium) at a concentration of 1 × 10⁻⁶ Colony Forming Units (CFU) mL⁻¹. After inoculation the seeds were dried at room temperature for 8 hours to be planted in the experimental plots. 15 days after sowing, the chemical fertilizer was applied to the corresponding treatments. The fertilizer was buried 10 cm from the corn stem in the form of Urea (N) and Triple Calcium Superphosphate (P₂O₅) in a single application.

Table 2 shows the Mean Squares and the statistical significance when submitting the grain yield and the other biomass components (t∙ha⁻¹) to the corresponding Analysis of Variance (ANOVA). No statistical differences were found (p = 95%); therefore, applying chemical fertilizers (Chem), alone or combined with biofertilizers (Chem-Bio) and even not applying any treatment (Control) is statistically the same. However, the information will be discussed later on when considering the arithmetic data of the investigation. However, it seems that further research is needed since the variables studied, under the specific conditions of this work, did not implicate any substantial change.

The CV’s (%) ranged from 0.014 in Partial Biomass Lot 2 to 18.23 in Partial Biomass Lot 1. Authors such as Pimentel (1985) [13] comment that the CV’s can be different depending on the type of experiments. Other authors such as: [14] [15] [16] Gómez and Gómez (1984) [14] ; Martínez (1988) [15] ; Patel et al. (2001) [16] suggest that when the CVs are greater than 30%, the experiments have low precision.

Similar statistical results were reported by Uribe Valle and Dzib Echeverria (2006) [10] and Uribe-Valle et al. (2007) [17] . They did not find any statistical differences between yields when corn was treated with Mycorrhizae + Azospirillum, a chemical treatment (N-P₂O₅-K₂O) (40-100-00) or a control (00-00-00) in a Luvisol of Yucatan, Mexico.

However, other authors in north Mexico like Díaz Franco et al. (2012) [18] have found important statistical differences suggesting that mycorrhizal inoculation alone was very competitive in relation to chemical fertilization. The

ns =Statistically no significant at p = 95%; EE = Experimental Error, CV = Coefficient of Variation.

combined inoculation of G. intraradices and A. brasilense, did not present any additive effect on corn growth. In addition, of the ecological advantage, it is more profitable by reducing the cost production of corn as compared to the use of chemical fertilizers.

Table 5 shows the Determination Coefficients (R²) when the GY’s were compared with the PB. The highest R² was obtained with Sac in both experimental lots with 0.75 and 0.84 for Lot 1 and Lot 2 respectively; while Chich showed very low Determination Coefficients of 0.19 and 0.27 in each lot. This indicates that the GY of Sac can be associated more intensely with the above ground PB than that of Chich.

The above analysis indicates that the GY may not always be highly associated with the production of PB but also depends on the genetic material and other factors that need further study. It has been found [23] that the dry matter and

grain yield in the corn hybrid H-562 (applying 160 kg N ha⁻¹) was mainly related to the size and duration of the photosynthetic biomolecular apparatus (Leaf Area and Duration of the Total Leaf Area) which induced a highest Growth and Assimilation Rate.

On the other hand, with the Vandeño corn variety, better response was obtained with biofertilizer and the application of nitrogen; but even when the dry matter increased this was not substantially reflected in the grain yield. This behavior is due to a higher expansion, duration and speed growth of the plant canopy [23] .

Referring to previous works of Ramirez et al. (2020) [24] related to the contents of amino acids Lysine and Tryptophan in both Chich and Sac, it would be very important to quantify the relationship between those amino acids and the variables studied in this work. But in a first glance, it seems that the HI’s of Sac, in both lots, are better associated to the amioacids than that of Chich. However, there is a trend for the aminoacids to decrease while the HI’s increase. This contrasting agronomic and biochemical behavior needs a further and deep understanding to improve the corn breeding programs.