Agronomic evaluation of genetic material and experimental design

25 maize genotypes were evaluated in the spring-summer (S-S) agricultural cycle in rainfed conditions of 2016, in the Experimental Field of Iguala, Guerrero, located at the following coordinates of 18° 21’ 45’’ north latitude and 90° 30’ 05’’ west longitude, at 735 meters above sea level and climate Awo (w) g, the driest of the hottest humid ones with rain in summer, average annual temperature of 26 °C, being May the warmest month with about 44 °C, the average annual rainfall is 977.15 mm (^{García, 2004}).

A randomized complete block design with three repetitions was used, the experimental unit consisted of four 5 m long grooves with 0.8 m wide grooves with one plant every 0.17 m, which produced a population density of 73 529 plants ha^-1. The sowing was established in rainfed conditions, the fertilization was with the formula 120N-60P-00K, 50% of N was applied and all the phosphorus in the sowing and the rest of the N at 45 dds. Atrazine + terbutrine was applied in pre-emergency, at a rate of 2 L ha^-1 to control weeds, 15% G Terfubos at a dose of 7 kg ha^-1 to control Phyllophaga spp. and Agrotis lineatus, 5% granulated permethrin at doses of one sack per hectare and monocrotophos at doses of 1 L ha^-1 to combat Spodoptera frugiperda Smith and Heliothis zea.

Agronomic characters and statistical analysis

It was measured in cm, plant height (AP) and cob height (AMz), cob position index (IPMz), AMz/AP divided, weighed in kg, plant (PP) and cob (PMz), cob index (IMz), PMz/PP, green forage yield (RFV), grain yield (RGr) was divided, calculated using the cob weight of the useful plot x percentage of grain x moisture percentage x conversion factor to kg ha^-1, dry fodder yield (RFS), was carried at constant weight for thirty days, dry fodder index (IFS), RFS/RFV was divided.

In the statistical analysis of the data it included: analysis variance, comparison of means with Fisher's LSD test (with a 5% level of significance) among the measured variables. The Statistical Analysis System (SAS) version 9.0 statistical program was used.

Results Analysis of variance and comparison of means

The analysis of variance (Andeva) indicated that the mean squares of the genotype factor were highly significant (p≤ 0.01) in five variables, two were significant (p≤ 0.05) and three showed no statistical significance. In experiments conducted in the field, coefficients of variation (CV) have been reported that range from 3 to 10%, 11 to 20% and 21 to 30%, classified as low, medium and high (^{Pimentel, 1985}).

In green forage yield they reported 6.6 to 20.8% (^{Baghdadi et al., 2012}), grain yield of 12.4 to 17% (^{Gómez et al., 2008}), dry forage yield of 5.06 to 23.2% (^{Gaytan et al., 2009}; ^{Peña et al., 2010}), it has also been suggested that CV greater than 30% should be discarded (^{Patel et al., 2001}). In the present investigation the CV ranged from 8.1 to 18.6%, which means that the results are consistent and reliable (^{Gordon and Camargo, 2015}).

The average plant height (AP) was 250 cm and fluctuated from 188.6 to 275.6 cm, results similar to those reported by ^{Shelton and Tracy (2013)}; ^{Manjarrez et al. (2014)}; ^{Gómez et al. (2016)}. Several researchers consider it important to measure plant height in maize to explore genetic diversity between plants (Nuñez et al., 2015).

The HV-65 interverietal hybrid showed a higher cob height (AMz); on the contrary, the synthetic variety VS-535 and the experimental hybrids H-H563xHCF₂-91 and H-H516xHCF₂-91 exhibited lower values (101, 103 and 104 cm) than the general average (122.2 cm), similar results were reported by ^{Palemón et al. (2012)} in interserietal maize crosses. In addition to the above, the height of the cob should be located around 92 cm so that the plant is balanced and avoid the stem or root finish due to the height and weight of the cob (^{Shelton and Tracy, 2013}).

In this work, the relative position of the cob was low in genotypes originating in Iguala (0.46), average in Cotaxtla (0.49) and high in Celaya (0.55). The experimental hybrid H-H562XHCF₂-91 stood out in plant weight with 35.8 kg and the lowest weight corresponded to the VS-536 (21.8 kg). The experimental hybrids outperformed the commercial control hybrids H-520, H-563, H-516, H-562, HV-65 and the control varieties VS-535, VS-536 and V537C.

In cob weight, ten experimental hybrids matched the H-516 control that produced the highest cob weight (12.58 kg); on the contrary, the HV-65 interval hybrid showed a lower weight (2.16 kg), when contrasting the extreme values, 10.42 kg of variation was found. The commercial hybrid H-516 had a higher position of the cob (0.44), which indicates that 44% of the biomass of the plant is represented by the cob. Unlike the HV-65, it exhibited a lower cob rate (9%).

Commercial control hybrids H-520, H-564C, H-565, H-563, H-562, HV-65 and varieties VS-535, VS-536 and V-537C, were statistically surpassed by 10 experimental hybrids in cob weight.

The green forage yield (RFV) fluctuated from 29 to 46 t ha^-1, with an average of 38.8 t ha^-1. The commercial hybrid control H-564C produced higher RFV, its cob was positioned at half the height of the plant (IPMz of 0.5) and relatively lower biomass invested in the cob (IPMz of 0.28), it is notorious that it had a low index of dry fodder (6%), this result indicates that its green fodder is heavy and with a higher liquid content; however, its RFV was similar to eight experimental hybrids and commercial hybrids witness H-565 and H-563.

The experimental trilinear hybrids H-H562xHCF2-91 and H-H565xHCF2-91 produced 46 834 and 46 683, kg ha^-1 and surpassed the commercial hybrids (H-520, H-516, H-562 and HV-65), varieties (VS-535, VS-536 and V-537) and seven experimental hybrids. The RFV of the synthetic variety VS-536 (29 083 kg ha^-1) with respect to the H-H562xHCF2-91 hybrid (46 834 kg ha^-1), 17 751 kg ha^-1 of variation was detected at a population density of 75 000 plants ha^-1. ^{Mandic et al. (2015)} reported that with 71 429 plants ha^-1 they obtained 67.51 t ha^-1 green forage yield, ^{Sharifi and Namvar (2016)} recommend 110 000 plants ha^-1 to obtain a higher RFV.

The commercial trilinear control hybrids H-563 and H-565 exhibited greater production of green fodder at a population density of 75 000 plants ha^-1, which have adapted to tropical climates, with a tortilla dough yield of 1.8 and 1.9 kg per 1 kg of maize (^{Gómez et al., 2013a}; ^2013b). The H-564C hybrid produced high RFV, good cob coverage, plant and cob health and dough and tortilla quality is good (^{Sierra et al., 2011}). Eight experimental hybrids with genetic potential for green forage production were detected and competed with three commercial control hybrids (H-563, H-564C and H565).

The simple 329x97 hybrid produced higher grain yield (7 183 kg ha^-1) and was similar to the experimental hybrids H-H562xHCF₂-91 and H-H565xHCF₂-91 outstanding in RFV and RFS and outperformed the commercial control hybrids (H- 516, H-520, H-562, H-564C, H-565 and HV-65). The synthetic variety VS-536 and the HV-65 varietal hybrid produced lower grain yield, plant weight and cob.

^{Vázquez et al. (2013)}, reported that with 80 000 plants ha^-1, the high quality maize yield components of protein were affected. ^{Lashkari et al. (2011)} evaluated 70 000 to 80 000 plants ha^-1, increased grain yield and decreased the number of grains per row, number of grains per cob and cob length of three hybrids. ^{Mandic et al. (2016)} evaluated 5.6 to 7.1 plants m^-2 and reported that grain yield increased 5.94% (790 kg) and decreased the diameter of the cob, number of rows per cob, weight of 1 000 grains, weight of cob (^{Safari et al., 2014}).

^{Haddadi and Mohseni (2014)} evaluated 75, 85, 95 and 105 thousand plants ha^-1, reported higher grain yield with 75 000 plants ha^-1. These studies indicate that forage production is tolerable at a certain level of grain loss to obtain higher yield of green forage with high densities.

The average dry forage yield (RFS) was 2.4 kg and ranged from 2 to 2.9 kg of dry matter. Commercial control hybrids H-520, H-562 and H-564C and eight experimental hybrids produced similar RFS. H-516, H563, H-565, HV-65 hybrids, varieties VS-535, VS-536 and V-537C and eight experimental hybrids were surpassed in RFS by experimental hybrids and three commercial hybrids.

The H-562 control trilinear hybrid is outstanding in grain yield, dry matter production, cob coverage and adapts in warm and semi-solid areas of the states of Tamaulipas, Yucatán, Chiapas, Guerrero and Sinaloa (^{Gómez et al., 2008}) H-562 expressed a high rate of dry fodder (8%); that is, the hybrid produces fodder with lower liquid content and higher dry matter content.

Conclusions

Genotypes with acceptable green and dry forage yield were identified in a warm environment of Iguala, Guerrero. In addition to fodder, experimental hybrids with desirable grain yield and which the farmer considers of economic importance were identified. The progenitors that were used as females H-H-516, H-H-562 and H-H-565 when crossing the HCF2-91 line, generated trilinear hybrids with agronomic characteristics, outstanding grain yield and forage components. The trilinear hybrid H-H565xHCF2-91 stood out in most of the variables, it was registered with the SNICS as H-568. The H-563 control showed genetic potential in forage related characteristics. The experimental hybrid HEC-734286, had a favorable response in green, dry and grain forage yield. The hybrids generated in Celaya, Guanajuato with cooler weather responded to the town of Iguala with warmer weather. Commercial and experimental hybrids with forage characteristics were identified.