In southern Tamaulipas and other regions of the Mexican tropics, the yield of soybeans under temporary conditions is often limited by lack of moisture during the reproductive stage of the crop. This is due to variations in the amount and distribution of rainfall, and the coincidence of the period of pod formation and grain filling, with the time of greatest probability of precipitation being in September and October. On the other hand, the sensitivity of the soybean to the short photoperiod in summer “late” plantings, causes that the vegetative cycle, the plant growth and the period of grain filling decrease, which also affects the yield (^{Maldonado and Ascencio, 2014}).

According to ^{SIAP (2016)}, the area planted with soy in Mexico in 2015 was 250 thousand hectares, which increased in relation to 2014 that was 211 thousand hectares. However, production was lower in the first (341 thousand tons) compared to the second (350 thousand tons), mainly due to the decrease in yield (1.36 t ha^-1), since in 2015 there was drought in the largest soybean producer in the country, the yield in the previous year was 1.88 t ha^-1 (^{SIAP, 2016}). According to ^{Maldonado et al. (2013)}, a similar phenomenon occurred in 2011, where drought reduced soybean yield in southern Tamaulipas to 0.9 t ha^-1.

Insufficient soy production and the significant increase in its demand, positions Mexico as the third most importing country of this oil after China and the European Union. Of the 3.4 million tonnes imported in 2012, 4 million tonnes are currently imported to meet the needs of the oil and protein processing industry (^{USDA-FAS, 2016}), which represents a currency leak of more than 1 500 million dollars.

Therefore, one of the objectives of genetic improvement of soybean in the INIFAP - The Huastecas Experimental Field, is the generation of high productivity varieties that develop the reproductive stage during the time of greatest probability of rainfall in the region, with a long pod formation and grain filling period (R4-R7), and a high grain filling rate in steps R5-R7. Based on these criteria the Huasteca 600 soybean variety was generated, with application in southern Tamaulipas and other regions of the humid tropics of Mexico.

The Huasteca 600 variety was developed from the hybridization of lines H88-1880 (currently Tamesi variety) x H88-3868 (^{Maldonado and Ascencio, 2012}), which was carried out at the INIFAP-Experimental Field The Huastecas in the spring-summer cycle (S-S) 1994. The parents are lines adapted to tropical environments, reason why they are classified in the group of maturity IX. The line H88-1880 originated from the crossbreeding of the Santa Rosa strain x H80-2535, while the line H88-3868 was generated from the crossing of H80-2352 x Santa Rosa, both progenitors were developed in the area of soy genetic improvement of the Annual Oilseeds Program of INIFAP.

In the subsequent S-S cycles the F1 plants were obtained, the segregating population F2 was advanced by the modified pedigree method or seed per seed or single pod (^{Fehr, 1978}; ^{Fehr, 1993}). The selection of individual plants was carried out in the F3 generation, then the seed was multiplied and the F4 was advanced, finally the H98-1052 (Huasteca 600) was obtained in the F5 generation during the spring-summer cycle 1998.

Performance evaluations and agronomic and phenological characterizations were performed in different environments in the southern region of Tamaulipas under temporal conditions. The preliminary performance test was conducted in 1999 and 2000, the regional uniform test from 2001 to 2003, and the uniform tropical tropic test from 2004 to 2010. The validation with soy producers was carried out in 2011 and 2012, which allowed corroborating the yield potential and agronomic behavior of H98-1052 at semi-commercial level.

Huasteca 600 is of medium to high size with erect stem and good branching, of determined growth habit, since 71% of its final height obtains it to flowering. The leaflets of the leaves are ovoid, of medium size and medium green color. The flowers are violet. The seed is of medium size, the weight of 100 seeds is of 15.7 g, of subespheric form, yellow color and with hilio coffee. It has pods with two and three grains. Its pubescence is brown.

In the Table 1 presents the main agronomic characteristics of the Huasteca 600 variety, compared to the Huasteca 200 control. Characteristics were recorded at optimum plantings in the middle of July and applying the descriptive criteria in the vegetative (V) and reproductive (R) strains developed by ^{Fehr and Caviness (1977)} for soybean cultivation. It is a variety with 48 days at R2 (full bloom beginning), four days before Huasteca 200, allowing it to develop the pod formation and grain filling period (R4-R7) at the time of greatest rainfall probability in September and October. In addition, this period is five days greater than that of Huasteca 200, considered an advantage because it uses more moisture to obtain the yield.

Table 1 Agronomic characteristics of the Huasteca 600 compared to the Huasteca 200 (control). INIFAP-The Huastecas Experimental Field. 

The presented 119 days at R8 (physiological maturity). The final height of the plants is 79 cm, achieving adequate growth to express their yield potential when sown in the optimum season in southern Tamaulipas (June 15 to July 20). It produces the first pods at 14 cm from the ground, in Huasteca 200 is at 16 cm, height suitable for mechanical harvesting without problems of grain losses.

The seed is of medium size, on average 100 seeds weigh 15.7 g, contributing this characteristic to the yield, and its conservation during the storage. It has a good level of resistance to bending and weeding. Due to its medium to high size, it can be planted in rows 76 to 80 cm apart or narrower, at a density of 250 thousand to 300 thousand plants ha^-1.

From 2004 to 2010 the Huasteca 600 response to the most common foliage and stem diseases in southern Tamaulipas was evaluated, observing that it has a good level of resistance to “frog’s eye” (Cercospora sojina Hara) and tolerates “mildew “[ Peronospora manshurica (Naum) Syd-ex Gäum], “ammunition shot” [ Corynespora cassiicola (Berk. and Curt.) Wei] and “anthracnose” [Colletotrichum truncatum (Schw.) Andrus and W. D. Moore]. In most of the evaluation environments, there were no symptoms or moderate damage of less than 10% in the leaf area, without affecting seed yield and quality.

It was also evaluated with respect to the preference of “velvetworm” (Anticarsia gemmatalis Hübne) and “false soybean meter” (Pseudoplusia includens Walker) defoliating larvae, and Huasteca 600 showed no greater damage than the variety commercial control, considering the same pressure of insects. Therefore, it has no greater preference than commercial varieties to such defoliants and tolerates up to 30% damage in the vegetative stage, without affecting yield.

Certified seed of the Huasteca 600 variety can be produced under the environmental conditions of the southern region of Tamaulipas, applying the technology available for this purpose; it can also be kept in cold rooms from year to year under controlled temperature and humidity conditions (18 °C and 70% RH). For the production of high quality seed and its subsequent conservation it is recommended to apply the technology package available at INIFAP-The Huastecas Experimental Field.

The industrial quality of soybeans is determined by the proportions of protein and oil in the seed, and the fatty acid profile. It is desirable for the soybean oil to have low percentages of palmitic and stearic acids and a high percentage of oleic acid; and to a lesser extent linoleic and linolenic acids, the latter induces a rapid deterioration of the oil due to oxidation. In the case of the Huasteca 600, the oil content (20.63%) and protein (36.91%) is acceptable, since they amount to more than 57%; with respect to fatty acids, palmitic and stearic add up to 13.61%, slightly higher proportion than the control variety; the content of oleic acid is 22.31%, also slightly higher than that of the control (Table 2).

Table 2 Analysis of the oil and protein content in the seed of the Huasteca 600 soybean and the Huasteca 200 control. INIFAP-The Huastecas Experimental Field. 

Los resultados para aceite y proteína se expresan en (%) en base seca. ¹ = se determinó por el método modificado NMX-F-615-NORMEX-2004; ² = se determinó por el método modificado NMX-F-608-NORMEX-2011; ³ = se determinó por el método AOAC 969.33 edición 17 cromatografía de gases utilizando los estándares de referencia necesarios.

In relation to the percentages of linoleic and linolenic acids, the first one is 54.31%, similar to that of Huasteca 200, while the second one is 8.42%, slightly lower than that of the control (Table 2). These values are within acceptable levels by the oil and protein industry in the country, and give Huasteca 600 a slight advantage in quality with respect to the Huasteca 200.

Regarding the average yield according to the uniform regional and humid tropic trials from 2004 to 2009, Huasteca 600 produced 2 988 kg ha^-1, which is 25% higher than that of the Huasteca 200 variety, which was 2 442 kg ha^-1 (Table 3).

Table 3 Yield of Huasteca 600 compared to Huasteca 200 (control). INIFAP-The Huastecas Experimental Field. 

The variety Huasteca 600 adapts to regions of warm humid and subhumid climate, with rainfall of 800 to 1000 mm annual, average temperature of 25 to 27 °C, photoperiod of 13:32 to 13:11 h light and it prospers satisfactorily in soils Vertisols. These conditions are found in southern Tamaulipas, mainly in the municipalities of Altamira, Gonzalez and The Mante. It can also be planted in the east of San Luis Potosí and north of Veracruz, since the conditions of climate and soil are similar to those of the south of Tamaulipas. According to the agronomic and phenological characteristics and its response to the short photoperiod of the tropics, Huasteca 600 can be adapted to other regions of the southern and southeastern states of Mexico, but it is recommended to validate previously its agronomic and productive behavior.

In the S-S cycles 2011 and 2012, the validation of Huasteca 600 was carried out at a semi-commercial level with soy producers in southern Tamaulipas. The results (Table 4) indicated that the new variety exceeded 1 to 31% the yield of the commercial varieties used as a control, which corroborated its productive potential to favorably impact the soybean crop in the region.

The Huasteca 600 soybean variety is the intellectual property of INIFAP and is registered in the National Catalog of Varieties of Plants (CNVV) of the National Seed Inspection and Certification Service (SNICS) with the final registration SOY-011-181115.

The original and basic seed of Huasteca 600 is produced at the INIFAP-Experimental Field The Huastecas under the supervision of the generators of the variety, in order to preserve its genetic identity. To guarantee the purity and quality of the seed in its different categories: basic, registered and certified, and to protect the interests of soy producers, seed production is carried out with the supervision and regulation of the SNICS. The organizations and companies producing certified soybeans interested in Huasteca 600 should address their request to the Headquarters of the Experimental Field of Huastecas to acquire the basic seed.