Four producer plots with an area between 25 and 50 hectares were selected. These plots had a very similar sowing date (October 15 to 30, 2017), the soil type was Vertisol [30] and the sowing method was with a mechanical seeder, it was supplied only with rainwater and residual soil moisture. Fertilization was done very sparingly (approximately 100 kg of ammonium phosphate per hectare) and hybrid seed was used in all of them. The plots were identified as Alfredo V. Bonfil 1 and Alfredo V. Bonfil 2; Melchor Ocampo and Kikab, which was related to the name of the localities where the farmers who owned these plots resided.

To sample the yield, five 8 m² sites were selected, one at each vertex of the plot and one in the center.

The harvest was carried out from February 6 to 20, 2018. For the statistical analysis of the yield, a Completely Random Design was used, with four treatments and four repetitions. The treatments were the localities and the repetitions were the samplings carried out. The comparison of means was carried out with the Duncan DMS method (p = 0.05).

Five hybrid materials are used: DKS-32, DAS-4430, SYN5515, 85P20 and AMBAR. Five fertilization treatments: control (without fertilizer), 50, 100, 150 and 200 kg∙ha⁻¹. Ammonium phosphate was used because this fertilizer is the most widely used source of fertilization for crops in the region [31] [32] [33]. Finally, five planting densities were also considered: 100, 150, 200, 250 and 300 thousand plants per hectare.

It was planted in the locality called Edzná Experimental Site (19˚36'34.97''N; 90˚13'17.32''W and 24 m altitude), owned by the National Institute of Agricultural Forestry Research (INIFAP). The soil was Vertisol type, whose chemical analysis registered a phosphorus content of 1.40 ppm; and a nitrate content of 33.4 ppm that were considered as low and high, respectively [34]. The crop was supplied with the moisture retained in the soil, three rains that added 45 mm and two auxiliary irrigations.

The sowing date was January 16, 2018 and the harvest from May 5 to 8, 2018.

Weed control was carried out in two ways: the first was chemically and for this, 2,4-D amine (0.5 l∙ha⁻¹) was applied, when the plant was 20 days old. The second weed control was done mechanically after 35 days, using an iron implement that removed the soil and was powered by a tractor. The pests were controlled with Espinetoram (0.075 l∙ha⁻¹) in the case of Spodoptera frugiperda, and with Imidacloprid (0.5 l∙ha⁻¹) in the case of the yellow aphid (Melanaphis sacchari). All the fertilizer used in the different treatments was applied at sowing.

As an experimental plot, there was a row of five meters long and the separation between rows was 80 cm. A randomized block design with two repetitions was applied. The data were statistically analyzed as factorial AxBxC, using the Infostat software. Fisher’s LSD method (p = 0.15) was used to compare the means.

The investigation was carried out in the town of Valle de Quetzalcoatl (19˚13'- 54.55"N; 90˚15'08.19"W), which is located in the municipality of Champotón. Crop rotation was studied in order to take advantage of the soil in a more comprehensive and sustainable way, as well as the humidity from the rains, and the residual humidity retained in the soil, in order to obtain two crops of grains at a time. Year in the same place.

The crop rotation had as members the soybean crop sown in summer and sorghum sown in winter; six varieties of soybeans were used; Huasteca 200, Huasteca 300, Huasteca 400, Tamesí, Vernal and Luziania; as well as five grain sorghum hybrids: DKS-32, DAS-4430, SYN5515, 85P20 and AMBAR.

The plot had an approximate area of 2.0 hectares. Soybean varieties and sorghum hybrids were planted in the form of adjacent strips, with a spacing of 80 cm between rows and a length of approximately 200 m.

1) Soil preparation

Regarding the sowing season, soybeans were sown in July and sorghum in December. Soil preparation for planting was carried out as traditionally done by farmers in the region, preparing the soil through three steps of disc harrow at the beginning of soybean planting. Also included, every eight furrows, was a trench ditch 1.10 to 1.20 m wide and 35 cm deep, which functioned as surface drainage to dislodge excess water from the rains, which occurred abundantly from July to September. For the sorghum planting, the soil was not removed, only herbicide was applied to eliminate the weeds.

2) Planting density and fertilization

The sowing density was 250 thousand plants per hectare for soybeans, with approximately 32 kg per hectare of seed; and from 200 to 250 thousand plants per hectare in sorghum, with 8 - 9 kg of seed per hectare. The fertilization of both crops was carried out at the time of sowing. 100 kg of ammonium phosphate (DAP) per hectare were applied to soybeans, while 200 kg of this same fertilizer were applied to sorghum.

3) Weed and pest control

Weed control in soybeans was done with a pre-emergent herbicide, one day after sowing, for which Acetochlor (1.5 to 2.0 L per hectare) was used. Later, Imazetapril (1.0 L per hectare) was used to eliminate 10 cm tall weeds. In sorghum, Atrazine was applied to prevent the outbreak of broadleaf weeds, mixing with Glyphosate (2.0 L + 1.0 kg per hectare), to eliminate some grasses that had germinated and that still had very little development. This last herbicide application was made one day after sowing in both crops. Subsequently, to keep the crop clean until harvest, the weeds that emerged between the rows were fought mechanically using machinery in the case of large plots, and manually, in the case of the experiment. This was done both in soybeans between 35 and 40 days after sowing as recommended by Magallanes et al., [35]. The pests that appeared in soybeans were D. balteata in soybeans [36], which was fought with Dimethoate (0.75 L per hectare) and Stigmene acrea, which was fought with Espinetoram (75 ml per hectare). In sorghum, the pest that required control was the yellow aphid (Melanaphis sachari) and this was done with an application of Imidacloprid (0.5 L per hectare).

The main study variable was grain yield at 14% moisture. Some auxiliary variables were also recorded such as the height of the plant, the days until flowering (counted from sowing). At the time of harvest, the number of pods in soybeans was counted, as well as the total number of plants and effective plants (plants with panicles in the case of sorghum).

In the case of crop rotations, a randomized block design was used to analyze the sorghum and soybean yields obtained from the samplings. The data from days to flowering and days to harvest, of the soybean varieties, as well as the sorghum hybrids, were presented as an average and were used to know in more detail the life cycles of the cultivars involved.

As could be observed, the control without fertilizer and the dose of 50 kg of diammonium phosphate (DAP) per hectare were statistically equal in yield, and lower than the rest of the treatments. Likewise, the best fertilization dose was between 150 and 200 kg∙ha⁻¹ of DAP, with average yields of 4527 - 4562 kg∙ha⁻¹. These results were in agreement with those obtained by Rivero [24] and Stevens and Dunn [25]. The laboratory results, obtained prior to the study, had diagnosed a phosphorus deficiency in the soil, so this response to phosphoric fertilization was expected according to the studies of Sahrawat [27] and Gambaudo [29]. The magnitude of this response was estimated at approximately 1200 kg per hectare, which contributed a net profit of $2310 pesos per hectare at the current price of $3300 per ton of grain in 2020 in this region of the country.

The sowing density that reported the lowest yield was 100,000 plants per hectare, being statistically lower than the others. The best sowing density was 300 thousand plants per hectare, with 5176 kg∙ha⁻¹, these results differ from that reported by Berenguer and Faci [22] and Schatz et al., [19], who found no differences in yield to different plant densities and indicate that sorghum is a crop that compensates for the low population by emitting larger panicles and grains; but they coincide with that reported by Tabo et al., [23], Cicchino [38] and Stevens and Dunn [25].

Regarding the hybrids, the best of them was DKS-32, with 5794 kg∙ha⁻¹, followed by SYN5515, with 3791 kg∙ha⁻¹; the hybrid 85P20 registered the lowest yield (1755 kg∙ha⁻¹), presenting at the same time a greater affectation of the yellow

*DAP = ammonium phosphate; **Quantities with the same letter are not statistically different. Duncan test (DMS p = 0.05).

aphid, a fact that coincided with the Perales report [39]. Likewise, this pest was reported for the first time in 2015 by producers of the entity and agrees with the reports by Rodríguez and Terán [40] and Cortez [41].

The plant’s response to fertilization, in its phenology, was manifested in a shortening of the interval of days elapsed from sowing to flowering when the results of the best fertilization dose were compared with those of the treatment without fertilizer (Figure 2). This fact could be attributed to the effect of phosphoric fertilization according to the reports by Mixquitla and Villegas [42] and Álvaro [43], who point out that phosphate accelerates maturation and seed production, which induces growth, more vigorous and faster maturation of the plant when the phosphorus content in the cell is adequate.

In the treatment without fertilizer DKS-32 and SYN5515 (the early cycle hybrids) had an average of 73 days from sowing to flowering, however, in the best doses this interval was shortened by 14 days, reaching the flowering time to 57 days. Regarding the DAS4430, 85P20 and AMBAR hybrids (the intermediate cycle hybrids), they had an average of 88 days from sowing to flowering in the treatment without fertilizer, but at the best dose this interval was shortened by 23 days, reaching flowering time at 65 days. This reduction in days to flowering was associated with an increase in grain yield according to the average that was recorded in the different doses of fertilizer (Figure 3), a result that coincides with that indicated by Wade and Douglas [21] and Tabo et al., [23].

In both species, important phenotypic characteristics were manifested that must be taken into account in crop rotation. Soybean varieties, such as Huasteca 400 and Luzianía registered 41 days from sowing to flowering, very similar to the Huasteca 300, Vernal and Tamesí varieties that registered 46 days; being the Huasteca 200 variety the latest of all, with 55 days (Table 2). As it could be

observed, all the soybean varieties registered variations in their phenology with respect to that indicated by Magallanes et al., [35], this consisted of a shortening in the interval of the days from sowing to flowering. The soybean varieties were harvested between 117 and 141 days after sowing. In sorghum, the materials with the earliest cycle, DKS-32 and 5515, flowered between 53 and 54 days after sowing, while DAS-4430, 85P20 and Ambar, the materials with a slightly longer cycle, they flowered between 56 and 59 days. The literature indicates that the physiological maturity of the grain in sorghum occurs from 25 to 45 days after flowering [44], in such a way that the maturity at harvest took place approximately 35 days later. Of this stage, that is, 120 to 127 days after sowing (Table 3), indicating that none of the hybrids had a late cycle, which was perhaps influenced by the high temperature of the months of April and May in the entity, which goes 30˚C to 34˚C maximum.

The average yield of the soybean varieties in this study was 1758 kg∙ha⁻¹ (Figure 4), which is considered lower than the averages reported for the state of Tamaulipas [35], and the state of Campeche [31]. The statistical analysis registered a coefficient of variation that was considered high (33.41%), indicating that there were some factors out of control that could have affected the investigation; however, there were highly significant differences between the varieties, which indicated that at least one of them had a different behavior from the others. The low yields indicate that the excess moisture that prevailed in the early stages of crop development had a negative effect on grain yield. It was evident that the plants produced fewer roots and fewer foliage, fewer pods and smaller grains; The formation of nodules in the root was also affected, as indicated by Steduto et al., [45], and nutrient absorption was reduced.

Sorghum in rotation with soybeans had an estimated average yield of 5816 kg∙ha⁻¹, which was comparable to those obtained by producers who sow sorghum in monoculture, on the sowing date considered most appropriate. The analysis of Figure 5 showed that DKS-32 and SYN5515 were the hybrids with the highest yield, with 5985 and 5950 kg∙ha⁻¹, respectively, although they only outnumbered the hybrid DAS4430, which obtained 5642 per 325 kg∙ha⁻¹. This general performance of the yield was higher than that reported by Villalobos [33] and was attributable in part to the density of sowing and fertilization, which were higher than those used commercially by producers, a fact that was confirmed by comparing these results with those obtained in the exploration of the sorghum yield in commercial plots such as, for example, in the town of Alfredo V. Bonfil 1, whose data were described in previous paragraphs.

The accumulated precipitation in the year in the study locality was 1248 mm, as indicated in Figure 6; of this amount, soybeans received 821 mm from sowing to harvest, while sorghum only received 109 mm. This explains why soybeans suffered due to excessive soil moisture and sorghum suffered from drought, although in the end this fact was overcome. Therefore, in the climatic conditions in which the research was carried out, the sorghum-soybean crop rotation could be carried out with more success using the sorghum hybrids DKS-32 and SYN5515, and the Huasteca 300 and Vernal soybean varieties. The results of the climatic behavior in the study area indicate that the soybean sowing date should be modified, starting sowing on June 16 and ending on July 15 so that it is less affected by excessive humidity. From the soil and thus obtain higher yields. On the other hand, in sorghum, the sowing date should be from November 1 to 30 to prevent it from suffering too much water stress and to also try to take better

advantage of the moisture contained in the soil, as well as the low rainfall of the months from January to March so that the plant develops in better conditions. In addition to the absence of rains in this part of the year, there is another negative climatic factor for the development of the plant, which is the high temperature of the months of March, April and May, which occurs in this region of the country [46], which undoubtedly affected the performance of the sorghum hybrids and allowed the selection of those with the best performance [23] [47] [48].