Dulcina: a new variety of sweet sorghum for Tamaulipas and central Nuevo León

Mexico has optimal agroecological conditions for the production of sweet sorghum. The states with the largest potential area are Veracruz, Campeche, Tamaulipas, Tabasco, Guanajuato, Sonora, Sinaloa, Nuevo León, Michoacán, Chihuahua, and Quintana Roo (Ramírez-Jaramillo et al., 2020). The state of Tamaulipas is the largest producer of sorghum in Mexico; in 2021, it grew 1 000 000 ha, 80% under rainfed conditions (SIAP, 2022).

Rising prices in the global market of fossil fuels and growing demand and its instability make renewable fuels economically viable (Khawaja et al., 2014). Another important factor is that oil reserves are finite (Serna et al., 2011). To reduce the world’s dependence on fossil fuels, renewable fuel alternatives, such as bioethanol production, have been developed. Sweet sorghum is a potential source for bioethanol production.

Compared to other crops, it provides high biomass yield and high sugar/ethanol yield, and it has the ability to grow in marginal areas (Tang et al., 2018). The generation of new genetic materials is required for the development of a bioenergy industry that allow the migration towards a more sustainable system in the medium term (Moreno-Hernández et al., 2018). Since 2006, the Faculty of Agronomy of the Autonomous University of Nuevo León has carried out research in genetic improvement of sweet sorghum.

During the autumn-winter (A-W) 2009-2010 cycle at the Marín Campus. Dulcina was obtained by genetic recombination and selection (Table 1). The germplasm that gave rise to this variety was the fertility-maintaining elite-grain line SBB-25, which was developed in Mexico by the National Institute of Forestry, Agricultural, and Livestock Research (INIFAP, for its acronym in Spanish)-Río Bravo Experimental Field, it is the progenitor of the following commercial hybrids: RB-4000, RB-Patrón, and RB-Huasteco (Williams-Alanís et al., 2017). The sweet and forage sorghum variety Rox Orange or Waconia was generated by the Wisconsin Agricultural Experiment Station, USA (Ali et al., 2008).

The F₁ cross of SBB-25 x Rox-Orange was made during the A-W/2009-2010 cycle and the F₂ seed was obtained during the S-S/2010 cycle. During the cycles of A-W/2010-2011, S-S/2011, A-W/2011-2012, and S-S/2012, in the F₂, F₃, F₄ and F₅ generations; the method of selection of furrow per panicle or pedigree was applied by self-fertilizing tall plants with thicker stems without lodging and with tolerance to foliar diseases, with a sugar content in the juice between 15 and 20.5 °Brix.

To complete the formation of the varieties during the A-W/2012-2013 cycle, the seed was increased by self-fertilizing plants with similar phenotypic characteristics. The best 10 varieties (visual selection) were evaluated in experiments, which were planted in four environments in southern Tamaulipas (Cuauhtémoc Station) and four environments in central Nuevo León (Marín), where the commercial varieties Keller (USA) and RB-Cañero (INIFAP) served as controls.

At a later date, a third trial was designed and evaluated in three environments in northern Tamaulipas, which was compared to the INIFAP controls. In the evaluations of the three localities, the experimental variety (SBB-25 x Rox Orange) 17-1-1-1 stood out, which was called Dulcina. It has compact panicles, with small grains, orange in color, 26.2 cm in panicle length, 16 cm in exsertion, with an intermediate cycle (74.4 and 87.1 days to flowering).

In central Nuevo León, it had a later cycle than Keller, the tall plants (227 to 230 cm) were similar to the commercial controls. Williams-Alanís et al. (2017) mentions that the tallest, sweet sorghums are the most productive. The sugar content in the stem juice was 6.6 to 20.4 °Brix, ranging from -3.4 to 13.3% compared to Keller, the best commercial control (Tables 2, 3 and 4). In northern Tamaulipas, Dulcina (20.6) was higher in °Brix than the controls RB-Tehua (17.6) and RB-Cañaveral (18.2). Elangovan et al. (2014) states that the characteristics of juice production and °Brix are very important for the production of bioethanol.

Dulcina presented values of TPWKH between 37 685 and 44 407 kg ha^-1, SWKH between 29 431 and 35 203 kg ha^-1 and JWKH between 15 300 and 22 136 L ha^-1. They were above the control by 8.5 to 26.1%, except for TPWKH in southern Tamaulipas, which was 11.1% lower. Bioethanol production ranged from 2 614 to 3 925 L ha^-1, higher than the best control by 6.1 to 32.3%, equivalent to 191 to 959 L ha^-1 more.

In norther Tamaulipas, Dulcina (3 925 L ha^-1) was higher (p≤ 0.05) than the commercial controls, RB-Cañaveral (2 966 L ha^-1) and RB-Tehua (2 788 L ha^-1), in bioethanol production (Table 4); bioethanol production was estimated according to the formulas described by Rakhmetova et al. (2020). Theoretical production of bioethanol L ha^-1= (°Brix sugar 100) x (0.65 L bioethanol 1 kg sugar) x (0.85) x (stem weight kg ha^-1).

In southern Tamaulipas (Table 5), Dulcina was more tolerant to lodging than Keller and RB-Cañero. Leaf blight [Helminthosporium turcicum (Pass.), Leo and Suggs and leaf anthracnose [Colletotrichum graminicola (Ces.) G.E. Wilson] occurred, and Dulcina was tolerant and was superior (p≤ 0.01) to Keller.

The management of the crop should be carried out in accordance with the regional recommendations for commercial sorghum production of the irrigation technology package. All SNICS indications (sowing, isolation, demixig, etc.) must also be complied with in terms of seed production.

Dulcina is recommended for the lowlands (0-800 masl) of central Nuevo León and Tamaulipas. It was registered on January 13, 2021, with the breeder’s title No. 2606 in the National Catalog of Plant Varieties (CNVV, for its acronym in Spanish). UANL has seed available for the increase of the variety.