Research PaperRedefinition of sweet sorghum harvest time: New approach for sampling and decision-making in field

- The highest sweet sorghum sugar yields occur at three phenological stages.
- Brix can be used as a harvest indicator, in field, for sweet sorghum.
- There are specific internodes which represent Brix of the whole stem.

Sweet sorghum (Sorghum bicolor (L.) Moench) has been studied as an alternative raw material for ethanol production. A bottleneck of the crop is the determination of its harvest time in the field. Currently, stem maturation, measured through the Brix (soluble solids content) of intermediary internodes and the identification of grain maturity are used as harvest indicators. However, it is believed that this methodology is not suitable for this purpose, since these strategies have resulted in unsatisfactory industrial yields. The present work aimed to determine the ideal harvest time for sweet sorghum and identify which internode or segment of internodes better represents Brix of the stem juice. The experiment was conducted under field conditions, using the sweet sorghum cultivar BRS 511. It was set in a randomized block design with five replications and seven treatments. The treatments consisted in the sweet sorghum phenological stages, which were pre-flowering, flowering, milk dough, soft dough, mealy dough, hard dough and senescence. Were evaluated fresh (FBY) and dry biomass (DBY) yields, and moisture content (MC). It was quantified juice extraction (JE), juice yield (JY), tons of Brix per hectare (TBH) and total sugars in the juice (TS). Brix of the juice from each internode and from the whole stem (without segmenting the internodes = 'real' Brix) were evaluated. The highest Brix of stem juice (SB) was registered at the hard dough stage, with 16°Brix. The multiplicative index TBH, which combines SFBY, JE, SB and allows the estimation of sweet sorghum industrial yield, exhibited the highest value between the soft to hard dough stages, with an average of 9.40 t °Brix ha−1. Regarding the internodes, the highest Brix values were observed at the ones located at the stem middle-third. However, Brix measured in this section was not representative of Brix obtained from the whole stem. Regardless of the phenological stages, the internode number 2 was the only one whose Brix had not differed from the whole stem Brix ('real' Brix). Analyzing only the soft, mealy and hard dough stages (harvest prediction), Brix of the internodes 2 and 3 had not significantly differed from the stem real Brix. The sweet sorghum BRS 511 harvest time is comprehended between the soft to hard dough stages, in which were found the highest industrial yields parameters. For cultivar BRS 511, in field-Brix sampling could be used as a harvest indicator and it must be done at the internodes 2 and 3, which better represents the real Brix obtained from the whole stem.