Stalk Size Affected by Planting Method in Sweet Sorghum
V. H. Teetor, C. Schmalzel, D. T. Ray
School of Plant Sciences, University of Arizona, Tucson AZ USA
Current efforts in Arizona to grow Sorghum bicolor (L.) Moench for biofuel
production have focused on maximizing biomass and sugar yields, and to allow
harvest over a greater period of time in order to supply optimum feedstock to
ethanol processing facilities. The varieties tested have been either bred for the
southeastern United States (harvested between 140 and 160 days after planting)
and/or production of shorter-season lines bred in Texas (harvested at about 110
days after planting). Our previous work has shown that as intra-row plant density
decreases, stalk diameter increases, thus the stalks are thicker and heavier. This is
both good and bad in that thicker stalks are able to store more juice/sugar and are
less apt to lodge, but they result in a greater biomass to be transported and
processed. The 2014 experiment was designed to try and understand the
relationship between planting density and stalk diameter.
Seeds of eight varieties were sown on June 6, 2014 in mechanically (Normal) and
manually in hills (Hill) arrangements. Normal rows were planted with a tractordriven grain drill at 18 seeds m-1. Hills were planted as a cluster of three to five seed
every 0.5 m. In this split-plot design, planting arrangement was the main plot and
variety the sub-plot, with five replications. Each variety was harvested 30 days after
half of the plants were flowering. Dates ranged from September 24 (110 days after
planting) to November 22 (169 days after planting). A 3.05m section from each of
two harvest rows (4-row plots) were cut manually and weighed. A subsample of 15
plants was weighed with and without leaves and panicles. Stem diameters were
recorded before the stalks were passed through a roller mill and juice collected and
weighed. Juice samples were analyzed by HPLC with a differential refractometer.
Theoretical yields of biomass, sugar, juice, and ethanol were calculated.
For all varieties, stalks in the Hill plantings were significantly thicker than those in
the Normal row plantings. Weights of the 15-stalk subsamples and juice were also
significantly higher for the hill arrangements. There were no significant differences
(p=0.4305) for plot weight between the Hill and Normal planting treatments. So
even though the plants in the Normal planting were smaller, they compensated in
total biomass because there were more plants per area.
There are different mechanized planting machinery that could be used to plant in
Hills, but in Arizona stand establishment will probably continue to be as in our
Normal treatment. Simply, it is easier to harvest, transport and press smaller
diameter plants. However, these results are important for subsistence farmers in
arid environments where they have to be stingy with finite resources.