Sorghum Weed Management As Affected By Plant Population, Row Spacing and Herbicide Programs.

Weed control remains a major challenge for economically viable sorghum production in North Carolina because sorghum is highly sensitive to weed competition during early growth stages. Moreover, herbicides able to suppress grasses are extremely limited due to sorghum sensitivity. Besides grass weeds, Palmer amaranth (Amaranthus palmeri) is one of the broadleaf weeds that may be the most problematic in sorghum production. As previously demonstrated by different studies, a potential response to improve weed control in sorghum production would be to narrow the spacing between rows and to increase the density at which sorghum is planted. Separate field studies have been conducted in 2012 and 2013 at the Upper Coastal Plain Research Station (Rocky Mount, NC), at the Caswell Research Farm (Kinston, NC), at the Central Crops Research Station (Clayton, NC) and at Clarkton, NC, to determine which row spacings and which plant populations would increase crop competitiveness sufficiently to allow the reduction of POST herbicide applications. The experiment has been conducted in a factorial arrangement of treatments in a randomized complete block design with row spacing (19, 38, and 76 cm), plant population (40,000, 80,000, 120,000, 160,000 plants, and 300,000 plants per acre), and herbicides (non-treated, PRE application of S-metolachlor at  1412 g ai.ha^-1 + atrazine at 1824 g ai.ha^‑1, and PRE application of S-metolachlor at 1076 g ai.ha^-1 + atrazine at 1390 g ai.ha^-1 followed by POST application of 2,4-D amine at 333 g ai.ha^-1 or acetochlor at 840 g ai.ha^-1) as main factors. Sorghum was rated for the percentage of Palmer amaranth, sicklepod (Senna obtusifolia), and large crabgrass (Digitaria sanguinalis) control 4 weeks after PRE,  and 1, 3 and 7 weeks after POST. Weed density and biomass were evaluated before harvest as well as yield at the harvest. In 2012, overall, Palmer amaranth density increased with wider row spacings. For 19 cm rows, its density decreased from 8 plants.m^-2 to 2 plants.m^-2 when sorghum population increased from 80,000 to 300,000 plants per acre. Its biomass has been primarily affected by plant population and reached a maximum dry weight of 1420 g.m^-2 for 76 cm rows and 40,000 plants per acre, and a minimum of 400 g.m^-2 for 19 cm rows and 300,000 plants per acre. Palmer amaranth control averaged 98% for both herbicide strategies at Clarkton and Rocky Mount. At Kinston in 2012, sicklepod control averaged 42% for PRE herbicide alone and 66% for PRE herbicide followed by POST herbicide whereas heterogeneity of large crabgrass infestation prevented to draw conclusions about the efficacy of the two herbicide strategies. Sicklepod control tended to increase with plant population for both the 19 and 38 cm row spacings. For 76 cm row spacing, percentage of control at a given date remained similar between the different plant populations. Sorghum yields were shown to present significant differences at Clarkton and Rocky Mount in 2012. The highest yields were associated with the combination of narrow rows (19 cm) and high plant densities (120,000 to 160,000 plants per acre). For both the 38 and 76 cm row spacings, 80,000 plants per acre produced optimal yields.