Normie W. Buehring

Sicklepod (Senna obtusifolia) Response to Shading, Soybean (Glycine max) Row Spacing, and Population in Three Management Systems1

Abstract: Studies were conducted in 1997 and 1998 at the Northeast Mississippi Research and Extension Center to investigate the effects of row spacing (76, 38, and 19 cm), soybean population, and three weed management systems on sicklepod growth and seed production. The cultivars ‘Hartz 5088RR’ (glyphosate-tolerant) and ‘Hutcheson’ (a conventional cultivar) were used in two separate studies. The average soybean populations over cultivars and year were 245,000 (low), 481,000 (medium), and 676,000 (high) plants/ha. The three weed management systems were: no (untreated), one, and two herbicide applications. In the glyphosate-tolerant system, one or two postemergence (POST) applications of glyphosate were used, whereas in the conventional system, flumetsulam plus metolachlor preemergence was used alone (single) or followed by chlorimuron POST (sequential). Reducing soybean row spacing from 76 cm, coupled with increased soybean population, reduced sicklepod population up to 80%. Except for Hutcheson in 1998, reducing row spacing and increasing soybean population also reduced sicklepod seed production in both the untreated and the single applications. A single herbicide application reduced sicklepod population up to 68% from untreated plots. However, except for Hartz 5088RR in 1998, the sequential application did not further reduce sicklepod population. In a shading study, partial shading increased sicklepod height but reduced dry weight. However, as shading level increased from 65 to 80 and 95%, height was also reduced. Nomenclature: Chlorimuron; flumetsulam; glyphosate; metolachlor; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby #3 CASOB; soybean, Glycine max (L.) Merr. Additional index words: Glyphosate-tolerant soybean, sequential herbicide applications, seed production. Abbreviations: MG, maturity group; POST, postemergence; PRE, preemergence; WAP, weeks after planting.

Sicklepod (Senna obtusifolia) Control and Soybean (Glycine max) Response to Soybean Row Spacing and Population in Three Weed Management Systems1

Studies were conducted with one glyphosate-resistant and three conventional soybean cultivars to determine the effects of reducing row spacing from 76 to 38 and 19 cm, in combination with increased soybean population and herbicide applications (single and sequential), on sicklepod control and soybean yield. ‘Hutcheson’ was the only cultivar, and only under adverse conditions (1998), for which the 19-cm rows provided greater sicklepod control than the 38-cm rows. It was only under optimum conditions (1997), and only with conventional cultivars and sequential herbicide applications, that the 19-cm–row yield of 3,350 kg/ha was 21% greater than the 38-cm–row yield in medium population (455,375 plants/ha), and 21 and 64% greater than the 19- and 76-cm–row yield in low populations (241,000 plants/ha), respectively. In 1998, the same treatment showed similar late-season sicklepod control (> 80%) and a yield of 1,890 kg/ha, with no difference between the 19- and the 38-cm rows; but the yields were 15 and 24% greater than the yield of the narrow (19 and 38 cm) and that of the 76-cm row in low populations, respectively. Low populations with sequential applications in narrow rows and in the 76-cm row showed similar late-season sicklepod control (59 to 70%) and yield in 1998; but in 1997, the narrow rows showed 18 and 32% greater late-season control and yield than the 76-cm row, respectively. In 1997 and 1998, for ‘Hartz 5088RR’ (glyphosate resistant) with medium population and narrow rows, the single and sequential applications provided similar late-season sicklepod control (> 80%) and yield, and at least 10 and 24% greater yield than the narrow and the 76-cm rows in low populations, respectively. Nomenclature: Glyphosate; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby #3 CASOB; soybean, Glycine max (L.) Merr. Additional index words: Chlorimuron, flumetsulam, glyphosate-resistant soybean, metolachlor, sequential herbicide applications. Abbreviations: MG, maturity group; POST, postemergence; PRE, preemergence; WAP, weeks after planting.

EFFECT OF PLANTING DATE, NITROGEN RATE, AND HYBRID ON SUNFLOWER

Sunflower is a potential crop for the southeastern United States for production of cooking oil or biodiesel. In 2007, we evaluated the effect of planting date (PD, 20 April, 20 May, and 20 June), nitrogen (N) application rate (0, 67, 134, and 202 kg ha−1), and hybrid (‘DKF3875’, ‘DKF2990’, ‘DKF3510’, and ‘DKF3901’) on sunflower productivity and oil profile in four Mississippi locations, Newton, Starkville, and two sites in Verona. There was a trend of increased oleic acid concentration with earlier planting dates, especially in hybrids with lower oleic acid concentration. Earlier planting dates of ‘DKF3901’ and ‘DKF2990’ (the hybrid with the lowest oleic acid) actually had 200 to 300 g kg−1 higher concentration of oleic acid when grown in Mississippi vs. the original seed of the same hybrids used for planting and produced at a more northern latitude. This and a recent study in Mississippi suggest that modern hybrids could provide ample yields even when the N fertilization is relatively low.

Diversifying Soybean Production Risk Using Maturity Group and Planting Date Choices

Due to the long growing season for soybean production, producers in the Mid-southern US can plant from late March to June. They also have a range of maturity group (MG) choices, affecting the length of the growing season, that are physiologically and economically viable. A producer’s decision of what to plant and when constitutes two potential decision variables that can be freely manipulated to not only maximize profit, but also reduce economic risk. Early maturing MG III and IV soybean cultivars planted early or mid-season typically are highest yielding and thereby the preferred choice of producers. However, planting part of a producer’s acreage at later dates and using later maturing MG VI soybeans may offer producers similar returns (as observed with early planting using early maturing cultivars) at a meaningfully reduced level of risk.