Abstract: Glyphosate-resistant (Roundup Ready®) soybean is widely grown to provide broad-spectrum weed control following sequential applications of glyphosate. Studies were conducted during 1998 and 1999 at Keiser, AR, and in 1999 at Pine Tree, AR, to examine the effect of glyphosate-resistant soybean populations on the number of glyphosate applications needed to maintain 90% or greater control of all weed species and on the gross profit margin from weed management. Soybean was seeded in 19-cm rows at 12 rates ranging from 185,000 to 1,482,000 seeds/ha. Each seeding rate received a single application of 0.56 and 1.12 kg ai/ha glyphosate once weeds were 5 to 7 cm tall, followed by repeat applications when control of any species fell below 90%. Soybean yield and gross profit margin at Keiser were similar both years, and a quadratic curve best described the relationship between seed yield and seeding rate, with maximum predicted and observed yield at 988,000 seeds/ha. Soybean seeding rate affected light interception by soybean and the number of glyphosate applications needed to maintain 90% control of all species. By the later stages of vegetative development, soybean at Pine Tree achieved ≥95% light interception, maximizing yield at all seeding rates. Late-season light interception at Keiser ranged from 88 to 99%, causing soybean yield to vary by seeding rate. Three applications of 0.56 kg/ha glyphosate were needed at seeding rates of 185,000 and 247,000 seeds/ha, whereas higher seeding rates sometimes required only a single application for season-long weed control. Application and herbicide costs were offset by reduced seeding costs; thus, predicted gross profit margin from weed management was optimized at the lowest seeding rate of 185,000 seeds/ha.

Nomenclature: Glyphosate; soybean, Glycine max (L.) Merr. ‘Delta King 5961 RR’.

Additional index words: Economic analysis, light interception, photosynthetically active radiation, Digitaria sanguinalis, Echinochloa crus-gall, Euphorbia maculata, Ipomoea lacunosa, Sesbania exaltata, DIGSA, ECHCG, EUPMA, IPOLA, SEBEX.

Abbreviations: DAE, days after emergence; LAI, leaf area index; PAR, photosynthetically active radiation.