David L. Holshouser

Herbicide Effects on Visible Injury, Leaf Area, and Yield of Glyphosate-Resistant Soybean (Glycine max)1

The failure of glyphosate to control all weeds throughout the entire growing season has sometimes prompted growers to use herbicides other than glyphosate on glyphosate-resistant soybean. Field studies were conducted in 1999 and 2000 to investigate potential crop injury by several herbicides in glyphosate-resistant soybean and to determine the relationships between soybean maturity group, planting date, and herbicide treatment on soybean injury, leaf area index (LAI), and yield. Glyphosate-resistant soybean generally recovered from early-season herbicide injury and LAI reductions; however, some treatments reduced yield. Yield reductions were more common in double-crop soybean than in full-season soybean. In full-season soybean, most yield reductions occurred in the early-maturing ‘RT-386’ cultivar. These yield reductions may be attributed to reduced developmental periods associated with early-maturing cultivars and double-crop soybean that often lead to reduced vegetative growth and limited LAI. Reductions in LAI by some herbicide treatments were not necessarily indicative of yield loss. Further yield reductions associated with herbicide applications occurred, although soybean sometimes produced leaf area exceeding the critical LAI level of 3.5 to 4.0, which is the minimum LAI needed for soybean to achieve maximum yield. Therefore, LAI response to herbicide treatments does not always accurately indicate the response of glyphosate-resistant soybean yield to herbicides. Nomenclature: Glyphosate; soybean, Glycine max (L.) Merr. Additional index words: Double-crop soybean, full-season soybean, soybean maturity group. Abbreviations: EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase (EC 2.5.1.19); fb, followed by; LAI, leaf area index; PRE, preemergence; POST, postemergence; WAP, weeks after planting.

Wheel Traffic to Narrow-Row Reproductive-Stage Soybean Lowers Yield

Soybean producers need to know the loss associated with sprayer tire damage to fully determine the cost and benefit of reproductive-stage pest management strategies. Experiments were conducted to determine the yield loss resulting from wheel traffic applied at the R4-stage to full-season and double-crop soybean planted in three row spacings. Wheel traffic reduced yield when planted in 7.5- and 15-inch row spacing. Traffic did not reduce yield of soybean planted in wider rows, but non-irrigated soybean planted in 36-inch row spacing yielded equal to or less than trafficked soybean planted in narrow rows. Drilled soybean tended to compensate for damaged rows better than 15-inch soybean, but only when environmental conditions were conducive for compensation. Water stress during the time of traffic treatment reduced soybeans ability to compensate for damaged rows. When prorated to 45- to 120-ft booms, yield loss ranged from 6.4 to 1.0%. These data provide information that can be used to refine reproductive-stage pest management decisions in soybean.

Uniform Stand and Narrow Rows are Needed for Higher Double-crop Soybean Yield

Double-crop soybean producers do not always realize a yield benefit when they convert from a 15-inch planter to a 7.5-inch grain drill. Poor seed singulation with drills was suspected to limit narrow-row yield response. Experiments were conducted to determine if a precision drill with 7.5-inch row spacing and accurate soybean seed singulation would improve stand uniformity and/or increase yield over a standard grain drill with poor seed singulation or a vacuum-meter planter with 15-inch row spacing and accurate soybean seed singulation in a doublecropped soybean system. The effects of planter speed on stand uniformity and yield were also investigated. Stand uniformity with the precision drill was equal to the vacuum-meter planter and better than the standard drill. Soybean yield with the standard drill was equal to the vacuum-meter planter in three of four years and less than the vacuum-meter planter in one year. Soybean yields were greater when planted with the precision drill than when planted with the vacuum-meter planter in 2 of 3 years, and averaged 10% over three years of study. Planting speeds of 5 or 7 mph did not affect stand uniformity or yield. The Implications of Stand Variability and Yield Narrowing row spacing from 30 or more inches to 20 or fewer inches consistently increases double-crop soybean yield (3,4,6,7). Yield benefits of narrowing row spacing to less than 15 to 20 inches are less clear. Comparing 10versus 20-inch row spacing in Louisiana, Boquet et al. (5) measured a 11 and 5% increase in yield with the 10-inch rows when soybean were planted on June 15 and July 1, respectively. Averaged over 56 North Carolina locations, 10-inch row spacing yielded 5% greater than 20-inch row spacing (7). Soybean planted in 7.5-inch rows yielded greater than in 15-inch rows in three of five Maryland sites (13). In contrast, soybean grown in 7.5-inch rows did not yield greater than 19inch rows when planted in June or July in an Arkansas study (2). Double-crop soybean growers in Virginia have commented that they receive little to no yield benefit from narrowing rows below that which can be obtained with a narrowrow (15 to 24 inches) planter (D. L. Holshouser, personal communication, 2001). Some of the discrepancy between the Arkansas data or Virginia grower experience and other research may be in the type of planter used. When yield increases occurred, either the same planter was used for both row spacings or plots were hand-planted to insure uniformity between plots. In the Arkansas study, soybean in 7.5-inch rows was planted with a grain drill and a rotary-plate planter was used for the 19-inch row spacing. Likewise, growers use grain drills when narrowing row spacing to 7.5 to 10 inches. Vacuum-meter, finger-pickup, and rotary-plate planters will meter, or singulate, seed well, while seed singulation with grain drills that use fluted wheels is usually not as good. Gaps created by poor seed singulation with drills may be responsible for the lack of yield response. This implies that uniform placement within a row is important in realizing higher soybean yield with 7.5- to 10-inch row spacing.