Preemergent Herbicide Evaluations
on Onions in Starr County, April 2000By O. Montemayor, R. Zamora, L. Brandenberger, and R. Wiedenfeld
herbicide study on direct-seeded onion was conducted at Starrco Farm, located in Starr County, Texas, in the fall of 1999 through the spring of 2000. The objectives of this study were to determine the efficacy and phytotoxic effects of a newer compound that could be applied on a pre-plant-incorporated basis for the control of weeds in direct-seeded commercial onions.Location soil type is a Rio Grande Silt Loam. Onions were established as four lines on raised beds on 40-inch centers, and were direct-seeded on October 14, 1999 to the cultivar ‘Texas 1015 Y’ immediately following the application of herbicide treatments. Drip irrigation was used to supply crop water needs. Crop cultural practices were similar to those carried out on the remainder of the field for commercially produced onions.
Treatments included in the study were: Prefar 4E (Bensulide) at 5 lbs ai/acre pre-plant incorporated (PPI); Prefar 4 E at 5 lbs ai/acre PPI + Dual Magnum (S-Metolachlor) at 0.63 lbs ai/acre over the top of the existing crop after crop establishment (Post-establishment PE); Authority 75 DG (Sulfentrazone) at 0.04 lbs ai/acre PPI + Dual Magnum (S-Metolachlor) at 0.63 lbs ai/acre over the top of the existing crop PE; Authority 75 DG at 0.05 lbs ai/acre PPI; Authority 75 DG at 0.07 lbs ai/acre PPI. All PPI treatments were applied October 14, 1999 prior to planting, and were incorporated with a PTO-powered rotovator 1 to 2 inches deep prior to planting. Post-establishment applications of Dual Magnum were applied on November 24, 1999. The crop was in the two- to three-leaf stage of growth at that time. All treatments were applied with a hand-held spray boom calibrated to deliver 20 gallons of spray materials per acre.
Phytotoxicity and efficacy data were recorded on November 12, 1999, with phytotoxicity recorded as the percent damage to the crop, and efficacy recorded as percent overall control of weedy species. Counts of three different weed species were made on November 19, 1999, and included a number of Palmer Amaranth (Amaranthus palmeri S. Wats.), Horse Purslane (Trianthema portulacastrum L.), and Tumble Mustard (Sisymbrium altissimum L.). Plant population counts of onions in each plot were recorded on December 7, 199, and bulb counts of four different bulb size classes were recorded on April 18, 2000 prior to harvest. The trial was arranged in a randomized block design with four replications.
Phytotoxicity ratings recorded on November 12, 1999, 29 days after treatment (DAT) ranged from a low of 1.3 to a high of 36.3 percent damage (See Table 1 below). Crop damage consisted primarily of reduced stands in the affected plots. The untreated check, Authority + Dual Magnum, and Authority at 0.05 and 0.07 lbs/acre had significantly lower damage than either of the Prefar treatments. Authority + dual Magnum, and Authority at 0.05 and 0.07 lbs/acre had 6.3, 3.8, and 12.5 percent damage, respectively. Counts of onion plants on December 7, 1999, 54 DAT, support the observations that were made in the November phytotoxicity ratings. Plant counts ranged from a high of 118.3 for Authority + Dual Magnum to lows of 69.5 and 75.0 plants per plot, respectively, for Prefar + Dual Magnum, and Prefar alone.
Efficacy ratings were taken on November 12, 1999, 29 DAT, and varied significantly for overall control of weed species (See Table 1 below). Ratings ranged from a low of 15.0 for the untreated check to 95.0 percent control. Prefar + Dual Magnum, Prefar, Authority + Dual Magnum, and Authority at 0.05. and 0.07 lbs/acre rates resulted in 95.0, 93.8, 45.0, 54.4, and 58.8 percent control, respectively. Weed population counts were recorded on November 19, 1999, 36 DAT, for three different weed species (See Table 2 below). Counts of amaranth and purslane for all herbicide treatments were significantly lower when compared to the untreated check, but did not vary significantly for tumble mustard. The Prefar + Dual Magnum, and Prefar alone treatments resulted in the lowest number of Palmer amaranth counted, with 6.5 and 1.5 plants per plot, respectively, compared to 112.3, 66.8, 75.5, and 64.8 plants per plot for the untreated check, Authority + Dual Magnum, and Authority at 0.05 and 0.07 lbs/acre rates, respectively. Horse purslane counts were lowest for Prefar + Dual Magnum, and Prefar alone treatments. The untreated check had 49.8 purslane counted per plot compared to 1.3, 0.8, 21.0, 15.1, and 13.8 for Prefar + Dual Magnum, Prefar, Authority + Dual Magnum, and Authority at 0.05 and 0.07 lbs/acre rates, respectively.
Onion bulb counts were recorded for four different size categories on April 18, 2000, and yield estimations were based on these counts. Overall, few differences between treatments were observed (data not shown). The untreated check, Authority + Dual Magnum, and Authority at 0.07 lbs/acre had the highest estimated yields of 3- to 4-inch onions with 1,066.3, 1,019.3, and 1,027.1 bags/acre, respectively. Prefar + Dual Magnum, Prefar, and Authority at 0.05 lbs/acre had respective yields of 725.3, 627.3, and 989.9 bags/acre. The number of onion plants that had an emerged seed stalk per plot were counted on April 18, 2000, and a percentage of flowering plants was determined from it (See Table 2 below). Percent flowering plants was significantly higher for the untreated check compared to Prefar + Dual Magnum, which were 19.8 and 2.7 percent, respectively.
Overall, the Authority treatments resulted in less crop damage when compared to treatments that utilized Prefar, although weed control for the three weed species was less for Authority treatments. The effects of damage from Prefar were supported by onion plant-count data and, to a lesser extent, a tendency toward reduced yield. Although yields did not vary significantly between treatments, control of weeds within the plots by a commercial crew was carried out in the latter part of November, and is probably the cause for this effect. Based on the study’s results, it appears that Authority has potential for use as a pre-emergent herbicide for commercial onion production, but further study is warranted, particularly to determine if higher rates of Authority would increase weed control while still maintaining an acceptably low level of crop damage.
We would like to acknowledge James Peterson for providing an area for this study, and for his time and efforts to make this project successful.
