Field experiments were conducted at the Oakes Research Extension Center near Oakes, ND, in 2007 on an Embden loam and in 2008 on a Hecla sandy loam with a previous crop of hard red spring wheat (Triticum aestivum) each year. A split-factor design was used with tillage as the main plot and herbicide treatments as sub-plots, replicated four times. Individual plots were four paired-rows of ‘Teton’ (Petoseed Co., Santa Maria, CA, USA) onion with main rows measuring 40.6 cm from center-to-center and 6.4 cm between each pair.
2.1. Tillage and Herbicide Treatments
Conventional and strip tillage treatments were conducted 15 October 2007 and 13 October 2008. Conventional or non-inversion tillage consisted of two passes of a field cultivator that disturbed the soil to an average depth of 30 cm. In 2008, the conventional tillage treatment required an additional field cultivator pass 21 April to reduce soil clods and create a suitable seedbed. Strips for the strip tillage treatment were made using a strip tiller (Maverick; Yetter Manufacturing Inc., Colchester, IL, USA) equipped with coulters. Strips were 15.2 cm wide and tilled to a depth of 15 to 20 cm.
Herbicide treatments included PRE herbicides DCPA (Dacthal W-75; AMVAC Chemical Corporation, Los Angeles, CA, USA) or pendimethalin (Prowl H2
O; BASF Corporation, Research Triangle Park, NC, USA) and POST herbicides bromoxynil (Buctril; Bayer CropScience, Research Triangle Park, NC, USA) or oxyfluorfen (GoalTender; Dow AgroSciences, Indianapolis, IN, USA). For comparison two checks: the standard grower practice for weed control and a hand-weeded check were included. The standard grower practice for weed control consisted of DCPA applied PRE at 11.2 kg ai ha−1
. Weekly hand-weeding began once the onion row could be seen. The other DCPA treatment examined the lowest herbicide rate on the label. DCPA and pendimethalin were applied one week after planting at 8.4 kg ha−1
and 0.8 kg ai ha−1
, respectively. POST herbicides were applied at micro-rates, which consisted of four weekly applications at 70.1 g ha−1
, which corresponded to 0.25 and 0.5 times the lowest labeled rate for bromoxynil and oxyfluorfen, respectively. Micro-rate applications began when broadleaf weeds reached the cotyledon to one-true-leaf stage, which corresponded to the loop-stage for onion both years. An overview of the herbicide treatments and timings are shown in Table 1
. A CO2
-pressurized backpack sprayer was used to make all micro-rate treatment applications. The backpack sprayer was equipped with TeeJet 8002 flat-fan nozzles (Spraying Systems Co., Chicago, IL, USA) spaced at 45.7 cm along a 1.5-m-wide boom, which sprayed four paired-rows at a time and was calibrated to deliver 187 L ha−1
2.2. General Procedures
Onion were seeded at a population of 617,760 and 704,250 seeds/ha on 20 April 2007 and 23 April 2008, respectively and grown in accordance to production recommendations, including soil fertility, overhead irrigation, as well as insect and disease management practices [12
]. Five in-season nitrogen applications (32% urea-ammonium nitrate) were applied at 2 weeks intervals, beginning at the onion three-leaf growth stage. During the 2007 growing season, the fungal leaf disease purple blotch (causal agent, Alternaria porri
) was detected and controlled with applications of the fungicides azoxystrobin (Quadris; Syngenta Crop Protection LLC, Greensboro, NC, USA) at 146 g ai ha−1
and pyraclostrobin (Headline; BASF Corporation, Research Triangle Park, NC, USA) at 219 g ai ha−1
in rotation. Onion fly maggots (Delia antique
) were also detected in 2007 and were controlled with chlorpyrifos (Lorsban; Dow AgroSciences, Indianapolis, IN, USA) at 1120 g ai ha−1
. In 2008, fungicide applications of azoxystrobin (146 g ha−1
) and pyraclostrobin (219 g ha−1
) were made in rotation, similar to that denoted for 2007.
Grass weeds were controlled as needed with clethodim (Select; Valent USA Corp, Walnut Creek, CA, USA) at 140 g ai ha−1 plus a crop oil concentrate (Herbimax; Loveland Products Inc, Greeley, CO, USA) at 1% v/v. All treatments received a POST application of bromoxynil and oxyfluorfen at 280 g ha−1, and 1120 g ha−1, respectively, at the onion two- to three-leaf growth stage for mid-season weed control. If onion in the POST herbicide treatments reached the two- to three-leaf growth stage before micro-rate applications were completed, the mid-season application was delayed one week after the last micro-rate application. This occurred in 2007. Fertilizer, fungicide, and standard herbicide applications were made with a tractor mounted sprayer equipped with TeeJet 8002 flat-fan nozzles, spaced at 48 cm along a 2.8-m-wide boom; the sprayer was calibrated to deliver 170 L ha−1.
Three broadleaf weed species, common lambsquarters (Chenopodium album), redroot pigweed (Amaranthus retroflexus), and hairy nightshade (Solanum physalifolium) were evaluated. Broadleaf weed control was evaluated by counting plants within a 1 m2 quadrate in the onion row 7 d after each micro-rate herbicide application, but prior to the sequential application.
Onion bulbs were hand harvested, allowed to cure, and then graded according to the United States Department of Agriculture’s (USDA) standards as cull, small, medium, and large [13
]. Bulbs were harvested from each middle two paired-rows on 5 September 2007 and 24 September 2008; harvested area was equal to 2.5 m2
. The USDA standard diameters of grades were: cull, less than 2.5 cm; small, 2.5 cm to 5.7 cm; medium, 5.7 cm to 7.6 cm; and large, greater than 7.6 cm. Split and diseased bulbs were graded as culls, regardless of diameter.