Safflower is an annual oilseed crop well adapted to the semiarid regions of the US Great Plains and Canada. Safflower possesses a deep taproot system, which can extend to a depth of 2 to 3 m into the soil, and is more tolerant to drought stress compared to other oilseed and small grain crops [1] [2] . Therefore, safflower would be a potential fit as a second crop in the dryland winter wheat-fallow rotations in this region [3] [4] . In 2015, Montana ranked second among the safflower producing states, with 13% of the total US safflower production [5] . In the absence of weed interference, safflower grain yields can exceed 2000 kg∙ha⁻¹ when grown after winter wheat [2] . However, safflower is a poor competitor with weeds, and weed control is one of the major production challenges for successful adoption of this crop [4] [6] . Safflower seedlings remain in the rosette stage for 3 to 4 weeks after emergence and the canopy closure occurs late in the season; hence, early-emerging weeds species can easily outgrow and shade the crop [4] . A season-long weed interference in safflower can reduce grain yields by 93% [4] [6] .

Trifluralin, EPTC, ethalfluralin, and S-metolachlor are the preemergence (PRE)/ preplant incorporated (PPI) herbicides labeled for use in safflower [1] [7] . Trifluralin (applied PPI) was the first herbicide labeled for use in safflower [8] . It is effective on some annual grasses, but does not adequately control Brassica species, kochia, and Russian-thistle. These weed species are particularly troublesome in safflower production areas of this region, including Montana [1] [4] [6] . Furthermore, the need for soil-incorporation of trifluralin, ethalfluranlin, and EPTC for optimum weed control often limits their use in no-tillage semi-arid cropping systems of this region. S-metolachlor applied PRE controls annual grasses and few broadleaf weeds such as pigweeds (Amaranthus species) [9] [10] . During 1980s, sulfonylurea (SU) herbicides such as chlorsulfuron, metsulfuron, and thiameturon (now thifensulfuron) were tested, particularly for postemergence (POST) broadleaf weed control, in safflower [4] [6] . Previous studies found that safflower exhibits variable tolerance to these SU herbicides, and moderate to severe injury may occur if applied to safflower plants less than 15-cm tall [4] [11] . This is a major limitation in using these SU herbicides for early-season POST weed control, especially for weeds such as kochia and Russian-thistle, which can emerge very early in the spring before or with safflower seedlings in the US Great Plains [6] [12] . Thifensulfuron is the only POST herbicide currently labeled for broadleaf weed control in safflower in the US [1] . However, the widespread occurrence of ALS-resistant kochia and Russian-thistle in Montana and several other states in the US Great Plains [13] renders thifensulfuron ineffective for controlling these weed biotypes in safflower.

Therefore, there is a need to investigate alternative herbicide programs for improved broadleaf weed control in safflower, especially kochia and Russian-thistle, and to facilitate registration of new products for use in safflower. The objective of this research was to investigate effective PRE soil-residual herbicide programs for improved crop safety and season-long broadleaf weed control in safflower.

Field experiments were conducted in 2015 and 2016 at the Montana State University Southern Agricultural Research Center (45.92''N˚108.25''W) near Huntley, MT. Soil at the test site was Fort Collins clay loam, fine-loamy, mixed, superactive, mesic Aridic Haplustalfs, with a pH of 7.8% and 2.1% organic matter. Safflower was planted with a no-till drill intoa field that had been fallowed last year. A pre-plant burndown application of glyphosate (Roundup Powermax®, Monsanto Company, Saint Louis, MO 63,167) at 1260 g∙ai∙ha⁻¹ was applied in the spring to kill existing weeds prior to safflower planting. Plots were fertilized with Nitrogen-Phosphorous-Potash as per Montana State University and North Dakota State University recommendations for safflower production [1] . Safflower variety “MonDak” (Safflower Technologies International, PO Box 907, Laurel, MT 59,044) was planted on April 25, 2015 and May 2, 2016 in 30-cm rows at a depth of 2.5 cm with a seeding rate of 22 kg∙ha⁻¹ to obtain 20 plants∙m⁻¹ of row. This variety has an average plant height of 56 cm and possesses a pure white seed (normal hull), with an average seed yield of 2158 kg∙ha⁻¹ and oil content (rich in oleic acid) of 35.4% [1] . Safflower seedlings emerged approximately one week after planting in both years. Experiments were conducted under dryland conditions. Monthly mean temperature and accumulated rainfall at the test site in 2015 and 2016 are shown in Table 1.

Treatments were arranged in a randomized complete block design with four replications, and plots were 3 m wide by 9 m long. Preemergence (PRE) herbicide treatments listed in Table 2 were applied on April 25, 2015 and May 2, 2016. A non treated check was included for comparison. All herbicide treatments were applied with a CO₂-pressurized backpack sprayer calibrated to deliver 187 L∙ha⁻¹ at 186 kPa using XR11002 flat-fan spray nozzles. Each year, the test site had a natural uniform infestation of kochia and Russian thistle.