Volume 60, Number 1, April 2023

Common ragweed (Ambrosia artimisiifolia) is a troublesome summer annual broadleaf weed in sugarbeet in Minnesota and North Dakota. Growers attending the 2022 sugarbeet growers’ seminars reported common ragweed as their second most troublesome weed following waterhemp (Amaranthus tuberculatus). Past experiments investigating chemical control options reported targeting common ragweed less than 5-cm with repeat glyphosate plus clopyralid applications at 1,010 g ha^-1 plus 53 g ha^-1, respectively, provided 92% control. Repeat applications of clopyralid plus glyphosate were more effective on both small (≤5 cm) and larger (≤10 cm) common ragweed; however, common ragweed 15-cm or greater were too large for POST control in sugarbeet. Recent greenhouse evaluation of common ragweed sourced from fields with weed control failures confirmed that the application of glyphosate alone is no longer an effective mode of action for common ragweed control. In addition, certain common ragweed populations from 2021 also demonstrated alarming tolerance to clopyralid; however, clopyralid eventually provided common ragweed suppression at 158 g ha^-1. A field experiment conducted in 2022 considered common ragweed control from a more concentrated formulation of clopyralid (Stinger HL) at different rates, timings, and tank mixtures. Results suggest common ragweed populations are adapting to sugarbeet herbicides and sizes ranging from 5- to 10-cm are becoming increasingly difficult to control. Clopyralid rates to 105 g ha^-1 plus glyphosate provided unacceptable control when applied to common ragweed greater than 5-cm. Targeting common ragweed less than 5-cm with repeat clopyralid applications at 79 g ha^-1 or greater plus glyphosate at 1,050 g ha^-1 provides the greatest common ragweed control in sugarbeet. Timing herbicide applications to common ragweed size rather than sugarbeet growth stage is crucial.

Rinskor active (flopyrauxifen- benzyl) belongs to the arylpicolinate class of chemistry, a new structural class of the synthetic auxin (Group 4) herbicides.  Rinskor is currently registered as Loyant® herbicide in the US for use in rice where it has broad spectrum activity on grasses, broadleaves, and sedges.  Rinskor has a low use rate (<30 g ai/ha), a favorable environmental and toxicology profile, rapid degradation in the soil and plant tissue, and little persistence in the environment.  Due to its favorable profile, Rinskor received a “reduced-risk” review from EPA and a residue tolerance exemption in October 2019.  Corteva is exploring potential opportunities to expand Rinskor into other crops including sugar beet.  In Europe, Rinskor has been evaluated on sugar beet as part of a POST program where 3-4 POST applications are made in sugar beet.  Results indicate that sugar beet has acceptable tolerance to Rinskor when applied from cotyledon to 6 leaves and that Rinskor provides good control of weeds in the Chenopodiacea, Apiaceae or Umbellifera families.  In the United States, sugar beet growers can use glyphosate, a valuable tool in managing weeds on glyphosate-tolerant sugar beet.  However, glyphosate resistance has continued to spread and additional weed control options in sugar beet are limited.  Previous work has indicated that Rinskor provides excellent control of difficult-to-control weeds such as common lambsquarters (Chenopodium album) and other broadleaf weed species.  In 2021, two trials were conducted in ND/MN utilizing Rinskor in sugar beet to evaluate crop tolerance and control of glyphosate-resistant waterhemp (Amaranthus tuberculatus).  Initial results from this work indicated that 0.5-1.0 g ai/ha of Rinskor applied prior to 10-leaf sugar beet as the potential target rate range and application timing.  In 2022, several trials were conducted throughout the sugar beet growing regions of the US.  Rinskor applied alone caused visible crop response to sugar beet and the addition of glyphosate, ethofumesate, and s-metolachlor increased crop response.  Two applications of a program treatment including Rinskor provided good to excellent control of common lambsquarters, waterhemp, Palmer amaranth (Amaranthus palmeri), kochia (Kochia scoparia), and common ragweed (Ambrosia artemiisifolia).  Future work in the United States will continue to define the use rate and program treatments required to manage weeds with Rinskor in sugar beet.

A sugar beet growth rate study was conducted in Alberta using a high root yield variety and a high sugar content variety. The objective of this study was to measure sugar beet root yield increases and sugar accumulation in both sugar beet varieties. Tests were conducted at 4 different locations between 2018 and 2021. Varieties were harvested approximately bi-weekly starting on July 15 and were measured for six consecutive treatment dates during the growing season ending in early October. Sugar loss to molasses was not measured due to instrumentation limitations for early harvest dates; therefore, sugar was referred to as gross sugar and not extractable sugar. For gross sugar per tonne and percent sugar, site years were analyzed separately due to an interaction that occurred between fixed and random factors. Root yield growth averaged approximately 340kgs/acre per day or 2.4 tonnes/acre per week for the high root yield variety and 310kgs/acre per day or 2.15 tonnes/acre per week for the high sugar content variety over the entire harvest period. Gross sugar per acre (GSA) increased 70kgs/acre per day or 492kgs/acre per week for the high root yield variety and 66kgs/acre per day or 460kgs/acre per week for the high sugar content variety. Gross sugar per tonne (GST) increased 1.0kgs/tonne per day or 6.9kgs/tonne per week for the high root yield variety and 1.2kgs/tonne per day or 8.1kgs/tonne per week for the high sugar content variety. Gross sugar per tonne was significantly (P<0.05) higher for the high root yield variety on the first harvest date, which was unexpected. On the second harvest date, the high sugar content variety had numerically higher GST. For the remaining 4 harvest dates, the high sugar content variety produced significantly (P<0.05) higher GST. This result provided strong evidence that the higher quality variety started slower but eventually surpassed the high root yield variety in quality. Maximum root yield and gross sugar per acre production occurred for both varieties between August 1 and August 14. Gross sugar per tonne and percent sugar accumulated the fastest between July 15 and July 31 for both varieties and slowed considerably between the second-last and last harvest dates. It was concluded that varieties vary in their sugar accumulation profiles over time and across multiple years.

Glyphosate-resistant (GR) waterhemp (Amaranthus tuberculatus) and common ragweed (Ambrosia artemisiifolia) are weed control challenges in sugarbeet in North Dakota and Minnesota. Sugarbeet growers control GR weeds using soil residual chloroacetamide herbicides (group 15) for waterhemp control and clopyralid (group 4) for common ragweed control. However, growers need additional effective herbicides to improve the consistency of broadleaf weed control in sugarbeet. Florpyrauxifen-benzyl (Rinskor) is a mimic auxin herbicide (group 4) which controls susceptible plants by disrupting plant growth processes. Rinskor is labeled as Loyant for postemergence grass, sedge, and broadleaf weed control in rice at 30 g ha^-1 in Arkansas, California, Florida, Louisiana, Mississippi, Missouri, South Carolina, Tennessee, and Texas. Experiments conducted in Europe suggest sugarbeet may tolerate Rinskor to 2 g ha^-1. Experiments to examine crop safety and selective weed control from Rinskor were conducted at six locations in Minnesota and eastern North Dakota in 2021 and 2022. In 2021, Rinskor at 0.5, 1, 2, and 4 g ha^-1 was applied at the 2, 6, or 10 sugarbeet leaf stage and in 2022, repeat applications of Rinskor at 0.5, 1 and 2 g ha^-1 alone or in mixtures with glyphosate, ethofumesate and S-metolachlor were applied at the 2-lf and 6-lf stage. Waterhemp control in 2021 and 2022 and common ragweed control in 2022 were compared to standard sugarbeet weed control programs. In 2021, sugarbeet tolerated Rinskor at 0.5 and 1 g ha^-1 at the 2- and 6-lf stage, but sugarbeet did not tolerate the 10-lf stage of application. In 2022, sugarbeet tolerated repeat Rinskor applications at 0.5 and 1 g ha^-1 at the 2- and 6-lf stage. Sugarbeet root yield from Rinskor at 0.5 and 1 g ha^-1 at the 2-lf and 6-lf stage was the same as the untreated control in 2021, but Rinskor at 1 g ha^-1 repeated at the 2- and 6-lf stage reduced root yield in 2022. Rinskor did not affect % sucrose content in either 2021 or 2022. Rinskor alone at 0.5 or 1 g ha^-1 only provides suppression of waterhemp or common ragweed, and will therefore need to be mixed with other sugarbeet herbicides including the chloroacetamides to achieve 95% or greater waterhemp control and clopyralid to achieve 95% or greater common ragweed control.