Conventional soybean producers in the southern and mid-southern regions have increasingly adopted early planting systems in recent years. Many soybean producers in the southern and mid-southern regions like planting their soybneans early because it allows them to harvest earlier, making the planting of the subsequent winter-wheat rotation easier. However, metribuzin causes increased damage to soybeans and decreases weed control in early-planted soybeans (Goddard 2024). Furthermore, metribuzin applications
led to significant yield losses in early planted soybeans in Mississippi (Poston et al. 2008), while 80% of metribuzin in the soil can be dissipated within 30 days (Fouad et al. 2023).

In traditional soybean production systems where soybeans are planted in mid to late May, preemergent herbicides like metribuzin only need to last a month to prevent the germination of pigweeds such as waterhemp or palmer amaranth that germinate in June. However, a second later dose of metribuzin on top of early planted soybeans to extend the control of pigweeds is not possible currently because post-emergence metribuzin applications heavily damage many soybean cultivars. Fortunately, a greenhouse screening of injury ratings (Figure 1) by Dr. Revolinski in conjunction with Dr. Vieira identified MGs 4, 5, and 6 soybean germplasm highly tolerant to post-emergence metribuzin applications (Table 1) and a major genetic marker explaining up to 54% of the phenotypic variation (Figure 2). While these markers and genetic resources may be useful for developing metribuzin tolerant lines for traditional soybean systems, the MGs used in the previous study are not suitable for early planted soybeans where the producers are planting soybean early so that they can harvest sooner.

To aid in the implementation of weed management strategies in early planted soybeans, QTLs associated with metribuzin tolerance need to be introgressed into late MG-3 to early MG-4 advanced breeding lines. While a large effect QTL was identified in MGs 4-6, early-planted soybeans rely on late MG-3 or early MG-4. Thus, identifying and incorporating genetic resources within these MGs are needed to develop a proper early-planted cropping system.

Early-planted soybeans are becoming increasingly popular among soybean growers in the south and midsouth due to the ease of moving into the next crop of the rotation. However, little consideration is made for the management of weeds in early-planted soybeans. With increasing resistance to glyphosate, glufosinate, and synthetic auxins in broadleaf weeds, growers are increasingly relying on pre-emergent herbicides such as metribuzin to manage weeds in soybean production systems. Having the option to add a second metribuzin application on top of the soybeans would be greatly beneficial for managing troublesome pigweeds (waterhemp and palmer amaranth) in early planted soybeans because the pre-emergent applications of metribuzin are likely to be mostly degraded by June when waterhemp and palmer amaranth germinate (Figure 3). If pigweed populations are glyphosate-, glufosinate-, and synthetic auxin-resistant, then management of these pigweeds relies on metribuzin which will be inactive in early planted soybean systems by June when the weeds germinate. Our work would allow for an extended residual activity of metribuzin in the soil by allowing an over-the-top application of metribuzin in soybeans, thus maintaining the control of pigweeds through the time in which pigweeds germinate.

As such, our proposed work fits perfectly into the Nationally Integrated Weed Management research area because the project will allow for improved weed management options in early planted soybeans. Additionally, the project also fits into the Tools & Technology for Soybean Improvement research area because the project is working to improve soybean tolerance to an herbicide using natural variation. Because of multiple herbicide (glyphosate, glufosinate, group 2s and synthetic auxins) resistance in palmer amaranth and waterhemp, metribuzin is one of the only herbicides that is still effective for managing pigweeds when multiple herbicide resistance is present. Without effective metribuzin regiments in early planted soybeans, early planted soybeans will likely become economically unfeasible when multiple herbicide resistant pigweeds invade those fields.

Goal: Using the natural genetic variation conferring post-emergence metribuzin tolerance, develop metribuzin-tolerant soybeans suitable for early planting, thus allowing for increased metribuzin use in early-planted soybeans.
Objective 1: Screen late MG-3 and early MG-4 soybeans from GRIN for tolerance and map QTL conferring the tolerance.
Objective 2: Perform full doses responses on a subset of lines that are identified as highly tolerant or highly susceptible to post-emergence metribuzin applications. Advanced breeding lines and commonly grown commercial varieties will also be included in the dose responses.

For this project the KPIs will be monitored based on the activities outlined in the objectives. The KPIs include i) the screening of 196 late MG-3 or MG-4 soybean lines for postemergence metribuzin tolerance in two trials separated by time, ii) determination of the current postemergence metribuzin tolerance levels of advanced breeding lines and commercial soybean lines using a dose response study, iii) development of 5 bi-parental populations between highly mtribuzin tolerant lines and high-yielding lines that will be developed into 250 metribuzin tolerant high-yielding lines that will be entered into multi-environment yield trials.

Updated August 1, 2025:
Characterizing and Utilizing Metribuzin Tolerance in Soybeans to Improve Weed Management Strategies in Early Planted Soybeans – Semi-annual report (2025-2026)
PI: Samuel Revolinski (University of Kentucky)
Co-I: Caio Vieira (University of Arkansas)
Project Goal
The identification of genes or markers that can be utilized in development of MG3 commercial soybean lines and to begin the introgression of those genes into agronomically viable MG3 soybean lines for early planted soybeans.
Objectives
1. Screen MG3 soybean accessions from the USDA GRIN for post-emergence metribuzin tolerance to identify genetic markers correlated with metribuzin tolerance in soybeans.
2. Perform dose responses for post-emergence metribuzin tolerance with advanced breeding lines.
3. Integrating metribuzin tolerance genes into superior soybean lines.
Milestones
1. Obtaining MG3 GRIN lines to screen for post-emergence metribuzin tolerance. Completed.
2. Obtaining planting trays, cones and soil to run post-emergence metribuzin trials. Completed.
3. Running post-emergence metribuzin trials (7 trials total to cover all genotypes). In progress.
4. Using previously identified markers to identify crosses with soybean lines with good agronomic properties. Completed.
5. Performing crosses to develop good agronomic lines of soybeans with high post-emergence metribuzin tolerance. In progress.
6. Identifying soybean lines to perform dose response studies for post-emergence metribuzin tolerance in soybeans. Completed.
7. Performing dose-responses to characterize metribuzin tolerance in highly tolerant and highly susceptible soybean lines. To begin shortly.

Screening MG3 soybean GRIN accessions for tolerance to metribuzin.
We have successfully ordered and obtained 1811 MG3 soybean lines from the USDA GRIN system. So far, we have screened 292 varieties of soybeans for tolerance to 400 g ai ha-1 of post-emergence metribuzin. Since we have ordered supplies to screen and created tags for the 1811 soybean tags, we will be able to screen 582 soybeans each month for metribuzin tolerance. By screening such a large panel of genotyped soybean lines, we will have increased power for identifying genetic markers for metribuzin tolerance.

Dose responses for advanced breeding lines.
Once strongly tolerant and susceptible lines are identified in a few of the GRIN accessions, advanced breeding lines and the few GRIN accessions will have dose response curves performed for post-emergence metribuzin tolerance. The advanced breeding lines are going to be obtained from the University of Arkansas soybean breeding program by August 14th so that dose responses can be performed.

Integrating metribuzin tolerance genes into superior lines.
The UA Soybean Breeding program is screening dozens of pre-commercial materials with the molecular marker identified on chromosome 3. These are being used to develop new breeding populations in Summer 2025, which should be advanced to homozygosity until Spring 2027. In addition, the same samples are being shared with Revolinski's group for tolerance confirmation through phenotypic screening.

Preliminary Results
To test out the trial design and identify commercial lines to include in the dose-response analysis, 220 commercial (121) and specialty soybean lines (99) were screened for postemergence metribuzin tolerance. Additionally, this information will allow us to determine what the mean tolerance of commercial lines currently is to inform crosses and later breeding objectives for the development of highly metribuzin tolerant commercial lines. With 400 g ai ha-1 applied we used the same trial set-up that will be used for the larger MG3 GRIN panel we found that the experimental design and dose would be practical for screening all of the MG3 GRIN accessions.

Figure 1. Mean injury rating (1-5) histograms of commercial and non-commercial soybean lines with 400g ai ha -1 metribuzin applied to them post-emergence where an injury rating of 1 is completely tolerant and an injury rating of 5 is completely susceptible.
Population Mean Injury Rating Min Injury Rating Max Injury Rating
Commercial 3.0 1.3 5
Non-Commercial 3.4 1.1 5
Combined 3.2 1.1 5
Table 1. Summary statistics of the trial by commercial or non-commercial soybean lines.

Since the mean injury of commercial lines of soybeans is significantly (p<0.01) lower than the non-commercial lines (Table 1 & Figure 1) it is likely that indirect selection (breeders using metribuzin to control weeds in head rows) has already selected for increased level of metribuzin tolerance in commercial lines. However, a non-commercial line actually had the lowest average injury rating indicating that the non-commercial lines (Like GRIN accessions) likely have genes that could be bred into commercial lines to increase their metribuzin tolerance even more. The repeatability of the trial was 0.69 indicating that much of the post-emergence metribuzin tolerance is controlled genetically. Overall, we are confident that it will be possible to identify many metribuzin tolerance conferring genes in GRIN panel that can be bred into commercially viable lines.

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Having the option to add a second metribuzin application on top of the soybeans would be greatly beneficial for managing troublesome pigweeds (waterhemp and palmer amaranth) in early planted soybeans because the pre-emergent applications of metribuzin are likely to be mostly degraded by June when waterhemp and palmer amaranth germinate (Figure 3). If pigweed populations are glyphosate-,glufosinate-, and synthetic auxin-resistant, then management of these pigweeds relies on metribuzin which will be inactive in early planted soybean systems by June when the weeds germinate.
Our work would allow for an extended residual activity of metribuzin in the soil by allowing an over-the-top application of metribuzin in soybeans, thus maintaining the control of pigweeds through the time in which pigweeds germinate.