A. Procedures:
For objective 1, a random field survey was initiated to collect seeds of tall waterhemp, Palmer amaranth, horseweed, and common lambsquarters from NY soybeans fields. Matured seeds of all target weed species were collected from soybean fields.
Discriminate-Dose Experiments: Separate discriminate-dose experiments will be conducted to determine the response of each weed population to glyphosate (Roundup PowerMax), cloransulam (FirstRate), atrazine, fomesafen (Flexstar), mesotrione (Callisto), glufosinate (Liberty), 2,4-D (Weedone LV6), and dicamba (Xtendimax) herbicides. Seeds will be sown on the surface of commercial potting mix in flat trays placed in a greenhouse in Ithaca, NY. After emergence, single flat trays will be maintained with approx. 18 seedlings per population. Experiments will be arranged in a randomized complete block design with 3 replications (three flat trays per herbicide from each population). Greenhouse will be maintained at 26/23 ± 3 C day/night temperatures and 16/8 h day/night photoperiod. Herbicide applications will be made to 8- to 10-cm tall seedlings inside a spray chamber. Appropriate adjuvants will be used as defined by each herbicide label.
A discriminate dose (field-use rate) of Roundup PowerMax @ 32 fl oz/a, FirstAct @ 0.6 oz/a, AAtrex @ 32 oz/a, Flexstar @ 16 oz/a, Callisto @ 3 fl oz/a, Liberty @ 32 fl oz/a, Enlist One @ 32 fl oz/a, and Xtendimax @ 22 fl oz/a will be used. Percent control on a scale of 0 to 100, 0 being no injury/control and 100 being complete control/plant death will be visually assessed at 14, 21, and 28 days after application (DAA). The percentage of survived plants in each population to all tested herbicides will be calculated. Plants that survived the discriminate dose screening will be transplanted to bigger pots (10-L capacity) and allowed to grow in the greenhouse. The progeny seeds (F1) obtained from those survivors will be used for the dose-response studies to further characterize the level
of resistance for each herbicide tested.
Whole-Plant Dose-Response Experiments. Dose-response experiments will be conducted to characterize the level of resistance in confirmed herbicide-resistant (HR) tall waterhemp, Palmer amaranth, horseweed and common lambsquarters populations compared to a known susceptible population. Plants from each selected population will be grown in 10-cm diam plastic pots containing commercial potting mixture as explained previously. Experiments will be arranged in a randomized complete block design with 12 replications (one plant per pot for each herbicide dose per population) and will be repeated. Doses ranging from 1/4- to 8-times the field-use rates of each tested herbicide
will be used when pigweed seedlings reach 8- to 10-cm tall.
Data Collection and Analyses: Data on percent control (chlorosis, stunting, bleaching, necrosis, and desiccation) and mortality on a scale of 0 to 100, 0 being no injury/control and 100 being complete
control/plant death will be recorded at 14, and 21 DAA. The dead plants will be harvested at 14 DAA and live plants will be harvested and dried at 21 DAA. Dry weights will be determined at 28 DAA. Data on percent control and shoot dry weights from each dose-response experiments will be regressed against doses of each tested herbicide using a 3-parameter log-logistic model. Nonlinear regression parameter estimates will be determined using R software. Based on LD50 and GR50 values, the resistance index (referred as R/S ratio) for each confirmed HR tall waterhemp and Palmer amaranth
population will be estimated by dividing the LD50 or GR50 value of resistant population by the LD50 or GR50 value of susceptible population.
For objective 2, On-farm field studies will be conducted to determine the performance of PRE and POST herbicide programs for crop safety and control of glyphosate-resistant weeds (tall waterhemp, horseweed, etc.) in Enlist soybeans. Multi-location (with variation in soil type and environmental conditions) field experiments will be conducted at the Cornell University’s owned Musgrave research farm in Cayuga county and on grower’s fields.
Experiments will be set up in a randomized complete block design, with 3 to 5 replications. An Enlist soybean variety will be planted by following standard agronomic practices (seeding rate, planting time, row spacing, fertilization). PRE and POST herbicides that are currently labelled in soybeans for tall waterhemp or horseweed control will be evaluated (Table 1). All herbicides will be applied at full labeled use rates with recommended adjuvants. The selected POST herbicides will be applied at the recommended growth stage of soybeans. Herbicide treatments will be applied with a CO2-operated backpack sprayer, calibrated to deliver 15 GPA of spray solution.
Data Collection and Analyses. Data on glyphosate-resistant tall waterhemp or horseweed plants per m-2 will be recorded in the center of each plot prior to POST treatments. Data on percent visual control and crop injury will be monitored 20, 30, 60 days after PRE herbicide application, and at soybean harvest. Prior to soybean harvest, the survived tall waterhemp plants per m-2 will
be collected from the center of each plot to determine the shoot dry weight and seed production. Soybeans will be harvested to record yields, and to assess the grain quality. All data will be subjected to ANOVA using PROC MIXED in SAS. Means will be separated using Fisher’s Protected LSD test at P <0.05.
B. Justification:
Palmer amaranth, tall waterhemp, horseweed, and common lambsquarters are most troublesome weed species in agronomic crops. These weed species demonstrate greater competitive ability and cause huge economic yield losses in soybeans. A single female plant of tall waterhemp plant can produce on average 2,000,000 which is almost three times more than average seed produced by a female Palmer amaranth (Keely et al. 1987; Massinga et al. 2001). In the United States, tall waterhemp and Palmer amaranth have evolved resistance to many herbicides: ALS-inhibitors (chlorimuron, imazaquin, imazethapyr, pyrithiobac etc.), EPSPS inhibitors (glyphosate), HPPD inhibitors (mesotrione, tembotrione, topramezone etc.), long-chain fatty acid inhibitors (S-metolachlor etc.),
microtubule inhibitors (pendimethalin, trifluralin, etc.), photosystem-II inhibitors (atrazine, simazine etc.), PPO inhibitors (lactofen, fomesafen etc.), and synthetic auxins (2,4-D, dicamba, etc.) (Heap 2024; Kumar et al. 2019; 2020). In addition, horseweed and common lambsquarters have also evolved resistance to ALS and EPSPS inhibitors (Heap 2024).
Weeds resistant to multiple herbicides such as ALS-inhibitor (many herbicides), EPSPS inhibitor (glyphosate), and PS II-inhibitor (atrazine, simazine etc.) are increasing in the northeastern U.S., including New York (Aulakh et al 2020; Heap 2024). More recently, occurrence of glyphosate-resistant Palmer amaranth and tall waterhemp are increasingly evident in New York cropping systems and other northeastern United states (Aulakh et al. 2020; Kumar 2024). In addition, glyphosate-resistant horseweed is also present across northern counties of the NY state. Declining herbicide options coupled with ever increasing problem of glyphosate-resistant weeds further complicate the management of these weed species in predominant cropping systems. Due to limited POST herbicide options, attaining a season-long control of glyphosate-resistant tall waterhemp, Palmer amaranth or horseweed is challenging. Therefore, alternative integrative herbicide strategies (including PRE and POST programs) are urgently needed to control glyphosate-resistant tall waterhemp, Palmer amaranth and horseweed in NY soybean.
Soybean cultivars with resistance to glufosinate, glyphosate, and 2,4-D (Enlist) as well as glufosinate, glyphosate, and dicamba (Roundup Ready 2 Xtend) have recently been commercialized. The development of these multiple herbicide-resistant traits in soybeans would allow POST applications of glufosinate, dicamba or 2,4-D for managing broadleaf weeds, including glyphosate-resistant biotypes. Increased misuse of glyphosate plus dicamba or 2,4-D–resistant soybean technologies will likely result in evolution of dicamba and 2,4-D–resistant weed biotypes. To mitigate the risk of further evolution of multiple herbicide resistance to dicamba or 2,4-D-resistance in pigweeds, horseweed or common lambsquarters, it is important to understand the baseline distribution and frequency of resistance to commonly used herbicides among NY tall waterhemp, Palmer amaranth, horseweed, and common lambsquarters populations.