With the increasing number of weed species developing resistance to commonly used postemergent herbicides, soil applied preemergent herbicides are becoming an essential component of an effective integrated weed management (IWM) plan. Including a preemergent herbicide within an IWM plan can help diversify modes of action and decrease reliance on a POST only herbicide program. Soil applied preemergent herbicides are reliant on rainfall to become activated in the soil. However, too much rain can result in leaching or runoff of soluble soil applied herbicides, thereby decreasing their efficacy.
Throughout much of the US, we are seeing an increase in both the frequency and intensity of extreme precipitation events, and these trends are projected to continue throughout the 21st century. Increasing extreme spring rainfall events are likely to increase populations of problematic weed species, due largely to reduced efficacy of weed management tactics. Developing an integrated weed management approach that can effectively suppress weeds in years with extreme spring precipitation will be essential to maintain crop yields, prevent the evolution of herbicide resistance, as well as decrease herbicide runoff and leaching.
We will examine how extreme rainfall events influence the efficacy of residual herbicides varying in mode of action (group 14 vs group 15) and solubility, and whether a cereal rye cover crop can increase or decrease weed control efficacy when intense rain events occur.
The objectives and hypothesis of our proposed research include:
Research Objective 1. Evaluate how variable precipitation influences the efficacy of residual herbicides varying in solubility.
Hypothesis 1. Residual herbicides that are less soluble will see less of a loss of efficacy in response to extreme rainfall events.
Objective 2. Evaluate whether cereal rye surface residues can enhance weed control efficacy when used in combination with either Group 14 or Group 15 herbicides when extreme rainfall events occur.
Hypothesis 2. Cereal rye management tactics will increase weed control within each herbicide treatment level, regardless of extreme precipitation scenario.

Our proposed work encompasses two research objectives investigating how extreme precipitation affects weeds and weed management tactics used individually or integrated. Weed management tactics include both residual herbicides (varying in mode of action and solubility) and cereal rye surface residues.

We selected two S-metolachlor is a commonly used pre-emergent herbicide used to control weeds in both corn and soybean, as well as a number of other crops. S-metolachlor also has a relatively high-water solubility (530 mg L-1) and low sorption to soil particles, making it an ideal candidate for a precipitation study.
We will establish a field experiment at PSU’s R.E. Larson Research Center in Rock Springs, PA to examine the effect of varying extreme precipitation scenarios on weed emergence under management practices that vary in use of a cover crop and residual herbicide. The experiment will be established as a split-split plot randomized complete block design with four replications. Main plots will include two levels of cover crop: 1) No cover crop, and 2) cereal rye.
Split-plots will include six residual herbicide treatments:
1. None
2. Outlook (high solubility)
3. Dual (medium solubility)
4. Zidua (low solubility)
5. Authority/Spartan (high solubility)
6. Valor/Chateau (low solubility)

Split-split plots will include Simulated precipitation treatments. Our precipitation treatments include:
1. Ambient rainfall
2. Intense precipitation (5 inches) in one day
3. Frequent & Intense precipitation
(2 events of 2.5 inches of rain in one week)

Agronomic Management. Cereal rye (130 kg seed ha-1) was planted in fall 2021, and will be terminated with glyphosate in spring. Prior to cereal rye termination, above-ground biomass will be collected from two 0.50 m2 quadrats per plot, sorted into cereal rye and weeds, then oven-dried, and weighed. The no cover crop (NoCC) plots will be maintained weed-free until soybean planting with broad-spectrum herbicides as needed. One day prior to soybean planting, all plots will receive a burndown herbicide to kill any existing vegetation, and ensure only weeds emerging after planting and residual herbicide application are being evaluated. At this time, the residual herbicide will be applied (S-metolachlor at 1.75 kg ai ha-1) in the designated treatments. Soybean (420,000 seeds ha-1) will be planted in 30 in. rows, and standard fertility (NPK) will be applied based on soil tests. At time of soybean planting, a ZRX roller-crimper system will be used to flatten cereal rye in both cover crop treatments.
Weed species establishment. In order to evaluate the effect that varying precipitation has on the efficacy of weed control tactics alone or in combination, we will include three weed species that will be planted in 0.5m2 area in the center of each plot prior to residual herbicide application: Smooth Pigweed, Giant foxtail, and Marestail.
Prior to residual herbicide application, 500 viable seeds of each weed species will be sown into a quadrat in the center of the subplot. This is to reduce any edge effects resulting from the precipitation additions. After S-metolachlor application, emerged seedlings of all four weed species will be counted and carefully pulled weekly and then immediately sprayed with glyphosate to remove that weekly cohort. This will continue for ten weeks after soybean planting.
To impose the precipitation treatments, we will use a low-intensity sprinkler, which consists of a PVC pipe frame and in the center an inverted cone spray tip, and sprayer nozzle mounted on a PVC sprinkler arm with a water pressure control mechanism (ball and throttling valve, and pressure gauge).

Research Objective 1. Evaluate how variable precipitation influences the efficacy of residual herbicides varying in solubility.
Objective 2. Evaluate whether cereal rye surface residues can enhance weed control efficacy when used in combination with either Group 14 or Group 15 herbicides when extreme rainfall events occur.

Results from our proposed research will: 1) quantify the effect of extreme precipitation events on residual herbicide efficacy; 2) evaluate the potential for cover crops to enhance weed control in response to extreme precipitation; and 3) improve the capacity for IWM to increase agricultural resilience to a changing climate by preventing pest outbreaks associated with extreme weather, thus increasing overall yield stability.
Results from the research objectives will be distilled into a general ‘rule of thumb’ to aid farmers and agronomic professionals in understanding roughly how much precipitation will result in loss of weed control efficacy of residual herbicides varying in solubility, with inferences drawn to other residual herbicides.
Project results will be disseminated to a wide audience through multiple mechanisms, including: 1. Extension events and workshops; 2. factsheet; and 3. educational video.

Updated October 17, 2022:
Our proposed work included two research objectives investigating how extreme precipitation affects weeds and weed management efficacy. Weed management tactics included both 5 residual herbicides (Outlook, Dual, Zidua, Authority/Spartan, Valor/Chateau, plus a no residual herbicide control) and cover crop mulch (with or without a cover cro mulch). All of these are commonly used pre-emergent herbicides used to control weeds in soybean, as well as a number of other crops. The residual herbicides selected vary in solubility.
The experiment occurred at PSU’s R.E. Larson Research Center in Rock Springs, PA and was established as a split-split plot randomized complete block design with four replications. As a split plot we applied three levels of precipitation treatments (Ambient rainfall; Intense precipitation (5 inches) in one day; Frequent & Intense precipitation (2.5 inches of rain per event in two events in a single week).
We planted cereal rye (130 kg seed ha-1) in Fall 2021, which was then terminated in late Spring 2022 with glyphosate. Prior to cereal rye termination, above-ground biomass was collected from two 0.50 m2 quadrats per plot, sorted into cereal rye and weeds, then oven-dried, and weighed. The no cover crop plots were maintained weed-free until soybean planting with broad-spectrum herbicides. Prior to soybean planting, all plots received a burndown herbicide to kill any existing vegetation. Soybean (420,000 seeds ha-1) were planted in 30 in. rows, and standard fertility (NPK) was applied based on soil tests. At time of soybean planting, a ZRX roller-crimper system was used to flatten cereal rye in both cover crop treatments. At this time, the residual herbicides were applied.
To evaluate the effect that varying precipitation has on the efficacy of weed control tactics alone or in combination, we planted three weed species: smooth pigweed, giant foxtail, and marestail). 325 germinable seeds of both foxtail and smooth pigweed, along with 0.5 g of marestail seed plus flowering structures, were sown into a 0.5 m2 quadrat in the center of the subplot.
The precipitation treatments were imposed within one week of soybean & weed seed planting and residual herbicide application. The precipitation was in addition to ambient rainfall, and was applied using rainfall simulators. To do this, we used a low-intensity sprinkler, which consists of a PVC pipe frame and in the center an inverted cone spray tip, and sprayer nozzle mounted on a PVC sprinkler arm with a water pressure control mechanism (ball and throttling valve, and pressure gauge. After herbicide and precipitation application, emerged seedlings of all four weed species were counted and carefully pulled every other week. Weed emergence counts continued until soybean canopy closure.

Status Update:
We successfully constructed 8 new rainfall simulators (in addition to the 16 we built using soybean board funds last year) to imposed the precipitation treatments (see Figure 1). The experiment was implemented in a timely manner according to our protocol described in the soybean board grant. We are still in the process of counting emerged seedlings (this will be our last week), as well as analyzing the data.

Updated December 31, 2023:

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Often, integrated management tactics are highlighted as the best approach to increase the resilience of a farming system to weather variation and extreme scenarios. However, our research shows that the integrated approach (cover crop surface residues combined with residual herbicides) did not improve weed control when extreme precipitation events occurred, and may have lowered weed control efficacy in certain scenarios (e.g. when more soluble residual herbicides were used, such as Outlook). This research highlights the importance of evaluating integrated approaches across a range of environmental conditions and weather scenarios. However, it is important to note that even within our extreme rain treatments, we still saw that residual herbicides remained overwhelmingly effective. Among the Group 15 herbicides, both Zidua and Dual remained effective, providing over 80% control of pigweed, however control of foxtail was more variable especially when combined with a cereal rye cover crop. With both Outlook and Dual, we saw greater loss of efficacy with greater intensity of extreme rainfall (5x1) compared to greater frequency (2.5 x 2), and this was exacerbated when combined with cereal rye surface residues.

Results from the research objectives will be distilled into a general ‘rule of thumb’ to aid farmers and agronomic professionals in understanding roughly how much precipitation will result in loss of weed control efficacy of residual herbicides. These guidelines can signal to farmers when early scouting is essential because a timely post-emergent herbicide application will be crucial to maintain adequate weed control. Adaptive management decisions in response to loss of efficacy of pre-emergent residual herbicides could include earlier post-emergent herbicide applications and addition of soil-applied products to extend residual weed control. The proposed research will increase the ability of soybean growers to adapt to unpredictable weather events through integrated weed management tactics (cover crops and preemergent herbicides).