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 opportunities for and 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.

How much precipitation does it take before we can expect to see a loss of preemergent herbicide weed control? Our proposed research seeks to address this question by examining the effect of intense rain events (0, 1, 2, and 3 inches) on the efficacy of Dual II Magnum (S-metolachlor) control of common summer annual weeds within no-till soybean. We will also examine whether a cereal rye cover crop can increase or decrease weed control when intense rain events occur.

The objectives of our proposed research include:
Research Objective 1. Quantify the effects of increasing intensity of extreme spring precipitation on the weed control efficacy of integrated tactics (cover crop + residual herbicide) compared to either tactic alone.
Hypothesis 1.1. Increasing the intensity of extreme precipitation events will decrease weed control provided by S-metolachlor (measured as the percent reduction in weed emergence in herbicide-treated compared to untreated treatments; see Figure 2).
Hypothesis 1.2. Cereal rye management tactics will increase weed control within each herbicide treatment level, regardless of precipitation.
Research Objective 2. Evaluate the effect of increasing intensity of extreme precipitation on the relative persistence of S-metolachlor at key time points after herbicide application.
Hypothesis 2.1. We will find lower emergence of weeds planted (in the greenhouse) in soils collected from treatments with S-metolachlor compared to without, but this effect will be less pronounced at increasing intensity of extreme precipitation events.
Research Objective 3. Evaluate how increasing intensity of extreme spring precipitation events affect seedling emergence timing of problematic agricultural weeds across individual (herbicide or cover crop) and integrated (cover crop + residual herbicide) weed management tactics.
Hypothesis 3.1. Increasing the intensity of precipitation events will affect weed emergence timing, and this effect will be more pronounced with a cover crop mulch.

Our proposed work encompasses three research objectives investigating how extreme precipitation affects weeds and weed management tactics used individually or integrated. Weed management tactics include both a residual herbicide (S-metolachor) and cover crop mulch (terminated early or late in the spring). 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 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. The split plots (2 m X 2 m) will consist of a full-factorial combination of the residual herbicide (two levels: no herbicide (‘NoHerb’) and with S-metolachlor (‘S-metolachlor’)) and six levels of precipitation treatments (see Table 4, and further description below).
Agronomic Management. Cereal rye (130 kg seed ha-1) was planted in fall 2020, 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 treatmentsSoybean (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 (S-metolachlor or cover crops) alone or in combination, we will include two weed species that are generally effectively controlled with S- metolachlor (smooth pigweed and giant foxtail). Prior to S-metolachlor 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.
Simulated precipitation treatments. Our precipitation treatments include:
Median Precipitation Control: We will ensure our median precipitation control closely resembles an ‘average’ year by using a combination of rainout shelters (in wet years) and supplemental irrigation (in dry years) to moderate water close to the 50th percentile of both frequency (# of days per week that receive rain) and intensity (amount of rain in any daily rain event) based on precipitation data at each site for the previous 30 years.
All other precipitation treatments will receive the same amount of simulated precipitation as the Median control, but then will also receive additional precipitation varying in intensity. To examine the effect of increasing intensity of extreme events on weed control efficacy, we will apply either 1, 2, or 3 in. of precipitation in a single day event, three days after S-metolachlor application (days after herbicide application, DAHA) in addition to the rainfall included in the median precipitation control.
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). For both the heavy and extreme precipitation events, the rainfall simulator will be continuously moved from plot to plot so that the designated daily precipitation amount is delivered over a 12-hour period as opposed to all at once.
Soil sampling for residual herbicide longevity (Obj. 2). To determine whether S-metolachlor is still actively suppressing weeds in the soil after the simulation of extreme precipitation treatments, we will perform weed emergence assays with soil collected within all combinations of cover crop, herbicide, and precipitation treatments (directly adjacent to area overseeded with weed seeds in the field, see Figure 3) at two timepoints: 10 and 20 days after S-metolachlor application. Five soil cores per subplot will be collected to a depth of 5 cm, and soil will be brought to the greenhouse for a residual herbicide assay. Soil will be placed in a 7.5 cm pot and planted with 50 seeds of smooth pigweed and 50 seeds of giant foxtail and irrigated as needed. Over the course of two weeks the number of seedlings per pot will be recorded. Utilizing soil collected from the field at multiple collection timepoints and performing the weed emergence assays in the greenhouse instead of the field will enable us to infer that suppression is due to the persistence of S-metolachlor actively controlling weeds in the soil and not from the cover crop or crop canopy suppressing weed emergence.

Research Objective 1. Quantify the effects of increasing intensity of extreme spring precipitation on the weed control efficacy of integrated tactics (cover crop + residual herbicide) compared to either tactic alone.
Research Objective 2. Evaluate the effect of increasing intensity of extreme precipitation on the relative persistence of S-metolachlor at key time points after herbicide application.
Research Objective 3. Evaluate how increasing intensity of extreme spring precipitation events affect seedling emergence timing of problematic agricultural weeds across individual (herbicide or cover crop) and integrated (cover crop + residual herbicide) weed management tactics.

Results from our proposed research will: 1) quantify the effect of increasing frequency and intensity 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 S-metolachlor, 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 August 26, 2021:
Evaluating the Effects of Intense Precipitation on the Efficacy of Weed Management in Soybeans

Project Update, 8/26/21

Our proposed work included three research objectives investigating how extreme precipitation affects weeds and weed management efficacy. Weed management tactics included both a residual herbicide (S-metolachor) and cover crop mulch (with or without a cover cro mulch). 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.
The experiment occured 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. The split plots (2 m X 2 m) will consist of a full-factorial combination of the residual herbicide (two levels: no herbicide (‘NoHerb’) and with S-metolachlor (‘S-metolachlor’)) and four levels of precipitation treatments (0, 1, 2, and 3 inches of simulated rainfall in a single day event).
In Fall 2020, cereal rye (130 kg seed ha-1) was and then terminated in late Spring 2021 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 (NoCC) plots were maintained weed-free until soybean planting with broad-spectrum herbicides as needed. 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 herbicide will be applied (S-metolachlor at 1.75 kg ai ha-1) in the designated treatment
To evaluate the effect that varying precipitation has on the efficacy of weed control tactics (S-metolachlor or cover crops) alone or in combination, we planted two weed species that are generally effectively controlled with S- metolachlor (smooth pigweed and giant foxtail). Prior to S-metolachlor application, 500 viable seeds of each weed species were sown into a 0.5 m2 quadrat in the center of the subplot.
Approximately one week after S-metolochlor application, the precipitation treatments were imposed using 16 rainfall simulators which applied either 1, 2, or 3 in. of additional precipitation in a single day event. This precipitation was in addition to ambient rainfall.
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.
Weed Control Efficacy: After S-metolachlor application, emerged seedlings of all four weed species were counted and carefully pulled weekly. Weed emergence counts continued until soybean canopy closure.

Status Update:
We successfully constructed 16 rainfall simulators to imposed the precipitation treatments (see Figure 1 in attached report).

We are still in the process of analyzing the data. However, so far our data suggests that s-metolachlor successfully suppressed emergence of giant foxtail (Figure 2 in attached report) and smooth pigweed (Figure 3 in attached report) compared to the no herbicide treatments, even under the most extreme precipitation scenario (3 inches). Interestingly, without the cereal rye cover crop, we see greater weed suppression with s-metolachlor at 1 and 2 inches of additional precipitation compared to both the 0 and 3 inches of rain. The loss of efficacy with the 3 inch precipitation event suggests that higher levels of rainfall we likely will see a loss of s-metolachlor efficacy.

Across all levels of simulated precipitation, we found the most consistent suppression of pigweed emergence when both s-metolachlor and a cereal rye cover crop were present.
The cereal rye cover crop suppressed emergence of smooth pigweed when no additional precipitation was added. Interestingly, without the cereal rye cover crop (in both treatments with and without the s-metolachlor), we found that the imposed precipitation treatments decreased the emergence of smooth pigweed seedlings.

View uploaded report

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 S-metolachlor, with inferences drawn to other 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).