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 March 30, 2022:
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 occurred 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 (‘NoHerbicide’) and with S-metolachlor (‘s-metolachlor’)) and cover crop (with a cereal rye cover crop (‘Rye) and without (‘NoRye’). As a split plot we applied 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 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). We successfully constructed 16 rainfall simulators to impose the precipitation treatments
After S-metolachlor application, emerged seedlings of both weed species were counted and carefully pulled weekly. Weed emergence counts continued until soybean canopy closure.

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Our findings show that precipitation at the levels included in our study (3-inches of added precipitation, totaling 4 inches in the week after S-metolachlor application) had little effect on weed control efficacy of either S-metolachlor or cereal rye surface residues. Both S-metolachlor and cereal rye surface residues effectively controlled smooth pigweed regardless of added precipitation. The cereal rye surface residues did not provide any additional weed control of smooth pigweed when S-metolachlor was used. S-metolachlor effectively controlled giant foxtail, and S-metolachlor efficacy was not dependent on level of precipitation or cereal rye surface residues. Cereal rye surface residues were less effective at controlling giant foxtail compared to smooth pigweed (see attached figures).
Our research findings show that cover crop surface residues are not likely to exacerbate the potential effects of extreme rainfall on the efficacy of residual herbicides, at least not at the levels of precipitation included in our study.
Future work will examine whether extreme rainfall events and cover crop surface residues affect the efficacy of other preemergent residual herbicides, as well as whether these effects vary in other soil types and under greater levels of precipitation.