Grazing, Forage and Cover Crop Effects on Weed Dynamics During Transition to Organic Production in a Semiarid Environment

Project Director: José Franco, Savanna Institute

Project Overview

Transitioning to organic requires three years of chemical-free production before crops can be considered for organic certification. This three-year transitional period poses several significant challenges for producers, including weed control.

Cover cropping and grazing can provide effective, natural methods for managing weeds during this transitional period, although the extent of weed control varies by cover crop species, cover crop mix composition, soil type, and climatic conditions. Currently, there is a limited understanding of how specific cover cropping mixtures and grazing perform under reduced-tillage conditions for weed control in semi-arid areas.

This three-year study, conducted at the USDA Northern Great Plains Research Laboratory (Mandan, ND), evaluated the impacts of six unique cover crop mixes (grazed and ungrazed) on weed community, density, canopy cover, and seedbanks in a reduced-tillage cropping system undergoing the transition to organic.

Farmer Takeaways

Integrating grazing into cover cropping systems during the transition to organic did not influence weed seedbank density, weed population, or weed canopy cover.
Perennial cover crop mixes may provide stronger weed control than annual cover crop mixes. In this study, the perennial treatment outperformed many of the other cover crop treatments, resulting in a significant decrease in weed seedbank density, and weed density for both broadleaf and grass weed species.
Cover cropping reduced the weed seedbank density.
Weed community composition changed over the transitional period, resulting in a shift towards grass and perennial weed species.

Project Objectives and Approach

Experimental Set-Up

A three-year experimental trial was established at the USDA Northern Great Plains Research Laboratory (Mandan, ND).
- Grazing treatments (main-plot) included: (1) grazed and (2) ungrazed
- Cover crop treatments (sub-plot), which included both warm- and cool-season species and were designed to fulfill distinct functional roles, included:
  1. Weed Suppression (WS) – designed to control weeds and reduce the weed seedbank
  2. Soil-Building (SB) – designed to improve soil fertility
  3. Pollinator (PO) – designed to support pollinators with flowering plants
  4. Multipurpose (MP) – designed to balance weed control, soil health, and pollinator support
  5. Three-Year Rotation (3Y) – a full-season cover crop mixture followed by a sequence of annual crops harvested for grain
  6. Perennial (PE) – a mix of grass and alfalfa grown as forage

Table 1. Objective-based cover crop treatments by year in a 3-year organic transition study on a sandy-loam site in the northern Great Plains (adapted from original article)

Cover crop mixes were seeded in late spring/early summer and either (1) grazed in late summer/fall (grazed treatment), or (2) roller-crimped, mowed, or hayed (ungrazed treatment).
- - Grazed plots utilized a mob grazing technique (high-density, short duration). Plots were grazed for 3-7 days by cattle in 2017/2018 and by goats in 2019, with an average grazing pressure across all years ranging between 12,723 and 15,263 kg animal body weight/hectare.
A secondary cover crop was seeded in fall/winter (see Table 1), and a final round of grazing/no-till termination took place the following spring, before the next round of cover crop mixes were seeded.
In 2019, oats were planted as a cash crop.

Data Collection:

Weed community, weed density, and canopy cover was monitored two- and six-weeks after planting.
Soil weed seedbank density was measured at the beginning and end of the organic transition phase (2016 and 2019) using 10 soil cores (to a depth of 10cm) from each plot. Soils were sieved and placed in irrigated containers inside a greenhouse. Emerging seedlings were counted and identified by species.

Key Findings

Weed Seedbank Density

Averaged across all treatments, weed seedbank density was lower at the end of the study than at the beginning, indicating that, regardless of grazing or the composition/intended function of each cover crop mixture, cover cropping was effective at reducing the number of viable weed seeds in the soil.
Neither grazing nor cover crop treatments provided effective control of yellow foxtail (an aggressive, annual grass weed). Seedling counts for this weed increased across all treatments, suggesting that certain weed species may be more difficult to manage during the organic transitional period, and that alternative weed management strategies may be needed.

Weed Density & Community Composition

Weed density (plants/m^2) increased over the three years of evaluation, particularly in the first year.
In 2017 and 2018, weed density was significantly lower for the perennial cover crop treatment than for at least three of the other cover crop treatments.
- - These results suggest that perennial cover crop mixes may provide stronger weed control than annual cover crop mixes.
No significant effects of grazing or grazing x cover crop interactions were observed for total weed density of any weed species, suggesting that grazing and no-till termination practices (i.e. roller-crimping, mowing, haying) provided similar levels of weed control, and that the composition of cover crop mixes in grazed systems did not significantly alter grazing preference for or against weeds.
The composition of the most abundant weed species varied across years. The most abundant species at the beginning of the study were pennycress (broadleaf), purslane (broadleaf), and redroot pigweed (broadleaf). At the end of the study, the most abundant weed species were purslane (broadleaf), redroot pigweed (broadleaf), and yellow foxtail (grass).
- - Over the three-year transitional period, the composition of grass weed species increased, and the composition of broadleaf weed species tended to decrease. Additionally, the weed community composition shifted towards perennial weeds.

Weed Canopy Cover

Cover crop treatments significantly influenced weed canopy cover percentages, although these effects varied by year.
No significant effect of grazing or its interaction with cover crop mixes was observed for weed canopy cover.

Resources

Cardoso, A. S., Hendrickson, J. R., Liebig, M. A., Kronberg, S. L., Clemensen, A. K., Archer, D. W., Nieman, C., Kohmann, M. M., & Franco, J. G. (2026). Grazing, forage and cover crop effects on weed dynamics during transition to organic production in a semiarid environment. European Journal of Agronomy, 178, 128134.

Location

North Dakota

Collaborators

John Hendrickson, USDA-ARS Northern Great Plains
Mark Liebig, USDA-ARS Northern Great Plains
Scott Kronberg, USDA-ARS Northern Great Plains
Andrea Clemensen, USDA-ARS Northern Great Plains
David Archer, USDA-ARS Northern Great Plains
Christine Nieman, USDA-ARS Dale Bumpers Small Farms
Marta Kohmann, University of Wisconsin-Madison
Abmael Cardoso, University of Wisconsin-Madison

Region

Plains

Topic

Soil Health, Weed Management, Transitioning to Organic, Cropping Systems, Livestock Feeding

Year Published

2026