Lanceleaf Ragweed

​Lanceleaf ragweed (Ambrosia bidentata) is a summer annual weed from the sunflower family (Asteraceae), native to North America, and commonly found in disturbed areas, pastures, and rangelands. It grows 1 to 4 feet tall, with erect, much-branched stems and rough, triangular leaves that have two sharp teeth at the base, giving it a distinctive appearance. Its inflorescence is larger than that of common ragweed, with downward-curving bracts between flower clusters. Lanceleaf ragweed thrives in heavy, untilled, or poorly maintained soils and is a concern for livestock producers due to its low palatability and potential to cause allergies in humans through pollen production.

Natural Control with Regenerative Grazing
Regenerative grazing, a form of managed rotational grazing, can effectively control lanceleaf ragweed by leveraging livestock to enhance ecosystem health, improve soil, and reduce weed pressure. Below are key strategies for controlling lanceleaf ragweed naturally using regenerative grazing, based on available information and principles applicable to similar ragweed species like western and common ragweed:

1. Maintain Dense Grass Canopy:
   • A dense grass canopy shades out lanceleaf ragweed, limiting its access to sunlight, which is critical for its growth. Regenerative grazing involves moving livestock frequently to prevent overgrazing, allowing grasses to recover and maintain a competitive leaf canopy. This reduces ragweed establishment, as it requires light to germinate and thrive.
   • Implementation: Divide pastures into smaller paddocks and rotate livestock every 1–3 days, depending on grass recovery rates. Ensure rest periods of 60–90 days between grazing events to allow grasses to regrow and develop deeper roots, which outcompete ragweed.

1. Rest and Recovery Periods:

1. • Resting pastures during the latter half or the entire growing season allows grasses to build strong root reserves, which compete with ragweed for resources. Resting has been shown to reduce western ragweed by 50–60%, and similar results are expected for lanceleaf ragweed due to its annual nature and reliance on open soil.
   • Implementation: Plan grazing schedules to allow full plant recovery (e.g., 60–90 days) before reintroducing livestock. Avoid grazing the same pasture the same way each year to disrupt ragweed’s life cycle.

1. Improve Soil Health:

1. • Regenerative grazing enhances soil organic matter and microbial activity through even manure distribution and trampling, which supports healthier grass growth and reduces conditions favorable to ragweed. For example, regenerative grazing has increased soil organic matter from 2–4% to 4–6%, improving nutrient availability and water retention, which benefits grasses over weeds.
   • Implementation: Use livestock to naturally fertilize pastures by rotating them strategically. Avoid overgrazing, which compacts soil and favors ragweed establishment in disturbed areas.

1. Mechanical and Manual Support:

1. • While regenerative grazing is the primary strategy, mowing or hand-pulling lanceleaf ragweed before it flowers can prevent seed production. Mowing when plants are 4–6 inches tall can significantly reduce populations, especially in dry seasons. Hand-pulling is effective for small infestations in gardens or pastures, as lanceleaf ragweed’s shallow taproot makes it easy to remove before seed set.
   • Implementation: Mow in mid-June if ragweed is present, and pull plants in small areas before they produce seeds (potentially 30,000–60,000 per plant). Combine with grazing to ensure grasses remain competitive.

1. Enhance Biodiversity:

1. • Sowing native seeds after ragweed control can increase pasture diversity and resistance to reinvasion. For example, studies on giant ragweed show that sowing native annual, perennial, and woody species after eradication accelerates native plant recovery, which can outcompete ragweed.
   • Implementation: After grazing or mowing, sow native grasses or forbs like yellow-rattle (Rhinanthus minor) to weaken aggressive species and create space for desirable plants. This can be done on bare soil patches to enhance competition.

Additional Considerations

• Timing: Focus control efforts in mid-spring to early summer when lanceleaf ragweed is small (2–4 inches tall) and most vulnerable to grazing or mechanical removal. This prevents seed production, which is critical for long-term control.
• Grazing Management: Avoid continuous grazing, which allows livestock to overgraze preferred grasses, opening space for ragweed. Regenerative grazing’s planned rotations ensure even grazing pressure and minimize soil disturbance.
• Challenges: Lanceleaf ragweed’s ability to thrive in poor soils and its high seed production (up to 60,000 seeds per plant) make it persistent. Consistent management over multiple seasons is necessary to deplete the seed bank.

Limitations
Regenerative grazing is effective but may not completely eliminate lanceleaf ragweed, especially in heavily infested areas or where grazing distribution is uneven. Combining grazing with occasional mowing or hand-pulling may be necessary for severe infestations. By implementing regenerative grazing with strategic planning, you can naturally suppress lanceleaf ragweed while improving pasture health and biodiversity.
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