Alfalfa

Alfalfa (Medicago sativa) is a deep‑rooted, perennial legume used widely for forage and soil improvement. Growers choose it as a cover crop to provide multi‑year living cover, fix nitrogen, build soil structure with a penetrating taproot, and to leave a large nitrogen credit to the following crop while supplying high biomass. USDA Climate Hubs, UMN Extension Benefits of Alfalfa as a Cover Crop

• Erosion Control: Perennial alfalfa maintains year‑round cover that is highly protective. In Minnesota, perennial forages (alfalfa, clovers, grasses) were about twice as effective as winter oilseeds at reducing soil erosion and nitrogen loss; conservation‑tillage trials show cover crops can cut soil loss from 8 tons/ac to about 0.9 ton/ac compared with conventionally tilled corn with no cover. In wind‑erosion‑prone systems, alfalfa grasslands reduced wind‑driven soil loss by roughly 50% versus annual cropland (24 vs. 51 t/ha/yr). UMN Extension, 2024, SARE—Managing Cover Crops in Conservation Tillage Systems, recent research: Agronomy (2025)
• Weed Suppression: A vigorous alfalfa canopy with longer cutting intervals keeps the stand competitive and suppresses many annual weeds; cultural tactics like harvest scheduling and irrigation timing further reduce summer annual grasses. UC IPM—Integrated Weed Management in Established Alfalfa, UC IPM—Harvest Scheduling and IPM
• Soil Structure Improvement: Alfalfa develops a strong taproot system that can explore soil well beyond 20 feet, creating macropores and stable aggregates that improve tilth and trafficability. Cover crop roots and residues enhance aggregation and soil organic matter. UMN Extension, USDA Climate Hubs
• Water Management: Deep roots and continuous cover increase infiltration and reduce runoff; in a multi‑year study sampling soils to 24 ft, rotations including alfalfa had 55% less nitrate‑N and 26% less deep soil water compared with continuous corn, indicating less leaching risk and improved water use. USDA Climate Hubs, recent research: Journal of Environmental Quality, 2023, UNL CropWatch summary
• Disease/Pest Break: As a multi‑year non‑host phase, alfalfa interrupts many annual‑crop disease and insect cycles; it also hosts beneficials, and practices like border‑strip harvesting in alfalfa increase parasitoids and predators that suppress pests in neighboring crops. SARE, UC IPM—Harvest Scheduling and IPM
• Nitrogen Management: As a legume, alfalfa fixes large amounts of N. Well‑grown stands commonly fix on the order of 100–150 lb N/ac per year (more in high‑yield systems), with additional N in roots and crowns; first‑year corn after alfalfa typically needs little or no fertilizer N. Book‑value N credits commonly used in the Upper Midwest are 150/100/50 lb N/ac for good/fair/poor alfalfa stands for first‑year corn (and 75/50/0 for second‑year). OSU Extension, UMN Extension, Penn State Extension
• Biomass Production: Alfalfa produces substantial dry matter. In Minnesota establishment trials, 12–18 lb/ac seeding rates produced 3.4–3.6 tons dry matter/ac in the first production year; mature irrigated stands in the West commonly achieve higher totals where seasonal ET is 40–60 inches (about 6 inches of water are required per ton of hay), implying 6–10 tons/ac potential where water is not limiting. UMN Extension, UC ANR, OSU Extension

Planting and Management Advice

• Seeding Rate (drill and broadcast):
   
  • Drill: 12–20 lb/ac of pure live seed (PLS) for a pure alfalfa stand; 12–15 lb/ac is commonly recommended in the Upper Midwest. UMN Extension, Iowa State Extension
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  • Broadcast: 20–25 lb/ac (B), or increase seed 20–30% over the drilled rate to compensate for lower seed‑to‑soil contact; cultipack after broadcasting. NC State Extension—Table 1, SARE—Cover Crop Management
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  • Inoculation: Use the alfalfa‑specific rhizobium (Sinorhizobium/Ensifer meliloti) if seed is not pre‑inoculated. Penn State Extension, Alabama Extension
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• Seeding Depth: 0.25–0.5 inch on medium/fine‑textured soils; up to 0.75 inch on sands. Firm seedbed so a footprint is no deeper than ~0.5 inch; seeing ~10% of seed on the surface after drilling indicates correct depth. UMN Extension, Iowa State Extension
• Soil Type and pH: Best on deep, well‑drained loam to silt/clay loam; avoid sites with ponding or high water tables. Optimum pH is 6.5–7.0 (lime 6–12 months pre‑seeding where needed); alfalfa does not tolerate acid, water‑logged, or saline soils. UMN Extension, UNL NebGuide, OSU Extension
• Planting Time (seasonal windows by region):
   
  • Upper Midwest/Northern Plains: Early spring (as soon as soils allow) or late‑summer seedings; for Minnesota, late‑summer is typically July 25–Aug 15 (allow 6–8 weeks before hard frost). UMN Extension
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  • Northeast: Early April–late May or late July–mid‑August. UMass Extension
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  • Southeast (NC): Mountains (best) Jul 25–Aug 10; Piedmont/Tidewater Sep 15–Oct 15 (possible to Oct 31); Coastal Plain Sep 1–30. Drill 15–20 lb/ac; broadcast 20–25 lb/ac. NC State Extension—Table 1
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  • South (AL): Fall seeding windows ~Aug 15–Oct 1 (North AL) and Oct–Nov 1 (South AL); spring late Feb–Apr 1; allow 6–8 weeks of growth before first hard frost. Alabama Extension
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  • Great Plains (OK): Late August–early September (extend into early October with irrigation to ensure emergence and fall growth). OSU Extension
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  • Arid West (NM): Spring seedings are feasible; often mid‑ to late‑summer (early Aug–late Sept) is preferred; allow 6–8 weeks before a 28°F hard freeze. NMSU Extension
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• Termination (methods and timing):
   
  • Chemical: Most effective programs use 2,4‑D and/or dicamba, often with glyphosate where needed (not effective on glyphosate‑resistant alfalfa). Target ≥4 inches of actively growing regrowth for good translocation; observe plant‑back restrictions (typical ranges: 2,4‑D ~7–14 days to corn; dicamba 0–30 days, product dependent). UW–Madison IPM, UMN Extension, Penn State Extension
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  • Mechanical: Aggressive tillage that completely severs roots (e.g., moldboard plow or overlapping chisel sweeps) can terminate stands; fall termination often enables earlier spring planting. UMN Extension
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• Rotational Considerations:
   
  • Autotoxicity: Do not reseed alfalfa into alfalfa for at least one year after termination, due to autotoxic compounds that inhibit seedling root growth; thickening old stands with alfalfa is not recommended. Rotate to a grass crop (e.g., corn) to capture the N credit. UMN Extension—Reviewed 2023
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  • N Credits: For first‑year corn after alfalfa, adjust N rates using stand‑based credits (commonly 150/100/50 lb N/ac for good/fair/poor stands in many states; second‑year credits 75/50/0). Validate locally. UMN Extension
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• Water Requirements and Drought Tolerance:
   
  • Water use scales with yield; about 6 inches of available water are required per ton of hay. Deep taproots (>20 ft where soils allow) access subsoil moisture, making alfalfa more drought‑tolerant than shallow‑rooted legumes, though high yields still require ample water (CA seasonal ET commonly 35–60+ inches). OSU Extension, UC ANR, UMN Extension
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Practical notes

• Seedbed: Aim for a very firm surface (footprint ≤0.5 inch) and excellent seed‑to‑soil contact; calibrate small‑seed boxes carefully. UMN Extension, Penn State Extension
• Nurse/companion crops: On sloping or erosive sites, a low‑rate small‑grain nurse crop can protect seedlings but must be managed to avoid competition. Iowa State Extension
• Inoculation and fertility: Inoculate if not pre‑inoculated; ensure pH and P/K are non‑limiting before planting. Penn State Extension, UMN Extension

This guidance emphasizes actionable numbers from university extension, USDA/NRCS, SARE, and recent peer‑reviewed studies so you can dial in alfalfa as a high‑value, multi‑year cover where continuous living cover and a strong N credit are priorities.