Several growers have approached me asking: Can I plant new alfalfa into my existing stand to improve thinning fields? After reviewing research and visiting a few fields that tried it and saw low germination, thin, and patchy stands, here’s my guidance. It depends, but the odds are often not in your favor, especially without addressing the original cause of the stand loss.
Start with: Why did the alfalfa disappear?
Before reseeding, take time to evaluate why the stand is declining in the first place. Was it due to poor drainage, low-lying areas, persistent pests, compacted soils, repeated harvest stress, or drought? In many cases, the same problems that caused the stand to thin out in the first place will continue to affect any new seedlings unless actively addressed. You can tell a field is declining when it has fewer than 55 shoots per square foot, as yields begin to drop by about a tenth of a ton for every shoot below that mark, especially in older stands where depleted soil moisture limits production.
It’s also important to note when plant losses began. Was it early in the stand's life due to poor establishment, or did it decline gradually after multiple years of cutting? Each scenario affects how successful any attempt to thicken the stand will be.
Overseeding into a young alfalfa stand
If your alfalfa stand is less than a year old, there is still a window of opportunity to successfully thicken it by interseeding new alfalfa seed. At this early stage, autotoxicity—the natural process where mature alfalfa produces compounds that hinder new alfalfa seedlings is minimal. Likewise, the level of competition for sunlight, water, and nutrients from established plants is lower.
In these younger stands, filling in bare patches or missed rows can work if done promptly. The key is ensuring the soil is not compacted, and there's sufficient moisture to support germination and early root development. Using a no-till drill to place the seed directly into the soil helps improve success.
However, even in these cases, success depends on careful management. Make sure weeds and insects are under control, and the stand isn't already struggling from other underlying problems.
Overseeding into an established or older stand
Once an alfalfa stand is more than one year old, the risks of overseeding increase significantly. The older the stand, the more barriers new seedlings face. The biggest challenge is autotoxicity. Mature alfalfa plants release water-soluble compounds that inhibit the germination and root growth of new alfalfa seedlings nearby. These compounds can linger in the soil and make it difficult for new plants to survive, even if they germinate.
Competition is another issue because a mature alfalfa plant is more than a thousand times larger than a seedling, and it aggressively outcompetes new seedlings for sunlight, moisture, and nutrients. Unless the stand is extremely thin with less than one plant per square foot, it is unlikely that new seedlings will establish successfully.
Additionally, older fields often have poor seedbed conditions due to soil compaction, pest presence, and weed pressure. Diseases and insect pests, already established in the old stand, may quickly attack the new seedlings.
Research conducted by the University of California Agriculture and Natural Resources and the University of Nebraska–Lincoln Extension supports these observations. In one study, seedlings overseeded into mature alfalfa stands survived the first year, but most died by the second year, and the few remaining contributed little to the overall yield. Another study showed that even when suppression was attempted using herbicides like paraquat, seedlings struggled to establish due to surviving mature plants. Glyphosate plus 2,4-D was more effective at completely killing the stand, allowing for better reseeding success, but only when enough time had passed for autotoxic compounds to break down.
So, what should growers do?
If your alfalfa stand is thinning and older than one year, planting new alfalfa into the same stand is rarely effective unless the old stand is fully terminated first. This means killing it completely with glyphosate and 2,4-D, then waiting at least two to three months (and preferably a full growing season) before reseeding. This break allows for toxic compounds to degrade and gives moisture levels a chance to recover.
Fields with sandy, well-drained soils may require shorter waiting periods, as rainfall and microbial activity can leach and break down toxins more quickly. In contrast, heavier soils and irrigated fields benefit from waiting longer.
Caution: Even when reseeding is carefully timed and executed, yields from alfalfa re-established in the same field are often lower than yields from fields that were rotated with a different crop. For this reason, rotating out of alfalfa to cereals, which also helps take advantage of the nitrogen left behind by alfalfa, is often a more productive and reliable option.
Another effective option to boost a declining alfalfa field is to overseed with alternative forage species that complement alfalfa rather than compete with it. For detailed guidance on suitable companion forages, including tips on species selection, seeding rates, and optimal timing, see “Overseeding Options for Reviving Thin Alfalfa Stands.”
References
- Anderson, B., Holman, T., Baltensperger, D., Volesky, J. (2010). Reseeding or thickening thin alfalfa stands. University of Nebraska–Lincoln Extension.
- Canevari, M., Putnam, D., Lanini, W., Orloff, S., Reed, B., Vargas, R. (2000). Overseeding and companion cropping in alfalfa. University of California Agriculture and Natural Resources Publications.
