Just as thinning hair is a hazard for humans, a thin alfalfa stand is a common hazard of getting old!  

But thin alfalfa stands can occur in young stands as well. So what is the wisdom of overseeding alfalfa into alfalfa to improve the stand life and to correct stand deficiencies?

Figure 1. Thinning alfalfa stands can be a challenge for alfalfa growers – overseed to correct the stand or plow under? This stand was at about 5 plants/ft², on the margins.  Numbers of stems during re-growth may be more important than numbers of plants (see table 1 below).

Can overseeding (planting alfalfa into existing stands) be done?

Maybe, but it's a high risk practice for several reasons.

• First, alfalfa is a slow growing perennial and has a hard time competing with existing plants (1,000 x its size) and weeds for light, nutrients, and water. Additionally, soils in established fields are not conducive to germination.
• Second, if the cause of the stand loss is not corrected, the original cause of plant loss is likely to repeat itself.
• Third, alfalfa produces autotoxins that reduce development of seedling plants, and so suppress growth of young alfalfa seedlings in existing stands.

However, this can work under some circumstances which we will discuss.   Here are a few examples in our experience that may illustrate this.

Example #1. Correcting an Older Deteriorating Stand. Back in 1996-1998, a Yolo County grower's field had a number of ‘bare spots' in the field, and it was less than satisfactory stand after 2 years of production (see Table 1 below for common acceptable stand densities).   We did a trial in this field where the grower seeded alfalfa and we monitored seedling survival. The alfalfa stand was lightly harrowed and alfalfa seeded into the stand. The grower had good emergence, and when the seedlings emerged, we counted the number of seedlings/ft² where there were no plants (open areas), where there was a moderate stand at 6 plants/ft², and where there was a strong stand at 10 plants/ft² for 1.5-years. In all three situations, the alfalfa seedlings did not persist long-term (See Figure 2). There was a little more survival in open areas, but after 2 years, even these seedlings did not persist, and the stand was once again disappointing. The new seedlings were also spindly and non-productive and probably didn't contribute much to yield.  The reasons for this are 1) competition and autoxicity in the high density areas, and 2) open areas with the most stand loss were generally low areas frequently with standing water, which (once again) killed the struggling seedlings.

Figure 2: Overseeding alfalfa into a 2-year old alfalfa stand, and survival of the seedlings where there were no alfalfa plants, where there were 6 alfalfa plants/ft², and were there were 10 plants/ft², Yolo County, 1996-1998.

Example #2. Correcting a New Stand that had been damaged by frost. In 2013, a new fall seeding at Davis, CA exhibited early good growth, but the young seedlings were frozen out during an unusually cold windy November, much to out disappointment; the stands were well below our 6-10 plants/ft² minimum threshold for a good stand. Then, due to the ongoing CA drought, we had a very open January, and conditions were good for field operations, so we 1) controlled weeds with Roundup, 2) Overseeded about 22 lbs/acre without tillage with an aggressive grain drill with proper seeding depth, and 3) sprinkler irrigated (and were assisted with a few rains). These seedlings did quite well, and when warmer weather returned in Feb-March, growth was excellent, and this stand is now in its 4^th year of production.   The fact that this was Roundup-Ready greatly helped the weed aspect of this operation since there were no residual herbicides to suppress the seedlings. The key was that it was a young stand, and we planted early, not in the heat of spring-summer.

Example #3. Mitigation of Flood of Disease Damage. We have seen some success with overseeding alfalfa into alfalfa for first year stands that are lost due to flooding or disease. For example, during some of the excess rainfall year of 1997-98, we saw newly-planted seedling alfalfa stands that were flood damaged in the winter over portions of the field or an entire field. As soon as the water came off, more alfalfa seed was flown onto moist clay-loam soils and the resulting stand was perfect. In another case, we observed a seedling field that died from seedling diseases due to cold, wet weather. This field was lightly harrowed and planted again when conditions were warmer, and a good stand resulted. Since the diseases were transitory and weather-related, this succeeded. The key issue here is that these were young stands and planting was late winter/early spring.

Example #4. Overseeding Under Desert Conditions. Farmers in the Imperial and Palo Verde Valleys and in Arizona routinely overseed existing alfalfa stands after about 2-3 years of production. They get lots of stand losses in these areas because they harvest 8-9 times, have tremendous heat and salt stress and traffic damage, have heavy cracking soils, and grow highly non-dormant varieties that have high yields but low persistence. These are typically very successful at establishment and during the first several cuttings. Keep in mind that deserts of CA and AZ have very favorable conditions for seeding in Oct-January, with over 70 degree temperatures during the day and sunny conditions. However, these stands frequently disappear again, the same result as in Example #1 where seedlings are often (once again) killed by standing water, salinity, traffic, or whatever killed the plants in the first place, but sometimes these renewed stands do fine.

Example #5. Importance of Overseeding Grasses or Clovers. If you're interested in improving your alfalfa stand, try overseeding with grass forages such as orchardgrass, high quality fescue or ryegrass, oat hay or berseem clover. This is a lot less risky than seeding alfalfa into alfalfa, and has been a highly successful practice. These annual or perennial forages are planted in the late fall or wintertime and will grow fast (pushing through the dormant established alfalfa) and fill in weak areas of alfalfa stands. Know your market for mixed hay because it will not likely meet the needs for high quality hay for the dairy industry, though alfalfa grass hay mixtures are favored for horses. Grasses typically will typically overtake the alfalfa over time.

A full discussion of overseeding alfalfa can be seen in the publication: Overseeding and Companion Cropping in Alfalfa http://anrcatalog.ucanr.edu/Details.aspx?itemNo=21594 and in a chapter on 'Old Stand Management' in our Alfalfa Manual on-line chapters.  http://alfalfa.ucdavis.edu/IrrigatedAlfalfa/  

Conclusions

Thus overseeding alfalfa into existing alfalfa can work in some cases, but is fraught with risk.  

Consider:

• Overseeding alfalfa into young stands to correct early problems can succeed. Consider herbicide limitations and optimizing timing, light tillage or no-till, and seeding depth for good establishment in established stands.
• Overseeding alfalfa into older stands is typically less successful since often the early cause of the stand depth is not addressed. In spite of autotoxicity, seedlings can germinate under mild conditions, but frequently this is a less-than-successful practice.
• Consider overseeding grasses or clovers to extend the life of the stand and create a ‘mostly grass' hay product.

We'd like to hear your experiences with overseeding alfalfa – has it been similar to ours?