The problem, caused by production of compounds by existing alfalfa plants that reduce seed germination or forage productivity in new seedlings, may be more severe in sandy soils.

However, irrigation has been used successfully in some areas to leach the autotoxic factor out of the root zone. In research testing, the old alfalfa stand was killed and plots were irrigated heavily before planting the new stand.

“Environmental selection appears to be the source of alfalfa’s autotoxic trait,” Undersander says. “Alfalfa originated around the coast of the Mediterranean which was a hot, dry desert when perennial alfalfas were evolving. Development of the autotoxic trait may have reduced competition for scarce soil moisture from nearby new seedlings.”

Researchers have not conclusively identified the chemical(s) responsible for autotoxicity in alfalfa; however, some of the compound’s characteristics are known to scientists. It is extractable from fresh alfalfa herbage and not a product of microbial action, water-soluble and more concentrated in alfalfa herbage than in roots. It also delays germination, inhibits alfalfa root growth, causing swelling, curling and discoloration of the root, and lack of root hairs and reduces alfalfa root growth more than it reduces seed germination.

“The best evidence suggests that the chemical medicarpin may be a primary cause,” Undersander says. “However, some characteristics observed for toxicity don’t seem to be associated with medicarpin.”

Alfalfa producers are well aware that autotoxicity causes stand failure if it’s planted too soon after an old alfalfa stand. However, if the new alfalfa stand is well established but not yielding or persisting well, producers often overlook presence of autotoxicity.

“Research suggests the negative effects of autotoxicity may linger,” Undersander says, “resulting in unnoticed long-term reductions in plant stands and yield.”

Research has also revealed that soil texture plays a significant role in autotoxicity. Extracts containing the autotoxic chemical that were made from alfalfa topgrowth passed more rapidly through leaching columns of sandy soil than through columns containing silty clay loam. Fractions of the leachate that passed through the columns were collected and added to petri dishes containing alfalfa seeds.

“Approximately 50 percent more water was required to move the extracts through the silty clay loam compared to the sandy soil,” Undersander says. “The autotoxic effect on root growth was stronger and reduced root growth to a greater degree in the sandy soil. However, it persisted longer in the silty clay loam.”

Results of the testing imply that short-term autotoxicity may be more severe in sandy soils. The autotoxic factor is also more easily leached out of the root zone of sandy soils with irrigation. Further research is needed to determine the amount of rainfall or irrigation necessary for leaching the autotoxin out of different soil textures to allow shortened rotation intervals.

“Work in New Hampshire and Michigan showed that alfalfa could be successfully established two to three weeks after the old stand was killed with herbicide,” Undersander says. “The studies documented the presence of autotoxins if reseeding was done less than two weeks after killing the old stand.” However, this study did not consider the possibility of yield reductions.

Additional studies suggest that yield penalties in reseeded alfalfa vary from region to region. Studies in Midwestern states consistently demonstrated the greatest plant stands in the longest rotations. A replicated study at Davis, California, found no differences in stand or yield with two-week and four-week replanting. Immediate reseeding or a one-week rotation did result in reduced stand.

“In western Nebraska, sandy soils are under pivot irrigation and growers want to return a field to alfalfa production quickly to reduce erosion,” Undersander says. “They have effectively used a back-to-back system that destroys the old stand in late spring, usually after the first cutting. Either tillage or herbicide is used to remove the old stand. Foxtail millet is seeded to obtain a quick hay crop and no-till alfalfa is planted back in the millet stubble in mid- to late-August. In those areas, on non-irrigated soils, 30 days out of alfalfa is usually sufficient, but at least half a growing season is preferable.”

Tests involving reseeding alfalfa to thicken it and using high rates of seed-treatment fungicides were recommended in the past. Successive testing suggests frequent irrigation used in the studies may have contributed more to the success of the alfalfa growth than the fungicides.

“In previous research we observed that old alfalfa plants appear to develop a zone of influence around them in which establishment and growth of new seedlings are inhibited,” Undersander says. “If the effect and size of this autotoxic zone could be determined, thickening old stands might be possible when old plant density declines below a critical level.”

Experiments to determine the size of the autotoxic zone around alfalfa plants have shown seedling density and yield of alfalfa was strongly inhibited within an 8-inch radius of the old alfalfa plant. That means the old stand would have to have a plant density of less than 0.8 plants per square foot before new plants could be successfully established between the zones. It’s unlikely that new seedlings emerging within the autotoxic zone would contribute to yield.

“Management strategies that eliminate existing alfalfa plants and residue can improve establishment of new alfalfa,” Undersander says. “Time required for breakdown of alfalfa residue in soil and subsequent removal of the autotoxic chemical from the root zone may vary with soil type and management. It appears that with light soils or irrigation we may be able to reseed in four to five months or less. Reseeding before 12 months on heavier soils will result in reduced alfalfa yield for the life of the stand.”

Undersander notes that alfalfa rotation choices are limited, but there are some options.

• Plant a summer small grain, red spring wheat or spring barley, and have it custom-harvested. This results in some cash income and allows for transport to existing markets. Identify markets before utilizing this option.

• Alternative forage options:

– Oats or triticale with small grains.
Seed at one to two bushels of the small grain and 50 pounds of peas. This is a fast-growing crop that will produce two to four tons of high-quality dry matter. It should be harvested at the boot stage of the oats for dairy and soft dough stage for beef and dairy heifers.

– Italian ryegrass seeded at 20 pounds per acre will produce high-quality forage for the dairy or horse industry.

– Italian ryegrass seeded with crimson or red clover.
Seed this blend at 12 pounds ryegrass and 6 pounds clover. It will produce a high- quality dairy feed with some legume that dairymen are familiar with. It is also excellent for the horse industry.

– Foxtail millet or sudangrass seeded at 20 pounds per acre can produce a high-quality dairy feed, though most dairymen would resist feeding it to the milking cows.

– Teff grass seeded at 5 pounds per acre is a warm-season grass that will produce a fine, leafy hay good for the horse market as well as for beef and dairy heifers.

Nitrogen fertilizer is not needed for any of these options due to the legume credits from the alfalfa.  FG

PHOTO: Staff photo.