Repairing New Alfalfa Seedings

Repairing New Alfalfa Seedings

by Joel Bagg, Forage Development Specialist & District Sales Manager, Quality Seeds Ltd

There are many poor newly established alfalfa stands in Ontario this year due to multiple factors, including a very dry May following by an extremely wet June. Fields where excessive rainfall ponded on heavier textured, poorly drained soils were especially affected. Brillion seeders can work well, but drills generally work better in extreme soil conditions, such as too loose or too wet. 2015 was one of those years. Some of these fields will require reseeding or overseeding to enable an acceptable forage stand and future yields.

Too Dry

Although new forage seedings were seeded early this spring into excellent soil conditions, extended dry weather without rain delayed the germination. Some seeds initially germinated, but then died from lack of moisture. Delayed germination has the same effect as delayed seeding by allowing weeds to establish and compete with forage seedlings. Forage germination and emergence was very uneven, particularly where the seedbed was not firmed with adequate packing and proper seed placement. Where a companion crop was seeded, such as oats or oat-pea mixtures, significant competition for moisture during that early dry period with the new forage seedlings has negatively affect establishment in some cases.

Too Wet

Heavy rains following the first week of June flooded and ponded for extended periods in some of the new stands, killing many potential plants. Where this occurred, many surviving seedlings are showing symptoms of aphanomyces root rot and other fungal seedling diseases. These plants generally appear stunted and have little potential to do well. Also, where cereal companion crops were harvested as forage, considerable compaction, rutting and traffic damage to new seedlings in tire tracks occurred.

Poor Weed Control

To complicate matters, both the dry May and following excessively wet weather challenged our ability to do effective weed control. 2,4-DB sprayed in late-May and early-June during the hot, dry weather suppressed some alfalfa seedling growth, with some symptoms of injury. The risk of injury to alfalfa seedlings is greatly increased when 2,4-DB application is made outside of the 1 to 4 trifoliate stage window, especially when the weather is hot and dry. Many new alfalfa seedings had very uneven emergence, which in addition to rain delays, made it challenging to stage 2,4D-B applications at the desired 1 to 4 trifoliate stage. In extreme cases, some direct seedings did not receive any herbicide at all due to wet soils, resulting in excessive weed competition with new forage seedlings.

Assessing New Seedings

How many alfalfa seedlings are required for a normal or acceptable stand? At a 15 lbs / acre seeding rate, we are seeding about 75 seeds / square foot. Normally, about 60% of these have emerged a month after seeding giving us about 45 seedlings / square foot. The following spring, about one-half of these have survived, about 20 – 25 plants / square foot. The following year we typically are left with about 8 – 9 plants with crowns large enough to have more than 50 stems / square foot. (Mature pure alfalfa stands with less than 40 stems / square foot should be rotated.) In dry years, germination is sometimes delayed, but stands often eventually become better than expected. This is not necessarily the case in wet years when seeds and germinated plants have died.

New seedings normally have 25 seedlings / square foot or more the seeding year. Healthy alfalfa stands have an amazing ability to compensate for low plant density by increasing the number of stems per crown. At this point, under the circumstances we have had so far, in a predominately alfalfa stand, you want to see at least 15 healthy seedlings / square foot to have an acceptable yield potential. These stands may have to be rotated earlier than normal. Assess seedlings for plant health and signs of aphanomyces root rot. Aphanomyces is an alfalfa seedling disease in wet soils that stunts the plants for the life of the stand. Unlike most other seedling diseases, metalaxyl (Apron) seed treatment does not control aphanomyces. (“Aphanomyces Root Rot In Alfalfa http://fyi.uwex.edu/forage/files/2014/01/ARR.pdf)

What Are The Options?

Each field should be walked and assessed on an individual basis. Variability within a field can make the decision making challenging. Patching small areas within fields can be difficult, depending on their size and shape. It is often easier to go over whole fields. The use of a no-till drill to overseed into a thin new seeding is much more effective than broadcasting or using a conventional drill. Driving a no-till drill across the stand will kill a few plants, but with low seedling counts the gain should be much more than the loss. Of course, soils must be fit so there is good seed-to-soil contact with minimal traffic damage. If some healthy alfalfa is present but needs to be thickened, a reduced seeding rate should be used that is inversely proportionate to what is there (down to about 10 lbs / acre). Grass seed can also be included if required.

There is no alfalfa autotoxicity to be concerned about the year of seeding. Alfalfa needs about 6 weeks of growth after germination to survive the winter, and will generally survive if a crown develops before a killing frost. Similar to summer seeding, as a general guideline alfalfa should be over-seeded before the following dates:

  • 2900 CHU areas - August 20th
  • 2500 - 2900 CHU areas - August 10th
  • < 2500 CHU areas- July 30th.

Grass species can usually be successfully seeded up to 3-4 weeks later than these dates.

There are a number of scenarios, depending on the situation:

1.  Cereal Companion Crop Removed

  • If the alfalfa stand is thin but the field is fairly free of weed pressure, the best option is to simply over-seed with a no-till drill.
  • In excessively weedy fields, a glyphosate burndown may be required before reseeding.

2.  Direct Seedings

These fields are typically very weedy.

  • If the weed pressure isn’t too high, clipping to prevent weeds from going to seed before overseeding with a no-till drill may be an option.
  • A glyphosate burndown may be required in excessively weedy fields, and possibly followed by some vertical tillage to break down residue.

Crop Insurance is available through Agricorp that offers establishment protection for new forage seedings. (www.agricorp.com )

For more information, refer to:

“Successful Forage Establishment” http://fieldcropnews.com/?p=9535 and

“Summer Seeding Alfalfa” http://fieldcropnews.com/?p=3316.