April 16, 2021

Fluffy seedbed is an enemy for planting alfalfa

WOODLAND, Calif. — The best fields to plant alfalfa are well-drained with deep soils.

“Deep soils are where you get the best advantage from its deep root system,” said Kim Cassida, Michigan State University Extension forage and cover crop specialist.

When selecting a field to plant alfalfa, it is important there is no herbicide residual left in the soil from the previous crop.

“Some of the newer herbicides used for soybeans might have a soil residual of 18 months,” said Cassida during an Alfalfa Livestream webinar hosted by Hoard’s Dairyman.

Alfalfa toxicity is an important consideration.

“Alfalfa produces toxins that are deadly to its own seedlings, but won’t harm other plants,” Cassida said.

The toxins damage the root of the seedlings.

“You might get a failed establishment or you might get autosuppression where you have plants, but their roots are damaged,” Cassida said. “It may look like you have a stand doing OK, but the lifetime stand productivity will be reduced because the plant can never repair the root damage and that’s why you can’t thicken an alfalfa stand during its lifetime.”

It is also why there is a waiting period before trying to establish a new stand after termination.

“That waiting period could be from two weeks to two years because the biggest problem with predicting autotoxicity is we don’t know what the compounds are that are doing this so we can’t measure it,” Cassida said.

Autotoxicity disappears over time, and it will disappear more rapidly with a lot of water.

“It also disappears faster in sandy soils, and it breaks down faster under tillage than in a no-till system,” Cassida said.

The best time to plant alfalfa is in the spring or late summer.

“In the spring typically you have more water available and you might be able to get one to two cuts in the seeding year,” Cassida said. “But the disadvantages are a lot more weeds, insects and disease that you have to contend with.”

In the spring, alfalfa should be planted so the second trifoliate leaf stage is after the last killing frost date.

“Before the second trifoliate stage, the alfalfa seedling is pretty tough when it comes to cold weather, but after that it is quite susceptible to cold injury if the temperature gets below 26 degrees,” Cassida said.

There will be a lot less weed and disease pressure if the alfalfa is planted in the late summer. However, Cassida said, there is an increase risk of drought, there will be no harvest during the seeding year and there is a risk of winter coming early.

“Alfalfa has an interesting aspect of seedling growth called contractile growth where in the early stages of development the crown is above the ground,” Cassida said.

“While it’s above ground the crown is susceptible to freezing damage and it takes six to eight weeks before the contraction pulls the crown underground to protect it from frost.”

A study at Purdue University looked at the difference between planting coated or uncoated alfalfa seed, with both types of seeds planted at 21.8 pounds per acre.

“The surviving seedlings were at 29.5 per square foot for the coated versus 30 seedlings per square foot for the uncoated,” said Don Miller, director of product development for Alforex Seeds. “At the end of the summer it was essentially the same stand.”

With a coating, Miller said, the seed weight is a little heavier, which is an advantage with broadcast seeding.

“The coating can pull moisture to the seed and with the coating we can get more rhizobium numbers on the seed,” Miller said.

“You will need to calibrate your planter if you’re switching from uncoated to coated seed because the seed flows faster,” he said.

There are a wide variety of planters that can be used for seeding alfalfa including grain drills, Brillion packer types, airflow spreaders and no-till drills.

“If you’re planting in rows, use a planter with press wheels for good seed to soil contact,” Miller said. “Keep the row spacing narrow to discourage weed competition.”

For broadcast planting, Miller said, roll and pack before and after seeding to provide better seed to soil contact.

“Also increase the seeding rate 10% to 20%,” he said. “When using a drill, be sure to check the seeding depth.”

“Anytime we think about depth of planting we need to think about the seedbed,” said Earl Creech, professor and Extension agronomist at Utah State University. “The enemy of a new alfalfa seeding is a fluffy seedbed.”

Creech uses the rule of boot to evaluate a seedbed for alfalfa planting.

“Walk on the seedbed and if the heel and sole make an imprint in the soil, but the arch does not, then the seedbed is ready for an alfalfa planting,” he said. “When the arch makes an imprint, the soil is too loose.”

Most failures of alfalfa planting are the result of seed planted too deep, Creech said.

“Typically we target to plant alfalfa one-quarter to one-half of an inch deep,” he said.

To evaluate different seeding rates, Creech talked about a research trial conducted at six locations across the United States where researchers planted alfalfa at 6, 10, 14 or 18 pounds per acre.

“Once we hit the 10-pound rate we didn’t see any improvement of yield over the five years of the trial,” Creech said.

“In year one, we had more plants per square foot at the higher seeding rate than the lower seeding rate,” he said. “At year five, all the fields ended up in the same place because alfalfa is a self-thinning plant.”

Creech highlighted several reasons farmers should increase the seeding rate for alfalfa.

“Increase the rate if there is a poor seedbed that is cloddy, powdery or fluffy, if you have poor equipment, if the soil moisture is variable or if you have high amounts of residue on the soil surface,” he said.

For more information, go to www.alforexseeds.com.

Martha Blum

Field Editor