Choosing the right inoculant strategy

After some disappointing results this season, one pulse crop specialist says it’s time to take a hard look at these practices

Growers of soybeans and other pulse crops need to know both if nodules are present in acceptable number, and if they’re functioning properly.

Manitoba Agriculture pulse crop specialist Dennis Lange says it’s a good time to think about just what’s the right approach regarding inoculants for pulses.

“This year I’ve had a few calls on peas and soybeans from western Manitoba where they are finding very poor nodulation,” Lange told the Co-operator in a recent interview.

He says that while it is fresh in the minds of farmers, they should think about what will work best for them to avoid this in the future.

“Nodulation,” for the uninitiated, refers to the bacterial growths (nodules) on the roots of crops that have been infected with inoculants (a live rhizobia bacteria treatment). The rhizobia feed off the plant roots, and the process takes nitrogen from the air and carbohydrates from the root to create fertilizer that in turn feeds the plants.

When the nodules are functioning properly during the growing season they are blood red inside. The roots usually start nodulating in the second or third trifoliate and once they hit full flower, there should be lots of nodules on the plant (approximately 10 per root).

“But the number of nodules is not as important as if they are functioning,” Lange said. “You can have five nodules on a plant and if they’re nice and pink inside and the field is nice and green then you’re good. But we’ve had years when you start digging up roots and you’re not finding any,” he says. “That becomes a problem because the plant eventually runs out of nitrogen.”

Avoiding poor nodulation requires a well-thought-out strategy that starts with determining which treatments or treatment is best for a given field. There are a number of things that growers must consider while developing their inoculants strategy.

Firstly as most farmers know, the same rhizobia that creates nodules on soybeans, will not create them on fababeans, or lentils. So, it’s important that farmers make sure the bacteria are matched up to the crop that’s being grown.

The next consideration is the method of delivery. There are three different methods to apply inoculants: a liquid application, a peat application and a granular application.

“Typically the applications are done at seeding time,” Lange said.

The peat is a sticky powder that adheres to the seed. The liquid is sprayed on the seed and it dries. The granular inoculant is poured in the furrow beside the seed and eventually it will find the roots and infect them. Each has its benefits and its drawbacks in terms of cost, durability and ease of use.

Liquid inoculant scores high in the ease-of-use category because it can be applied to the seed before planting, but it can also be more prone to desiccation. The peat and granular methods are less prone to desiccation and offer higher rhizobia survival rates. Peat is a little messier to apply. Granular is the most expensive, but most durable of the three. Deciding which formulation to use will depend on soil moisture, nitrogen levels, field history and economic factors.

The final consideration is whether multiple applications are advisable.

“The biggest thing that I encourage growers to do, is to know your history,” says Lange. “Once you’ve grown soybeans multiple years on a field, the bacteria become more natural.

“The bacteria are not normally found in Manitoba but when applying these bacteria every year you plant soybeans, you get some natural carry-over. If you have grown soybeans multiple times on a piece of ground, you can probably use a liquid, single-treat inoculant.”

Lang recommended that farmers check the Soybean Fertility Fact Sheet, that can be downloaded from the Manitoba Pulse & Soybean Growers Association website. It has a checklist that will determine whether any given field is a candidate for single inoculation or not.

“If you use just a liquid on seed and it’s a really wet year and you don’t have a history of soybeans on the field, you will see some nodulation failures on some fields,” says Lange. “It’s never widespread. But every year, I always get a few calls.”

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