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Three Ways To Meter Seed

Performance of type A and B systems improves significantly with the addition of corrugated inlet tubes and deflector

vanes at the secondary splitters.

As airseeders have evolved over the years, manufacturers have increased the precision of their machines. And as working widths have increased, getting seed and fertilizer from the meter on the tank to the openers is a much more complex task. Here’s a look at the common distribution systems in use on today’s air seeders.

Each manufacturer has it own design variation, but it is possible to describe all of them as falling into three basic categories, labelled types A, B and C. Each forces seed and fertilizer to travel a slightly different route from the grain tank to the seed openers.


Type A systems meter grain into a large-diameter primary tube that carries all the seed to a primary splitter. Here seed is separated randomly into a series of smaller-diameter secondary lines that carry it to a second splitter, for another random separation. From there, individual lines lead off to the seed openers that place it in the ground.

With this system, seed undergoes two random flow divisions. That means an increased possibility of variation in the number of seeds that make it to each opener. Some openers may get more seed than others.

Engineers call the amount of difference in the number of seeds reaching each opener the coefficient of variation. Independent organizations, like the Alberta Ag Tech Centre, consider a coefficient of variation of 10 per cent or less to be very good for establishing even plant stands.

“When you are relying on randomness, it does increase variability,” says Garth Massie of Morris Industries. The Morris design, along with those used by some other manufacturers, relies on a modified type B distribution system, which is a little different.


Type B systems have only one random split in the seed flow. The object of this system is to improve the coefficient of variation. Seed from the grain tank is metered into a series of primary hoses, each one leading to its own splitter on the seeder frame. That means there is only one splitter on each line. From that splitter, seed flows directly to the openers. Massie says eliminating one random split is part of the reason the current Morris system is capable of a very accurate (+/-six per cent) variation between openers.


The final design, type C, does all the distribution right at the metering system under the grain tank. Seed is metered directly into individual lines, and they carry it all the way to the openers. There are no random splits in the system.


A 2004 research paper published online by the Alberta Ag Tech Centre provides a look at the performance of four different airseeders from that era using the type A system. According to that paper, variation of canola seed and fertilizer rates delivered to the openers ranged from eight to 29 per cent.

For type B systems, again using fertilizer and canola, the rate of variation ranged from nine to 21 per cent for the different brands tested. The type C machine tested, however, delivered less than 10 per cent variation among openers.

But manufacturers have refined their designs since then, which has improved those numbers significantly. Lawrence Papworth, manager of the Ag Tech Centre, says the type B design is the most common one airseeder manufacturers use today. “The B system is basically a compromise between the (other) two,” he says.

With the very large working widths now demanded by producers, using a type C system with an individual line running to every opener would require a massive network of plumbing to get an individual hose to every opener. That could increase the likelihood of problems, such as trouble with kinked lines when the seeder is folded up for road transport.

The B system minimizes that problem because it uses fewer feet of hose. And it also eliminates one random split of seeds, helping improve consistency in the amount of seed delivered to each opener.

But according to the Ag Tech Centre’s research, performance of type A and B systems improves significantly with the addition of corrugated inlet tubes and deflector vanes at the secondary splitters. These features create turbulence

inside the hoses that keep seed and fertilizer suspended more evenly throughout the airflow. Without them, product tends to travel along the bottom of a hose and against the outside edges at corners. That means both type A and B systems are now also capable of achieving an accuracy rate better than 10 per cent.

Rob Fagnou, spokesman for Bourgault, says the type A system Bourgault uses offers very good seed distribution. He points to independent testing at places such as Prairie Agricultural Machinery Institute (PAMI) that bear out that claim.

As well, that type of metering and distribution system also offers some advantages. Fagnou notes that when product volumes

are low inside the grain tank, seed or fertilizer may not flow evenly over the metering device. As seed or fertilizer often remains a little higher on the tank sides than in the centre, the centre of a meter may not get any product at all. That could cause some primary lines to run empty leaving unseeded strips in the field when the volume of seed in the hopper is low. When product is metered into a single line, that problem doesn’t occur, although the overall volume of seed being metered out may still be reduced.


Whatever type of system on your seeder, Fagnou cautions growers to keep fan speeds set correctly. That is essential in getting undamaged seed delivered properly to openers.

As a starting point, fan speed should be set based on implement width, application rate and ground speed. “From this initial fan speed the operator may need to increase or decrease the fan speed to achieve the correct setting for the specific operating conditions,” says Fagnou. “A good rule of thumb is to have the air kit clean out in three to four seconds from the time the main clutch is shut off. Clean-out times greater than this indicate that the air kit is close to plugging and clean-out times less than this indicate excessive fan speeds.”

Scott Garvey specializes in writing about tractors and farm machinery technology for publications in Canada and Great Britain. He’s also a former affiliate member of the Society of Automotive Engineers (SAE). He farms near Moosomin, Sask.

About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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