Are higher seeding rates warranted?

By Brenda Frick, Ph.D., P.Ag.

Organic farmers often seed cereals at higher rates than are recommended for farmers who are not managing organically. Is this a good idea? Like most things, the optimal seeding rate is a balance between costs and benefits. This balance may be different for organic producers than it is for their neighbours.

The cost of increasing the seeding rate includes, of course, the cost of additional seed. This is often a greater burden for organic farmers, as they must use higher priced organic seed.

Organic producers may view the cereals in their rotations as opportunities to suppress weeds. Higher seeding rates increase competition. Where there are few crop plants, weeds gain a greater proportion of the resources. At higher crop densities, crop plants cover the ground more quickly and shade out weeds. Their roots are distributed more evenly, and they can access more of the water and nutrients. Greater competition with weeds can be effective in all cropping systems, but is less of an advantage if herbicides are used, if fields are relatively weed free or if they are under-seeded. A higher seeding rate can also compensate for crop losses during post emergent mechanical weed control.

At higher seeding rates, crops compete more with each other as well. Individual plants in a dense stand may tiller less and have fewer seeds per plant, than individual plants in a less dense stand. This compensation may mean that higher seeding rates than recommended do not result in higher yields. However, fewer tillers may mean that the crop matures earlier, and more uniformly. This may be an important consideration if crops are seeded late.

At higher seeding rates, plants may mature more quickly, but they may also run out of limited resources more quickly. Kernel weights and protein may be lower. Nutrients applied in the form of green manures and composts tend to be available a lower rates than chemical fertilizers, but because the breakdown of organic matter in the soil is slow, these nutrients tend to be available throughout the season.

A group of organic producers in southwest Saskatchewan conducted seeding rate trials last year. Each trial was seeded on a green manure plowdown. Kirby McCuaig seeded three different rates of Kamut (83, 122 and 174 lb/ac). Dave Montgomery seeded spring wheat at 75, 90 and 110 lb/acre. Danny Rempel seeded flax at 40, 50 and 60 lb/ac. Dwayne McGregor seeded durum at 1¾, 2, 2½ and 3 bu/ac.

With both Kamut and wheat, the lowest seeding rate was the weediest. Yields were similar with the higher two rates. In the durum, there were few weeds and highest yields occurred at the lowest seeding rate. Weed numbers were similar in the different flax treatments, but flax yields were highest at the highest seeding rate. Of course, these trials are all a single year's data, and should be interpreted as preliminary. They do suggest, though, that higher seeding rates may be more useful in the presence of weeds.

The Organic Agriculture Centre of Canada conducted a national seeding rate study last summer with the collaboration of organic farmers. Farmers from Saskatchewan, Manitoba, Ontario, Quebec and the Maritimes were asked to seed 4 different rates of wheat: the recommended rate for their area, 1¼x that rate (the Canadian Organic Growers recommendation), 1½x and 2x that rate. Martin Meinert, a producer from southwestern Saskatchewan, collaborated on this project. In his trials, lighter seeding rates had more weeds. Heavier seeding yielded more and allowed plants to resist sawfly damage more easily.

Researchers Roxanne Beavers and Andy Hammermeister are currently analyzing data from the national seeding rate study. They have found that doubling the seeding rate rarely doubles the number of crop plants. Weed numbers and weed biomass weights were sometimes, but not always reduced by higher seeding rates. For some trials, such as Martin Meinert's, yields increased strongly with seeding rates. For others, the relationship was much weaker.

Scientists have much more confidence in the trends they discover if they are consistent across fields and remain the same in different years. This is essential in the development of recommendations that might be used in a variety of years or locations. It also allows scientists to more effectively learn about underlying mechanisms for the results. Researchers at OACC are asking farmers to help them repeat their experiment in 2004.

For further information, or to volunteer as a collaborator on the project, please contact Roxanne Beavers or Brenda Frick.


Brenda Frick, Ph.D., P.Ag., is the Prairie Coordinator for the Organic Agriculture Centre of Canada at the College of Agriculture, University of Saskatchewan. She welcomes your comments at 306-966-4975 or via email at brenda.frick@usask.ca .

Roxanne Beavers can be reached at 902-843-2318 or rlbeavers@nsac.ns.ca.

This article first appeared in The Western Producer, and is published here on the OACC website with permission.