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 higher seeding rates obviously includes the cost of additional seed, which is often a greater burden for organic farmers because they must use higher-priced organic seed.
However, the advantages are also real, especially when suppressing 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 or if fields are relatively weed free or under seeded. A higher seeding rate can also compensate for crop losses during post emergent mechanical weed control.
At higher seeding rates, crops also compete more with each other. Individual plants in a dense stand may tiller less and have fewer seeds per plant than individual plants in a less dense stand. While this may mean that higher seeding rates do not result in higher yields, fewer tillers may also mean that the crop matures earlier and more uniformly, which may be important if crops are seeded late.
Plants may mature more quickly at higher seeding rates, 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 at 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 southwestern 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 pounds per acre. Dave Montgomery seeded spring wheat at 75, 90 and 110 lb. per acre. Danny Rempel seeded flax at 40, 50 and 60 lb. per acre. Dwayne McGregor seeded durum at 13/4, two, 21/2 and three bushels per acre.
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 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 in Saskatchewan, Manitoba, Ontario, Quebec and the Maritimes.
They were asked to seed four rates of wheat: the recommended rate for their area, 11/4 times that rate, which is the Canadian Organic Growers recommendation, 11/2 and two times 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, who are analyzing data from the national seeding rate study, 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 Meinert’s, yields increased strongly with seeding rates. For others, the relationship was much weaker.
Scientists have 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 more information or to volunteer as a collaborator on the project, please contact Roxanne Beavers or Brenda Frick.
Frick is the Prairie co-ordinator for the Organic Agriculture Centre of Canada located at the University of Saskatchewan. Frick can be reachedat 306-966-4975, at email@example.com, or www.organicagcentre.ca.The opinions expressed in this column are not necessarily those of The Western Producer.