News
Developing pea seeding rate guidelines

Field peas is an important crop in many cropping systems across the prairies.
A wide range of pea varieties are available in different market classes. These
varieties also differ in growth habit, branching and several other characteristics.

Researchers at the University of Saskatchewan are comparing several pea varieties
to determine how much difference there is in the growth habit, and whether or
not this difference may allow for different seeding rates and better weed competition.
Traditionally, the recommended seeding rate for all field pea varieties is the
same, at 75 to 85 plants per square metre.

"In an earlier trial, researchers here discovered differences in the branching
between different pea varieties," says associate professor, Dr. Steve Shirtliffe.
"Some varieties can really branch out, similar to canola, filling in the
space and increasing competition with weeds. Therefore, we're trying to find
out if pea varieties that have increased basal branching could require lower
seeding rates." Researchers hope the results could lead to revised guidelines
for the economically optimum field pea seeding rates and potentially reduce
production costs for growers.

This three year project, initiated in 2005, has only one year of preliminary
data analyzed so far. Graduate student, Joshua Spies is doing the work on this
very large experiment. The project is comparing several field pea varieties
from several different market classes, including green, yellow, maple and silage
varieties. Varieties with low and high basal branching were selected in each
market class for comparison and contrast.

"A range of seeding rates is being compared, from very low to very high
rates, to determine the range of differences between the varieties," explains
Shirtliffe. The rates vary from much lower than growers would normally use,
to rates much higher. Seeding rates of 10, 30, 90, 120 and 150 plants per square
metre are being compared. Several measurements are being calculated, including
plant density, branching at lower nodes, biomass, light interception at maximum
canopy, pod development, maturity, disease rating, lodging scores, yield and
harvest index.

"One of the components of the research is to determine the optimum seeding
rate that maximizes yield," says Shirtliffe. "This concept is based
on what we call the 'Law of Constant Final Yield', which means there is a certain
seeding rate or plant population that provides the maximum yield, and going
beyond that rate won't give you any more yield. At that rate, instead of getting
more yield, you will tend to get more plants with fewer branches and fewer pods."

By comparing different varieties at different plant densities, researchers
can capture the yield density response. This response can be put on a graph
showing how the yield of the variety changes at different plant densities and
help identify the constant final yield. Table 1 outlines the varieties being
evaluated in the project.

"So far, the one year of data available suggests the differences in branching
between the varieties isn't as large as we hoped to find, although there are
definitely some differences," explains Shirtliffe. "It appears there
may be some differences in branching between the market classes, but we will
need to wait for the full three years of data to confirm this preliminary observation."
As well, the plots with lower densities seemed to branch a lot more than the
higher seeding rates and higher densities, which showed very little branching
at all.

CDC Sonata, a forage field pea variety, was one that did perform quite differently
than the others. "CDC Sonata seemed to reach maximum yield at a very low
rate and had the ability to easily branch and fill out," says Shirtliffe.
"This may mean that growers seeding field peas as a forage may not have
to seed as heavy a rate as regular field peas."

Growers can look for more firm recommendations after the final harvest and
data analysis in the fall of 2007. "Right now, we can safely say that growers
using the current seeding rate recommendation are getting more than enough seed,"
says Shirtliffe. "If they get good emergence, then the recommended seeding
rate provides more than enough seed in all cases to maximize yields. Unless
there are other problems, there doesn't seem to be any need for increasing the
seeding rate beyond the recommended rate."

Shirtliffe adds that with the differences they are noting, they expect to be
able to refine the recommended seeding rates, perhaps for each different market
class. Of course growers will have to take geographic location, climate and
environmental conditions into account, along with seeding rates. "This
may allow growers to adjust their seeding rates down, which would save them
money. Lower seeding requirements would also allow for faster multiplication
of seed on the farm, helping breeders make new seed varieties available much
quicker." Watch for final research results and possibly new seeding rates
in early 2008. -30-