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Optimizing dryland pea yields

Seeding date, seeding rate, field selection and time of weed removal critical.

November 26, 2007  By Bruce Barker

Pulse crops can be an integral part of crop rotations, especially when their
nitrogen-fixing benefits are considered. According to Mark Olson, provincial
pulse extension specialist with Alberta Agriculture, Food and Rural Development
(AAFRD) in Edmonton, pulses could easily increase in acreage if farmers could
more dependably grow the crop. "At around 700,000 acres of pulses, Alberta
growers aren't even at one-quarter of the pulse acreage potential," says

Olson says that getting more yield is something that is within the reach of
most growers. In fact, a 2000 pea survey by Terry Buss, formerly of AAFRD, showed
the average pea yield went up only slightly from 1986 to 2000, despite better
performing varieties and increased agronomic knowledge. Yield stability was
also disappointing. In the time-frame since then, growers still do not seem
to be optimizing yield.

Target early seeding date
Field peas is a cool season crop, and can germinate at low temperatures and
withstand light frost. On the other hand, it cannot withstand high temperatures
during flowering – one of the reasons why yields were disappointing for
some growers in 2006.

Use a metre stick rather than a square foot or metre tool to estimate
plant populations.
Photo Courtesy Of Reduced Tillage Linkages.

Olson says if the field peas crop is sown early, flowering will usually occur
during the cooler part of the summer. He cites a greenhouse study that compared
three day/night temperatures. The researcher found that temperatures of 27 degrees
C daytime and 17 degrees nighttime had 46 to 66 percent fewer pods and 50 to
70 percent less seed weight compared to growth temperatures of 17 degrees/ 7
degrees C.

"The majority of growers try to seed early, which means the last couple
of weeks in April or the first week of May," explains Olson. "That
generally means peas are seeded first."

Target optimum plant population
While targetting plant populations has been discussed numerous times in recent
years, it is worth repeating. When determining seeding rate, consider the target
plant population of seven to eight plants per square foot (75 to 85 plants per
square metre).

"This is one of the weakest links in field pea production. More often
than not, the optimum plant population is not being met consistently. Problem
pea fields that agronomists encounter are often those with low plant populations,"
says Olson.

The recommendation for seven to eight plants per square foot is supported by
several studies in the 1990s conducted by Bob Park and Ken Lopetinsky of AAFRD.
For smaller seeded varieties like Radley and Express, a plant population of
seven optimized yield. However, for the larger seeded varieties, increasing
plant populations to nine plants per square foot provided a benefit.

More recently, research in 2002 found that under low moisture conditions and
relatively weed free conditions, yields may be optimized at plant populations
as low as 4.5 to seven plants per square foot. Olson cautions though, that at
the lower end of that range, peas may suffer loss in yield potential if weed
competition is significant and good moisture conditions exist.

"There will always be some weed competition and if there are problems
with stand establishment at lower seeding rates, then you can lose yield,"
he says. As a result, he recommends the target plant population of seven to
eight plants per square foot.

Another key consideration when looking at plant stands and the ability of the
crop to compete with weeds is the amount of light getting through the pea canopy
to enable weed growth. A study in Manitoba in 1996 found that semi-leafless
varieties allow more light through the crop canopy, so plant populations must
be higher to provide better competition with weeds.

Olson says that growers should always test seed for germination and vigour
when determining seeding rates. He says growers should not rely on an average
thousand kernel weight for a seed lot. Rather, they should count out the seeds
and weigh them on the farm.


"Year in and year out, when examining regional field pea trial data, there
is as much as a 25 percent difference in seed size within the same variety depending
on where it is grown in the province," cautions Olson. "Those are
huge variations."

To emphasize that point, consider that a seed which is 25 percent smaller could
result in a target plant population dropping from six down to 4.5 plants per
square foot. Olson also advises growers to throw away their square foot or square
metre measuring tools and use a yard or metre stick to estimate plant populations.
Simply measure the number of plants in 12 inches of seedrow and convert it to
plants per square foot.

For a 12 inch row spacing, the number of plants in one foot of row equals the
number of plants per square foot. At a nine inch row spacing, multiply the number
of plants in 12 inches of row by 1.333; for eight inch row spacing, by 1.5.
(The factor is calculated by dividing 12 inches by the row spacing: i.e. 12
divided by 9 = 1.333).

Olson disputes the notion that higher seeding rates cost too much. He says
surveys have shown that most growers (more than 80 percent) do not use certified
seed except on a small acreage to multiply up a variety for use on their own
farm. The increased cost of seeding half a bushel, or 30 pounds per acre more
of brown bag seed, is approximately $3.00 per acre – a cost that can be
easily recovered in increased yield.

Field selection
While field selection seems like a very basic choice, Olson says it impacts
on everything from weed control to fertility management. Fields with Canada
thistle, sowthistle, dandelion, nightshade and narrow-leafed hawk's beard should
be avoided, since they are not adequately or easily controlled in field peas.
Nightshade causes extra problems since the 'berries' are the same size as pea
seed and contain toxic akaloids. Growing a pea crop on a cereal field with a
pre-harvest glyphosate treatment the previous fall can help reduce these weed

Summerfallow fields, heavily manured fields or fields with applied nitrogen
should not be used for pea production: excessive vegetative growth may occur.

Herbicide residues are a concern to pea growers and record keeping is critical.
Olson says that herbicides used in the previous years in Group 2 (Assert, Ally,
Everest and Sundance, for example) and Group 4 (Banvel, Lontrel and Tordon,
for example) are products that can cause germination problems in field peas.
Looking back to the year before is simply not enough if products with longer
lasting residual activity have been used.

"Many top growers are looking for alternatives or alternative ways to
avoid using residual herbicides altogether so as not to restrict marketing and
crop choices in the following year," comments Olson.

Time of weed removal
Early weed removal is critical to maximize field pea yield. "The research
has been presented 100 times before, but it bears repeating because it cannot
be overstated," says Olson.

Research by Neil Harker, at Agriculture and Agri-Food Canada at Lacombe, showed
that removing weeds later cuts yield, and that full season competition without
weed removal resulted in yield losses of 40 to 70 percent. On average, if weeds
were not removed one week after crop emergence, yield decreased by seven percent.
Two weeks after emergence, pea yields dropped another 12 percent and three weeks
after emergence, pea yields dropped by an additional 13 percent for a whopping
26 percent yield decrease by waiting four weeks after emergence to spray.


Olson says that despite the proven benefits of spraying early, some growers
miss the optimum spraying window. It may be due to weather conditions, but he
explains that incorrect growth staging and a lack of understanding of field
peas growth also may be the cause. And peas can grow rapidly once out of the
ground, as well.

"Under good growing conditions, field peas can grow two nodes per week,
moving out of the optimum stage for spraying quite rapidly," explains Olson.

He also cautions that the internodal distance can vary from variety to variety,
as well as with environmental conditions, so plant height is not a good indicator
of spray timing. He also notes that prairie researchers count nodes differently.
Alberta follows the European growth stage model and ignores the scale leaf.
In Saskatchewan and Manitoba, scale leaves are counted as nodes, so what is
the third node in those provinces might be counted as the first node in Alberta.
As a result, the recommendations in each provincial crop protection guide is
slightly different.

For example, in Saskatchewan, Odyssey is recommended for application at the
one to six above-ground node stage (one to six true leaf stage) of field peas.
In Alberta, application is recommended at the one to six node stage. Both recommendations
are for the same growth stage, but growers have to recognize how to count leaf

"This is where crop scouting can make a difference. We need to get out
in the field and not only assess weeds, but ensure that herbicides are applied
at the right stage," says Olson.

These four main cropping decisions are largely responsible for optimizing field
pea yield. Olson says that by focussing on these decisions, growers will go
a long way to solving many of the problems that arise when assessing disappointing
pea yields.


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