Soil Temperatures and Weed Control

Is it better to start early, or wait for a stronger start in warmer soil?

by Megan Madden

Deciding when to seed pulse crops can have an effect on germination, plant stand, weed competition, and ultimately, yield.

According to Saskatchewan Pulse Growers’ 2018 Saskatchewan Pulse Crops Seeding and Variety Guide, peas, lentils, and faba beans should be seeded earliest, as they can germinate in soils as low as 5°C. Desi chickpeas should be seeded into lightly
warmer soil temperatures of 7°C but large Kabuli varieties, dry beans, and soybeans do not germinate and emerge well in temperatures this low. Seed these crops into warm, moist soils (10°C at the depth of seeding). Today’s direct seeding allows
producers to seed shallower because residue on the soil surface keeps moisture closer to the surface, with no need to chase moisture at depths in some years.

Some simple measures in the fall can help improve soil temperature earlier in the spring. Ensure your combine is evenly spreading chaff and chopped straw so seeding equipment will not plug in heavy residue areas, and so soils will warm evenly.

A recently completed study by Ramona Mohr of Agriculture and Agri-Food Canada in Manitoba found residue management influenced soil temperature at seeding at all of their trial sites. Mohr’s summary states “the average soil temperature of treatments where straw was retained on the soil surface was approximately 1°C to 3°C lower than those where straw was removed. Despite these early-season differences, residue management had limited effects on plant stand, soybean yield, and seed quality, including test weight and seed weight in 2015.” If fall management is not adequate, a heavy harrow can be used to slightly blacken the soil in the spring, increasing the warming ability for seeding.

Timing decisions will also impact factors beyond germination and emergence. Dale Risula, Provincial Specialist, Special Crops with the Saskatchewan Ministry of Agriculture (SMA) says, “Seeding date can also have an impact on how that plant competes with weeds. Warm season plants start slower, which means they compete less with our cool season adapted weeds.

For example, chickpeas are less tolerant of a cool spring, with the Desi varieties being slightly better suited than the Kabuli.”

Clark Brenzil, Provincial Specialist Weed Control in the Crops and Irrigation Branch for the SMA says that pulse crops have a critical weed control period defined as the growth stages in the crop that must be kept weed free to prevent yield loss.

“The critical period for most crops is based on yield loss due to weeds of no more than five per cent, since it is the level of statistical relevance that is often used. In other words, the crop has to be weed-free during these critical stages to prevent a yield loss of more than five per cent,” Brenzil explains. “For example, there is no real advantage to controlling weeds in a soybean crop before the first trifoliate. Weeds not controlled in soybeans between the first and third trifoliate stage can cause upwards of 50 to 60 per cent yield loss, whereas yield losses outside of this range generally result in less than five per cent yield loss, much of this at the early end of the range.”

Pulses are generally poor competitors, and with limited in-crop weed control options, pre-seed burnoff timing can also affect crop yield. Brenzil says that there is a disadvantage to waiting to do burnoff until just prior to late-seeded crops, particularly
this year when moisture resources may be limited. “Do your burnoff as soon as you can get in the field,” he recommends. “When dandelions are just beginning to bloom is the best time to control them for optimal yield.”

The benefit of early pre-seed weed control also applies to winter annual weeds. Brenzil adds that this recommendation comes from a series of research projects completed by Rick Holm and Ken Sapsford at the University of Saskatchewan, that studied
timing of pre-seed burndown treatments with respect to the yield of wheat. These studies found that early dandelion removal during the burnoff treatment resulted in better crop yield, when compared to later burnoff treatments.

Timing of seeding will play a role in the decision to use a seed treatment with your pulses. Brenzil says that seed placed into cold soils benefit from a seed treatment to protect the seed viability, as the longer the seed sits in the soil, the more susceptible it is to seed rot.

Risula encourages producers to test their seed beyond the standard germination test to determine pathogens present. He recommends always treating Kabuli chickpeas, as they are sensitive to seed- and soil-borne pathogens, and have high seed-to-seedling transmission for seed‑borne Ascochyta.

While seed size in itself is not a reliable indication of seed vigour, it can impact seeding rate which, in turn, will affect plant stands and ultimately competitiveness. Risula recommends using the thousand kernel weight to calculate appropriate seeding rate to target plant densities that allow the crop to compete with weeds.

“Even a relatively uncompetitive crop like lentils will suppress weed growth and produce higher yields if seeded at higher densities,” says Brenzil.

There is no magic formula for when to plant which of your pulse crops, but utilizing a combination of these strategies will ensure your crop is set up for success.