New research shows cover crops can help mop up excess soil moisture, says Ranjan Sri Ranjan, an irrigation, drainage, and water management expert.
The University of Manitoba professor wants to better understand the movements of water, both frozen and unfrozen, in the root zone. To do that, he used finger-sized probes to measure moisture and soil temperature over the fall, winter and following summer of 2005- 06 at five different depths at the University of Manitoba’s long-term cropping trial plots in Carman.
Plots seeded to an oats and berseem clover cover crop were compared to an oat crop alone, with fallow as a control.
“Overall, the soil moisture in the cover crop plot was 34 per cent lower than in the oats alone,” Ranjan told attendees at the recent ManDak Zero- Tillage Association annual workshop.
“Fall freeze-up was delayed in the cover crop, and it also froze to a shallower depth.”
The plots with oats and berseem clover had less soil moisture in the fall, thawed out earlier, and the canola in them the following summer showed more growth, possibly due to the 18 to 20 per cent lower soil moisture in the spring, and the fact that it was 0.2 to 1.2 warmer at a depth of 0.4 metres. The ground froze to 0.6 metres in the berseem clover/oats plot, compared to 0.8 metres in the oats alone, he added.
However, if the spring turns out to be dry, the cover crop might adversely affect available moisture for the subsequent crop, he added.
As an interesting side note, similar research at the Manitoba Zero-Till Research Association (MZTRA) farm near Brandon in 2009 showed that even though the temperature dipped to -40 C for three weeks, the soil surface of native prairie grass plots never froze, possibly because the grasses had trapped heavier snow cover.
“The soil covered in native prairie grass remained warm, so in springtime, when the snow melts, it is able to infiltrate
in, so you get less runoff water,” said Ranjan.
Forty years of Environment Canada weather data shows there is enough heat and water for late-season cover crops on the Prairies at the tail end of the growing season most years, said Martin Entz, professor of natural systems agriculture at the University of Manitoba.
In Pierson, for example, winter wheat on average matures on August 8, and in three out of four years, it matures on Aug. 12.
On average, there are 609 growing degree days, and in three years out of four, there are at least 514.
“Once you get 500 growing degree days, you can grow cover crops,” said Entz.
The average precipitation at Pierson was 120 mm during Aug. 8 to freeze-up, and 70 mm in three out of four years. Precipitation in the 15-day period after winter wheat maturity averaged 15 mm, or at least 10 mm in three of four years. This shows there is sufficient moisture to germinate cover crops either drilled in after harvest or broadcast with a special attachment on the combine.
Fall cover crops are very efficient water users because with lower air temperatures, plants don’t transpire as much water to stay cool. For example, in summer, alfalfa or grasses produce about 12 kilograms/acre of dry matter for each millimetre of precipitation, but fall growth is much more efficient, and could exceed 20 kg/acre/ mm.
“You know that with your volunteer crops, such as canola, which grow extremely well on very little water,” said Entz.
Available nitrogen is also a limiting factor in fall, he added, but that’s not a problem if a legume is grown. As a bonus, this gives legumes a competitive advantage over other plants.
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“Onceyouget500 growingdegree days,youcangrow covercrops.”
– MARTIN ENTZ