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Flax underperforms on canola stubble

Where are you going to plant flax? A demonstration at the East Central Research Foundation (ECRF) at Yorkton, Sask., suggests that canola stubble should be avoided.

“Flax relies heavily on mycorrhizal associations to explore the entire soil volume around it for phosphorus,” says Mike Hall, research coordinator with the ECRF and Suncrest College (formerly Parkland College) at Yorkton. “Anything that disturbs mycorrhizal populations, such as tillage or growing a preceding non-mycorrhizal crop like canola, is likely to cause a phosphorus (P) deficiency for flax, which may not be compensated for by higher rates of in-season P.”

Hall headed up a Saskatchewan Agriculture ADOPT (Agricultural Demonstration of Practices and Technologies) demonstration trial in 2022 at Yorkton. Wheat and canola were grown in 2021, a drought year leaving high residual nitrogen (N) in the top 24 inches (60 cm) at 104 pounds N per acre (116 kg/ha). The plots were split with one-half cultivated after harvest, and the other left with standing stubble. Phosphorus rates were also compared at zero, 25, and 50 lbs P2O5/ac (zero, 28 and 56 kg/ha). Forty pounds (18 kg) of N were also side-banded along with the P fertilizer.

Flax was sown on May 24, 2022, into the wheat or canola plots. Growing conditions were much better in 2022, with May rainfall well above the long-term average. Over the growing season, 12.8 inches (325 mm) of rainfall occurred compared to the long-term average of 10.7 inches (272 mm). There was some hail on June 23, but the flax was young enough to recover and produced an average of 35 bushels per acre (2,197 kg/ha) across all treatments. 

Crop emergence was similar between all treatments of stubble type, cultivation system or rate of side-banded P, averaging 30 plants per square foot (302 plants/ m2). Crop vigor was assessed on June 23, 2022, which can be a reflection of better P uptake. The statistically highest vigor was on wheat stubble at a 7.8 out of 10 rating compared to canola at a 6.5 rating. Standing stubble also had statistically higher vigor at 7.5 for standing stubble and 6.8 for the cultivated treatments. There was no difference in vigor between the fertilizer P treatments.

Yield was higher on wheat stubble
The yield results mirrored the vigor ratings with the highest yield on wheat stubble and standing stubble plots. On average, flax yield was 10 per cent higher yielding on wheat stubble and six per cent higher yielding on standing stubble. Flax yielded 39 bu/ac (2,456 kg/ha) on standing wheat stubble compared to 33.4 bu/ac (2,100 kg/ha) for flax grown on cultivated canola stubble.

“We were hoping that canola and cultivated canola would be the worst treatments, and increasing rates of phosphorus wouldn’t be enough to compensate for the lack of mycorrhizal root association,” says Hall. 

There was no difference in yield with increasing P fertilizer rates. However, there was a higher yield trend as P2O5 was increased from zero to 50 lbs/ac for flax sown on wheat stubble, with yield increasing from 35 bu/ac (2,207) to 38.5 bu/ac (2,421 kg/ha).

“Perhaps the flax was better able to utilize the added phosphorus when grown on wheat stubble due to better mycorrhizal root associations,” says Hall.

Higher rates of P couldn’t improve canola stubble yields
Digging deeper into the data, Hall says that the lower yield of flax grown on cultivated canola stubble could not be compensated for by increasing the rate of phosphorus. For example, side-banding 50 lbs P2O5/ac produced a flax yield of 33.7 bu/ac (2,119 kg/ha) when grown on cultivated canola stubble, which was lower than 37.4 bu/ac (2,351 kg/ha) for flax receiving no added P when seeded on standing wheat stubble.

“All the data supports the concept that flax grows better on standing wheat stubble compared to cultivated canola stubble due to better mycorrhizal root associations aiding the uptake of phosphorus,” says Hall.

Maturity was estimated in September 2022 by calculating the percentage of brown bolls. On average, flax grown on canola had a higher percentage of brown bolls than on wheat stubble, indicating earlier maturity, and there was no difference in maturity on stubble type. Hall says the delayed maturity on wheat stubble wasn’t expected and could be due to other factors such as more soil moisture, a better seedbed, higher residual N or less disease.

Flax sown on wheat stubble had earlier maturity as P rates increased but not on canola stubble. Added phosphorus was also found to hasten maturity but only with the high rate on standing stubble.

“Added phosphorus should hasten maturity. Perhaps it was only evident for flax on wheat stubble because it was more immature than the flax on the canola stubble,” says Hall. “It is also possible that the better mycorrhizal root associations with the wheat stubble facilitated better up-take of the applied phosphorus.”

Hall concludes that the demonstration showed that mycorrhizal root associations can be important for flax, and that growing flax on standing wheat stubble can help nurture these associations.

“The study successfully demonstrated that flax grows better on standing wheat stubble and the worst on cultivated canola stubble,” says Hall. “However, other factors in addition to better phosphorus uptake were also likely involved in creating this difference.”