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Crop Advisor’s Casebook: Irregular strips of struggling canola

A Crop Advisor's Solution from the April 4, 2015 issue of Grainews

Plants in the affected areas had smaller, thinner leaves. The newest-formed leaves out of the growing point in the problem-area plants were slightly cupped and starting to yellow.
Spencer McArthur is a sales agronomist with Richardson Pioneer Ltd. at Balgonie, Sask. photo: Supplied

In early July I received a phone call from Dwane, a grain farmer at Davin, Sask., who was seeing a peculiar pattern forming in one of his canola fields. Dwane told me that despite some heavy June rains, his early canola establishment had been excellent — he had achieved his target plant density and the crop was off to a fast, vigorous start. But as the crop progressed throughout June, Dwane began to notice strips developing in one field where the canola was lagging in maturity and was noticeably less vigorous.

“I measured out an isolated strip and it was 80 feet wide, the exact width of my drills,” Dwane explained. “Something must have gone wrong at seeding time, but I just can’t narrow down what is happening out there.”

As the crop progressed throughout June, strips developed where the canola was lagging in maturity and noticeably less vigorous. photo: Supplied

I decided that it would be best to meet Dwane at the problem field so we could go through the diagnosis together. The strips were evident as soon as I arrived at the field, but I observed that they were in an irregular pattern throughout the quarter, and that halfway through the field the strips abruptly ended. Dwane already had an answer for that. “I had both my drills working in the same field, and I think the strips are due to one of my drills not working properly,” he said.

My first thought was that one of the drills had placed the seed at a level that was either too shallow or too deep, since seeding at proper depth is a crucial component to getting canola off to a fast start. Lower plant vigour or a lag in maturity could definitely be attributed to depth issues, especially if the seed was placed too deep and emergence was delayed.

When I dug up plants from both the affected and unaffected areas, I found that the distance from the point of germination to where the stem turned green (where the plants broke the soil surface) was exactly same in both areas, indicating that seeding depth was consistent and therefore not the issue.

Dwane wondered if perhaps bad seed was to blame. That one was easy to rule out, as the same seed-lot had been used in both drills. Also, the strips abruptly ended halfway through the field even though the drills had been filled with enough seed for the entire piece, meaning the seed had not changed from start to finish.

Having ruled out both seeding depth and seed quality, I turned my attention to fertility. Dwane explained to me that he had applied a strong fertility program to his canola at seeding, and that the fertilizer rates used in this field were the same as those with his other canola fields, which all looked normal. He had placed seed-placed nutrients to meet the crop’s phosphorus and sulfur needs and there had been a side-banding nutrient application to match the nitrogen requirements. The fertilizer rates used by Dwane were more than adequate for meeting his yield targets.

The fertilizer rates used by Dwane were more than adequate for meeting his yield targets. photo: Supplied

I wasn’t convinced that a nutrient deficiency wasn’t at work here, so I performed a close examination and comparison of the plants in the affected and unaffected areas. The canola plants were in the rosette stage in both areas, with the buds just becoming visible at the growing point.

There was less ground cover within the problem strips, but that was mainly due to the plants in the affected areas having smaller, thinner leaves. The plants in the affected area were also a lighter green colour, in contrast to the dark blue-green tinge of the unaffected canola nearby. I also noticed that the newest formed leaves out of the growing point in the problem-area plants were slightly cupped and starting to yellow.

I had a strong suspicion as to what was going on, so I removed newly formed leaves from plants in both affected and unaffected areas and sent them to a lab for tissue testing. The results provided the answer for what was limiting Dwane’s canola and causing the strips in his field.

Crop Advisor’s Solution: Insufficient sulphur and phosphorus harms canola

The symptoms I observed in the affected plants were a clear sign of nutrient deficiency. The strip pattern in the problem field led me to believe that the seed-placed fertilizer had not been metering on the drill at planting, causing the plants to be low in phosphorus and deficient in sulphur. Fortunately for Dwane, the seed-placed fertilizer must have started to metre correctly again so that the whole field was not affected.

Sulphur has low mobility in canola plants; therefore, deficiency symptoms usually show up in the younger leaves and stems. Phosphorus deficiency symptoms in canola are not as definitive and are more difficult to diagnose. Phosphorus is mobile within the plant and deficiency symptoms appear first in older tissues as the plant transfers reserves to newer growth. Because the tissue test samples were taken from the newest formed leaves, the lab results confirmed the sulfur deficiency but there were no definitive conclusions on plant phosphorus content.

However, from past soil sampling I knew that the phosphorus levels on this field were generally lower, so it wasn’t surprising to see stunted plants that were lagging in maturity in those areas where no phosphorus fertilizer had been applied. Past soil sampling had also revealed that soil in this area was quite sandy, and that due to its lighter texture it was generally more susceptible to nutrient leaching. As a water mobile nutrient, sulphur was more than likely washed out of the rooting zone during one of the many heavy rainfall events we had experienced in June.

Rescue treatments of sulphur have been shown to be quite effective in reducing or correcting sulfur deficiency symptoms, especially if the application is made prior to flowering. Foliar sprays generally can’t provide enough sulfur to correct a severe deficiency, so I recommended that Dwane apply a top dressing of a granular product such as ammonium sulfate over the affected strips. I felt this would be his best management strategy for salvaging as much yield as possible from his crop in the problem field.

For phosphorus, top-dressing to correct a deficiency is usually not as effective. Phosphorus is utilized most effectively when it is placed near the rooting zone for uptake; due to its relative immobility in the soil, it is a poor candidate for a top-dressing rescue treatment.

Spencer McArthur is a sales agronomist with Richardson Pioneer Ltd.

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