LETHBRIDGE – The results of a seven year study suggest that under proper conditions, farmers may be able to grow canola and peas more often in a rotation than now recommended.
Stu Brandt, an Agriculture Canada researcher at Scott, Sask., told Alberta Agriculture’s Agronomy Update conference in Lethbridge that current recommendations to grow canola or peas once every four years in rotation are based primarily on weed and disease control requirements.
“Some of the questions beginning to emerge were whether or not some of the better disease resistance, weed control, fungicides and the improved yield potential of some of the varieties that we’re dealing with were able to overcome these restrictions,” Brandt said.
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“We set this experiment up using a variety of rotations, everything from continuous canola or continuous peas, two year rotations with canola or pea every second year in rotation with wheat, a three year canola-wheat-pea, as well as more typical four year rotations – canola-wheat-pea-wheat.
“Even when we were setting up these experiments, one thought was you couldn’t grow canola and peas in the same rotation unless there was a three year separation between each of those crops.”
Brandt set up the long-term rotations at Scott and Melfort, Sask.
Scott is in the dark brown soil zone while Melfort is in the thick black soil zone.
The researchers at Scott and Melfort set up duplicate rotations using conventional open pollinated blackleg-susceptible varieties and a herbicide tolerant hybrid that had good blackleg resistance.
As improved varieties were developed, the researchers updated their hybrids.
Brandt selected Westar as the open pollinated variety because it was typical of varieties available at the time.
“One of the things very obvious from the study is, rotation length or growing canola less frequently in the rotation, it was a highly effective tool in minimizing blackleg infection,” Brandt said.
“As we go from continuous Westar to Westar every second, third or fourth year, there is a steady decline in the incidence of blackleg.
“The other thing that’s obvious is that going to varieties with a high level of blackleg resistance, we were able to dramatically reduce the incidence of that disease and mask some of the impact of rotation on that disease.
“One conclusion we can draw from this is that the rotation recommendation in place may have contributed to this disease. If you really wanted to avoid this disease with something like Westar, maybe you could only grow canola every five or six years.”
Brandt said the researchers saw a decline in yield as soon as they decreased the rotation length at Melfort when growing Westar. At Scott, the Westar yields were highest in the three year rotation. This may have reflected the better moisture conditions in Melfort, which may have favoured greater disease as rotations shortened.
The researchers also noticed that the highest yields obtained with the hybrid variety were in the three year rotation, when the canola grew on field pea stubble. Brandt felt that probably reflected a nitrogen benefit after the peas.
Brandt said there appeared to be a cumulative effect with disease incidence.
“The longer we get into this more intensive rotation, the lower the canola yields will be, relative to more appropriate rotations.”
Brandt said he also observed that the canola cultivar did not affect subsequent wheat yields in any year since he initiated the study.
“If you think of producing 40 to 50 percent more yield and removing 40 to 50 percent more nutrients with the hybrid cultivar as opposed to Westar, you would expect there would probably be some impact in reduced yields or in higher nitrogen fertilizer requirements. Yet our residual soil nitrogen has not differed between cultivars,” he said.
When focusing on field peas, the researchers grew subplots with fungicides and no fungicide and applied fungicides regardless of disease or moisture conditions.
“That was one crop where we tended to see the best response to the fungicides. At Scott, we didn’t see a big yield reduction, even with continuous peas, but we did tend to see the highest yields in the four year rotations. In Melfort, we saw a big reduction in field pea yields in the continuous rotation. The reductions got larger and larger as we progressed,” Brandt said.
When the researchers looked at the most recent three year period of the rotations , 2004 to 2006, they observed more separation between rotations.
“Without fungicides, there is a general tendency for yields to decline as we grow peas more intensively in the rotation. With fungicides, we can partially overcome that. We still saw quite a good response to fungicides in the continuous pea rotation,” he said.
“In years where our yield potential was less than 1,500 kilograms per hectare (600 kg per acre), we saw little if any response to fungicide, whereas if we had yield potentials in excess of 2,000 kg per hectare (800 kg per acre), the response to fungicide has been more than sufficient to offset the cost of that fungicide.”
The researchers have done preliminary economics on the study and found that it’s tough to make continuous canola look good in any situation.
“Under conditions where canola prices are relatively low, the four year rotation tends to provide the best returns. As canola prices move up to the $7 a bushel range, there’s not a big difference between the three and four year systems. At high canola prices, the canola-wheat rotation starts to look more favourable.”
“More intensive rotations may be possible with some of these newer varieties. But we have to ask whether more intensive rotations will increase the risk that blackleg resistance will break down.
“And considering that genetic resistance is such a powerful tool in managing this disease, we have to ask if we’re prepared to take that risk,” he said.
“I think more intensive rotations with pea are somewhat feasible. We need to ensure we always have at least a one year break between pea crops. We should consider foliar fungicides for peas when yield potential is high.”
