[deck]Crop rotation can grant growers major benefits over the long-term. However, it demands time and careful management—and the willingness to take some risks.[/deck]
Many growers see crop rotation as a mandatory practice for growing healthy tubers, and aren’t surprised at the continuous stream of research indicating its benefits for improving soil and limiting disease.
But to say rotation is complicated would be a vast understatement—so many variables, and information, crowd the process that making crop rotation decisions is often a difficult process.
Growers must decide whether they have the land, or can rent sufficient land, to make proper rotations feasible; they must factor in the availability of crop-specific planting and harvesting equipment; and they must assess which crops to seed based on their benefits for the main potato crop, their compatibility with soil and topography, and ultimately, their market value. For many growers, their final decision on crop rotation must hinge on the latter—but what makes sense for the pocketbook may have unexpected impacts on the land.
Surprising Findings
One of the longest-running studies of potato rotation in Western Canada, led by Ramona Mohr, has been ongoing in Carberry, Man., since 1998. Researchers have looked at two-year, three-year and four-year rotations of, respectively, potato-canola, potato-wheat, potato-canola-wheat, potato-oat-wheat, potato-wheat-canola-wheat and canola underseeded to alfalfa-alfalfa-alfalfa, and each rotation’s effects on disease, soil and crop yield and quality.
“This whole study had several aspects to it—certainly yield and quality were going to flow out of this, but we were also [looking at] soil quality issues and monitoring weeds, as well as looking at other microbial aspects other than pathology,” says Byron Irvine, an agronomist and the research manager of the Brandon Research Centre.
“From the beginning we had a good idea that the most risky rotations would be the shorter rotations. When this started there was belief that Brassica species would suppress disease. The canola trials were key,” he adds.
But the canola trials presented one of their most surprising findings. In the article Lessons from the Carberry Potato Rotation Study, the researchers note that by 2007, the PC rotation was exhibiting a lower yield than the other rotations, and more than 75 per cent of these plants were affected by vascular wilt. Though research studies emitting from the United States and elsewhere have indicated that Brassica species are disease-suppressing in rotation with potato, the Carberry study has shown something else over the long-term.
“What we found was that rotation length made a difference, as did the type of crop,” notes pathologist Debra McLaren. “The two-year rotations were not sustainable, and interestingly, 13 years after establishment, the three-year rotation with canola is starting to show disease at about the same level as the PW rotation. Highest disease levels are still evident in the PC rotation. It hasn’t given us the disease suppression we had expected, but preliminary results from 2010 suggest that the development of disease-suppressive soils over the long-term is not out of the question.”
Irvine says that some of the studies that pointed to Brassica species as excellent rotation crops might have used canola as green manure, which would account for the positive results. But according to Irvine, canola may not be ideal in rotation for more reasons. “It’s often difficult to get canola to establish well immediately after potato, so in some ways that’s the worst case scenario for a two-year rotation. It’s probably not a rotation that I would recommend, but I think you’ll continue to see canola in rotation here unless disease becomes a lot worse,” he says.
In terms of the best-case scenario for crop rotation from a disease standpoint, Irvine and McLaren both recommend cereal crops in at least a three-year rotation. They make the recommendation cautiously, however, asserting that growers must judge their own circumstances before making rotation decisions. “Rotation strategies are one of those things that are climatic and soil specific, and certainly we understand principles can be translated to different situations, but growers should have these conversations with local agronomists,” says Irvine.
Soil and Water
Studies have shown that rotation can improve not only disease rates, but the overall health and water and nutrient-holding capacity of the soil. A potato crop contributes very little organic matter or humus to the soil, greatly increasing the need for expensive inputs unless rotations are used and other best management practices observed. Beside its effect on yield, a lack of available nutrients in the soil can lead to lower quality tubers and thus lower marketability.
Additionally, poorly structured soil can also become compacted under the wheels of heavy farm equipment and may cause the soil to lose some ability to retain water.
Bert Tupling farms 1,250 acres of potatoes in a 3,500 acre operation in southwestern Ontario with his two sons, Andrew and Aaron, and uses two-year and three-year rotations. He also rotates potato varieties, using different varieties in each field to ensure that the land “does not become too accustomed” to any single variety. For Tupling, crop rotation has made a huge difference in yield and overall soil health on his operation.
“The number one benefit in my book is yield. That yield is gained from more organic matter built in the soil,” he says. “That organic matter, in our particular soil, has to do with the absorption or retention of moisture. In terms of the soil health, it allows more aeration, and thus oxygen [to reach the plants].”
According to Paul Adriaansen, who grows potatoes on 2,500 acres near Wellwood, Man. and maintains a three- to four-year rotation, the practice makes common sense. “I see crop rotation as a necessary tool for responsible farming, not a hindrance at all. Almost all of our irrigation rotates with the potatoes. Quality disease-free potatoes is the key to success in the marketplace for us, as well as for the processors who represent our joint product.”
Adriaansen says he always uses cereals in the rotation after potatoes. “We have had very good success with rye after potatoes on light land, low inputs and as of late good returns,” he says. Despite the researchers’ findings for canola in rotation, Adriaansen has found canola to be a valuable rotation crop for its impact on the soil. “We like canola before potatoes on the heavier soils—it seems to mellow the soil structure for planting. Many people in our area choose this for their rotation,” he says.
With regard to soil moisture, Adriaansen has had less luck with pulse crops in rotation. “In the bean rotation, soil organics are not restored, making the soil have less ability to absorb moisture. The result of this is a much harder time with clods of soil at harvest,” he says.
The Future of Rotation
According to the results of a study prepared by the Eastern Canada Soil and Water Conservation Centre called Crop Rotation Systems for Potato Production in Atlantic Canada, multiple factors limit the adoption of crop rotation as a standard best management practice. Among them are limited land availability, land tenure discouraging growers from investing in sustainable practices, farm debt and the long transition period involved in establishing an effective rotation.
As the Carberry researchers point out in the study Effect of Crop Yield and Quality in an Irrigated Potato System, rotation has little effect on yield and disease control for the first three to four years, and it may take even longer before the effect of rotation becomes predictable. “During the transitional phase of the rotation… few treatment effects were evident,” the authors of the study note. “In fact, it was not until the tenth year after treatments were imposed and beyond that strong and consistent effects of rotations occurred annually.” This time lag can be costly for growers.
However, Tupling believes the cost of land is the biggest factor preventing more growers from using crop rotations. “You’ll hear farmers say, ‘What’s the point of me growing alfalfa when I only gross 100 dollars an acre with that land?’” he says. “Land prices in this particular area are probably between $7,000 and $10,000 an acre. At that price, to grow alfalfa for a livestock operation is not profitable. They can’t sell the rotational crop for enough money.”
Nevertheless, many growers across Canada believe the risk to be worth it. And Tupling sees the risk itself as inevitable for any profitable operation. “Rotation comes with challenges in marketing, challenges in how to handle additional residue—you have to do it more than once,” says Tupling. “Everything you do, you have to have a market, and if you don’t have one you have to go find one.
“Farmers, in order to survive, have had to learn the little challenges that come up. Part of that is taking risks, and knowing how far in the risks to go,” he says.
