Features Business & Policy Research
Rotation research update

Mohr and her colleagues have been studying this for a long time. In 1998, they initiated a 14-year experiment in Carberry, Manitoba involving six crop rotation treatments (see table on page 6), with irrigation only used on the potato crops to reflect the production practices of the area.

“At the time the study was started, pressure on the limited amount of land that was under irrigation in Manitoba, and the high value of potato relative to other crops had created interest in the use of two-year rotations,” Mohr notes. “However, we also looked at three and four year rotations to reflect standard practices.”

Canola was included as a rotational crop to test if it would reduce disease potential in potato. Alfalfa was included as a rotational crop in one four-year rotation to assess the impact of a perennial legume on disease incidence, weed populations and soil properties, in comparison to a four-year rotation of annual crops only. Wheat and oats were also included to reflect commonly grown cereals in the region.  

“We found that in the first nine years, no single rotation or group of rotations consistently out-performed the rest, but this changed as the rotations matured,” says Mohr. “During 2002 to 2009, canola-containing rotations produced higher yields in 2002, 2004 and 2005, but not beyond 2005. From 2007 to 2009, two-year rotations produced a markedly lower yield than three and four-year rotations but then yield differences appeared to diminish.”

Between the three and four-year rotations, no statistical differences were noted although P-C-W and P-C-A-A were most often among the higher-yielding rotations.

“In general, rotation had little effect on tuber quality factors such as hollow heart, fry colour and specific gravity,” Mohr adds, “likely because factors that influence quality such as variety, nutrient management and irrigation, as well as environmental factors like temperature, were consistent among rotations within a given year.”

The presence of canola promoted some diseases (such as ‘potato early dying’ PED wilt disease) while suppressing others such as fusarium dry rot. “It appeared that one of the Fusarium species that causes Fusarium Head Blight in wheat may also have infected potato,” notes plant pathologist Debbie McLaren.

Having a perennial alfalfa crop in the rotation appeared to increase the potential for sclerotinia, even in four-year rotations, as both canola and alfalfa are sclerotinia-susceptible. In terms of weed incidence, having alfalfa in the rotation did result in an increased occurrence of alfalfa, sweetclover and medic in the other crops.  Short rotations did not increase the weediness of the potato crop.

“With soil erosion, the potential for this is always higher following potato in rotation as crop residue cover is below the level required to control it,” says soil scientist Alan Moulin. “We found that soil organic carbon did not change during the first seven years of the study.”

Analysis of the later years is currently underway, but no additional soil organic carbon data is available at this time. “Soil microbial communities were found to differ between soil depths, among rotations, and among phases within a rotation,” adds soil microbiologist Marcia Monreal.

Higher wireworm larvae populations were measured where wheat preceded potato and in most cases, a higher percent of damaged tubers was observed in this rotation. “We found lower wireworm populations where canola had been grown prior to potato,” says Mohr. “This is likely due to the application of thiamethoxam (Helix Xtra) to canola seed and the production by canola of glucosinolate degradation products, both of which may deter wireworms and click beetles. Wireworms also have a preference for crops other than canola.”

Looking at the study from an overall economic perspective, because potatoes are a high-value crop, shorter rotations and potato yield were the biggest factors driving profit.

“The P-C rotation therefore had the highest net revenue from 1999 to 2005 because of potato value and yield, not canola,” says economist Mohammad Khakbazan. “However, after 2005, the economic performance of two-year rotations declined due to lower yields.”

P-C-W was also among the higher revenue-generating rotations, while P-C(A)-A-A and P-W-C-W tended to have lower economic returns due to the low frequency of potato in rotation. Averaging across the period 1999 through 2010, there was no difference in net revenue among potato rotations, but a trend toward higher net revenue for potatoes in two-year compared to three-year and four-year rotations. “Results suggest that, despite the production risks associated with two-year rotations in the longer-term, an economic argument can be made for using two-year rotations in the short-term due to the high value of the potato crop,” Khakbazan notes.

Disease, specifically PED, appeared to be the main driver influencing marketable potato yield as neither weed pressure nor declining soil quality were closely associated with yield declines in the two-year rotations.

“The Potato Rotation Study at Carberry was discontinued in 2011 after fourteen growing seasons,” says Mohr.  “In 2012, the entire experiment was planted to soybeans, and soybean yield and quality will be measured to determine the effect of the various rotation treatments.”

SIDEBAR

Rotations used
potato-wheat (P-W), potato-canola (P-C), potato-canola-wheat (P-C-W), potato-oat-wheat (P-O-W), potato-wheat -canola-wheat (P-W-C-W), potato-canola (underseeded to alfalfa)-alfalfa-alfalfa (P-C-A-A).

Researchers
Ramona Mohr, Byron Irvine, Mohammad Khakbazan, Debra McLaren, Marcia Monreal and Alan Moulin at the AAFC Brandon Research Centre in Brandon, MB; Dale Tomasiewicz and Alison Nelson at the Canada-Manitoba Crop Diversification Centre in Portage la Prairie.