Wrestling with Crop Rotation Decisions
By Greg Stewart, OMAFRA Corn Specialist

In a season where no crop seems to stand out as a profit producer, it is difficult to get excited about pondering crop selections or sequences for this fall or next year. However, a year like 2000 reinforces the need for sticking with crop rotations that build soil structure, improve water infiltration and lend themselves to reduced tillage systems in order to provide some protection against soil erosion.

Unfortunately, the most profitable crop rotations in the short term are being increasingly positioned as less sustainable and even less profitable when a slightly longer time frame is considered. Several studies have positioned the corn-soy-soy rotation as being the most profitable rotation in the U.S. Cornbelt. Unfortunately this rotation has some obvious weaknesses:

Multiple Year Soybeans
The dominant use of soybeans in long-term rotation studies has produced some compelling numbers. One of these studies was conducted at the Woodslee Research Station (soil type: Brookston clay loam) by Agriculture and Agri-Food Canada researchers Susan Weaver, Al Hamill and Jianhua Zhang. They compared crop rotations of continuous soybeans, corn-soys, and corn-soys-wheat (among others). Soybean yields declined quite dramatically over the course of the nine-year experiment in the continuous soybean plots. This trend was less noticeable in the corn-soy rotation, and soybean yields were most consistent in the three-year rotation including wheat. At the conclusion of the experiment all plots, regardless of the previous rotations, were planted to corn. When examined as groups, all rotations which included winter wheat resulted in significantly higher corn yields than those rotations that included only corn or soybeans.

More recently, Doug Young, agronomist with Ridgetown College, University of Guelph, has compiled some interesting crop rotation results. These rotation studies were initiated in 1994 and include four distinct crop rotations. The rotations and average soybean yields obtained over the last three years are illustrated in Table 1. Continuous soybeans lag significantly behind the other non-monoculture options, with highest yields resulting from the three-year rotations.

The message is fairly clear. The idea of swamping your crop rotation with soybeans to the point of continuous soybeans spells trouble. What is more difficult to get a good handle on is the impact of the popular crop rotation that has two years of soybeans back-to-back in a four-year rotation. Work done in Wisconsin showed that second-year soybeans often yielded quite well relative to first-year soybeans and certainly better than beans in the third consecutive year. Probably the biggest threats to two years of soybeans are reduced soil structure and increased soybean cyst nematodes. Results would indicate that low populations of soybean cysts can multiply rapidly in that second year of soybeans and cause real problems with soybean yield potential. Meanwhile, Tony Vyn's previous work from the University of Guelph indicated that two years of soybeans in a four-year rotation often resulted in poorer soil structure and greater erosion potentials than other less soybean-intensive rotations.

Repeating the Red Clover Song
Ken Janovicek, crop scientist at the University of Guelph (Department of Plant Agriculture), has taken a closer look recently at the value of red clover in a rotation. He compared crop yields over the past six years between two crop rotations. These rotations were the same, corn-corn-soy-winter wheat, with one exception: the wheat was underseeded to red clover in one set of plots and not in the other. These rotations had been in place since the early 1980's. Table 2 illustrates the yield advantages for the rotation including red clover for each crop in the four-year sequence. Janovicek calculates that at average prices there is a $63/ac increase in revenue over the four years. Add to this a reduced nitrogen cost of $12/ac in year one, (first-year corn) and you have an $85 benefit to the clover at an establishment cost of approximately $15.

The trouble with keeping red clover in the province’s rotations has been one of uniform stand establishment (and maintaining the stand) in the winter wheat crop. As well, vigorous stands of red clover have often been blamed with poor control of perennial weed problems or of requiring additional fall tillage. The merits of red clover warrant researchers and producers coming up with solutions to these problems. Peter Johnson, cereal specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, declares that using single-cut red clover will give nearly all of the benefits of double-cut red clover, but it will be much easier to control in the fall and will interfere less with quackgrass and bindweed spraying.

Reality Check
Short-term considerations (i.e., survival) often outweigh the longer-term agronomic considerations. Growers feeling pressure to plant more restrictive rotations (such as back-to-back corn or back-to-back soybeans) should do so realizing their options, benefits and risks. Taking the appropriate soil samples this fall to determine your soybean cyst nematode risk can provide you with valuable information about the potential downside of additional soybeans in your rotations. Fusarium infection in winter wheat fields seems to be increasing risk and management demands. However, making the effort to include winter wheat and underseeded red clover somewhere in your rotations can build soil structure and increase the profit potential for other crops in the rotation even when weather or commodity prices force you into less than ideal rotation decisions.

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