The importance of Wheat in Rotation

By Steve Dvorak

“The Importance of Wheat in Crop Rotations”—at least that is what I intended to write this article about. But it would probably be more apropos to simply title this “The Importance of Crop Rotations”; for I see equally concerning crop rotation deficiencies in both the eastern and western parts of the Northern Plains region. Off to our east it, seems like fewer and fewer are able to resist the industry’s push towards the almost monoculture corn/soybean “rotation”. And out west, it is still not uncommon to find fields that have either been in one class of wheat or another, or barley (or maybe some forage oats thrown in once in a while to spice things up) for years on end. In the first case, more wheat is the solution; in the latter, it is the problem.

When the subject of the importance of crop rotation comes up, or when the benefits of rotating crops or certain crop sequences are discussed, what is most commonly being alluded to is the importance of biodiversity in a sustainable system—and the productivity responses that are observed when it is reintroduced.

Namely, diversified crop rotations are more productive (the yields of any particular crop in the rotation are usually higher (often times 15 to 20% higher) than when in less diverse rotations) due to these observed benefits:

  • interrupted pest cycles (diseases, insects, and weeds have less opportunity to “get in sync” with the same couple of crops planted repeatedly in a consistent (predictable) pattern).
  • greater water use efficiencies and more complementary water use patterns (seasonally as well as due to different rooting types and depths).
  • nutrient cycling (complementary and varied patterns of nutrient fixation, immobilization and mineralization). And
  • improved soil health/quality (primarily from improved soil microbial balance and activity as well as from the return of greater amounts of various crop residues contributing to the maintaining and/or building of soil organic matter levels.

Maybe now is a good time to make a couple editorial comments. First let me say that the unknown benefits of adding biodiversity (including animal impact) back into a cropping system may actually be greater in total impact than are the known benefits listed above. So great have been the empirical observations of doing so that many are trying to exploit this biodiversity “magic” through the planting of multi-species cover crop cocktails in their cropping patterns whenever the opportunity presents itself, or even as a planned practice. And one of the best such opportunities comes after an early wheat harvest (winter wheat is usually harvested a week to 10 days prior to spring wheat). Not only do cash crop yields and values usually increase, but many growers also accomplish this while using lower levels of inputs—especially herbicides, insecticides, fungicides, and fertilizer.

Secondly, I feel it prudent (without being accused of beating a dead horse) to add my echo to the current clamor for more judicious management of the more valued (and therefore more commonly utilized) herbicide technologies. Of course I’m talking about glyphosate and glyphosate resistant weeds. All wheat varieties are still non-GMO and therefore non-“Roundup Ready”. Hence, including wheat in one’s cropping system essentially forces you to do what you should have been doing all along: a) include crops with different times and patterns of competition against weeds in your rotations, and b) utilize different herbicides with different modes of action.

Now back to our initial observations about out-of-balance rotations in our region. In western geographies where wheat and fallow have been overly relied upon, I will make reference to a series of experiments conducted at the Agriculture and Agri-Food Canada research facility near Swift Current, Saskatchewan during the years 1993 through 1997. Perry Miller, et.al. found that “the average yields of wheat grown on four pulse crop stubbles (desi chickpeas, dry edible beans, lentils, field peas) was 21% greater than yields on wheat stubble…” and that “compared to wheat stubble, wheat grown on broadleaf crop stubbles had higher grain protein levels, increasing by 8 and 5% (note: NOT percentage points) for pulses (see above list) and oil seeds (mustard, safflower, sunflower), respectively.” Perry Miller has since moved to Montana State University and has continued his crop rotation work producing similar findings.

While focusing on the ever-increasingly popular corn/soybean “rotation” (I place it in quotes facetiously as I can hardly consider the simple flip-flopping of 2 different crop species from year to year a serious rotation), I will provide a more substantial case for the need to add a third crop (wheat) back into the mix.

I will begin first by referencing an Ohio State University study looking at the accumulated impacts on soil quality of adding crop diversity (just soybeans or both soybeans and wheat) to what had been continuous corn for a number of years. I start here because I think this (soil health) is one of, if not the most important reason, for adding wheat back into the crop mix.

The study was initiated in 2002, and the measure of cumulative effects on select soil parameters was made in 2007. The soil characteristics tracked were as follows: total microbial biomass, basal respiration and specific maintenance respiration rates of CO2, total organic carbon, active carbon, total nitrogen, aggregate stability, particulate organic matter, and total porosity at different depths.

What is important to note from this study is that after only two complete cycles of the corn/soy/wheat rotation, all soil measurements (except porosity) improved significantly with the composite Soil Quality Index (SQI) improving by 15%. The importance of this finding is further underscored when contrasted to the impact of just adding alternate years of soybeans to continuous corn: after six years (three complete cycles of that rotation), there were no significant differences in any of the individual soil parameters or in the composite SQI relative to continuous corn.

I would be remiss not to mention that the wheat in this trial was winter wheat and its harvests were followed by the seeding of a cowpea cover crop. There is no way of separating out which (the wheat or the cover crop) was more important in improving soil quality—I suspect it was a combination (synergistic?) of both.

Here are the summarized responses (due to adding wheat to a corn/soybean rotation) from a couple geographically and climatically different studies from within the Dakotas region (Dakota Lakes, Pierre, SD, 1994-2000; and CCSP, Forman, ND 2004-2010):

Average corn yields in the corn/soybean rotation in the Pierre, SD, study were 59.5 bu/A. They jumped to 67.7 bu/A when wheat was added (Corn/Soy/Wheat), a 13.9% response. In contrast, at Forman, ND, corn yields only responded marginally (+3%: 165.9 vs 161.0 bu/A) when wheat was added.

Relative soybean yield responses were similar in both studies (~6.8%) when wheat was added to the corn/soybean rotation (16.4 bu/A in the C/S rotation at Pierre, bumped up to 17.5 bu/A in the C/S/W rotation; 38.4 bu/A in C/S rotation at Forman, jumped up to 41.0 bu/A in the C/S/W rotation).

A perusal of additional crop sequence and crop rotation studies from deeper into the Corn Belt (MN, WI, IA, NE) seemed to offer up these trends:

  • Corn is pretty happy in a corn/soybean rotation and much additional yield response is not likely if extended diapause corn rootworm problems are otherwise adequately addressed.
  • Soybeans are becoming increasingly distressed as the corn/soybean rotation becomes institutionalized. They more frequently show additional plant health and performance responses as rotations are stretched back out to include soybeans at a frequency of no more than1 year in 3.
  • And the overall frequency of 1 in 3 years is further improved upon if a 3 or 4 year break between soybean crops can be interjected into the rotation periodically.

To help substantiate the last point, consider some of the other rotations that have been or are being looked at near Forman, ND, at the CCSP demonstration farm: an additional 1.5 bu/A (an additional 3.7%) of soybeans was achieved in the SW/WW/Corn/Corn/Soy/Soy rotation as compared to the SW/Corn/Soy rotation —same frequency of soybeans in the rotation, but now including a 4 year break between soybean crops at one point. That’s 42.5 bu/A soybeans in that rotation, versus 38.4 bu/A in the Corn/Soy rotation, for an overall advantage of over 4 bu/A or +10.5%.

Another rotation that is no longer being looked at at the CCSP farm, SW/WW/Corn/Soy/Corn/Soy, was actually projected to be the overall most profitable (over a period of many years) rotation ever to be studied at the CCSP farm in an economic analysis conducted by an NDSU Ag Economics grad student, Thomas Zimmerman.

And that touches upon the last topic that I need to discuss: economics. The economic analysis of the CCSP data basically points out that the additional yields in the corn and soybean phases when rotated with wheat more than make up for the occasional to frequent less profitable wheat phases—so that in the end, rotations that include wheat along with corn and soybeans are actually more profitable than rotations that just include corn and soybeans.

So in summary, from all points of value (economic, agronomic, edaphic, environmental, and sustainability), including wheat in Northern Plains crop rotations just makes sense—and should be as it once was—a “no brainer”. —Steve Dvorak