Crop rotations can deliver more than the obvious benefits.
For example, fungi, bacteria, viruses and insects are often economically damaging only to a specific crop.
“Change the species of the crop in the field and you change the environment for that pest,” says Tom Jensen of the International Plant Nutrition Institute in Saskatoon.
Rotations between crop species can interrupt the life cycles of many pests that overwinter in soil and field litter.
“A winter wheat will change the life cycle again,” he said. “Fall planted crops can provide their own early season weed control by jumping ahead of spring weeds and reaching a crop canopy stage before the weeds enter the picture and begin using up valuable nutrients placed there by the farmer and intended for the crop.”
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Agronomists say adding spring or fall tillage when moisture levels support it can also improve soil environments when managing weeds and other pests.
Cynthia Grant, a researcher with Agriculture Canada in Brandon, said the benefits of rotations also include nutrition.
“Some crops are better nutrient scavengers than others,” she said.
“They can use additional nitrogen that might have leached down below a shallow rooted crop’s root zone. They might be better at using phosphorus than a previous crop, so available material that wasn’t useful last year will be enough to carry another crop this year.”
For example, Jensen said flax uses phosphorus better than wheat.
“It is better able to acquire that (phosphorus). It might be due to more acidic rhizosphere near the roots of flax causing less soluble forms of (phosphorus) to be made available,” he said.
“If you plant a flax crop following wheat, it can use the remaining (phosphorus). If you plant wheat after flax, it can use (phosphorus) that is left in flax plant residues on the surface and in decaying roots.”
He said potatoes are another example of shallow rooted crops that can leave behind significant nutrients.
“Potatoes get large applications of (nitrogen, phosphorus and potassium),” he said.
“Following these up with a wheat or sunflower that are deep rooted can scavenge otherwise unavailable material, creating higher yielding crops for less money than producers might otherwise.”
Grant said planning rotations that produce healthier crops also increases the crop’s scavenging potential.
“Healthier plants root better, they are better able to take up nutrients and as a result become even healthier and yield significantly more due to their ability to resist stress and make use of water,” she said.
Jensen said crop rotations could enhance the ability of some plants to use mycorrhizzal fungi that infect their root systems to improve the use of soil nutrients.
“The fungi acquire the nutrients and they share them with the crop plants. As well, the combined soil contact of the plant roots and fungal hyphae are far greater than the plant would have alone. The plant gives the fungi something they need (photosynthetic sugars) and this produces a symbiotic relationship that results in more production for farmers.”
Farmers can take an active role in this symbiotic relationship by planting successive crops that make use of the same mycorrhizzal fungi.
Jensen said some crops don’t form these associations and can actually result in early season deficiencies in the year that follows them.
Wheat or corn that follows canola can suffer early season phosphorus shortages while cereals work to re-establish the mycorrhizzal infections and populations in the soil.
Grant said there is limited research about these relationships, but she feels there is significant potential for production improvements once they are better understood.
“There is more to rotations than we understand today,” she said.
“Sequestering of nitrogen or weed management is only the beginning. Improved soil condition, improved water holding, making nutrients available when plants can best use it all have a place in (crop rotations).”
