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9/29/2008 - 10/5/2008

Make the Right Tillage Management Decision

by Mahdi Al-Kaisi, Department of Agronomy, and Mark Hanna, Department of Agriculture and Biosystems Engineering

This year, as producers decide whether to till or not, special consideration should be given to high fuel prices. Generally, producers know what it takes to operate tillage implements and how much fuel it would take to finish the work. Conventional tillage plus planting and spraying in general would require approximately three to four gallons of fuel per acre compared to one gallon per acre for no-tillage. Field operations in general, including tillage and harvesting, along with nitrogen fertilizer use and crop drying consume most of the crop production energy.

Soil Compaction
If fuel cost is not enough reason for farmers to consider no-tillage, there are other benefits to consider when deciding whether to till and not to till. The risk of soil compaction and soil nutrient losses, whether through reduced soil tilth or potential soil erosion, is a loss that will add to the total cost of energy and farming input. These losses are real and well documented as reflected in yield, fertilizer energy costs, and environmental risks for soil and water quality.

conservation pratices near Red Rock Lake

Conservation practices for protecting water quality near Red Rock Lake in central Iowa. (USDA Natural Resources Conservation Service)


The decision to till at any time (fall or spring) needs to be carefully planned. When soil conditions are near field capacity, soil aggregates are "lubricated" by water and readily reposition themselves through the air spaces. This is especially true when heavy harvest or tillage equipment is used. In addition, equipment operators need to remember that soil compaction can occur during the application of manure or anhydrous as well when soil moisture exceeds field capacity (maximum amount of moisture retained by the soil). Under wet conditions, the use of heavy equipment, such as tractors, grain carts, and combines, can significantly change soil structure and cause soil compaction. Operating in wet conditions and especially doing extra tillage will increase fuel use per acre as well. 

Compaction near the surface, within the top three to six inches of the soil, is generally associated with the amount of surface pressure. Compaction below that is primarily associated with axle weight. For example, if soil a foot below the surface is at field capacity and the tractor's axle load is seven to eight tons or greater, compaction can occur at this depth, despite lower surface pressures.

To mitigate the risk of compaction, use controlled traffic lanes for harvest and avoid driving loaded grain carts randomly through the field. Most damage occurs in the first pass of the implement. Grain tank extensions on combines also add to the load on soil. Check tire size and pressure, since larger tires allow for better "flotation" and lower tire pressure reduces the load on the soil.

Residue Cover
To minimize overwinter soil erosion, work on getting even distribution of crop residue while harvesting. Doing so is critical in preventing soil erosion throughout most of the coming fall, winter, spring, and well into 2009 when next year's crop establishes a canopy. Any residue down to the finest material--straw, chaff--can potentially reduce erosion by stopping rain splash, slowing and trapping runoff, and allowing for better water infiltration. But it has to be in place to be effective.

Large combine heads tend to concentrate material, especially fine material, in a narrow swath behind the machine. Concentrated residues are not only less effective in stopping erosion throughout the field, they also insulate the soil surface from the sun, reduce seed- to-soil contact, and make it tougher to plant in the spring, inhibiting crop growth. Furthermore, doing a good job of straw and chaff spreading this year could eliminate the need for tillage passes this fall or next spring.

Have combine operators set up and run equipment so that straw and chaff spreaders or choppers operate properly and the combine distributes residue evenly. Corn residue is usually heavier and most corn heads do a good job of chewing up the stalks and dropping them back in place. But the challenge is greater in soybeans, where essentially the whole plant goes through the combine and residue becomes fragile.

Combine operators also should pay attention to the height of the crop stubble left in the field. Crop stubble can protect the soil by limiting exposure to wind and water erosion and trapping snow through fall, winter, and early spring. Of course, soybeans need to be cut near the ground to avoid grain loss, but operating the corn head higher leaves more stubble and fewer stalks are run through the machine.

Producers who want to establish a goal of 30 percent residue at planting time next year probably should not go to the field this fall with a tillage implement, particularly into soybean residue. Soybean fields should be left in no-tillage. The amount of soybean residue is not significant enough to cause any change in soil conditions, especially for those who are concerned about soil wetness. If there is such a concern, then a good evaluation of the field conditions and assessment for tile drains needs to be considered rather than continuous tilling, which increases the potential for energy loss, soil loss, yield loss, etc.

As Iowa producers start to make decisions for the 2009 growing season, it is important to reflect and evaluate the outcomes of the previous season during the conservation planning process. Observations of yield performance, residue cover, and soil conditions should be guiding principles that minimize the potential of repeating any conditions that caused problems in the previous year.

Above all, conservation tillage and no-tillage are great systems and have a positive impact on soil productivity and profitability, no matter what the weather does. These systems conserve energy, improve soil tilth and soil organic matter, and can reduce the capital costs associated with the tillage equipment used in conventional tillage. Conservation decisions made now can affect soil erosion over the next several years. Producers should use the time after harvest to gather information and make sound decisions about conservation systems.



Mahdi Al-Kaisi is an associate professor in agronomy with research and extension responsibilities in soil management and environmental soil science. Mark Hanna is an extension agricultural engineer in agricultural and biosystems engineering with responsibilities in field machinery.

Seed Quality at Harvest

by XB Yang, Department of Plant Pathology
During the past planting season, there were many reports of low soybean seed germination rates, which may have been the result of last year’s wide spread of Phomopsis. Severe  Phomopsis fungus infection can reduce seed quality. This season, several diseases (Cercospora, downy mildew, and white mold) were prevalent in different parts of Iowa; the causal fungi of these diseases can infect seeds. As harvest begins, it is time to learn about these diseases and check seed quality.

Cercospora leaf spot. This fall Cercospora leaf blight was wide spread in some parts of Iowa, particularly central and northeastern Iowa. Cercospora leaf blight is caused by Cercospora kukuchii.  In late-July and August, the disease is easy to identify by a mottled purple-to-orange discoloration of the uppermost soybean leaves. The leaves also have a leathery appearance. In September, when soybean plants are approaching maturity, infected leaves turn orange or bronze. This season, delayed planting and warm weather in the fall seemed to favor the development of this disease. In fields with high disease incidence, soybeans are not the color we are familiar with. From a distance, bronzed leaves of these affected plants can be mistaken for sudden death syndrome or pod and stem blight. Close examination shows that only top leaves are affected. Affected plants may show up in only portions of the fields.

If the disease was found in fields before September or the disease is severe in the fall, seed infection is more likely. Infected seeds have a purple discoloration called purple seed stain. The level of seed infection varies with the level of foliar infection.  Discoloration may not be present in soybeans where infection developed late in the season. Seed infection by Cercospora fungi may cause poor seed vigor and reduced germination. Beans with substantial amounts of discoloration should not be saved for seed because of the seedborne nature of the disease. Infected seeds carry the disease, which can cause foliar infections when planted.  

Downy mildew is a late season disease in Iowa. However, this summer the disease appeared as early as late July. The disease is prevalent in cool wet seasons. Infected soybean leaves have regular shape, small lesions defined by a few cells. The lesions are pale or light yellow in color on the upper surface of the leaves. On the underside of the infected leaves, the lesions are grey in color with turf like mycelium which can be seen without a magnifier. The lesions are found in the upper plant because the fungal spores are airborne. Defoliation can occur when the disease level is high. In this situation, downy mildew will infect the seed, creating white mycelium on the seed coat.

White mold occurred again in part of eastern Iowa this season, although its occurrence was less extensive compared with the last few years. This was due in part to the less dense canopy from delayed planting. This fungus also infects soybean seeds. Infected seeds are light, small in size, whitish with a shrunken appearance. The regular gravity-clean process used in seed processing plants separates infected seeds from healthy seed. Therefore, it should not be a production concern when you purchase seeds from a quality company. However, the fungal sclerotia, which are similar in color and shape of mice droppings, could be mixed with seed from a white-mold-infected field. Soybean saved for seed should be checked for sclerotia if while mold was present in the field.


Fall soybean leaf with leaf blight

Soybean field with leaf blight late in fall.



XB Yang is a professor of plant pathology with research and extension responsibilities in crop diseases.

This article was published originally on 10/6/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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