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Chapter 3 - Residue Management, No. 3, August-September 1985

Combine Residue Distribution

Roger Veseth

Uniform distribution of straw and chaff from the combine at harvest is important in any farming system. It is especially important with no-till or minimum tillage planting of the following crop because more of the residue remains on or near the soil surface.

The potential for problems with combine residue distribution has increased over the past few decades for several reasons. Two of the most significant reasons are wider combine headers and higher residue production with new wheat varieties. Advances in fertilizer technology have also increased grain and residue production.

Average combine header widths have nearly doubled from about 12 feet in 1950 to 20 to 24 feet or more today. Most standard combines are not adequately equipped to uniformly spread the higher concentrations of residue with these wide header swaths.

The introduction of new high-yielding semidwarf wheat varieties has also increased the potential level of residue in the combine straw and chaff rows. STEEP researcher Paul Rasmussen, USDA-ARS soil scientist at Pendleton, OR, compared the grain and residue production data of a tall wheat variety commonly grown in 1953 to a high yielding semidwarf variety 30 years later in 1983 near Pendleton (Table 1). Along with the 56 percent increase in grain yield, total residue production increased 19 percent. The largest component of the residue increase was chaff, which increased 698 pounds per acre. That is a 63 percent increase compared to the 8 and 18 percent increases for stems and leaves, respectively. Doubling of the combine header width, together with this higher residue production of semidwarf wheats, has increased residue concentration in combine straw and chaff rows by about 120 percent since 1953.

Clyde Douglas and Ray Allmaras, also USDA-ARS soil scientists, along with Rasmussen and other STEEP researchers at the Columbia Plateau Conservation Research Center near Pendleton, have been evaluating the effectiveness of different combine residue distribution systems. They measured the straw and chaff distribution on a total of 26 farmer-operated combines during 1983 and 1984 in routine harvest operations. These included cylinder and rotary types, with and without straw and chaff spreaders.

Table 1. Effect of winter wheat variety development on yield and residue production from 1953 to 1983 near Pandleton, OR (Rasmussen, USDA-ARS, Pendleton, OR).


Growth-habit and year









Grain yield 3,000 4,680 1,680 56
Total residue 6,600 7,862 1,262 19
Stems 4,290 4,639 349 8
Leaves 1,200 1,415 215 18
Chaff 1,110 1,808 698 63


In 1983, total harvested plus uncut winter wheat residue in the fields averaged 4.8 tons per acre. Residue rates of up to 9 tons per acre were measured in the straw and chaff rows directly behind standard combines not equipped with adequate residue spreading attachments. Chaff comprised about 65 percent of the 9-tons-per-acre residue in the straw and chaff rows. Combines with modified flail systems or the addition of low-cost chaff spreaders had relatively uniform residue distribution across the header width. Straw choppers alone did not improve residue distribution.

The researchers emphasize there are many problems associated with heavy straw and chaff rows that can be prevented with chaff spreaders or modifications of the present distribution system. These problems include:

1. Poor drill performance — Drill plugging; straw "tucking" in the seed row; uneven seeding depth.

2. Uneven seedling emergence — Poor seed/soil contact; less access to solar energy.

3. Slower growth — Shading; cooler and wetter soils.

4. Lower nutrient availability — Microbial immobilization of N, P and S in residue decomposition.

5. Favorable disease and insect environment —Pythium root rot and other diseases favored by the concentrated food source and cool, moist environment; increased disease inoculum carryover with slower residue decomposition.

6. Reduced herbicide effectiveness — Delayed germination of weed and volunteer crop seeds; herbicide interception and absorption.

7. Increased crop competition — Concentration of weeds and volunteer limit nutrient and moisture availability to the crop.

8. Increased rodent damage — Concentrated food source and cover; protection from predators.

Successful no-till and minimum tillage systems require uniform combine distribution of straw and chaff at harvest. Commercial chaff spreaders or modified flail systems are now available to fit most combine models. Contact your local combine dealer, County Agricultural Agent or Conservation District for more information. Many growers have also made their own shop combine modifications to improve residue distribution. Good combine residue distribution systems are well worth the small time and financial investment.


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Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
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