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Effect of weeds on soybean harvesting efficiency
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August 17, 1998 - Although the primary cost of weeds in crops is their impact on yields, weeds escaping control also may interfere with harvesting and increase weed populations in subsequent years.  While there is considerable research available on weed competition and a growing amount of information on the implications of weed seed production on future management issues, there has been little research investigating the effect of weeds on harvesting efficiency.  This article will summarize results of a recent paper appearing in Weed Technology (Ellis, J.M., D.R. Shaw, and W.L. Barrentine.  1998.  Soybean seed quality and harvesting efficiency as affected by low weed densities.  Weed Technol.  12:166-173.)

The research was conducted in Mississippi and evaluated the effect of low populations of common cocklebur, hemp sesbania, redroot pigweed, sicklepod and ivyleaf morningglory on soybean seed quality and harvesting efficiency.  Early maturing soybean varieties were studied since weeds pose a greater problem in this situation due to the weeds frequently being green at the time of harvest.   The weeds were established at populations ranging from 0.125 to 2 plants per meter of 30-inch soybean row (equivalent to approximately one to 16 plants per 26 row-ft).  Data collected included:  shattered soybean seeds per sq.ft immediately before and after harvest, time required to harvest plots, soybean moisture at harvest, damaged seeds, foreign material, test weight, and splits.  A Gleaner Model M-2 combine was used for harvesting.  I will only present information concerning cocklebur, pigweed and morningglory in this article.

Harvestable soybean losses were not affected by the weed populations in this study.  Combine speed decreased slightly as weed density increased.  Large weeds such as cocklebur reduced cylinder speed and forced the operator to reduce combine speed.  Pigweed, cocklebur or morningglory did not affect the number of soybean splits.  The presence of weeds resulted in an increase in the number of damaged soybean seeds.  For example, the percent of damaged seeds in plots with 0, 0.25 and 2 morningglories per m row was 13, 13.5 and 28%.  Increases in damaged seeds often can be attributed to increases in soybean moisture at high weed populations.

The effect of these weeds on % foreign material and soybean moisture in Table 1.  The presence of either of the three weed species at 1 plant per row-meter resulted in approximately a doubling in foreign material compared to plots with no weeds.  Cocklebur at one plant per row-m increased soybean moisture at harvest four percent, the effect of pigweed and morningglory on soybean moisture was slightly less than that of cocklebur.

Table 1.  Effect of three weed species and densities on harvesting characteristics of soybeans.

Species 0.25  weeds/row m 0.5 weeds/row m 1.0 weeds row m

% foreign material (5% in control plots)

Cocklebur 5.2 7.0 10.6
Redroot pigweed 5.3 6.8 8.5
Ivyleaf morningglory 6.7 7.8 9.9

% moisture (14% in control plots)

Cocklebur 14.7 16.0 18.6
Redroot pigweed 15.0 15.6 16.9
Ivyleaf morningglory 14.3 14.8 15.8
  % yield loss
Cocklebur 7 14 28
Redroot pigweed 10 17 25
Ivyleaf morningglory 4 8 17

SourceEllis et al.  1998.  Weed Technol.  12:166-173.

The weed populations used in these studies were selected to be below the damage threshold (population at which a significant yield loss occurs).  However, the researchers noted that yield losses occurred with all species studied and at most weed populations (Table 1).  For example, cocklebur at one plant per row-meter reduced yields approximately 25%. 

Although the primary concern with weeds is their impact on yields, other effects need to be considered.  This research documents some of the effects weeds may have on harvesting efficiency.  The research was designed to look at situations where weeds remained green at the time of harvest, so some of the reported effects may be larger than we would expect to see here in Iowa since many weed  speicies have dried down at the time of soybean harvest. Weeds that remain green, such as waterhemp, could be expected to have similar effects as reported here. 

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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