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ISSUE 15   August 9, 2001



A question often debated is yield of Roundup Ready soybean varieties compared to conventional varieties. Perhaps time and accelerated breeding has made this a mute point. Another question is the affect of glyphosate on soybean nodules. An article by Charles Benbrook suggests that glyphosate injures soybean nodulation and nitrogen fixation contributing to soybean yield drag.

Glyphosate is toxic to the bacteria in soybean that causes nodule development and nitrogen fixation when bacteria is grown in direct contact with glyphosate in the petri dish. Does glyphosate affect bacteria in roots of Roundup Ready (RR) soybean?

King et al. Agronomy Journal 93:179-186

Sprayed innoculated RR soybean inn N-free soil in greenhouse. Results of first set of experiments:

RU @ 1.5 qt/A at 5 days after emergence (DAE) follow by (fb) RU @ 1.5 qt/A at 10 DAE.
At 9 days after 2nd application = reduction in weight of nodules and slight reduction in shoot weight
At 30 days after 2nd application = no difference to untreated.

Results of 2nd set of experiments:

RU applied three times at 1.5 qt/A (18, 25, and 32 DAE) - a total of 4.5 qt/A.
At 8 days after 3rd application = no difference in shoot weight, nodule weights, and nitrogen concentration as the untreated.
Glyphosate did not affect bacteria in soybean nodule’s.

Results of soybean variety testing:

RU applied five times at 1.5 qt/A = at total of 7.5 qt/A of RU on five different soybean varieties.
At 8 days after last application = one variety had less total nodule weight but another variety had greater nodule weight compared to the untreated. Nitrogen concentration did not differ in all five soybean varieties. Shoot weight on one variety was slightly less but did not differ for the other four varieties. Three varieties had less root weight and root nitrogen concentration. Considering five applications, total RU rate applied, only nitrogen available was that fixed by nodules on roots, and short recovery time, it appears that glyphosate does affect nodule function. Two varieties showed no affect at all.

Result of glyphosate on soybean yield:

Field location where RU was applied twice at 1.5 qt/ each application on two RR varieties yielded the same as each other or with the hand-weeded. At two comparisons where the soybean were drought stressed yield was reduced. No differences occurred at the four other comparisons. Drought stress combined with two applications of RU appeared to reduce yields in some situations with RR soybean.

The overall conclusions were that glyphosate did not have long-term effects on nitrogen fixation on RR soybean under normal conditions. There may be some concern with use of glyphosate under drought conditions where yield is limited.

Source: Benbrook, C. 2001. Troubled times and commercial successes for Roundup Ready soybean: glyphosate efficacy is slipping and unstable transgene expression erodes plant defenses and yields. Northwest Science and Environmental Policy Center.




Nelson and Renner in Michigan (Agronomy Journal 93:428-434) evaluated growth, development, and yield of soybean when glyphosate at 2 qt/A was applied at the V5 stage of Roundup Ready (RR) soybean and conventional herbicides were applied at the V5 stage of conventional soybean varieties. Glyphosate did not affect vegetative growth, reproductive development, leaf area, or yield compared to nontreated RR soybean. Tank-mix of Galaxy+Harmony GT+Poast or Basagran+Cobra+Select caused 14 to 18% soybean injury 21 days after application (DAA). Herbicide tank-mixes caused delayed vegetative development 7 DAA, delayed reproductive development at 20 and 80 DAA, reduced season-long soybean height, and reduced aboveground soybean dry weight at 35 and 56 DAA compared with nontreated plants in a weed free environment.

Application at the V5stage was scheduled to reduce incidence of white mold, although, lack of canopy closure from soybean injury was thought allows late-germinating weeds to grow.



Dr. Jerry Doll, Weed Scientist at University of Wisconsin compared several weed ID books for content. Below is a synopsis:



# of species



Weeds of North Central States


-a "how to use the keys" section
-keys based on flower color, woody plants, annual brdlf and grass in corn and soybeans

Ontario Weeds


-shortcuts to jump ahead in keys
-excellent text descriptions
-rich in color plates at end of book

Weeds of the Northeast


-vegetative characteristic key
-shortcut ID tables
-10 "comparison tables"
-foldout grass ID
-20 brush species

Weeds of Nebraska & Great Plains


-key based on flower, leaf, and growth characteristic
-17 excellent figures of plant & flower parts, shapes, etc.
-excellent black/white drawings
-includes uses and values, poisoning, and historical data.

Weeds of Northern USA and Canada


-key to mature plants based on leaf characteristic and flower color
-key to seedlings based on cotyledon shape
-Quick ID feature
-family characteristic

Richard Zollinger
NDSU Extension Weed Specialist



The North Dakota Department of Agriculture has issued another Section 24(c) Special Local Needs registration for a glyphosate product in flax. Growers now have four product options for preharvest weed control in flax. These are:

1. Roundup UltraMax from Monsanto

2. Round Ultra RT from Monsanto

3. Glyphomax Plus from Dow AgroSciences

4. Gly Star Plus from Albaugh

Please note that the SLN registration for Roundup Ultra RT is only for the 2001 growing season, and that the registration expires on December 31, 2001. Supplemental labeling for each of these products should be available at chemical dealers.

Duane R. Berglund
Extension Agronomist

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