Being German, I can understand the desire of our local Pennsylvania
Dutch farmers to have clean, weed-free fields. After all,
what will the neighbors think if they see ragweed, lambsquarters,
pigweed or foxtail all over your farm?
In truth, however, the crucial question with weeds is, are they
just an aesthetic problem, or do they really interfere with
the growth of your crops? Many years of research in our Farming
Systems Trial here at The Rodale Institute have shown that
despite higher weed levels in the organic systems, we are still
able to get the same or higher corn and soybean yields as in
a conventional system using herbicides for weed control.
||"What will the neighbors think
if they see ragweed, lambsquarters, pigweed or foxtail
all over your farm?"
Nevertheless, several questions remain:
- Could we get even higher yields in the organic systems
if we had fewer weeds?
- Can we identify crop varieties that are particularly good
at tolerating higher weed levels?
In this article we will concentrate on the second question.
Over the past few seasons, we have been conducting corn and
soybean variety trials with the goal of identifying those
varieties that will work well for organic farmers—i.e.,
those that will produce competitive yields and high quality
grain in the presence of prolific weed populations.
We are now in our third year of comparing readily available,
“off-the-shelf” commercial varieties of food-grade
corn and soybeans classified for high yields and adaptability
for large-scale production. Among those varieties, we have
identified differential responses to varying weed levels across
crops, fields, and years.
Both corn and soybean varieties differ
widely in their ability to tolerate weed pressure.
To evaluate weed competition in each variety, we designate
two areas in each plot: one that is kept entirely weed-free,
and one that is treated according to our usual management
of 1-2 rotary hoeings and 2-3 cultivations. While the mechanical
management gets rid of most of the weeds, creating a weed-free
area usually requires additional weeding by hand, especially
to eliminate weeds growing within the crop rows.
At the end of the year, we compare yields from both areas
and decide which varieties are worth testing further and which
should be eliminated from the trial.
Findings so far
Our standard yield targets are 130 bushels per acre for corn
and 40 bu/ac for soybeans. Less than optimum weather in 2003
resulted in corn and soybean yields slightly below those levels
for all but two soybean varieties, Iowa 3006 and NC+ 3F43,
both of which achieved 46 bu/ac.
In 2004, by contrast, the weather was favorable and both
corn and soybeans did very well--soybean yields were around
46 bu/ac across all varieties, while corn yields averaged
nearly 220 bu/ac. Differences among varieties in yield impact
caused by weed pressure were much more pronounced in the poor-weather
year than in the favorable year.
Among corn and soybean varieties, yield differences between
hand-weeded and non-hand-weeded areas were highly variable,
especially in 2003, as the following two graphs illustrate.
Figure 1. Corn yield increase
in weeded plots in 2003 & 2004
For corn, yield differences ranged from 0 to 46 bu/ac, meaning
that for some varieties weeds had no effect on yields, while
in others they reduced yields by over 60 percent. Soybean
yield differences were between 0 and 18 bu/ac, translating
into weed-induced yield reductions of up to 54 percent. Corn
variety NC+ 68F32 and soybean variety Iowa 3006 showed the
lowest yield differences (under 10 percent) between weeded
and un-weeded areas for two years in a row.
Figure 2. Soybean yield increase
in weeded plots in 2003 & 2004
Weed tolerant vs. weed suppressant?
We consider varieties whose yields are minimally affected
by weeds to be weed tolerant. They should not be described
as weed resistant, since they were subject to weed populations
just as high as less-tolerant varieties.
In addition to testing for weed tolerance, we've wondered
if we could identify varieties that actually suppress weeds,
for instance by strong early growth and rapid canopy closure.
But although this seems theoretically possible, we have yet
to identify any varieties that exhibit this effect--each year,
weed biomass in the standard mechanical management subplots
has been essentially the same across all varieties.
Iowa 3006 had less than a 10 percent
yield loss due to weeds in both 2003 and 2004. Needless
to say, we now grow Iowa 3006 on most of our production
We did, however, see differences between corn and soybeans
in terms of weeds and their effect on yields. In corn, weed
biomass averaged about 2,000 pounds per acre dry weight in
both 2003 and 2004, but yields fluctuated by variety independently
of weed pressure in a particular subplot. For example, NC+
112E1 yielded 75 bu/ac at 2,200 lbs/ac of weeds, whereas NC+
68F32 yielded 119 bu/ac at 2,500 lbs/ac of weeds.
Soybeans, by contrast, were generally more sensitive to weed
pressure than corn. For soybeans, the higher the weed biomass,
the lower the bean yields. In 2003, average weed biomass was
around 1,500 lbs/ac and weed-induced yield losses averaged
35 percent across all varieties. In 2004, weed biomass was
less than 300 lbs/ac and yield losses were near zero for all
varieties. (Although it may sound like a lot, 300 lbs/ac of
weeds is a very small amount, comparable to what you can expect
with herbicide applications.) Low weed levels in the 2004
soybean plots were most likely due to preexisting low weed
pressure in that particular field.
Data from our Farming Systems Trial also support the idea
that economic weed thresholds are lower for soybeans than
for corn. For both crops, however, significant differences
in weed sensitivity among varieties are apparent.
Minimal yield losses in 2004 bring us back to the question
of the economic weed threshold, the weed density at which
a control practice is economically justified. As Matt Ryan
wrote in a previous
article, our results suggest that in some cases, weed
thresholds might be higher than expected. Since weed control
can be costly and time consuming, management tactics should
be applied with weed thresholds in mind.
So what does all this mean for your bottom line? Figures
3 and 4 show the differences between varieties and years in
terms of net returns for conventional, organic feed-grade
and organic food-grade corn and soybeans.
Figure 3. Net return for corn
conventional and organic production
As you can see, organic food-grade and even feed-grade crops
are many times more profitable on a net basis than conventional
crops of the same varieties. The organic price premium simply
dwarfs the reduction in yield sometimes (but not always) seen
in organic systems.
Figure 4. Net return for soybean
conventional and organic production
But the two graphs also indicate that within organic systems,
your choice of corn and soybean varieties can have a big impact
on year-end returns. In 2003, for instance, soybean variety
Iowa 3006 gave a net return of $705/acre--40 percent higher
than variety HP 204, which returned just $509/acre. Multiply
that difference across 100 acres or so and you're talking
Our results also suggest that while returns are higher in
favorable years, the choice of variety is more critical in
To conclude, the following two tables summarize the data
presented in the previous four graphs, while also presenting
the actual yield figures in the presence and absence of weeds,
as well as the weed biomass totals.The first two columns are
averages across all varieties; the remaining columns show
results for two specific varieties for each year.
Economic summary for corn varieties
Keep in mind that although the weed biomass totals appear
in some cases to vary a great deal between varieties (841
lbs/ac in Iowa 3006 in 2003, vs. 1635 lbs/ac in HP 204) these
differences were not statistically significant across all
replications. In other words, we have yet to see any consistent
weed-suppressant effect by any corn or soybean variety.
In terms of yields, you can see that for the most part, plots
kept entirely free of weeds did yield higher than plots in
which weed management was limited to standard mechanical cultivation.
In some cases the yield difference was slight; in others it
was dramatic--and in one case (the soybean variety Iowa 3006
in 2004) the yield was actually higher in the weedy plot!
Economic summary for soybean
As we move forward with these variety trials, our main criterion
for choosing one variety over another will continue to be
weed tolerance. Varieties that produce yields close to or
above our yield targets while showing less than a 10 percent
yield loss because of weeds are re-tested the following year.
Poor performers are dropped from the experiment after a single
For soybeans we also evaluate bean quality, since split or
stained beans reduce food-grade prices and therefore overall
returns. It's important to keep in mind, however, that varieties
with lower bean quality can still be highly competitive on
the organic feed market.
For the 2005 trials we planted the following varieties:
In the second year of testing: NC+ 68F32, GH 41X2,
In the first year of testing: NC+ 72H54, NC+ 4771,
NC+ 60N37, LG 2603W, LG 2609W, GH 61K9, GH 61F3.
In the second year of testing: Iowa 3006, NC+ 2F11,
and NC+ 2FN93.
In the first year of testing: NC+ 2F45, NC+ 1F44,
Iowa 2054, Iowa 2067, Iowa 2068, GH 292N, GH 327, Viking
2022, Latham 280, Wilken 3494, Wilken 3476N. (The Wilken
varieties are feed varieties serving as a control.)
the RI trials suggest the utility of farmers conducting
simple variety trials on their own farms, as Jeff
Moyer described in his April
This winter we'll analyze the results of these trials and
draw up a new list of varieties to test in 2006.
This work is important because it represents a new approach
to variety evaluation. Conventional agricultural research
has tended to assume near-complete weed control through the
use of herbicides, so the ability of crop varieties to tolerate
weeds has received little attention. While much work remains
to be done, our results show that a tremendous opportunity
exists to identify
and develop varieties specifically suited to organic growing