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
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
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
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
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
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 acreage.
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
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 varieties
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 real money.
Our results also suggest that while returns are higher in favorable
years, the choice of variety is more critical in unfavorable years.
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
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 varieties
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 year.
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, GH 44X2. 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.)
|Results of 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 conditions.