Fall 2020 - 39

CROP ROTATION
of crops grown on an organic farm it just becomes a very
messy, complicated problem, " he said. " You've got all
these different crops at different acreages ... and also just
the number of crops involved and the fact that they're
going in at various times, and some of them getting planted
multiple times over the course of the season and so forth. "
Mohler said he and Johnson used the DACUM, or
Develop A Curriculum, a systematic
process for " coaxing information "
out of workers about a particular
job, using a workshop setting.
" We thought that that might be
a way of getting information about
how growers actually do crop
rotation. We had all these contacts
in the organic farm organizations
(who) were part of the project,
and so we had them nominate
farmers who had a reputation for
the expertise in crop rotation, "
Mohler said. " So that's where
we started. And then you see the
problem is that it's not something
that has been effectively studied
experimentally; you can't just go
to the scientific literature and find
out much about crop rotation.
The experiments that have been
done are mostly things like a cornsoybean
rotation versus continuous corn, for example. "
AD HOC SEQUENCING
What Mohler and his colleagues found from the
growers was that almost no fixed crop rotations survive
unforeseeable fluctuations in weather, disease and prices.
" Unless the farm is growing very few crops, you can't
have a fixed crop rotation, because something always
interferes with your forward planning, " he said. " You may
plan ... but, you know, the weather intervenes. You can't
get the crop in the ground in a timely fashion, so then
you have to think, " Well, we have the field, it's a good
field, might as well grow something on it. So you grow
something else. Or you're planning on growing X but then
it turns out you get this market opportunity, and, 'Wow, I
could sell a lot of Y if I grew it this year!' and so then you
jump on it and grow Y.
" Well, then your rotation is all derailed. And so, one
thing that came out of (our study) was that the forward
planning horizon is usually just a year. "
Mohler said most organic growers do have a plan for the
next year - they need to know next year's cash crop to plan
their cover crop for the winter.
" You don't want to plant hairy vetch, for example, in the
fall on a field, if you're going to plow it up early to plant,
you know, early lettuce, because hairy vetch won't have
grown much and it won't have produced much nitrogen -
it's kind of wasting the seed, " he said.
A 'BIGGER FRAMEWORK'
But what he also found interesting was that although
some of the growers' crop rotations seemed ad-hoc, they
kept their options open by growing " within a bigger
framework, " taking a few acres out of cash crop production
some years.
" Usually the larger framework involves taking the fields
out of whatever your big money
maker is, maybe it'll be a year just
in cover crops, " Mohler said. " As
one grower told me, he said, it's a
whole lot easier to grow six acres of
vegetables on 12 acres of land than it
is to grow six acres of vegetables on
six acres of land. The same principle
applies to 600 acres of vegetables on
1,200 acres of land. "
Mohler admits, though, that the
" bigger framework " strategy isn't for
landlocked growers or those whose
farmland is simply so valuable they
can't grow anything besides cash
crops.
" In the Northeast, and to a large
degree in the Southeast ... a lot of the
vegetable farms are growing some
vegetables - they're not to a large
scale, " he said. " You know, six, 20
acres or something. But they have a
lot more land that's not in vegetables. And the reason is,
they're not scaled up to manage that large of a vegetable
area. But that allows them the possibility of these kinds of
rotations where you have part of the land in cover crops or
a sod crop.
" Now, you get out to the Willamette Valley, Oregon,
where I grew up, or Central Valley, California, and the
land is so valuable that you can't afford to do that, or at
least most growers can't. " One California grower he talked
to rotates with rice, just because most weeds can't tolerate
being in a flooded field.
ROTATION FOR WEEDING
Mohler's more recent work has focused on multi-year
strategies for getting rid of weeds, and he has coauthored
another book on that topic, " Manage weeds on your farm:
An ecological approach, " which is currently in press with
Sustainable Agriculture Research and Education (SARE).
Rotating between crops that are planted in the spring,
with crops that are planted in the fall, can change the
" competitive stress " on the weed community.
" Of course, the same practice also changes the times that
you're tilling the soil.
" If you have a weed that goes to seed late in the season,
and you plant a fall crop, your tillage is going to kill that
plant before it goes to seed, " Mohler said.
" If you change the length of the growing season of your
crop, you can change the weeds, " he said. For example, a
ORGANICGROWER.INFO 39
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Fall 2020

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