Agroecosystem Diversity and Stability - Agroecology: The Ecology of Sustainable Food Systems - page 229

collection of monocultures. Any value greater than 1 indi-
cates a yield advantage for the intercrop, a result called
overyielding . The extent of overyielding is given directly
by the LER value: an LER of 1.2, for example, indicates
that the area planted to monocultures would need to be
20% greater than the area planted to the intercrop for the
two to produce the same combined yields. An LER of 2.0
means that twice as much land would be required for the
monocultures.

When the total LER is less than 1.0, negative inter-
ference has probably occurred, especially if the LERs of
the component parts of the mixture are all lowered in a
similar fashion. In this case, the intercrop provides a yield
disadvantage compared to monocropping.
When analyzing LERs and partial LERs, confusion can
often arise about what constitutes an advantage and what
the magnitude of the advantage is. Avoiding confusion
requires the recognition that different circumstances call for
different criteria for evaluating an intercrop's advantage.
There are at least three basic situations (Willey, 1981):

Application and Interpretation of the LER

1. When combined intercrop yield must exceed the

yield of the higher-yielding sole crops. This situa-
tion may exist when assessing mixtures of very
similar crops, such as pasture forage mixes, or
mixtures of genotypes within a crop, such as a
multiline wheat crop. In such cases, partial
LERs are not important in determining advan-
tage as long as total LER is greater than 1.0,
because the farmer's requirement is mostly for
maximum yield, regardless of which part of the
crop system it comes from. The quantitative
advantage is the extent to which the combined
intercrop yield is increased and total LER
exceeds 1.0, as compared to the yield of the
highest yielding sole-crop.

Since the partial and total LER values are ratios, and not
actual crop yields, they are useful for comparing diverse
crop mixtures. In a sense, the LER measures the level
of intercrop interference going on in the cropping
system.
Theoretically, if the agroecological characteristics of
each crop in a mixture are exactly the same, planting them
together should lead to the same total yield as planting them
apart, with each crop member contributing an equal pro-
portion to that total yield. For example, if two similar crops
are planted together, the total LER should be 1.0 and the
partial LERs should be 0.5 for each. In many mixtures,
however, we obtain a total LER greater than 1.0, and partial
LERs proportionately greater than what would theoretically
be obtained if each crop were agroecologically the same as
the others. A total LER higher that 1.0 indicates the pres-
ence of positive interferences among the crop components
of the mixture, and may also mean that any negative inter-
specific interference that exists in the mixture is not as
intensive as the intraspecific interference that exists in the
monocultures. Avoidance of competition or partitioning of
resources is probably occurring in the mixture.
When the total LER is greater than 1.5, or when the
partial LER of at least one member of the mixture is
greater than 1.0, there is strong evidence that negative
interference is minimal in the intercrop interactions and
that positive interferences allow at least one of the mem-
bers of the crop mixture to do better in the intercrop than
it does when planted in monoculture.
The traditional corn-bean-squash intercrop discussed
in chapter 15 — with a total LER of 1.97 — provides a
good example of this situation (Table 15.3). The corn com-
ponent of the system expressed a partial LER of 1.50,
meaning that it actually produced better in the mixture than
when planted alone. The positive interference responsible
for this result may be mutualistic mycorrhizal connections
between the corn and beans, or a habitat modification by
the squash that enhanced the presence of a beneficial insect
and reduction of a pest. Although partial LERs for beans
and squash were very low (0.15 and 0.32, respectively),
their presence obviously was important for the yield
enhancement of the corn.

2. When intercropping must give full yield of a

“main” crop plus some additional yield of a
second crop . This situation occurs when the pri-
mary requirement is for some essential food
crop or some particularly valuable cash crop.
For there to be an advantage to the intercrop,
total LER must exceed 1.0 and the partial LER
of the primary crop should be close to 1.0 or
even higher. With the emphasis on a key crop,
the associated plants must provide some positive
intercrop interference. The corn-bean-squash
intercrop mentioned above is a good example of
this situation because the farmer is mainly inter-
ested in the corn yield. If some additional yield
is obtained from the beans and squash, even if
their partial LERs are very low, it is seen as an
additional bonus beyond the yield advantage
gained by corn. The quantitative advantage is
the extent to which the main crop is stimulated
beyond its performance in monoculture.

3. When the combined intercrop yield must exceed

a combined sole-crop yield. This situation
occurs when a farmer needs to grow both (or
all) the component crops, especially when there
is limited land for planting. For the intercrop to
be advantageous, total LER must be greater
than 1.0, but no member of the mixture can
suffer a great reduction in its partial LER in the