Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 337

land unit sown with an intercrop (Vandermeer, 1989). If LER 1, intercrop-

ping can produce a higher total yield from a given area than can sole cropping;
when LER

1, sole cropping provides more yield per unit area.
The land use efficiency of intercropping sorghum with pigeonpea, a prac-
tice used extensively by Indian farmers, is illustrated by data from Natarajan
& Willey (1980 a ). Sorghum yields under intercrop and sole crop conditions
averaged 4190 and 4470 kg ha 1 , respectively; intercrop and sole-crop yields
of pigeonpea averaged 690 and 1020 kg ha 1 , respectively. Based on these
values, 0.94 ha of sole-cropped sorghum (4190 4470) and 0.68 ha of sole-

cropped pigeonpea (690 1020) were needed to produce the same yields har-

vested from one hectare of the intercrop, and the LER value of the intercrop
was 0.94

1.62. In this case, yield of each component of the mixture
was reduced by competition from the associated crop, but total yield from
intercropping, on a unit area basis, was 62% greater than from sole cropping.
Although intercropping does not always provide yield advantages over sole
cropping, LER values exceeding one have been reported for many different
intercropping systems (Willey, 1979 a ; Ofori & Stern, 1987; Balasubramanian
& Sekayange, 1990).
In some situations, farmers are interested primarily in obtaining full yield
of one “main” crop, but sow other species into the main crop for additional
benefits: more food and fodder, improved soil conservation, better weed
control, or other purposes (Willey, 1979 a ). LER values typically are not calcu-
lated in these cases, but exceed 1 if the farmer manages the intersown minor
crop(s) so that competition against the main crop is prevented.For reasons dis-
cussed below, main crop yields may even be enhanced by intersown species.
For example, in an experiment conducted by Abraham & Singh (1984),
average grain yield of sole-cropped sorghum was 1.97 Mg ha 1 , whereas yield
of intercropped sorghum was 2.70 Mg ha 1 ; intercropped cowpea provided an
additional 0.40 Mg ha 1 of seed.
Greater yield stability may be a particularly important advantage of inter-
cropping systems. Using data from 51 cropping systems trials conducted in
India from 1972 to 1978, Rao & Willey (1980) calculated that for a disaster
income level of 1000 rupees ha 1 , growing only sole-cropped pigeonpea
would fail to produce enough revenue one year in five, growing only sole-
cropped sorghum would fail one year in eight, growing both sorghum and
pigeonpea as sole crops would fail one year in 13, but growing a
sorghum/pigeonpea intercrop would fail just one year in 36. Using similar
data, Trenbath (1983) calculated that to minimize a farm family's risk of
failing to produce an adequate number of calories for subsistence, less land
was needed with a sorghum/pigeonpea intercrop than with sorghum or

0.68