In field trials in 1987/88 near Pangbourne, England, wheat
(Triticum aestivum) and field beans (Vicia faba) were
grown in an organic farming system as sole crops and additive
intercrops. The sole crops were grown at 25, 50, 75, 100 and 150% of
the recommended density (RD) for conventionally grown crops. The
intercrops consisted of all density combinations of wheat and beans
from 25 to 100% RD in a factorial experiment. The grain yield of sole
cropped wheat and beans increased significantly as their density was
increased. The highest yield of both was achieved at 100% RD,
indicating that the conventional recommendation was the optimum when
applied to organically grown crops. Land equivalent ratio (LER) values
for the intercrops were significantly greater than 1·0 when the
wheat was sown at > 5% RD and beans at > 50% RD. The highest LER
of 1·29 was achieved when wheat and beans were both sown at 75%
RD. There was resource complementarity, expressed as relative yield
total (RYT) > 1·0, in all of the density combinations. There
was a significant decrease in resource complementarity with increasing
wheat and bean density. The nitrogen content of the wheat grain and
whole plant biomass was significantly increased when the density of
beans in the intercrops was increased; this was reflected in a
significant increase in grain protein at harvest. The total amount of N
accumulated by the wheat, however, decreased with increasing bean
density due to a reduction in the biomass of wheat. Beans also showed a
significant increase in %N as the density of the other component
increased and a decrease in total N accumulation due to reduced
biomass. All of the intercrops accumulated more N than the sole cropped
wheat, but did not exceed that accumulated by sole-cropped beans. The
biomass of weeds was greater under beans than under wheat. Weed biomass
in intercrops was significantly reduced when the density of wheat and
beans was increased, resulting in a lower weed biomass in the
intercrops than was achieved in either the sole cropped wheat or beans.
The N content of weeds was significantly reduced with increasing wheat
density but was significantly increased with increasing bean density.
The total amount of N accumulated by weeds per unit area was reduced
significantly by increasing the density of both components. The levels
of disease on the wheat were low, but mildew (Erysiphe
graminis) increased significantly as bean density increased. The
incidence of chocolate spot (Botrytis fabae) increased
significantly with increased bean density. The experiment demonstrated
that it was possible to harvest the crop with a combine harvester and
the wheat and beans can be planted separately mechanically, therefore
this system is suited to mechanized agricultural systems.
Weeds in fields with contrasting conventional and genetically modified herbicide–tolerant crops. I. Effects on abundance and diversity