Agriculture Reference
In-Depth Information
numbers, e.g. Zeleny values) are also influenced in different ways; available reports
indicate that mixtures show no negative effects or slight increases in baking volume
(Bayerische Landesanstalt für Bodenkultur und Pflanzenbau, 1987; Zoschke, 1987;
Jackson and Wenning, 1997).
Table 10.5. Yield, powdery mildew severity and the yield stability of barley cultivar mixtures
relative to their pure stands in five experiments (each with 6 to 11 repetitions) between 1987
and 1995 in Poland. Three-way mixtures and pure stands of four to seven cultivars were
compared and ranked within each environment. Ranks were regressed on the mean yield per
environment. The mean square error (MSE) of the rank regression of mixtures is expressed
relative to the MSE of the rank regression of pure stands. An MSE < 1 indicates higher yield
stability than the pure stands (data from Finckh
et al
., 1997)
Cereal
cv.
Year
Envts
Rel. yield
Rel. dis.*
Rel. MSE
Spring Barley (f)
a
7
87-89
7
1.03
0.81
0.56
Spring Barley (m)
6
88-89
6
1.03
0.85
0.48
Spring Barley (f)
5
91-93
11
1.02
0.70
0.79
Spring Barley (m)
4
91-93
11
1.02
0.60
0.67
Winter Barley (f)
5
93-95
10
1.02
-
0.97
Spring Barley (f)
6
94-96
9
1.01
0.77
0.82
Spring Barley (m)
4
94-96
9
0.99
0.74
0.52
Spring Barley (f)
5
94-96
9
1.01
0.68
1.07
* Disease data not from all sites
a
f =
feed barley; m
=
malting barley
10.4 BENEFITS OF DIVERSIFICATION IN TIME (CROP ROTATION)
Diversification can be implemented in time by sequential or rotational cropping of
species and varieties, which can have highly beneficial effects in controlling weeds,
diseases and pests. Crop rotations are fundamental in improving crop health in
various ways (Finckh, 2003). These can be divided into: time effects reducing
pathogen propagules in the soil or on crop residues; indirect effects via soil
microbial activity; direct suppressive effects of certain crops on certain pathogens.
While the presence of a pathogen is required to cause disease, the absence of a
pathogen is not necessarily required for a healthy crop. Rather, the balance between
beneficial and detrimental organisms usually determines the outcome. Rotations are
the key to reduced pesticide inputs and may enable ecological methods such as no or
reduced tillage to reduce erosion but, at the same time, may increase certain disease
problems. This can be dealt with by appropriate crop rotations and green manure
treatments (e.g. Teich, 1994; Davis
et al.
, 1996). For example, mulches and
minimum tillage are known to increase earthworm populations and these have been
shown to reduce Rhizoctonia bare patch (caused by
Rhizoctonia cerealis
) and take-
all (caused by
Gaeumannomyces graminis
) of wheat, two diseases for which no
genetic resistance is available (Stephens
et al.
, 1994 a,b,c). In cereal production in
