Agriculture Reference
In-Depth Information
An interesting example is row-mixtures of rice in China (Zhu
et al.,
2000). In
pure stands neck blast severities of about 35% were reported that were reduced to
2-5% in the mixtures. At the same time, yields in mixtures increased by 100- 200%.
As 35% severity of neck blast should not result in more than 35% disease loss
(Thinlay
et al.,
2000a) there must be additional benefits in this system. To
understand this it is important to know that in Chinese rice production, 200 kg or
more of mineral N are added routinely per ha and year. These fertiliser dosages
invariably result in lodging of the old traditional varieties which in addition are
susceptible to blast (M.R. Finckh, personal observation). Growing these varieties as
single rows between the much smaller and stiffer hybrids effectively prevents them
from lodging and allows them to make use of the large amounts of fertiliser. In
addition, the hybrids probably compete effectively for the fertiliser thus reducing the
dosage available to the landraces relative to the pure stands. To fully understand
these interactions, plants in pure stands and mixtures need to be analyzed for
nutrient content against a control treatment at lower fertiliser levels appropriate for
the traditional varieties.
10.3.5 Diversity and yield
The yield of cultivar mixtures in the absence of disease in most cases varies around
the mean of the components, with a tendency to be greater than the mean. Such
increases (e.g. Allard, 1960; Allard and Adams, 1969; Norrington-Davies and Hutto,
1972; Nitzsche and Hesselbach, 1983; Finckh and Mundt, 1992a,b) may be
explained at least partially by niche differentiation among the components.
Allelopathy and synergisms of unknown origin might also play a role.
In species mixtures, niche differentiation is an even more prominent feature.
Indeed, in almost all cases reported, the overall yield of species in mixtures exceeds
the mean of the species grown in monocultures, i.e. the
land use efficiency
is
increased (Liebman, 1995; see also Tilman
et al.,
2001).
Two kinds of competition act in mixtures: intra-genotypic and inter-genotypic
(Jolliffe
et al.
, 1984). This differentiation is important: if the average inter-genotypic
competition is less than the average intra-genotypic competition, the mixture will
yield more than the pure stand average. Because of the different competitive abilities
and density response thresholds of different genotypes, competition is usually
asymmetrical and difficult to measure (Spitters, 1983; Firbank and Watkinson,
1985). Competitive relationships among plants may also change over time, for
example, if intra-genotypic competition becomes greater than inter-genotypic
competition beyond a certain plant density, frequency, or developmental stage
(Finckh and Mundt, 1996). Frequency-dependent selection has been reported in
wheat cultivar mixtures in the presence and absence of disease but may not be
common (Finckh and Mundt, 1993). An example of negative yield interactions was
reported for cassava (
Manihot esculenta
) cultivar mixtures. There, inter-genotypic
competition was greater than intra-genotypic competition resulting in yield
depression in complete mixtures (Daellenbach
et al.
, 2005).
