Agriculture Reference
In-Depth Information
It is important to underline that strong competitive ability may not improve a
genotype's performance if it affects its harvest index adversely relative to other
mixture components (Hamblin, 1975; Hamblin and Rowell, 1975), or if it is more
disease susceptible than the others (Suneson, 1949). Highly skewed competitive
interactions may also be disadvantageous if seed from the crop is to be kept for
further multiplication (Harlan and Martini, 1938; Jennings and de Jesus, 1968;
Wolfe, 1991; Finckh and Mundt, 1993).
In the presence of disease, mixtures of cultivars frequently yield more than the
mean of the components grown as pure stands (Table 10.4; Baumer, 1983; Wolfe,
1987; Finckh and Mundt, 1992a; Czembor and Gacek, 1996; Gacek
et al.
, 1996a,c;
Finckh
et al.,
2000: see Wolfe and Finckh, 1997 and Finckh
et al.,
2000 for
reviews). Although the correlation between disease severity and yield is often clear
in pure stands, it is not always so in mixtures. This is because the correlation
between disease severity and yield on individual mixture components is often poor
(Brophy and Mundt, 1991; Finckh and Mundt, 1992b; Finckh
et al.,
2000). Thus,
there may be underlying interactions among component cultivars that can be
modified depending on the amount of disease on those components.
Table 10.4. Summary of yield (t ha
-1
) comparisons for a large number of field trials in
England over 11 years comparing the yields of different mixtures of three varieties of barley
with their components grown as pure stands. The positive effects of the mixtures were
considered to be due largely to restriction of powdery mildew because resistance was largely
effective and infection was consistently severe in the area (data from Wolfe, 1987)
(a) Mean yields
No. of trial
comparisons
Mean yield of
mixtures
Mean yield of
components
Per cent increase
in mixtures
(P<0.001)
122
5.61
5.20
7.88
(b) Ranking summary
Number of times mixture yielded more than only:
0 components
1 component
2 components
All 3 components
0
12
50
60
Generally, in contemporary use of cultivar mixtures, the components have not been
selected for performance in mixtures. It is unlikely, therefore, that they will perform
as well as lines selected for mixture use, although it is worth noting that even in
pedigree line breeding programmes, the F1 and F2 generations are usually grown as
populations. However, recent trials have shown that tests of two-way mixtures can
