Page 149 - Breeding and regulatory opportunities, Renaud

Broccoli crop improvement

under conventional conditions could perform among the top under organic

conditions due to better weed competitive ability. In order not to overlook

the best performing cultivars for organic management systems, they advised

combining the cultivar ranking results from trials from both management

systems (see also Reid et al., 2009 and 2011).

In our trials the open pollinated cultivars were the lowest yielding and least

stable across all trials. The small group of OPs in our trials tended to be early

maturing and demonstrated a narrow harvest window at prime quality, which

could have contributed to their lack of resilience to environmental variation.

Duvick (2009) found that the heterosis in maize hybrids contributed to their

overall vigour under stress conditions. However, the research of Ceccarelli

(1996) and Pswarayi et al. (2008) in the case of barley indicated that modern

cultivars were adapted to low stress, high yielding environments and did not

always perform favourably in higher stress, marginal conditions. In the case of

our trials, however, the organic management conditions were not necessarily

low input stress conditions in the strictest sense, as mean head weights were

comparable to conventional, and therefore high ranking hybrids were shared

across environments with the exception of some that dropped their high

ranking under organic conditions. We therefore must stress that we anticipate

that results could be diferent when growing conditions are less favourable for

crop growth.

4.4.3 Repeatability as afected by Management systems

Lammerts van Bueren et al. (2002) described organic growing conditions

as heterogeneous and sometimes lower input environments compared to

conventionally managed production environments where high levels of readily

available nitrogen can mask variation in soil quality conditions. Higher variability

in growing conditions under organic conditions may cause increased macro- and

micro-environmental variance relative to the genotypic component, and result

in lower heritabilities compared to more controlled conditions in high-input

conventional farming conditions. In the present study, we were able to estimate

the proportion of the genotypic variance relative to phenotypic variance,

but because we did not have a genetically structured breeding population,

could only estimate repeatability rather than broad sense heritability. The

argument commonly used to support selecting in optimal environments is

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