From: Oz <Oz@upthorpe.demon.co.uk>
Newsgroups: sci.bio.food-science
Subject: Re: What if we clone a ........ being?
Date: Thu, 10 Apr 1997 10:46:31 +0100

In article <334C8F9E.5856@coleg-powys.ac.uk>, Peter Joules
<pjoules@coleg-powys.ac.uk> writes

>I think that Luc's point was that, whilst selection for disease
>resistance is acceptable, in modern agriculture there is a tendency for
>any disease or pest which _does_ get hold of a crop to devastate the
>yield.  This would IMHO not be the case if there were a mixture of
>varieties present.

Firstly one should bear in mind that I can only speak for the UK, but I
believe that it is certainly typical of Europe and probably of the
agriculturally advanced world in general.

Typically for grain crops you will never find more than about 50% of the
countries crop in a single variety, and indeed under 40% is more
typical. The rest is in different, usually markedly different,
varieties.

Seed production is such that a country can switch away from a variety
that becomes highly susceptible to a disease immediately this becomes
apparent. For example Joss Cambier and Slepjner fell from circa 40% to
well under 10% in the year (ie next crop) following the breakdown.

The warning signs for both varieties was clear at least a year before
they suffered severe breakdown. In the case of Joss, it was ignored
since it was at that time thought that yellow rust outbreaks were
entirely weather related and one bad year should not follow another. In
the case of Slepjner the reduction had already commenced following clear
warnings the year before. Those growing it tended to be more alert and
applied the fungicides in good time to prevent the disease (mostly).

In practice the best varieties produce very significantly better yields
and/or agronomic advantages (probably around 10%) than the next best so
the cost of not using it is substantial.

Note that wheat is by far the most extreme of the crops for mass
varietal selection. Barley is significantly more variable in varietal
selection and other crops (except minority crops) are more variable
still.

As a result the severe breakdown of a crop resulting in significant
losses is very rare, as I say I know of only two examples in 25 years in
the UK (both in wheat), and it is probably going to stay that way since
farmers and advisors are well aware of the potential hazard and have
been so for some decades.

--
'Oz     "Is it better to seem ignorant and learn,
         - or seem wise and stay ignorant?"