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This article may be downloaded for personal use only. Permission to reproduce this article must be sought from Wiley-Blackwell. A definitive version of this article was published in Annals of Applied Biology, 161(2): 91-115, 2012, and may be viewed at http://dx.doi.org/10.1111/j.1744-7348.2012.00561.x (Please note: access via this link may only be available with a subscription).
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Copyright (2012) The Authors. Annals of Applied Biology.