[The Norfolk Turkey]

I don't know the answer to your question for sure, nwalch, but I think it's more likely to be the germination rate which is affected rather than the strength of any resulting plant 

Definately germination, once you get the little blighters out of the ground its all up to you and the weather. The seed contains the sugars the plant needs before it can get into the sunshine, as the seed gets older the coating alters, making it harder for water to get in and the shoots to get out and the sugars inside 'go off'.