German chemicals and pharmaceutical company Bayer has said it will undertake its largest ever acquisition with the €7.25bn (US$6.64bn) purchase of Aventis CropScience , a unit currently owned 76% by French biotech giant Aventis and 24% by Schering . The companies have been in negotiation since July, and the purchase price includes the assumption of debt accruing to Aventis CropScience. The liabilities for the controversial GE corn