Associated British Foods has decided to write down its UK bioethanol joint venture by £98m due to the slump in crude oil and bioethanol prices and the ongoing weakening of the euro against sterling.The FTSE 100 group, owner of Primark clothing stores as well as diverse international food and ingredients businesses, said it was taking the decision to impair its investment in Vivergo Fuels Limited, a wheat-fed bioethanol joint venture with BP and DuPont.It said interim results for the 24 weeks ending 28 February will include a non-cash exceptional charge of £98m.In a trading statement in mid-January ABF warned of a "marginal" decline in full year earnings due to the strength of sterling coming on top of the expected large profit reduction at its sugar business and a £128m writedown from Chinese agricultural operations.