ri pper ▁that ▁s que ez es ▁the ▁pl astic - cover ed ▁b ale ▁between ▁two ▁metal ▁parts ▁to ▁avoid ▁pun ct uring ▁the ▁pl astic . ▁Simple ▁fixed ▁versions ▁are ▁available ▁for ▁round ▁b ales ▁which ▁are ▁made ▁of ▁two ▁sh aped ▁pip es ▁or ▁tub es ▁spac ed ▁apart ▁to ▁slide ▁under ▁the ▁sides ▁of ▁the ▁b ale , ▁but ▁when ▁lifted ▁will ▁not ▁let ▁it ▁s lip ▁through . ▁O ften ▁used ▁on ▁the ▁tra ctor ▁rear ▁three - point ▁link age , ▁they ▁incorpor ate ▁a ▁trip ▁t ipping ▁mechanism ▁which ▁can ▁f lip ▁the ▁b ales ▁over ▁on ▁to ▁the ▁flat ▁side / end ▁for ▁storage ▁on ▁the ▁thick est ▁pl astic ▁layers . ▁ ▁Fer ment ation ▁▁ ▁Sil age ▁under go es ▁an a er ob ic ▁fer ment ation , ▁which ▁starts ▁about ▁ 4 8 ▁hours ▁after ▁the ▁sil o ▁is ▁filled , ▁and ▁converts ▁sug ars ▁to ▁ac ids . ▁Fer ment ation ▁is ▁essentially ▁complete ▁after ▁about ▁two ▁weeks . ▁ ▁Before ▁an a er ob ic ▁fer ment ation ▁starts , ▁there ▁is ▁an ▁aer ob ic ▁phase ▁in ▁which ▁the ▁tra pped ▁o xygen ▁is ▁consum ed . ▁How ▁closely ▁the ▁f od der ▁is ▁pack ed ▁determ ines ▁the ▁nature ▁of ▁the ▁resulting ▁sil age ▁by ▁reg ulating ▁the ▁chemical ▁re actions ▁that ▁occur ▁in ▁the ▁stack . ▁When ▁closely ▁pack ed , ▁the ▁supply ▁of ▁o xygen ▁is ▁limited , ▁and ▁the ▁att endant ▁acid ▁fer ment