, ▁ex cludes ▁air ▁from ▁all ▁but ▁the ▁top ▁layer ; ▁in ▁the ▁case ▁of ▁the ▁stack ▁extra ▁pressure ▁is ▁applied ▁by ▁weights ▁in ▁order ▁to ▁prevent ▁excess ive ▁he ating . ▁ ▁Equ ip ment ▁For age ▁har vest ers ▁collect ▁and ▁ch op ▁the ▁plant ▁material , ▁and ▁depos it ▁it ▁in ▁tr uck s ▁or ▁w ag ons . ▁These ▁for age ▁har vest ers ▁can ▁be ▁either ▁tra ctor - dra wn ▁or ▁self - prop elled . ▁Har vest ers ▁blow ▁the ▁ch aff ▁into ▁the ▁w agon ▁through ▁a ▁ch ute ▁at ▁the ▁rear ▁or ▁side ▁of ▁the ▁machine . ▁Ch aff ▁may ▁also ▁be ▁em pt ied ▁into ▁a ▁b agger , ▁which ▁puts ▁the ▁sil age ▁into ▁a ▁large ▁pl astic ▁bag ▁that ▁is ▁laid ▁out ▁on ▁the ▁ground . ▁ ▁In ▁North ▁America , ▁Australia , ▁north western ▁Europe , ▁and ▁New ▁Zealand ▁it ▁is ▁common ▁for ▁sil age ▁to ▁be ▁placed ▁in ▁large ▁he aps ▁on ▁the ▁ground , ▁rolled ▁by ▁tra ctor ▁to ▁push ▁out ▁the ▁air , ▁then ▁covered ▁with ▁pl astic ▁sheets ▁that ▁are ▁held ▁down ▁by ▁used ▁t ires ▁or ▁t ire ▁ring ▁walls . ▁In ▁New ▁Zealand ▁and ▁Northern ▁Europe , ▁' b unk ers ' ▁made ▁of ▁concrete ▁or ▁old ▁wooden ▁railway ▁t ies ▁( sleep ers ) ▁and ▁built ▁into ▁the ▁side ▁of ▁a ▁bank ▁are ▁sometimes ▁used . ▁The ▁cho pped ▁grass ▁can ▁then ▁be ▁dump ed ▁in ▁at ▁the ▁top , ▁to ▁be ▁drawn