4 , ▁du cks ▁account ed ▁for ▁ 9 5 % ▁of ▁fo ie ▁gr as ▁production . ▁The ▁bre eds ▁primarily ▁used ▁are ▁the ▁Mus c ovy ▁du ck ▁( C air ina ▁mos ch ata ) ▁( also ▁called ▁the ▁Barb ary ▁du ck ) ▁and ▁the ▁hy brid ▁cross ▁of ▁a ▁male ▁Mus c ovy ▁du ck ▁and ▁a ▁female ▁P ek in ▁du ck ▁( An as ▁plat yr h yn chos ▁dom est ica ) ▁called ▁the ▁Mul ard ▁du ck . ▁This ▁hy brid ▁is ▁ster ile ▁and ▁is ▁therefore ▁sometimes ▁referred ▁to ▁as ▁a ▁" m ule " ▁du ck . ▁ ▁Mul ards ▁are ▁estimated ▁to ▁account ▁for ▁about ▁ 3 5 % ▁of ▁all ▁fo ie ▁gr as ▁consum ed ▁in ▁the ▁US . ▁About ▁ 9 5 % ▁of ▁du ck ▁fo ie ▁gr as ▁production ▁from ▁France ▁comes ▁from ▁force ▁fed ▁Mul ards ▁and ▁the ▁remaining ▁ 5 % ▁from ▁the ▁Mus c ovy ▁du ck . ▁ ▁After ▁h atch ing , ▁the ▁Mul ard ▁du ck lings ▁are ▁sex ed . ▁ ▁M ales ▁put ▁on ▁more ▁weight ▁than ▁females , ▁so ▁the ▁females ▁are ▁sla ugh tered . ▁ ▁Phys i ological ▁basis ▁ ▁The ▁basis ▁of ▁fo ie ▁gr as ▁production ▁is ▁the ▁ability ▁that ▁some ▁water f ow l ▁have ▁to ▁expand ▁their ▁es oph ag us ▁and ▁to ▁gain ▁weight , ▁particularly ▁in ▁the ▁li ver , ▁in ▁prepar ation ▁for ▁migration . ▁ ▁Wild ▁ge