Thirty second-parity Chinese Holstein cows in mid-lactation with a body condition score of 2.860.29 , weighing 54357kg and producing 24.323.86 kg milk per day were randomly assigned to 1 of the 3 diets ( n = 10 per treatment ) : 1 ) low-concentrate mixed forage diet ( LCF ) with a concentrate to roughage ratio of 46 : 54 ; 2 ) high-concentrate corn straw diet ( HCS ) with a concentrate to roughage ratio of 65 : 35 ; 3 ) low-concentrate corn straw diet ( LCS ) with the same concentrate to roughage ratio ( 46 : 54 ) as LCF .