A New Insight On Determination Of Dietary Carobohydrate Structure And Balance Index For 8 To 10-month-old Holstein Heifers

Heifer herd is the basis of dairy cows in the dairy farm.Heifer rearing affects the lifetime productivity and profitability of dairy cows and also palys an important role in the development of dairy industry.Carbohydrate is the major component of the diet in dairy cows,which accounts for 60%to 70%of the dry matter content in the diet.Carbohydrate structure and balace index are the new indicator to measure the growth and development and rumen health of replacement heifers.The aim of this study was to determine the optimal dietary carbohydrate balance index(CBI)and peNDF/MG ratio which could provide scientific basis for heifer rearing.This study included two experiments.Experiment 1:Twenty-four healthy Holstein heifers of similar age(8 months)and body weight were randomly assigned to four groups of six heifers.The trial period was 75 days,comprising 15 days of adaptation and 60 days for sampling.The dietary carbohydrate balance index(CBI)was applied as the ratio of physically effective netural detergent fiber(peNDF)to rumen degradable starch(RDS).Treatment diets consisted of four CBI levels:diet 1(CBIs.0=0.80,CBI1.18=1.21),diet 2(CBI8.0=1.11,CBI1.18=1.53),diet 3(CBI8.0=1.36,CBI1.18=1.86),diet 4(CBI8.0=1.72,CBI1.18=2.29).The results showed that the ADG of heifers increased initially and then dropped as the CBI increased.When the CBI1.18 was 1.86 or CBI8.0 was 1.36,the ADG of heifers reached the maximum.The frame of heifers were in good health and the paunch girth increased significantly(P<0.05).The eating and chewing time increased significantly(P<0.05),while eating frequency decreased significantly(P<0.05).The digestibility of dry matter(DM),crude protein(CP),neutral detergent fiber(NDF)and acid detergent fiber(ADF)increased initially and then dropped(P<0.05).Ruminal pH increased significantly(P<0.05)and different CBI levels and pH values were strongly correlated(pH=6.427 + 0.361×CBI8.0,R2=0.377,P= 0.011;pH=6.348 + 0.307×CBI1.18,R2=0.384,P=0.011).Acetate concentration increased significantly(P<0.05)and the ratio of acetate to propionate had the increasing trend(P = 0.07)as the dietary CBI increased.It is concluded that the optimal dietary CBI1.18 was 1.86 and CBI8.0 was 1.36 for 8 to 10-month-old Holstein heifers.Experiment 2:Fifteen healthy Holstein heifers of similar age(8 months)and body weight were randomly assigned to three groups of five heifers.The trial period was 55 days,comprising 15 days of adaptation and 40 days for sampling.Carbohydrate composition was expressed as the ratio of physically effective netural detergent fiber(peNDF)to metabolizable glucose(MG).Treatment diets consisted of three levels:LR group(peNDF8.0/MG = 1.46;peNDF1.18/MG =1.90),MR group(peNDF8.0/MG=1.74;peNDF1.18/MG = 2.33)and HR group(peNDF8.0/MG = 2.08;peNDF1.18/MG=2.79).The results showed that the ADG of heifers tended to decrease as the dietary peNDF/MG ratio increased(P = 0.07).The concentrations of BUN,TG and CHOL increased significantly(P<0.05),while the HDL concentration decreased in serum(P<0.05).After feeding 2 h and 4 h,INS concentration in LR group was higher than HR group(P<0.05).Rumen fermentation pattern was affected by dietary peNDF/MG ration.Propionate concentration had the decreasing trend(P=0.07)and the acetate to propionate ratio and NGR increased significantly(P<0.05).Cellulolytic bacteria such as F.succinogene,R.albus and R.flavefaciens increased and R.amylophilus decreased(P<0.05)as dietary peNDF/MG ratio increased.In addition,the digestibility of DM,CP,NDF and ADF decreased significantly(P<0.05).The results indicated that dietary peNDF/MG ratio can affect the growth and development,blood biochemical parameters,rumen fermentation and apparent digestibility of heifers and the optimal dietary peNDF8.0/MG(peNDF1.18/MG)ratio was 1.46(1.90).In summary,the optimal dietary CBI8.0 was 1.36 and CBI1.18 was 1.86 for 8 to 10-month-old Holstein heifers.The dietary peNDF/MG ratio can be considered as a nutritional indicator to evaluate carbohydrate composition when formulating diets.When peNDF8.0/MG(or peNDF1.18/MG)ratio was 1.46(1.90),heifers will be in good state.