| The purpose of this experiment was to study the effects of different diets on digestibility,productivity,rumen fermentation and physiological metabolism of Holstein cows in the late lactation period under heat stress,and to explore whether high-level DCAB diet can be used as a nutrition strategy to relieve the heat stress of cows caused by high temperature and high humidity.This experiment was conducted in summer 2018 at huahai pasture of youzhuo animal husbandry co.,ltd in Ningxiang city,Hunan province.Eight healthy Chinese Holstein cows with similar body weight(521±38.7)kg,milk yield(22±2.4)kg/day,lactation age(219+20)and parity(2±0)were selected as experimental animals.A 2 x 2 random crossover design was adopted,with each test cycle lasting 21 days(14 days being the adjustment period and 7 days being the formal period),with a total of two test cycles and an interval of 3 days between the two test cycles to reduce or eliminate the residual effects.In the first test cycle,8cows were randomly divided into two groups: control group with 4 replicates(DCAB: 33.46meq/100 g DM;DCAB=Na + K-Cl);The high DCAB group had 4 replicates(DCAB: 50.76meq/100 g DM).The control group was fed with basal TMR diet,while the high DCAB group was supplemented with Na HCO3 and K2CO3 on the basal diet.During the experiment,cows were fed three times a day,milked three times and drank freely.The average daily dry bulb temperature in the cattle shed was 24.93 ～ 34.25℃ and the average temperature was 28.94 2.5℃ throughout the experiment period.The daily average wet bulb reading range is 19.87 ～ 28.88℃,and the average wet bulb reading is 25.61 2.24℃;THI ranges from 73.94 to 85.00 with an average of 80.12 3.35.The respiratory frequency of cows in the two DCAB groups exceeded 60 times /min,and the rectal temperature exceeded38.5℃.There was no difference between respiratory frequency and rectal temperature(P>0.05).There was no significant difference in the intake of dry matter and nutrients in dairy cow diets with different DCAB levels(P>0.05).However,compared with the low DCAB group,the apparent digestibility of DM,OM,energy,CP,NDF and ADF in the high DCAB group are significantly different(P<0.01),and the apparent digestibility range of crude fat(EE)is significantly different(P = 0.008).There was no significant effect on milk yield(MY)and milk components such as milk protein,milk fat,lactose and non-fat milk solids(P > 0.05).Milk production efficiency is not affected by DCAB level.However,the total solid content of milk in high DCAB group was lower(P=0.016),and the content of SCC in milk in high DCAB group was significantly lower than that in low DCAB group(P<0.05).In addition,different DCAB levels had no significant effect on hydrolyzed amino acids in milk(P>0.05).There was no significant difference in p H value of rumen fluid before and 2 hours and 6hours after ingestion(P>0.05).There was no significant difference in rumen fluid p H before and 2 and 6 hours after DCAB intake(P > 0.05).There was no difference in NH3-N concentration between 2 hours after intake and 6 hours after intake(P>0.05),while before intake,NH3-N concentration in high DCAB group was significantly lower than that in low DCAB group(P<0.01).The volatile fatty acid(VFA)of rumen fluid before intake at different DCAB levels had no significant effect on butyric acid concentration,valeric acid concentration and isovaleric acid concentration in the rumen of dairy cows(P>0.05).the total volatile fatty acid(TVFA)showed an increasing trend(P=0.055),which significantly affected the acetic acid concentration,propionic acid concentration and acetic acid-propionic acid ratio in rumen fluid before intake(p < 0.01).The concentrations of total volatile fatty acids(TVFA),acetic acid,propionic acid,butyric acid and valeric acid in rumen fluid 4 hours after ingestion were significantly increased(P<0.01),and the concentration of isovaleric acid and the ratio of acetic acid to propionic acid in rumen fluid were significantly increased(p < 0.05).Increasing DCAB level significantly affected the concentration of total volatile fatty acid(VFA)A,acetic acid,propionic acid,butyric acid,isobutyric acid,valeric acid and isovaleric acid in rumen fluid 6hours after ingestion(P<0.01),and significantly affected the acetic acid-propionic acid ratio in rumen fluid(P=0.022).DCAD increased s O2 in blood(P < 0.05),and the increase of DCAB level had no effect on hematocrit,bicarbonate concentration,total hemoglobin,carbon dioxide partial pressure and oxygen partial pressure(P>0.05).The number of leukocytes,lymphocytes and basophils in the blood of cows fed high DCAD diet was relatively low(P < 0.05),and there was no significant difference in the number of monocytes between the two diets(P = 0.117).Apart from significantly affecting the glucose concentration in milk cow plasma(P=0.03),it has no effect on other serum biochemical indexes.To sum up,the improvement of DCAB level in diet can relieve the heat stress of dairy cows to some extent. |
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