| In order to investigate the effects of dietary energy level on nutrients apparent digestibility, lactation performance, energy utilization efficiency and related physiological and biochemical indexes of Holstein cow in mid-lactation under heat stress and determine the suitable net energy requirement of heat-stressed dairy cow in mid-lactation, a single factor experimental design was used in the present study. According to the parity (2-3). DIM (169±27) and milk yield (25±3.1 kg/d) similar, healthy and free from disease principles, twenty-five Chinese Holstein cows in mid-lactation were randomly allocated to 5 groups (5 cows in each group) corresponding to 5 isonitrogenous (crude protein 16.1%) dietary treatments with net energy content [net energy for lactation (NE1), dry matter basis] of 6.15 (group 1),6.36 (group 2),6.64 (group 3),6.95 (group 4) and 7.36 MJ/kg (group 5), respectively. The experiment lasted for 45 days, including a 15-day pre-experimental period followed by a 30-day experimental period. The results showed as follows:1) There was a significant negative correlation between dry matter intake and dietary energy level (R2=0.9833, P=0.017), and group 5 was significantly lower than the other groups (P<0.05 or P<0.01). Gross energy intake (GEI), digestible energy intake (DEI), metabolisable energy intake (MEI) and NEL intake were increased linearly (P<0.01), fecal energy also increased linearly (P<0.01), while methane energy decreased linearly (P<0.01). with the elevated dietary NEL content. Apparent digestibility of nutrients (excepted calcium, phosphorus, and gross energy) reached the maximum in group 1 and the minimum in group 5. the differences were significant or extramely significant (P<0.05 or P<0.01).2) With the elevated dietary NE1 content, milk yield was not significant differences among groups (P>0.05). However, when expressed in terms of fat corrected milk (FCM). FCM increased linearly or quadratically (P=0.02), and milk yield in group 4 was the highest, and was increased by 18.6% compared with that in group 1 (group 1 had the lowest value). Milk fat percentage and milk energy reached the highest in group 4 (4.39% and 3.15MJ/kg. respectively), followed by group 5. Milk dry matter content increased linearly (P<0.01) with the increased of dietary energy level. However, milk protein and lactose content were not affected by treatments (P> 0.05).3) The efficiency of energy utilization in terms of DE/GE and ME/GE, and net efficiency of ME used for lactation (k1), were similar (P>0.05) among treatments, whereas the efficiency of ME/DE increased linearly (P<0.01), the gross efficiency of MEI for lactation (E1/MEI) increased quadratically (P=0.01), with the elevated dietary NEL content. Likewise, the efficiency of NEL intake for lactation (E1/NEL intake) also increased quadratically (P=0.02), however, when taking the energy retention in body mass into account, no differences in energy utilization (E1+Energy balance/NEL intake) were observed among groups(P>0.05), with the elevated dietary NEL content.4) The improving of dietary energy level reduced respiratory frequency, groups 4 and 5 were extremely significantly lower than groups 1 and 2 (P<0.01), and had no significant difference compared with group 3(P> 0.05), In group 4, serum cortisol concentration was significantly or extramely significantly lower than that in groups 1,2, and 5 (P<0.05 or P<0.01), serum lipopolysaccharide concentration was significantly or extramely significantly lower than that in the other groups (P<0.05 or P<0.01), and serun glucose concentration was significantly higher than that in groups 1 and 5 (P<0.05); the activity of serum creatine kinase in groups 1 and 4 was significantly or extramely significantly higher than that in groups 2,3 and 5(P<0.05 or P< 0.01),and group 4 increased by 32.87% and 25.40% compared with groups 2 and 3, respectively; the activities of serum cereal third transaminase and aspertate aminotransferase showed group 1>group 2>group 4>group 3>group 5. Strong correlation were found between dietary NEL content and 1) DM intake; 2) NEL intake; 3) milk energy (Ei); 4) E1/ME intake; 5) E1/NEL intake, as well as between NEL intake and FCM. The suitable net energy required for dairy cow producing 1 kg FCM of 4% ranged from 5.01 to 5.03 MJ, concluded from the above-stated regressions. Correlation between HP (MJ/kg0.75) and MEI (MJ/kg0.75) could be expressed as:Log (HP)=-0.4304+0.2963 MEI (n=15, R2=0.99, RMSE=0.18). Fasting heat production (FHP) was 0.3712 MJ/kg0.75 when extrapolating MEI to zero, therefore the net energy for maintenance (NEM) for heat-stressed dairy cow was 0.3712 MJ/kg0.75 under this current condition. |
PDF Full Text Request