Effects of Different Energy Levels on the Milk Performance and Blood Chemistry in Jersey Cows in Tibet

Tibetan plateau located in the southwest of China with an average altitude of over 4000 m (Tan et al., 2021). The area is characterised by an extremely harsh environment, namely, severe cold, hypoxia (Chen et al., 2021). Owing to the extreme coldand hight elevation, demand for fat by people in the plateau is great than those on the plains (Li et al., 2021). Butter were used to supply of fat to the herders in areas as remote as rural Tibetand as one of the traditional food, it occupied an importan place in the diet. Because of higher content of milk fatty, most Jersey milk is used to produce butter in Tibet (Brown and Waldron, 2013).
       
However, because of the special geographical environment and the long period of negligible or zero plant growth each year (7-8 months), the winter dormant period is a harsh period for bovine (Ding et al., 2007), these objective factors were one of the main reasons that leaded to the lower milk production and quality in Jersey cows. It is well documented that high dietary energy input are critical for effective lactation (Wang et al., 2014). Zhou et al., (2015) reported that milking performance increased in response to the higher levels of dietary energy. Alstrup et al., (2015) also reported that fat supplemented ration increased yield of milk and milk fat. However, intaking excessive energy for lactating cows increased insulin resistance compared with cows consuming adequate amounts of energy (Leiva et al., 2015). However, little is known about the effects of dietary energy on late lactation performance in Jersey cows. This current study was conducted to evaluate the milk performance, blood chemistryand hydrolysis amino acid in milk of Jersey cows fed different levels of dietary energy.

This work was conducted in the Rikaze city of Tibet in November 2020. The experiments were conducted according to the animal care guidelines of the Animal Care Committee, Institute of Subtropical Agriculture, The Chinese Academy of Sciences, Changsha city, Hunan Province, China (No. KYNEAAM-2006-0015).
       
A randomized complete block design was used in this study. Forty multiparous Jersey cows (225±35 kg) in the late stage of lactation were blocked into 4 groups to give 4 blocks. Throughout the trial, cows were housed in a tie-stall facility, fed twice daily (0500 and 1600 h) and had free access to clean water. The experiment lasted for 6 weeks, before starting the formal experiment, all Jersey cows were fed the same diets for 2-wk and then randomly assigned to 1 to 4 treatments with different levels of energy in diets for later 4-wk experimental phase. The four experimental diets were formulated to meet the nutrient requirements of lactating cows according to NRC (2001). Ingredients and Chemical composition of experimental diets were presented in Table 1.
 

       
The orts were collected and recorded once daily. The diets offered and left behind were recorded daily for calculating DMI, Weekly composites of the concentrates, forage and orts were obtained from daily samples of about 0.5 kg stored at -20°C until analysis. The DM and CP of concentrates, forage and orts were analyzed using the procedures of the Association of Official Analytical Chemists (AOAC, 2002). The NDF, ADF and CF content of above-mentioned samples were determined using a Fibretherm Fiber Analyzer (Gerhardt, Bonn, Germany) according to Van Soest et al., (1991) with addition of sodium sulphiteand alpha-amylase in the NDF analysis.
       
Cows were milked twice daily and individual milk yield was recorded at each milking. Milk samples were collected at 2 consecutive (p.m. and a.m.) milking midways through the 5^th week of the experimental phase for conventional analysis. Concentrations and yields of fat, protein, lactose, total solids (TS) and solids-not-fat (SNF) were computed as the weighted means from p.m. and a.m. milk yields on each test day.
       
Blood sample was collected on the last day of the formal experiment at 0500, 0700 and 1100 h, respectively. 10 mL of blood samples were collected every point-in-time from the coccygeal vein into Vacutainer tubes which including anticoagulation. After sampling, tubes were kept on ice and immediately transported to the laboratory for processing. Blood was centrifuged at 4000 × g for 10 min at 4°C and plasma was stored at -80°C until used for assay.
       
Milk samples of fat, protein, lactose, SNF and TS were analyzed by infrared methods (Foss North America, Eden Prairie, MN; Ag-Source, Verona, WI). The indexes of blood chemistry were analyzed by kits (Beijing Leadman Biochemical Co., Ltd, Beijing, China) using auto-biochemical analyzer (Beckman C × 4, Beckman Coulter, Inc. USA).
       
For the determination of hydrolysis amino acid in milk, 1 ml milk sample were taken and mixed with 1ml concentrated hydrochloric acid into a ampulla bottle, sealed with alcohol blast burner and then put it into bake oven for 24 h at 110°C. After 24 h and cooling, opened the ampulla bottle and transferred hydrolysate to 25 ml volumetric flask, wishing the ampulla with deionized water three times and transferred it into volumetric flask to scale. The solution was filter by micropore filtering (0.25 µm) befor determination. 20 µL was used to test the hydrolysis amino acid by a HITICHI L-8800 automatic amino acid analyzer (Japan).
       
Results of milk production, milk quality and blood parameters were statistically analyzed using ANOVA and the GLM procedure of the Duncan’s multiple range tests were used to compare differences among the three treatments. A P-value of less than 0.05 was taken to indicate statistical significance.

Milking performance
 
The milk fat content in group E4 was 38.35% greater (P=0.0129) than that of group E3. The milk protein content of E1 treatment was 10.69% and 5.07% greater (P<0.0001) than that of E2 and E3 treatments (Table 2), Milk lactose of group E4 was significant greater than that of E1 and E3 treatments (P<0.05) and the content of total solids and solids-not-fat of group E4 was the highest among the treatments and it was 16.22% and 12.67%, 7.55% and 3.41%, 16.22% and 12.67% greater (P<0.0001) than those of group E1, E2 and E3 treatments, respectively.
 

 
Hydrolysis amino acid composition
 
Concentration of Asp, Thr, Ser, Glu and Gly for group E1, were on average about 34.09%, 32.23%, 36.11%, 33.25% and 32.69% higher than that of E2 treatment (P<0.05), besides, there were also significant difference (P<0.01) in concentration of Val, Ile, Leu, Tyr, Lys, His, Arg and Pro for group E1 treatment than that of E2 treatment. Concentration of total AA was on average about 35.01% and 23.01% higher (P<0.01) for E1 treatment than that of E2 and E3 treatments, respectively (Table 3).
 

 
Blood biochemistry indexes
 
The plasma GPT concentration of group E1 and E4 was greater (P<0.0001) than that of group E2 and E3 (Table 4). For the plasma TC and LDH, the greatest value (P<0.0001) was obtained from group E2 among treatments. The TG concentration in plasma for E2 treatment was significant lower (P<0.05) than that of E3 treatment. The greatest value of plasma UN concentration was found in treatment E1 and it was 31.29%, 36.87% and 46.59% greater (P<0.0001) than that of E2, E3 and E4 treatments, respectively. The plasma GLU concentration of E3 treatment was 8.84%, 9.84% and 12.97% greater than (P<0.01) that of E1, E2 and E4 treatments, respectively.
 

 
Milk performance
 
High dietary energy input played a critical role in effective lactation (Wang et al., 2014). Zhou et al., (2015) have concluded that milk yield, milk protein and yield and milk lactose yield increased in response to higher levels of dietary energy. The difference were ascribed to the different composition of diet and the species of cows probably, besides, the difference may be also result from the special geographical environment. Boerman et al., (2015) suggested that a high-forage diet that supplemented with palmitic acid increased milk fat yield, it was similar to the results in the current study. Zhou et al., (2015) reported that an increased energy level were beneficial to milk protein production due to increased yields of microbial protein when cows are fed corn stover as a dietary forage source. Cadorniga et al., (1993) also reported that high-energy diets stimulate microbial protein synthesis. The different dietary forage source and the plateau hypoxia and low tempreture environment may be play a critical role in these difference and the partly energy maybe were used to maintain normal life activities of Jersey cows instead of MCP synthesis. Energy intake is associated with milk yield and milk solids production (Broderick, 2003). This agrees with the current study because the milk total solids content was greatest on group E4, the result in this study was also in accordance with the results reported by Reid et al., (2015).
 
Hydrolysis amino acid composition of milk
 
Many factors could affected the amino acid composition, which included dairy breed, parityand lactation stage and the content of amino acid in high milk protein was higher than that in low milk protein, the results in the current study was in accord with the reported by Zhou, 2017. However, few documents reported that how energy affected the content of amino acid in milk under the condition of the plateau hypoxiaand low temperature environment and more further works were necessary to illuminate the mechanism.
 
Blood biochemistry
 
The activity of GPT could reflect the change of metabolism in body (Yan et al., 2015). The result in the current study was in accordance with the results reported by Yan et al., (2015). TG and TC were important indexes that reflected body energy and fat metabolism and could reflect the fat digestive and absorption directly (Yang et al., 2013). Sun et al., (2013) suggested that the content of TG and TC in plasma increased with the increasing levels of dietary energy. The difference may have a relation to the lipid metabolism under the special geographical and climate environment in Tibet. There was a positive relationship between the concentration of milk UN and blood plasma urea nitrogen (Ciszukand Gebregziabher, 1994). Jaster et al., (1990) reported that the high dietary energy for calf starters would result in the decrease of UN content in plasma, the result in the present study was in line with the results reported by Jaster et al., (1990).
       
Changes of plasma Glu content with the increasing level of dietary energy in the present study were multifarious. This phenomenon may be related to the plasma insulin content. Researchers have reported that dairy cows increased insulin resistance largely due to deficient energy intake for the postpartum period (Sinclair, 2010) and excessive energy intake for non-lactating dairy cowsand lactating (Leiva et al., 2015). Plasma LDH was an important enzyme that related to glycolysis and played a critical role in energy metabolism. The dietary energy level of E2 promoted gluconeogenesis in liver, which may be the main reason for the increase of plasma LDH content.

This work was jointly supported by the Special Item of Regional Collaborative Innovation in Tibet Autonomous Region and Science (QYXTZX-LS2021-01), Hunan Provincial Science and Technology Department Project (2020WK4002) and Technology Department of Tibet (XZ202101ZD003N).