Effects Of Metabolizable Protein Levels And Supplemental Amino Acids On Lactation Performance And Nitrogen Metabolism In Chinese Holstein Dairy Cows

Series of experiments were conducted to study the requirement of metabolizable protein (MP) and the effect of amino acids (AA) balance on milk production and nitrogen metabolism in Chinese Holstein dairy cows. Metabolizable protein levels and amino acids balance were investigated in Expt.l for the typical diets of high-yield dairy cows collected from eight representative large-scale dairy farms in Zhejiang province of China. Experiment 2 was conducted to study the effects of ratio of rumen degradable protein (RDP) and rumen undegradable protein (RUP) on N conversion in Chinese Holstein dairy cow. In Expt.3, the effects of levels of MP were studied on milk production and nitrogen utilization in Chinese Holstein dairy cows. Different ratio of lysine to methionine in metabolizable protein was designed in Expt.4 to study the lactation performance, nitrogen and AA metabolism of Chinese Holstein dairy cows.In Expt 1, to investigate the limiting factors that affect the dairy industry metabolizable protein (MP) levels and amino acids (AA) balance were calculated for the typical diet of high-yield dairy cows collected from eight representative large-scale dairy farms in Zhejiang province of China. The MP level was too high for Chinese Holstein dairy cow and had a negative effect on reproduction of dairy cows. There was an imbalanced AA proportion of MP for the typical diet and limited supply of Lys and Met.In Expt 2, fifteen multiparous dairy cows were used in quintuple 3×3 Latin square to study the effects of ratio of RDP to RUP on N conversion in Chinese Holstein dairy cows. Three diets were formulated isonitrogenously with varying ratio of RDP/RUP. High ratio of RDP/RUP resulted in high urinary N excretion and total N excretion, but no significant dietary effects were observed on milk yield,4% FCM, milk composition, and nitrogen excretion in feces. Different protein degradability changed the partitioning of the N excretion between urine and feces. Reduction in ratio of RDP/RUP could improve the efficiency of N utilization in lactating dairy cows by reduced N excretion in urine and feces without impairing milk production.The objective of Expt.3 was to investigate the effects of levels of metabolizable protein (MP) on milk production and nitrogen utilization in Chinese Holstein dairy cows. Forty multiparous dairy cows (body weigh (BW)=590 kg; days in milk=135; average milk yield=30.2 kg/d) were assigned to treatments randomly within groups based on DMI and milk production. Animals were offered diets with different levels of MP:8.3 (Diet A),8.9 (Diet B),9.7 (Diet C), and 10.4 (Diet D)% of DM. The MP level in Diet A was designed to meet the requirement according to current CNSAPH, while that in Diet D was based on NRC-2001 model. The experiment lasted for seven weeks. Milk yield and milk compositions (fat, protein, and lactose) were recorded, and urea nitrogen concentrations in serum, urine, and milk were measured during the experiment. Milk yield and milk protein percentage increased as the MP increased up to 9.7% of DM, and then leveled off. Concentrations of nitrogen in urine, serum, and milk increased lineally as the MP amount increased, indicating decreased efficiency of N utilization. Milk lactose percentage and total solid percentage showed, no significant differences among four diets. It is concluded that the optimal dietary MP level was at 9.6% of DM for Chinese Holstein dairy cows producing 30 kg milk per day.In Expt 4, the objective was to evaluate the ratio of lysine to methionine in metabolizable protein on lactation performance and amino acids metabolism of Chinese Holstein dairy cows. Thirty-two multiparous (body weight=600 kg (SD 19)) and 28 primiparous (body weight=560 kg (SD 16)) Chinese Holstein cows in mid-lactation (average days in milk=120 (SD 7), milk yield=32.0 kg (SD 2.33)/day) were used in a completely randomized block design. A 2×2 factorial arrangement of treatment was adopted to provide 2 levels of supplemental L-Lysine-HCL (0 or+) and 2 levels of HMB (0 or+). Four isocaloric diets were formulated to meet the requirement for metabolizable protein (MP) but limited with Lysine and methionine for Chinese Holstein dairy cow. Treatment ML and C were designed to have a Lys: Met ratio in MP of 3:1, while the Lys:Met ratio in MP were brought to 3.5:1 and 2.5:1 for treatment L and M, respectively by supplementation of L-Lysine-HCL or HMB. Overall, no differences in intake of DM, OM or NDF were observed (p>0.05). Milk yield was significantly increased by supplementing lysine and methionine (p<0.05). However, no significant effects on milk composition were found by supplementing lysine or methionine. An interaction effects (lysine X methionine) were found on milk yield and milk protein yield, indicating that performance of unsupplemented cows may have been limited by methionine or lysine. Supplementing methionine or an interaction with supplementing lysine decreased the urea nitrogen in serum, urine, and milk, indicating that efficiency of nitrogen utilization could be improved by providing a pattern of absorbed AA that more closely matches the AA requirements for milk synthesis. No significant effect of proportion of Lys:Met were found on plasma concentration of total protein (TP), albumin (ALB), non-esterifed fatty acid (NEFA), cholesterol (CHOL), high density lipoprotein (HDL), triglyceride (TG) and low density lipoprotein (LDL). Plasma concentrations of glucose (GLU) and aspartate aminotransferase (AST) tended to be lower in cows consumed limited AA, while the concentrations of alanine amiotransferase (ALT) and TG were similar among the groups. Supplementation increased the level of arterial serum essential AA and total AA, and improved the AA uptake of mammary gland. It can be concluded that dairy cows have requirements for amino acids (AA) that must be provided in the diet. To maximize milk performance, requirements must be determined and matched with dietary supply. The optimal ratio of lysine to methionine in MP for maximizing milk protein and milk yield was 3:1.In summary, reduction in ratio of RDP/RUP could improve the efficiency of N utilization in lactating dairy cows by reduced N excretion in urine and feces without impairing milk production. Milk yield and milk protein percentage increased as the MP increased up to 9.7% of DM, and then leveled off. Concentrations of nitrogen in urine, serum, and milk increased lineally as the MP amount increased, indicating decreased efficiency of N utilization. The optimal dietary MP level was at 9.6% of DM for Chinese Holstein dairy cows producing 30 kg milk per day. Dairy cows have requirements for amino acids (AA) that must be provided in the diet. To maximize milk performance, requirements must be determined and matched with dietary supply. The optimal ratio of lysine to methionine in MP for maximizing milk protein and milk yield was 3:1. The MP level was too high for Chinese Holstein dairy cow and has a negative effect on reproduction of dairy cows in South China. And there was an imbalanced AA proportion of Lys and Met in MP and limited supply of Lys and Met for the typical diets.