Effects Of Forage Sources On Milk Protein Precursors And Lactation Performance In Dairy Cows And Approach To The Mechanism

With the development of dairy industry, the production level of dairy cows has been rapidly improved in China during recent years. However, the milk quality still needs to be improved. Deficiency of high quality forage and the unreasonable utilization of existing resources are the two important problems. A large amount of alfalfa has been imported annually, while huge amount of cereal straws and stover has not been fully utilized in animal feeds. It is necessary to establish the utilization system of feed resources that is suitable for China’s national conditions. Therefore, the objective of this study was to evaluate the effects of dietary forage sources on precursor of milk protein and lactation performance in lactation dairy cows and approach to the mechanism. This study contained two parts including four trials. In the first part, the effects of dietary forage sources (alfalfa hay, Chinese wild rye grass, and corn stover) were evaluated on milk performance and precursor of milk protein, and the possible mechanism were approached. In the second part, the effects were studied of supplementing starch and rumen-protected (RP) AA on lactation performance and prodcurtion of milk protein precursor in lactating dairy cows fed corn stover.Part one:Evaluation of the effects of dietary forage sources on production of milk protein precursor and lactation performance in early-lactation dairy cows and approach to the mechanism (Trial1and2)1. Effects of dietary forage sources on milk performance and N utilization in lactation dairy cows (Trial.1). In order to evaluate the effects of forage sources on milk performance, N utilization efficiency and blood parameters of lactation dairy cows, twelve primiparous Chinese Holstein dairy cows were used in a multiple3×3Latin square design. Cows were divided into3groups according to similarity in milk production and day in milk. Diets were isonitrogenous and isocaloric, with a forage to concentrate ratio of45:55(dry matter (DM) basis) and contained similar concentrate mixtures. Different forage sources were:(1) corn stover19(CS),(2) Chinese wild rye hay21(CWR), and (3) alfalfa hay17(AH). Dry matter intake (DMI) was not affected by the sources of dietary forage (P>0.05). Milk yield was higher for cows fed AH than those on CS (P＜0.1), with an intermediate value for CWR. Milk protein content was higher in the cows fed AH than CWR, with CS at an intermediate position. The contents of milk fat and lactose were not different among the treatments. The concentrations of urea N in the urine, blood and milk were decreased for cows fed AH, indicating an increased N conversion (P＜0.01). Plasma albumin, glucose, and nonesterified fatty acid (NEFA) was not affected by treatments (P＞0.05). It is indicated that feeding alfalfa could improve milk production, milk protein yield, and N utilization efficiency. No significant differences were detected on milk performance and N utilization efficiency between Chinese wild rye grass and corn stover as the main forage sources.2. Approach to the mechanism with which the dietary forage sources affect peoduction of milk protein precursor in dairy cows (Trial.2). In order to reveal the mechanisms with which the forage sources affect production of milk protein precursor in dairy cows, rumen fermentation characters, production of metabolizable protein (MP), and arteriovenous difference in free AA were investigated from the samples taken in Trial1. Rumen fluid was collected from oral cavity, and rumen fermentation characters were detected; rumen OM and CP degradation of experiment diets was determined by nylon bag method; Urinary purine derivatives were used to estimate the microbial protein yield; blood samples were collected from coccygeal. The rumen pH was not affected by treatments (P>0.05). Ammonia N concentration was higher for cows fed diet CS than those fed CWR and AH (P＜0.05), with no difference between CWR and AH (P＞0.05). Concentrations of total VFA in cows consumed diet AH were higher than those fed CS (P＜0.05), with CS being intermediate.it is indicateding that AH could improve the rumen fermentation, supply more energy to the cows, and benefit to the milk production. The MCP yield for AH was higher than that for CS and CWR (P＜0.05). This was consistent with the higher VFA concentration and lower ammonia concentration in AH. And there existed a significant regression of the milk protein yield to the microbial protein yield in lactation cows. Effective degradability value of OM and CP for AH was higher (P＜0.05) compared with the CS and CWR. The higher rumen OM degradation was largely attributed to higher part proportion of fast degradable fraction. It is indicated that high proportion of alfalfa hay in the diet could improve MCP synthesis in the rumen due to its high sufficient supply of rumen available energy to the rumen microbes. No effects of the treatments were observed on the intestinally absorbable dietary protein (IADP)(P＞0.05). Thus, compared with diet CS and CWR, the higher MP value was attributed to its higher MCP yield. There existed a significant regression of the milk protein yield to the microbial protein yield in lactation cows. Compared with diet CS, AH improved the absorption of essential and total AA. indicated suggesting the better AA balance in AH was better than in CS. Overall, a high proportion of alfalfa hay in the diet is beneficial for milk protein yield and milk protein production by increasing microbial protein yield. This can be attributed to the improving supply of rumen available energy and the utilization efficiency of AA. It is necessary to evaluate the effect of supplementing starch and AA on production of milk protein precursor and lactation performance in dairy cows fed corn stover.Part two:Effects of supplementing starch and AA on precursor of milk protein and lactation performance of lactating dairy cows fed corn stover and approach to the mechanism (Trial3and4)1. Effects of supplementing starch and AA on milk performance and N utilization in lactation dairy cows fed corn stover (Trial.3). In order to evaluate the effects of supplementing starch and AA on milk performance, N utilization efficiency and blood parameters of lactation cows fed corn stover, twelve multiparous (3.5±1.0parity) Chinese Holstein dairy cows were used in a3x3Latin square design. Cows were divided into3groups according to similarity in milk production, day in milk and parity. The basal diets were isonitrogenous and isocaloric, with a ratio of forage to concentrate of45:55(DM basis). Three diets contained the following forage ingredients (%on a DM basis):(1) alfalfa19, and Chinese wild rye grass6(AH);(2) corn stover22, alfalfa6, supplemented with starch8(CSS); and (3) CSS supplemented with RP Met and RP Lys (CSSAA). In diet CSSAA the proportion of Lys and Met in MP estimated by CPM dairy soft was7.2and2.4%, respectively. Dry matter intake (P>0.05) and milk yield (P>0.05) did not differ among the treatments. Supplementation of RPAA increased the1.4kg milk yield thus increased the milk efficiency (P＜0.10). The contents of milk protein, fat, and lactose were not different among the treatments. Milk protein yield was higher (P＜0.10) for cows fed AH than those fed CSS, with CSSAA at an intermediate position. The urinary concentration of urea N for CSS diet was higher (P<0.05) than for AH, whereas the CSSAA diet had a lower urea N concentration in the milk (P＜0.05) and blood (P＜0.05) than CSS. Lower N conversion (P＜0.01) was detected for CSS than AH and CSSAA, with no difference between CSSAA and AH. Plasma total protein, albumin, glucose, NEFA, and β-hydroxybutyrate was not affected by treatments (P＞0.05). The results indicated that the supplementation of starch to a corn stover diet could achieve a similar lactation performance as cows fed alfalfa, while RPAA supplementation further improved the efficiency of milk and N utilization. 2. Approach to the mechanism with which supplementing starch and AA affects peoduction of milk protein precursor in dairy cows fed corn stover (Trial.4). The effects of supplementing of starch and AA on rumen fermentation characters, MP, plasma AA concentration were studied. Rumen fluid was collected from oral cavity of cows fed in trail3. Detection of rumen fermentation characters, rumen OM and CP degradation, estimation of the MCP yield, blood collection and plasma free AA concentration detection were similar to trail2. Ruminal pH and total VFA concentration did not differ among the treatments (P＞0.05). Ammonia N concentration was higher for cows fed diet CSS than those fed AH (P＜0.05), indicating that starch supplementation in corn stover diet improved energy supply to the cows, resulting in similar milk yield with those cows fed AH. The OM degradation and MCP yield were not different between CSS and AH (P＞0.05), suggesting that with starch supplementation the available energy supplied from CSS approached to diet AH, resulting in similar MCP yield. No difference was detected in IADP between diet CSS and AH (P＞0.05). Thus, the MP supply was not different between diets CSS and AH (P＞0.05). No significant difference was detected with AA supplementation on rumen fermentation, MCP yield, and MP supply. However, AA supplementation increased the plasma concentration of essential AA and total AA. From Tthe above results, it is indicated that the enhanced production of milk protein precusors in corn stover diet supplemented with starch is attributed to the increased supply of energy available to the rumen microbes and subsequently increased microbial protein synthesis. Supplementations of AA further increased the supply of milk protein precusors to the cows, but also improve the balance of amino acids within the cows’body.In summary, high proportion of alfalfa hay in the diet is beneficial for milk protein yield and milk protein production by increasing microbial protein yield. This can be attributed to the improving supply of rumen available energy and the utilization efficiency of AA. Insufficient of rumen fermentation carbohydrate in corn stover induced low milk protein yield. Corn stover with both rumen fermentable carbohydrate and AA supplementation could increase the MCP synthesis and improve MP utilization, resulting in higher milk protein yield.