Effect Of Dietary Forage Sources And Energy Supply On Milk Protein Syntheses And Regulation In Lactating Dairy Cows

Alfalfa is an extremely important forage source in ruminant rations, supplying high amounts of CP, and a good amino acid profile for milk protein synthesis. With the development of Chinese dairy industry, the production level of dairy cows has been rapidly improved during recent years. The gap between high-quality forage supply and demand is increasing rapidly annually in China(Zhu et al., 2013). A large quantity of expensive alfalfa(8 × 107 kg in 2013) has been imported annually, resulting in China being the largest importer in the world. At the same time, large amounts of crop residue such as corn stover are produced each year. It is estimated that approximately 2.2 × 1011 kg of corn stover is generated annually in China, and most of it is not fully utilized. However, there is urgent need to solve the alfalfa deficiency problem and the low utilization of corn stover. This study contained 2 parts: the objective of the part 1 was to investigate nutrient digestibility, rumen fermentation, microbial protein synthesis, and milk protein production when alfalfa was replaced with corn stover for lactating dairy cows. The objective of part 2 was to evaluate the digestibility, rumen microbial protein synthesis and lactation performance of dairy cows when fed different levels of corn stover and two different energy sources(GC vs. SFC).(1)Effects of different forage sources on lactation production, milk protein synthesis and amino acid metabolism in dairy cowsThe part 1 was conducted to determine effects of substitutes for alfalfa and corn silage on nutrient digestibility, milk performance, and rumen microbial protein synthesis in lactating dairy cows. Thirty-two primiparous Holstein cows were divided into 2 groups and fed ad libitum a TMR containing different forage sources: MF included 17.30% alfalfa and 18.77% corn silage with a forage-to-concentrate(F:C) ratio of 36:64. In contrast, CS diet contained 36.07% corn stover with the same F: C ratio. MF and CS diets were designed according to common ingredient large-scale dairy farms and free-range farmers in China, respectively. The experiment lasted for 15 weeks with a 2-wk adaptation and 13-wk experimental period. The DMI was increased by 4 kg/d in cows fed MF compared to CS(P<0.01). Intake and apparent digestibilities of OM, CP, NDF, ADF and EE in cows fed the MF diet were higher than those fed CS. Milk yield, milk protein content and yield were higher in MF than in CS(P<0.01). Rumen p H were not different(P>0.05) between treatments. Ruminal ammonia nitrogen concentrations for MF diet was higher than for CS diet. Concentrations of rumen acetic acid(P<0.01) and total volatile fatty acids(P<0.01) were higher for cows fed CS, with no difference in concentrations of rumen propionate and butyrate. Cows fed MF diet had higher microbial protein yield(P<0.01) and metabolizable protein(P<0.01) than those fed CS. Concentrations of urea N in the blood(P=0.09) and milk(P<0.01) were decreased for cows fed MF diet. At the same time, higher N conversion(P<0.01) and milk efficiency(milk yield/DM intake) were detected for MF versus CS. Results indicated, when corn stover was used to replace alfalfa hay and corn silage for lactating cows, milk protein production was reduced by decreasing microbial protein yield, and milk protein production may be restricted by lower nutrient digestibility.At the same time, the objective of this part was to evaluate the difference of AA uptake and utilization in mammary gland. the results showed that: the amino acid profile of CS diet was not balance and provide a lower intakes of amino acid, which leaded to a lower supply of amino acid in artery. Therefore, the AA utilization of mammary gland of CS was restricted by the level of EAA in artery, the reason was the low level of nutrition and blood flow. There was no difference in AA uptake and utilization between CS and MF, which resulted higer milk protein yield of MF diet.Based on part 1, another experiment was designed to evaluate the restrictive amino acids sequence between MF and CS diet using essential amino acid profile of the milk protein as the ideal amino acid profile, and mapped the “Wooden –barrel Principle” based on the “milk protein score” of 2 diets. The results showed: the restrictive amino acids sequence of MF diet was Met(64.9), Phe(73.82), His(75.82),Val(92.90), Ile(96.41), which according the milk protein score of amino acid uptake of mammary. The restrictive amino acids sequence of CS diet was Met(55.47), Lys(76.75), Leu(77.2), Ile(82.62), Val(95.40), which according the milk protein score of dietary amino acid profile.(2)Effect of dietary energy source and level on rumen microbial protein synthesis, milk performance, ratio of functional bacteria and ruminal papillae morphology in lactating dairy cowsThis study was conducted to examine the effects of dietary energy source and level on feed intake, digestion, rumen microbial protein synthesis and milk production in lactating dairy cows fed corn stover as a forage source. Eight multiparous Holstein cows, four of which were fitted with rumen cannulas, were used in a replicated, 4 × 4 Latin square design with each period lasting for 21 d(14 d for dietary adaptation and 7 d for measurements). The cows were assigned to one of four dietary treatments: low-energy ground corn(LE/GC); low-energy steam-flaked corn(LE/SFC); high-energy ground corn(HE/GC); and high-energy steam-flaked corn(HE/SFC). Changes to ruminal energy degradation rates were induced by feeding the cows diets of either finely ground corn(GC) or steam-flaked corn(SFC) as part of the diets with the same energy level. Milk yield, milk protein content and yield, and milk lactose yield increased in response to increased levels of dietary energy, while contents of milk fat and lactose remained unaffected. Cows fed HE diets had a higher crude microbial protein yield, total-tract apparent digestibility and concentration of rumen volatile fatty acids than those receiving LE diets. Milk yield, milk protein yield, and microbial protein yield were also higher when SFC replaced GC as the main energy source for lactating cows fed LE diets. These results suggest that an increased dietary energy level and ruminal degradation rate are beneficial to milk protein production, probably due to increased yields of microbial protein when cows are fed corn stover as a dietary forage.The results showed that the ratio of rumen cellulolytic bacteria, hemi-cellulolytic bacteria, proteolytic bacteria, amylolytic bacteria, and fat utilization bacteria was increased when cows were fed the HE diets with an increased thickness of stratumspinout and a decreased thickness of basales. There was on difference of rumen cellulolytic bacteria, hemi-cellulolytic bacteria, proteolytic bacteria, amylolytic bacteria, and fat utilization bacteria on dietary energy source. However, when cows fed the LESFC diet, the ratio of general prevotella, P. brevis, R. flavefaciens, and A. lipolytica were greater and the ratio of R. albus was lower compared to LEGC diet.In summary, compared with alfalfa hay, corn stover has a lower content of nutrient, especially for rumen fermentation carbohydrate, leading to a lower MCP yield and energy supply and decreasing the milk protein yield. Furthermore, the amino acid profile of CS diet is not balance compared to amino acid profile of milk protein, which further reducing the milk protein synthesis. In view of the defects of CS diets, it is useful to improve milk protein production by increasing the dietary energy level. But, an increased ruminal degradation-replaced ground corn with SFC rate is beneficial to milk protein synthesis when cows fed low energy level dietary, with no difference on high energy level dietary.