| In this study, the effects of parity and lactation stages on blood milk composition precursor and related hormones as well as their correlation with lactation performance and milk composition were studied by determination of serum biochemical parameters, in order to supply theoretical basis for the regulation of milk synthesis, scientific evaluation of dairy cows’ nutritional status and the formulation of dairy diets.Trial 1:This experiment was conducted to study the effects of parity and stage of lactation on milk component precursor and related hormones in blood of Chinese Holstein cows. Sixty Chinese Holstein cows from parity 1 to 4 with different stage of lactation were selected and fed the same TMR diet. The blood samples were collected on every two weeks and then were evaluated for the concentrations of glucose(GLU), triglyceride(TG), non-esterified fatty acid(NEFA), β-hydroxybutyric acid(BHBA), total amino acid(T-AA), thyroxine(T4), growth hormone(GH), cortisol(COR), insulin(INS), prolactin(PRL) and progesterone(PRG). For each blood variables, outliers were identified and discarded. Data were then analyzed for their Gaussian distribution and variable with not normal distribution were log-transformed to adjust for lack of normality. Gaussian and transformed data were statistically processed by PROC MIXED(SAS 9.2; SAS Institute Inc.) using a linear model to assess the fixed effects of parity, stage of lactation and their interactions. The results were showed as follows: the INS and PRL concentration in different parity were significantly(P < 0.01), while the COR, GH, PRG, T4 and five MCP parameters were not significantly(P > 0.05); GLU concentrations increases gradually with lactation stage, and GLU of early lactation is significantly lower than other lactation stage(P < 0.05), while BHBA and NEFA of the early lactation was significantly higher than that of middle lactation and later period(P < 0.05), and at the same time T-AA in the middle of the lactation(101-200d) increased significantly(P < 0.05);blood hormones COR and GH, PRG, T4 and INS were influenced by stage of lactation(P < 0.01 or P < 0.05), but was not significantly on PRL(P > 0.05), COR of the early lactation was significantly higher than in mid and late lactation(P < 0.01), while GH, PRG, and T4 were significantly increased during late lactation(P < 0.01). There are differences in milk component precursor and related hormones levels of different parties and stage of lactation. The outcomes of this work will improve the accuracy of the biochemical profile as a tool for dairy practitioners to assess the metabolic status of lactating Holstein cows.Trial 2:To study the change rule of lactation performance, serum milk composition precursors and related hormones and their correlations, 35 dairy cows in parity 2 to 4 were selected and milk samples were collected and analyzed for milk protein, lactose and fat. Correlation analysis was conducted by PROC CORR in SAS 9.2. The results showed that milk protein percentage and blood T-AA showed a significant positive correlation(P=0.0081), milk fat percentage was positively correlated with NEFA and BHBA, while no correlated relationship was founded between milk lactose and glucose(P=0.1150). Serum GH content was negatively correlated with milk yield, milk fat percentage, serum NEFA and BHBA(P<0.0001), and positively correlated with milk protein percentage(R=0.6194, P<0.0001) and serum GLU(P<0.0001), no correlated relationship was founded for GH and milk lactose percentage(P=0.7317). Strong positive correlated relationships were observed between COR and milk yield, milk fat percentage, serum NEFA and BHBA(P<0.0001), but COR negatively correlated to milk protein percentage and serum GLU(P<0.0001). T4 showed high positive relationship with milk fat percentage and high negative correlation with serum BHBA and NEFA(P<0.0001). From the results above, we can concluded that interaction effects were existed between milk composition and their precursors, that is the change of one milk precursors would result in multiple milk components variation and the synthesis of milk constituent was regulated by a variety of hormones. |
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