Beeinflussung des Stoffwechsels von Deutsch Holstein Kühen durch die individuelle Nebennierenrindensensitivität
Excessive lipolysis can have severe effects on the health of periparturient dairy cows. Negative energy balance (NEB) as a consequence of high glucose demand of the mammary gland and insufficient feed intake, leads to mobilization of body energy reserves, especially the adipose tissue. Non-esterified fatty acids (NEFA) are produced, as result of mobilization, and are metabolized in different pass ways. NEFA can be oxidized into ketone bodies (β-hydroxybutyrate (BHB), acetoacetate and acetone) in liver and muscle tissue and used as energy source. Furthermore, it can be re-esterified into triacylglycerol (TAG) and stored in the liver. The udder can use NEFA, as precursor, for production of milk fat. Higher blood NEFA and BHB concentration can be measured in periparturient cows. Subclinical or clinical ketoses can develop in consequence of gross lipolysis and can be accompanied by diseases, such as fatty liver, metritis, mastitis, displaced abomasum and decreased immunity. Economic and welfare considerations make it important to prevent the development of these diseases. Earlier studies already established that cows with high body condition at calving have a higher rate of lipolysis and have a greater risk to develop aforementioned diseases. Body condition is influenced by herd level factors (management, feeding), as well as by animal level factors (age, parity). Genetics, with heritability of up to 0.6, has an impact on body condition, too. Body condition scoring (BCS) is used to estimate the condition of individual animals. BCS scale ranging from 1 to 5 with intercept of 0.25-0.5 is usually used in Germany, where BCS 1 represents a very lean and BCS 5 a very fat cow. Ultrasonographic measurement of backfat thickness can be used as an alternative and more objective method to determine the body condition. Unlike monogastric animals, ruminants have a low intestinal glucose uptake. Main glucose source in ruminants is the hepatic and renal gluconeogenesis, where volatile fatty acids produced in the rumen are used as substrates. Moreover, ruminants exhibit periparturient insulin resistance, which favors the insulin-independent glucose uptake of udder. The augmented glucose supply of the udder is essential, as it is required for lactose synthesis and has an influence on the total milk production via osmotic effect of lactose. Insulin resistance is described as decreased insulin sensitivity (insulin concentration necessary to cause a half-maximal response) or decreased insulin responsiveness (maximal effect of insulin). Insulin sensitivity can be assessed by using surrogate indexes, which are calculated from blood concentration of glucose, insulin and NEFA, and are well-established in human medicine. Glucose tolerance test (GTT) or insulin tolerance test (ITT, insulin challenge) can be used for similar purpose. During these procedures glucose or insulin is administered and glucose, insulin and NEFA concentrations are assessed. However, the hyperinsulinemic euglycemic clamp test is considered as the gold-standard method in human and veterinary medicine. This thesis paid a particular attention to the hypothalamic-pituitary-adrenal (HPA)-axis. The HPA-axis is activated by different stressors and as reaction, adrenocorticotropic hormone (ACTH) from anterior pituitary and cortisol from adrenal cortex is secreted. Consequently, insulin sensitivity is decreased and lipolysis is increased. The individual differences in cortisol response can be used to determine the reactivity of the HPA-axis. To this end, a stimulation test (ACTH test) is implemented by injecting ACTH and cortisol concentration is measured, as the concentration depends on the reaction of adrenal cortex to ACTH stimulus. It was determined by earlier study that cortisol concentration after ACTH injection and a stress stimulus are analogous. Thus, it can be assumed that the cortisol level during ACTH test reflects the individual sensitivity to stress. The aim of this study was to divide dairy cows into low and high responder animals according to the cortisol released during the ACTH test. The trial period lasted from d-42 until d106 relative to parturition. Milk production and performance data, as well as blood samples to quantify hormone and metabolic parameters, were collected weekly until d98. BCS was determined weekly, too. Liver lipid content and backfat thickness were determined at four time points (d-42, d3, d21 and d98). Moreover, some production parameters are calculated, like fat corrected milk yield, energy corrected milk yield, energy balance, feed efficiency, energy conversion ratio, and metabolic efficiency. Surrogate indexes for insulin resistance (QUICKI, RQUICKI, HOMA, logHOMA, rpHOMA) were calculated from blood glucose, insulin and NEFA concentrations between d-42 and d98. Intravenous GTT test was performed at the end of the trial period (d106). Statistical analysis was done by using software SAS 9.3. The Proc mixed procedure was used to compute the effects of time, group (low/high responder) and time by group interaction for all measured and calculated parameters. The low responder animals (low cortisol peak during ACTH test) were significantly fatter and mobilized more fat. No differences were found between low and high responder cows in regard to feed intake, milk production and energy balance. However, low responders had superior body condition, as indicated by higher BCS (group p = 0.0002) and backfat thickness (group: p = 0.019, time*group: p < 0.0001). The increased lipolysis in these animals was delineated by higher concentrations of NEFA (group: p = 0.077, time*group: p = 0.013) and BHB (group: p = 0.083), as well as by higher liver (group: p = 0.036) and milk fat content (group: p = 0.004). Group had no other effect on milk parameters. There is no difference in energy conversion ratio and metabolic efficiency between the two groups, but feed efficiency is higher in low responder cows than in high responder cows in early lactation. Additionally, low responder cows had higher insulin-like growth factor 1 (IGF-1) (group: p = 0.037) and lower growth hormone (GH) (group: p = 0.008) concentrations. No differences were observed between the groups in insulin sensitivity, surrogate indexes and GTT, except for NEFA concentration during GTT test, which tended to be higher in low responder group (group: p = 0.083). In conclusion, low responder cows are fatter and have a disposition toward lipomobilization syndrome. In order to prevent the diseases associated with this syndrome (fatty liver, metritis, mastitis and displaced abomasum), special care should be taken to insure, cows are in optimal condition at the parturition. Moreover, breeding high responder cows with more robust metabolism could be beneficial to dairy farming.
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