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The study shows the influence of late gestation and lactation on the metabolism of modern crossbreed sows of different body condition during.Body weight, backfat thickness and thorax girth were measured to determine body condition. A total of 43 sows was monitored, with 22 being primiparous and 21 multiparous. The sows were divided into two groups according to different backfat thickness: into one group of ,,thin sows", called RFD KI, 1 (< 2,5 em), and another group of,,thick sows", called RFD KL 2 (> 2,5 em). Body weight, backfat thickness and thorax girth were measured on 110 th day of gestation, 10 th day of lactation (except body weight) and at weaning. Blood samples were taken along with body condition measurements, plus additionally adittional on the 3 "d day of lactation. The samples were taken from the cervical venes. They were tested for concentrations of urea, creatinine, free fatty acids, triglycerides, beta-hydroxybutyric acid, glucose and insulin. All animals were clinically healthy During the study period.The results are as follows:• Body weight, thorax girth and backfat thickness changed significantly during late gestation and lactation.• There were no significant differences in backfat thickness between primiparous and multiparous sows in RFD KL 1, whereas in RFD KL 2 the multiparous sows showed higher backfat thickness as the primiparous ones.• During lactation sows lost backfat thickness, thorax girth and body weight. Body weight loss of RFD KL 2 sows was -9,4 % (-20,0 kg), that ismore than that of RFD KL 2 sows with -6,61 % (-14,5 kg). Thorax girht loss showed no significant difference, being-5,21 % (-7,0 em) in RFD KI, 1, versus -5,07 % (-7,0 em) in RFD KL 2. The loss of backfat thickness was 7,89% (-0,24 em), in RFD KL 2, versus -5,52% 8-0,1 em) in RFD U 1, wich is statistically not relevant seen over the hole period. However, between the 10 th day of lactation and weaning the loss of backfat for RFD KI, 2 with -6,59% (-0,23 em) was significantly higher than for RFD KI, 1 with-1,03% (-0,02 em) (p < 0,05).No significant differences in reproductive- and rearing performance could be be shown between the two groups.During lactation there was no difference in feed and energy uptake. Only in multiparous sows differences in energy uptake were significant (p < 0,05).In both groups late gestation and laktation lead to significant changes in concentrations of urea, kreatinine, free fatty acids, triglycerides, beta-hydroxybutyric acid, glucose and insulin.In RFD KL 1 sows the concentration of urea was significantly higher (p < 0,05) on the 10 0" day of lactation and statisticaly (p < 0,1) higher at weaning. These facts demonstrate the increased catabolic rate in RFD KI, 1 sows as compared to RFD KI, 2 sows.High kreatinine concentrations in late gestation are to be explained by the high metabolic rate in fetus.At the beginning of lactation concentrations of free fatty acids were high in all groups. For RFD KI, 1 sows a significant increase (p < 0,05) during the 2 "A half of lactation can be shown. At weaning sows of RFD KI, 1 showed significantly higher concentrations of free fatty acids vs. RFD KL 2 sows (p < 0,05). This can be explained to some extent by fat depot mobilization and on the one hand by FFS synthesis in lever and mamarry cells, exspecially out of dietary carbohydrates.An increase of beta-hydroxybutyric acid concentration was shown for RFD KL 2 sows in the 2 W half of lactation and in RFD KL 1 sows at the end of lactation. Howerver, at no time did this increase lead to a state of clinical ketosis.Concentration of triglycerides decreased significantly (p < 0,05) from the beginning of lactation till weaning. Increasees in estrogen and, later, in prolaktin concentration could, byis inhibiting Epoproteinlipase in adipose tissue, lead to increased concentrations of triglyceride. At weaning RFD KL 2 sows showed significantly lower concentrations than RFD KL 1 sows (p < 0,05).For RFD KI, 1 sows at weaning there is a highly significant correlation in concentration of triglyceride and free fatty acids (p < 0,001; r = 0,9). High triglyceride concentrations may be the result from an increased rate of free fatty acids, wich were out of reesterification and synthesis. They were derived either from (dietary) carbohydrates or from metabolized fat depots. This is demonstrated by the significant correlation of backfat change during the complete study period and by the concentration of free fatty acids at weaning (p < 0,05; r 0 0,7).Whis the beginning of lactation glucose and insulin concentration increased significantly (p < 0,05) in both groups, reaching the 3 "d day of lactation, then decreasing continually till weaning. High glucose concentration at the beginning of lactation are the result of a high concentration of estrogens, caused by the birth. In addition there is a pronounced secretion of glucagon due to oxytocin secretion caused by succling stimulus. Decreasing concentrations during the time of lactation can be caused by the direct transport of the synthesised metabolits (induced gluconeogenesis) into the mammary glandular. Insulin induces a higher glucose transport into the cell. In connection with the beginning of prolactin secretion there are more receptors in mammary glands, rsulting in hightened insulin uptake by mammary glanular cefireceptors during progressing lactation.At the end of lactation RFD KL 2 sows were able to mobilize more metabolic depots than RFD KL 1 sows. This is shown in massive loss of backfat thickness in the 2 "d half of lactation for RFD KI, 2 sows. RFD KL 1 sows mobilize the small metabolic depots in backfat not further more, but gain additional energie out of dietary carbohydrates. As lactation proceeds the metabolism of,,thick" sows was becoming more balanced.There are no statistical differencees between both RFD groups in feed and energy uptake, as well as rearing success, fitter size, fitter weight and individual piglet weight, although there were differences in backfat thickness and body weight loss. The differences in metabolic parameters starting with the 2"" week of lactation is not accombined by different feed and energy uptake and reproduktive performance and rearing success. They are caused by different body condition at the beginning of lactation and on the other hand by different metabolic activation and catabolism, due to synthesis out of dietary carbohydrates.,,Thick" and ,thin" sows are reaching their goal on differnet strategies.In the present study backfat measurement is not an effective indicator for predictions on fitter size and weight. It can be assumed that with a longer lactation period RFD KL 1 sows would have increased in catabolism, in order to guarantee lactation yield and to rear the piglets.To be more conclusive further studies are required. In studying the complete reproductive cycle, metabolic parameters, and sexual hormones in connection with the reproduktive performance schould be taken in account.A topographic-anatoin-fically exact defined point for the measurement of backfat thickness should be useful. In this study the piont used proved to be appropriate for serial testings.