Effect of peripartum dietary energy supplementation on thyroid hormones, insulin-like growth factor-i and its binding proteins in early lactation dairy cows

The objective of this study was to examine the effect of dietary energy supplementation on hormones that are considered to be the main signals of a shift in energy balance around parturition. Sixty dry cows, 15 days before calving, were chosen and divided into two eaqual groups: control and experimental (GLY). Both groups were fed a standard ration balanced in accordance to the stage of the productivereproductive cycle. Additionally, each cow in the GLY group was given glycerol based dietary energy supplementation (250 mL daily during the dry and 300 mL daily during the lactation period), which provided additional 9.30 MJ NEL during the dry and 13.95 MJ NEL during the early lactation period. Milk production was measured on days 30 and 60 of lactation and milk production was significantly higher in GLY compared to control group at day 60 of lactation (p<0.05). Service period and insemination index were used as reproductive outcome parameters. Average service period in the control group was significantly longer than in the GLY group (p<0.05). Average insemination index in the control group was not significantly different than the index obtained for the GLY group. Blood samples were taken before the begining of the experiment (15 days before parturition), and at days 7, 30 and 60 of lactation. Concentrations of thyroid hormones, IGF-I, relative abundance of IGFBP-2, IGFBP-3 and IGFBP-4, concentrations of total protein and albumin in the blood were measured. Results showed that at days 7 and 30 after parturition, T4 concentrations were significantly higher (p<0.001, respectively) in GLY than in the control group, while T3 concentrations were significantly higher in GLY group only at day 7 after parturition (p<0.001). IGF-I concentrations and IGFBP-3 abundance were significantly higher in the GLY compared to the control group in all three examined postpartum periods. IGFBP-2 and IGFBP-4 concentrations were higher in GLY compared to the control group in all three examined postpartum periods, but the difference was significant only on day 60 after parturition (p<0.01, respectively). Concentrations of total protein and albumin were significantly higher in GLY compared to the control group in all three examined periods after parturition. Based on these results it can be concluded that peripartum dietary energy supplementation prevent cows' exposure to severe negative energy balance, preserves synthetic activity of hepatocytes and consequently has a positive impact on milk production and reproductive performances in dairy cows.