Plasma Growth Hormone and Insulin During Early Lactation in Cows Fed Silage Based Diets

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Abstract
Peripheral plasma insulin, growth hormone and blood glucose were determined in 24 dairy cows during early lactation. Blood samples were obtained at 4-day intervals during the first 56 days of experimental lactations 1 and 3 of a 3-lactation experiment. The cows were divided equally among 3 groups and fed 1 of the following forage rations for 3 consecutive lactations: corn silage (18.2 kg/day) plus alfalfa-timothy hay ad lib; corn silage (18.2 kg/day) plus alfalfa-timothy silage ad lib; or corn silage ad lib. These diets were continued throughout the intervening dry periods except in the latter group where corn silage intake was limited if individual animals exhibited excessive body weight gain. Four animals in each group received prepartum supplemental concentrates. The remaining 4 animals received no prepartum concentrate supplementation. Following parturition, concentrates were fed in increasing amounts to a maximum of 14.6 kg/day during the first 56 days of lactation. Plasma insulin, growth hormone and blood glucose were not affected by forage diet or prepartum supplemental concentrate feeding. Plasma insulin and blood glucose increased and plasma growth hormone decreased with time after parturition. These changes were correlated with nutrient intake which increased during the experiment. Changes in plasma insulin and growth hormone are related to metabolic adjustments during early lactation.