Changes in concentrations of insulin, growth hormone and metabolites in plasma with spontaneous feeding in lactating dairy cows.

Changes in plasma levels of growth hormone (GH), insulin and metabolites and their relationships to spontaneous feeding in four lactating dairy cows were studied. Cows were fed a 60:40 (concentrate:silage) mixed ration through 100 days postpartum. Jugular blood samples were withdrawn at 10-minute intervals for 24 hours at day 90 postpartum; feeding behavior was monitored by time-lapse photography during this period. Insulin concentration increased immediately upon feeding and remained elevated for up to 50 minutes after meal termination, despite no significant change in plasma glucose levels. Plasma GH was depressed at 20 minutes postmeal and rose thereafter. Feeding appeared to dampen the fluctuations in free fatty acids (FFA) observed during premeal intervals. Time spent eating during a meal was not correlated with concentrations of these humoral factors at any premeal interval. The correlations of GH/glucose and GH/FFA were significant during eating, noneating and all other intervals during 24 hours. Correlation coefficients of other hormone/metabolite relationships were lower during eating as compared to non-eating periods. The consistent GH/metabolite relationships and rapid, feeding-induced changes in insulin, suggest their importance in the control of feed intake and energy balance.