Removal of ruminal contents followed by restricted feeding does not affect the frequency of luteinizing-hormone pulses in steers1

Our objective was to determine the effects of acute nutrient restriction on secretion of luteinizing hormone (LH) in serum of male cattle. Angus × Hereford steers (402 ± 17 kg) with rumen cannulas were randomly assigned to treatment. Control (n = 4) steers had ruminal contents completely removed and immediately replaced, whereas restricted (n = 4) steers had ruminal contents completely removed and only 15% (5 L) of rumen contents replaced. Beginning at 1600 h on d 0, control steers were fed 9 kg of prairie hay and 1 kg of a grain supplement each day, whereas restricted steers were fed 1.8 kg of prairie hay daily. Blood samples were collected twice daily for determination of plasma concentrations of glucose, nonesterified fatty acids, and urea nitrogen. Serial blood samples were collected at 10-min intervals for 8 h on d − 1, 0, 1, and 3 to determine serum concentrations of LH. Concentrations of LH (P < 0.01) were greater on d − 1 compared with after removal and replacement of ruminal contents on d 0 and 1. Amplitude of LH pulses was greater on d − 1 (P < 0.02) compared with d 1 and 3. Concentrations of glucose in plasma were not influenced by treatment or day, but concentrations of nonesterified fatty acids and urea nitrogen in plasma after d 0 were greater (P < 0.01) in restricted than control steers. It is concluded that secretion of LH in steers is resistant to rapid changes in nutrient availability.