Nitrogen metabolism and fertility in cattle: I. Adaptive changes in intakeand metabolism to diets differing in their rate of energy and nitrogen release in the rumen

The ruminal degradability, intake, and metabolism of diets differing in the ir relative rate of energy and nitrogen release in the rumen were character ized prior to their use in a study of the effects of high peripheral levels of ammonia on reproductive function in cattle. In a 2 x 2 factorial experi ment, replicated four times, 16 heifers were offered isocaloric and isonitr ogenous diets containing two sources of fermentable carbohydrate, fiber (sl ow energy release, SE) or starch (fast energy release, FE), and two rates o f nitrogen release, which were either synchronous (S) or asynchronous (A) t o that of energy release. Throughout the experiment, the amount of feed off ered was held constant, at a level equivalent to 1.5 x maintenance. Four ru minally fistulated sheep were used to determine the in situ degradability o f these diets. The 16 heifers were bled before feeding at 0800 and at 0900, 1000, 1100, 1200, 1400, and 1600 on d 0 (introduction to dietary treatment s) and on d 4, 7, 11, 14, 21, and 28. Diet refusals were recorded at hourly intervals after feeding. The rapidly degradable nitrogen fraction of the S E:A and FE:A diets was greater than that of the SE:S and FE:S diets. Post-p randial jugular plasma ammonia levels rose to a peak of around 300 mu mol/L in heifers offered the SE:A and FE:A diets but did not rise in heifers off ered the SE:S and FE:S diets. All feed offered was consumed within Ih on di ets SE:S and FE:S throughout the experiment. The proportion of feed consume d within 1 h of feeding declined from 100% on d 0 to around 70 and 56% by d 21 for heifers given the SE:A and FE:A diets, respectively. Peak postprand ial plasma ammonia levels were accordingly lower, at around 160 mu mol/L. P lasma urea levels averaged 7 mmol/L and were unaffected by dietary treatmen t. High plasma ammonia levels were associated with a suppression in the nor mal postprandial rise in insulin. There was no significant metabolic adapta tion to high-ammonia-generating diets, and heifers given these diets modifi ed their pattern of intake in an apparent attempt to avoid excessively high levels of plasma ammonia.