Oxidation of protein (OXP), carbohydrate (OXCHO) and fat (OXF) was investigated with 12 growing bulls treated with beta-agonist (L-644, 969) during two 6 weeks trials (Section A and B) at a mean live weight of 195 and 335 kg. Heat production and nutrient oxidation was calculated from gas exchange, with CO2 reduced for CO2 from fermentation processes, and nitrogen excretion in urine. The beta-agonist had no effect on the level of rumen fermentation as indicated by the same methane production for control and treated animals. Heat Production (HE, RQx) increased by the treatment of beta-agonist corresponding to the increment in the protein retention. OXP/HE,RQx was reduced to about 10% in treated animals, indicating that in order to supply amino acids for an increased protein deposition oxidation of protein is decreased. OXF/HE,RQx were markedly higher in treated animals, but as indicated by the same CH4 production the level of the short chain fatty acids (SCFA) production was the same. Therefore, it was concluded that the increase in OXF was not caused by an increase in SCFA but by a direct influence of beta-agonist on mobilization and oxidation of body fat.

Total microbial cell counts and the content of deoxyribonucleic acid (DNA) and adenosine triphosphate (ATP) were determined in ruminal and abomasal digesta and in faeces from heifers fed a basal diet of barley and straw supplemented with urea, casein, soybean‐protein concentrate or feather meal. The concentration of ATP in the rumen content varied independently of the DNA content and total cell count, but dependently upon the nitrogen sources, the highest ATP/DNA ratio being obtained with casein. The ATP/DNA ratio in faeces was only one‐tenth of that found in the digesta of the rumen and abomasum, indicating either an extremely low level of activity of the microbial cells or more probably a very large number of dead organisms. It is suggested that DNA and ATP might be useful indices of the microbial status in terms of biomass and metabolic activity. The total cell count should still be included in routine studies to determine the proportion of protozoa to bacteria.

The concentration of adenosine triphosphate (ATP) in rumen fluid from heifers fed high levels of nutrients continuously and after 12–24 h starvation has been determined in order to examine the influence of different physiological conditions. The conventional parameters for characterizing the rumen fermentation have been applied to control the effect of a different nutrient supply. The mean concentration of ATP, determined by the luciferin‐luciferase assay, was 55.3 mg ATP/l rumen fluid in a heifer fed continuously, twice as much as found in a starved heifer. The mean ratio of biomass/ATP changed from 683 at the high level of energy intake to 1208 in the period of starvation. Consequently, the concentration of ATP in the rumen micro‐organisms is markedly influenced by nutritional conditions. Finally, the applicability of ATP determinations in rumen microbiology is discussed.

Protein and energy utilization and quantitative retention of protein, fat and energy was investigated with 12 Red Danish bulls during two subsequent 6 weeks trials (Sections A and B) at a mean live weight of 195 and 335 kg respectively. Treatments were control (Group 1) and beta-agonist (L-644,969) treated animals (Group 2 and 3). Beta-agonist supplementation was 5 and 10 mg/d in Group 2 and 3 respectively in Section A and 10 and 20 mg/d in Section B. Measurements were performed by means of nitrogen and carbon balances and with use of indirect calorimetry. In each section the mean intake of dry matter, metabolizable energy and digestible protein was of the same magnitude for all groups. The beta-agonist had no significant effect on protein digestibility and metabolizability of energy, but daily live weight gain was significantly higher in the treated bulls. The utilization of digested protein was strongly influenced by treatment, with the highest values for Group 2 in both sections. The protein retention increased with 25% in Group 2, with the highest increment of 113 g/d in Section B. The fat retention decreased in treated animals, most pronounced in Group 3, where the reduction was about 50% with a mean decrease of 145 g/d in Section B. The total energy retention was not different between control animals and Group 2 in both sections, but there was a shift in energy retention from fat to protein. The higher levels of beta-agonist used in Group 3 generated the same increase in protein retention, but fat retention was markedly lower and consequently the total energy retention was reduced.