A rapid method for the gas-chromatographic determination of volatile fatty acids (VFA, C2 to C5) in rumen fluid with the aid of flame ionization detection is based on the direct analysis of the rumen fluid without previous extraction. After filtration of the rumen fluid, metaphosphoric acid is added to precipitate proteins, and formic acid (5 %) is used to eliminate the disturbing "ghosting" effect. The clear supernatant obtained after centrifugation is injected directly into the column. The analysis of one rumen sample requires about 8 minutes. For the quantitative determination, a nonautomatic integrator is used. The individual VFA are calculated by comparing with a standard. The recovery and precision of the gas-chromatographic techniques are discussed.The energy requirements of ruminants are mainly covered by absorption through the rumen wall of the short-chain fatty acids. The molecular proportion of the different VFA is closely related to energy utilization and the animal's production. For nutritionists, a rapid and accurate method for fatty acid analysis in rumen fluid is therefore of great importance. In recent years, gas chromatography has appeared to be the most suitable technique.James and Martin (1952) perfected the first gas-chromatographic separation of both straight-and branchedchain lower fatty acids. Because of their volatility, the lower fatty acids were analyzed as esters, and later the acids themselves were determined directly.

-Compositional data of 17 Belgian Blue double-muscled bulls with empty body weight between 276 and 669 kg were analysed. Body composition only changed slowly and linearly within the investigated live weight range. Water, protein, fat and ash in the empty body varied between 65.8 and 72.0%, 18.9 and 21.2%, 3.5 and 9.7% and 3.0 and 4.% respectively. The chemical fat content in the empty body was remarkably low. The percentage of accreted energy as protein always remained higher than 50% of the total accreted energy, which is much higher than generally reported in literature. Caloric values for protein and fat were: 22.91 (± 0.90) MJ . kg -1 and 38.74 (± 1.53) MJ . kg -1 respectively. Carcass protein contained on average (in %) 8.8 Asp, 15.6 Glu, 5.3 Pro, 7.7 Gly, 6.6 Ala, 7.5 Leu, 7.4 Lys and 6.5 Arg, while the protein in the non-carcass parts contained (in %) 8.0 Asp, 12.7 Glu, 8.8 Pro, 15.1 Gly, 8.0 Ala, 6.6 Leu, 5.6 Lys and 7.2 Arg. The protein composition proved to be rather constant for the considered live weight range.

The effect of 0, 6 or 60 g yeast (Saccharomyces cerevisiae) daily on health and performance was investigated in 90 double-muscled and 48 non-double-muscled Belgian White-blue bulls during 60 and 51 days, respectively, upon arrival at the experimental farm. The yeast was incorporated in the concentrate, which was fed at 2 kg d-1 during the first ten days and at 2.5 kg d-1 afterwards. Besides, maize silage was fed up to 8 kg d-1, while grass hay was offered ad libitum. Feed intake, growth rate and number of sick animals were not affected by the yeast addition. Morbid animals required a similar number of days with antibiotic therapy for each yeast level within each beef strain. Rumen samples taken from 6 non-double-muscled bulls in the last but one week showed a decreased concentration of volatile fatty acids when 6 g yeast was fed daily. Except butyric acid concentration, rumen fermentation parameters were not altered. Yeast supplementation during an adaptation period after purchase did not affect health status and growth performance.

-The effect of housing in indoor pens or outdoor hutches was investigated with 23 male and 22 female double-muscled calves of the Belgian White-blue breed during a 20-week rearing period. Calves in both housing systems were fed the same diet, consisting of a restricted amount of milk products, a maximum of 3 kg of concentrate per day and grass hay ad libitum. They were weaned when the weekly concentrate intake was 3.5 kg at least. Preweaning daily liveweight gain was higher for hutch-housed calves (0.58 kg) than for the indoor-housed animals (0.46 kg), without an effect on intake. This resulted in a better energy efficiency (14.6

-Urinary creatinine excretion (UCE) and chemical body composition of 18 Belgian Blue double-muscled bulls were determined. Partly due to the large fasted live weight (fLW) range (308-710 kg), kg water and protein in the empty body were quite accurately predicted (adjR 2 = 0.94 and 0.97) by UCE. The precision of the prediction of kg empty body fat was less good (adjR 2 = 0.77). However, the empty body composition of the bulls varied only slightly and linearly with increasing live weight so that the fLW was the best parameter to estimate empty body water and protein with an adjR 2 value of 0.99 for both. The empty body fat was estimated from fLW with an adjR 2 = 0.75. The equations predicting relative body composition from UCE had much smaller adjR 2 values, varying from 0.53 for % empty body water to 0.30 for % empty body protein. However, the coefficients of variation were much smaller. The prediction of the relative composition based on UCE was always somewhat better than based on fLW, but the improvement was not enough to justify the work involved in determining UCE. Conclusively, due to the small compositional changes, UCE did not significantly improve the estimation of the body composition from fLW. From the comparison of the results with other studies, it can be concluded that the double-muscled bulls have an extremely high urinary creatinine concentration, which cannot entirely be explained by their increased protein content.