EFFECT OF BEEF TYPE, BODY-WEIGHT AND DIETARY-PROTEIN CONTENT ON VOLUNTARY FEED-INTAKE, DIGESTIBILITY, BLOOD AND URINE METABOLITES AND NITROGEN-RETENTION

Six normal-conformation and six double-muscled Belgian White-blue bull s were involved in a 2 x 2 x 2 factorial experiment to investigate the effect of genotype (normal versus double-muscled), body weight (400 v ersus 600 kg) and dietary crude protein (135 versus 165 g per kg dry m atter) on voluntary feed intake, digestion and its consequences on the nutritive value, metabolites in blood and urine and nitrogen retentio n. The diet consisted of concentrate and maize silage (50:50, dry matt er basis) and was fed ad libitum. Double-muscled bulls showed a signif icantly Ion er feed consumption (67.1 g DM per kg W-0.75) compared to normal bulls (81.5 g). When adjusted for dry matter intake, digestibil ity was not affected bf beef type. Some feed components tended to be b etter digested when body weight (crude fibre and nitrogen-free extract ives) or dietary crude protein content (dry matter, organic matter, cr ude protein, nitrogen-free extractives and energy) were higher. Blood urea nitrogen was not dependent on genotype, but increased with body w eight and dietary protein. Creatinine concentration in the blood and d aily creatinine excretion in the urine were highest in double-muscled bulls and heavier animals. Urinary 3-methylhistidine excretion was not different between double-muscled and normal bulls, bur increased with body weight. This was confirmed by the fact that nitrogen retention r elative to intake was similar for both beef types and decreased with a higher body weight. A negative effect of stress on feed intake, muscl e protein degradation and nitrogen retention in double-muscled bulls w as not excluded.