THE USE OF UREA DILUTION FOR ESTIMATION OF CARCASS COMPOSITION OF HOLSTEIN STEERS AT 3, 6, 9, AND 12 MONTHS OF AGE

Every 3 mo for a 2-yr period, two weaned Holstein steer calves (94.5 k g) were randomly assigned to each of four slaughter age groups (3, 6, 9, and 12 mo). Urea dilution was performed before slaughter, and urea space (US) was calculated as total volume and as a percentage of body weight (BW) and empty body weight (EBW). The relationships between US (kg, % EBW and % BW), BW, and EBW and carcass soft tissue composition (protein, fat, moisture, and ash) were studied. One- and two-pool mode ls were fitted using the urea dilution data and the coefficients of th ose equations (zero time, A + B), and the intercepts of compartments A and B were used to estimate body volume. Body weight and EBW effectiv ely predicted the amount of water, fat, and protein in the carcass sof t tissue. Equations expressed in kilograms were more accurate than tho se expressed as percentages. Urea space overestimated body water, prob ably because of the fast rate of urea disappearance in plasma. Correla tion coefficients between US and carcass soft tissue water (kg) based on the pooled data ranged from .74 at 6 min to .48 at 42 min after inf usion. The biexponential models coefficients explained more of the var iation of carcass soft tissue composition than US; correlation coeffic ients using volume B and the soft tissue composition (in kg) with pool ed data were .78 (water), .68 (fat), .69 lash), and .76 (protein). The relationships between A and soft tissue composition were weaker (wate r .59, fat .51, ash .58, and protein .59). The highest correlation coe fficients were obtained when A + B was used for water, fat, ash, and p rotein (.83, .70, .74, and .81, respectively). Equations combining BW, EBW, and two-model coefficients (A, B, A + B) explained much of the v ariation of soft tissue composition. No significant benefit was found in using the urea space at various times after infusion over BW or EBW alone to estimate carcass soft tissue composition in Holstein steers.