Theron S. Rumsey

Composition analysis of beef rib sections by dual-energy X-ray absorptiometry.

Dual-energy X-ray absorptiometry (DXA) was used as a non-invasive method to measure composition of 9, 10,11 - rib sections obtained from 80 beef carcasses. The weights of the rib sections ranged from 2986 g to 6394 g. Each rib section was scanned twice by DXA, once in a horizontal or standing rib position (DXA-1) and once in a vertical position (DXA-2). After scanning, each rib section was dissected into fat, muscle and bone. The intramuscular fat content of the longissimus muscle was measured chemically. Dissected lean and fat values were adjusted for the amount of intramuscular fat. DXA lean values were adjusted for amount of bone (less bone mineral content). Both DXA measurements consistently overestimated the amount of fat relative to dissection. The DXA-1 scan position measured more lean and less fat than did DXA-2. The correlation (r) between the amount of dissected lean and DXA-1 and DXA-2 lean was 0.91 and 0.85, respectively and between the amount of dissected fat and DXA-1 and DXA-2 fat was 0.94 and 0.92, respectively. The high correlations between DXA and dissection results indicate that DXA could be used as a non-destructive method for evaluating composition of cuts of beef; however, attention must be given to orientation of the cut during the DXA scan.

Effects of growth-promoting implants on morphology of Longissimus and Semitendinosus muscles in finishing steers.

Growth-promoting implants lead to increased muscle accretion in ruminants. To elucidate the effects at a cellular level, muscle fiber distribution and cross-sectional area (CSA) of longissimus (LM) and semitendinosus (ST) muscles were compared in implanted and control steers. Sixty-four Charolais steers were assigned to one of four treatments (16 steers/treatment): (1) no implant, (2) Synovex-S® (estradiol benzoate+progesterone), (3) Ralgro® (zeranol) or (4) Revalor-S® (trenbolone acetate+estradiol-17β). The experiment was carried out using four slaughter groups (SGRP). Sixteen steers each were slaughtered after 48, 104, 160 and 175 days (four steers/treatment) on trial. Steers on an implant treatment were first implanted at 15 months of age (day 0) and re-implanted at 56 and 112 days. Muscle fibers in the LM and ST (for both live biopsy and post-mortem samples) were characterized as either slow-twitch oxidative (SO), fast-twitch oxidative-glycolytic (FOG) and fast-twitch glycolytic (FG) fibers. Fiber distribution was minimally affected by SGRP in these physiologically mature steers. Implantation with Synovex did not alter fiber distribution in either muscle compared with control steers. Both Synovex-implanted and control steers showed a decrease of FG and an increase of FOG fibers in the LM from day 0 to SGRP 2 followed by an increase of FG and a decrease of FOG fibers. Ralgro- and Revalor-implanted steers had an almost constant fiber distribution in the LM throughout the experiment resulting in higher precentages of FG fibers in SGRP 2 (P<0.05) than SYN or CON steers. Biopsy samples of the LM muscle which were excised 51 days (SGRP 1-3) or 65 days (SGRP 4) before slaughter proved to be suitable for the determination of fiber distribution in live animals. Fiber area increased in post-mortem samples of both muscles from SGRP 1-3 in all treatment groups followed by a plateau. Implantation with Revalor led to an additional increase in fiber area from SGRP 3 and 4 (P<0.05). Synovex did not affect fiber area compared with control steers whereas Ralgro and Revalor implants led to larger fibers in SGRP 3 and 4, respectively. It can be concluded that some growth-promoting implants result in noticeable differences in muscle hypertrophic responses which coincide with their different effectiveness to enhance lean mass accretion.