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Department of Animal Sciences, The Ohio State University, Wooster 44691
2 Correspondence:
1680 Madison Ave. (phone: 330-263-3900; fax: 330-263-3949; E-mail:
loerch.1{at}osu.edu).
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Abstract |
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 Top Abstract IntroductionExperimental ProceduresResults and DiscussionImplicationsLiterature Cited |
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Key Words: Age Groups • Beef Cattle • Carcass Quality • Feedlots
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Introduction |
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TopAbstractIntroductionExperimental ProceduresResults and DiscussionImplicationsLiterature Cited |
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Increased intramuscular fat deposition for bulls may have been a consequence of their consuming a high concentrate diet for a longer period of time and a gradual increase in IGF-1 concentration as their testicles grew, rather than large fluctuations in IGF-1 concentration exhibited by steers in response to implantation (Schoonmaker et al., 2002). As implanted steers reached physiological maturity, muscle growth was not stimulated to the same degree as observed in bulls when they reached physiological maturity (Schoonmaker et al., 2002). Consequently, delaying feedlot entry until 205 or 365 d of age in bulls may compromise intramuscular fat distribution within the longissimus muscle to a greater extent than steers implanted more than once because of a continued increase in longissimus muscle area and a lack of early intramuscular fat deposition. Our objective was to determine the effect of age at feedlot entry on cattle performance and the dynamics of lean growth and intramuscular fat deposition in bulls and implanted steers.
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Experimental Procedures |
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TopAbstractIntroductionExperimental ProceduresResults and DiscussionImplicationsLiterature Cited |
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All calves were penned individually in a totally enclosed feedlot barn. Pen construction consisted of metal gates, and a slatted concrete floor. Pens were 2.6 x 1.5 m, giving each animal 3.9 m2 of space. On a DM basis, cattle were fed a 70% concentrate, 30% corn silage receiving diet containing 19.3% CP (Table 1
) for the first 14 d after feedlot entry. Initially, all calves were fed 4.5 kg of DM, and intake was gradually increased during the 14-d receiving period. Early-weaned calves were fed a 70% concentrate, 30% corn silage intermediate diet containing 15.9% CP (DM basis) from 124 to 201 d of age. From 202, 216, and 385 d of age, respectively, calves that entered the feedlot at 111, 202, and 371 d of age were fed an 85% concentrate, 15% corn silage finishing diet containing 15.7% CP (DM basis). Cattle were given ad libitum access to feed. Feed was delivered once daily beginning at 0800, and feed refusals were recorded daily for each animal. Initial and final weights were determined using the average of weights taken before feeding on two consecutive days. Interim weights were taken every 28 d before feeding. Average daily gain, DMI, and feed efficiency (gain/feed) were determined for each 28-d period, as well as for the entire trial. Feed samples were collected every 7 d throughout the trial and analyzed for DM according to the procedures of Goering and Van Soest (1970). Monthly composites of feed were analyzed for N content using a combustion-type N autoanalyzer (Leco FP-2000, Leco Corporation, St. Joseph, MI).
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At each feedlot entry date, cattle were scanned between the 12th and 13th rib using a PIE 200 ultrasound machine (Classic Ultrasound Equipment, Classic Medical Supply, Tequesta, FL) to determine fat thickness and longissimus muscle area. Calves were also measured at the hip at each feedlot entry date to determine hip height. Calves were removed from the trial on an individual basis when they reached a predetermined fat thickness of 1.27 cm (as measured by ultrasound). Hot carcass weight, fat thickness, percent kidney, pelvic, and heart fat, longissimus muscle area, and USDA quality and yield grades were determined by qualified Ohio State University personnel 48 h after harvest. Steaks from the 13th rib were cooked to an average internal temperature of 71.7°C, and peak Warner-Bratzler shear force was used as a measure of tenderness according to AMSA (1995) recommendations. The 9-10-11th rib section, and the longissimus muscle from the 6-7-8th rib was removed from the right side of each carcass. Rib sections (9-10-11) were deboned, longissimus muscles (6-7-8) were trimmed of external fat, and both were ground three times, and subsampled for determination of moisture, N, and ether-extractable lipid (AOAC, 1984). A conversion factor of 5.72 (Sosulski and Imafidon, 1990) was used to convert N to protein. Final empty body composition (using the 9-10-11th rib) of the edible carcass was determined using the procedures of Hankins and Howe (1946) and the equations of Garrett and Hinman (1969). Procedures and regression equations developed by Hankins and Howe (1946) accurately predict carcass composition over a wide range of weights (Nour and Thonney, 1994).
Data were analyzed using the GLM procedures of SAS (SAS Inst. Inc., Cary, NC) for a randomized design with a 3 x 2 factorial arrangement of treatments. The model included effects due to age at feedlot entry, castration, and the age at feedlot entry by castration interaction. Residual mean square was the error term, and animal was the experimental unit.
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Results and Discussion |
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TopAbstractIntroductionExperimental ProceduresResults and DiscussionImplicationsLiterature Cited |
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). When in the feedlot, yearlings gained BW faster (P < 0.01) than calves placed in the feedlot at 111 and 202 d of age, contributing to their shorter stay in the feedlot. However, when measured from 111 d of age until harvest, ADG was greatest (P < 0.01) for early-weaned calves, intermediate for calves placed in the feedlot at 202 d of age, and lowest for yearlings. This reflected the varying amounts of time cattle in these groups were fed a high-energy diet. Early-weaned cattle were 58 d younger (P < 0.01) than cattle that were placed in the feedlot at 202 d of age; yearlings were 143 d older (P < 0.01) compared to cattle that entered the feedlot at 202 d of age. A smaller, mature size and faster overall gains for early-weaned cattle contributed to this response, as well as the fact that early-weaned steers had fewer days from last implant to slaughter than both steers that entered the feedlot at 202 d of age and steers that entered the feedlot at 371 d of age (69, 100, and 150 d from final implant to slaughter, respectively). In addition, the time of year and weather conditions when cattle were in the feedlot may have contributed to the age difference at harvest. Early-weaned cattle finished prior to the stress of hot weather in the summer, whereas cattle that entered the feedlot at 202 and 371 d of age finished during and after the heat of summer, respectively. Early-weaned cattle were, however, placed on feed during the summer months of the previous year. Gill et al. (1993a,b) reported similar results when comparing calf-fed and yearling systems. Fluharty et al. (2000) and Schoonmaker et al. (2001) also demonstrated that early-weaned steers gain slower in the feedlot but faster overall and were younger at harvest than steers weaned at approximately 205 d of age. Increased gains prior to 202 d of age (P < 0.01) led to increased gains, overall, for cattle that entered the feedlot at 111 d of age compared to cattle that entered the feedlot at 202 and 371 d of age. Hip height was similar (P > 0.30) among age groups at 111 and 202 d of age, but at harvest, yearling calves were 4.6 cm taller than calves that entered the feedlot at 202 d of age, and calves that entered the feedlot at 202 d of age were 3.5 cm taller than early-weaned calves (P < 0.01). Total DMI, when in the feedlot, was similar (P > 0.22) among age groups, but daily DMI was the lowest for early-weaned calves, intermediate for calves that were placed in the feedlot at 202 d of age, and highest for yearlings (P < 0.01). When in the feedlot, early-weaned calves had the greatest efficiency, followed by calves that entered the feedlot at 202 d of age, and yearlings had the lowest feed efficiency (P < 0.01). Lower daily DMI and greater feed efficiency for younger calves is in agreement with Myers et al. (1999), Story et al. (2000), and Schoonmaker et al. (2002) and may have occurred because maintenance energy requirements were low, and as a result, a higher proportion of their energy intake was available for gain (NRC, 1996).
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A weaning status x castration interaction existed for ADG (P < 0.08) from 202 d of age until harvest and for harvest weight (P < 0.06). Bulls that were early-weaned gained 8.3% slower (1.55 vs 1.69 kg/d), and were harvested at a lower weight (509 vs 526 kg) than implanted steers that were early-weaned. This is in contrast to Schoonmaker et al. (2002), where bulls that were early-weaned gained slower, but no difference existed for harvest weight. Bulls and steers that entered the feedlot at 202 d of age gained similarly from 202 d of age until harvest (1.68 vs 1.69 kg/d), and harvest weight was not different (586 vs 578 kg); however, yearling bulls gained 5.6% faster (1.29 vs 1.22 kg/d) and were harvested at a heavier weight (714 vs 650 kg) than implanted yearling steers. Timing of the implant in steers in relation to feedlot entry and growth of testicles in bulls may have contributed to this difference in the pattern and extent of growth. Early-weaned steers were initially implanted 57 d after being placed on a higher energy diet in the feedlot; steers that entered the feedlot at 202 d of age were initially implanted at feedlot entry, and yearling steers were not initially implanted until 291 d of age, 80 d before feedlot entry. At this point in time, testicle development was in an advanced stage in yearling bulls. For cattle that enter the feedlot at approximately 205 d of age, previous reports (Lee et al., 1990; Hunt et al., 1991) demonstrated that nonimplanted bulls had lower ADG during the growing phase and similar to slightly lower ADG during the finishing phase than implanted steers. Steers in these reports were castrated at 6 to 7 mo of age and implanted separately with trenbolone acetate (200 mg; Finaplix-200, Roussel Hoeschst) and estradiol 17ß (24 mg; Compudose, Eli Lilly and Company) at 8 to 9 mo of age. At 11 to 12 mo of age, steers were either left unimplanted or received a lone trenbolone acetate implant (200 mg; Finaplix-200, Roussel Hoeschst). Henricks et al. (1988) reported that nonimplanted bulls (weaned at approximately 205 d of age) gained 15.6% faster than steers castrated at 6 to 7 mo of age and implanted successively (d 0, 70, 126) with Revalor-S starting at 9 to 10 mo of age. Bulls and steers in that trial consumed bermudagrass pasture and hay supplemented at 1.5% of BW with a 14% CP supplement.
At 111 and 202 d of age, hip height did not differ between bulls and steers (P > 0.28); however, steers were 2.3 cm taller (P < 0.04) than bulls at harvest. Daily DMI, total DMI, and feed efficiency were similar (P > 0.16) between bulls and steers during all phases of the trial. This is in contrast to Schoonmaker et al. (2002) and Hunt et al. (1991). Schoonmaker et al. (2002) reported that total DMI was greater for bulls than implanted steers from 115 d of age to harvest because they were in the feedlot longer. In that study, daily DMI and feed efficiency were not different between bulls and implanted steers when measured from 115 d of age to harvest; however, at a young age (111 to 201 d) daily DMI and feed efficiency were greater for nonimplanted bulls compared to implanted steers, and at a later age (202 d to harvest) they were greater for implanted steers compared to bulls (Schoonmaker et al., 2002). Hunt et al. (1991) observed that, when fed a 25% concentrate diet, nonimplanted bulls, weaned at approximately 205 d of age, consumed 15.2% less DM and were 8.2% more efficient than implanted steers during the finishing phase.
The pattern of fat deposition and increase in longissimus area was altered by age at feedlot entry. Similar to Schoonmaker et al. (2001), early-weaned cattle at 202 d of age were approximately 0.23 cm fatter (P < 0.01) and had approximately a 5.8 cm2 larger (P < 0.01) longissimus area compared to calves that were placed in the feedlot at 202 or 371 d of age (Table 3). This may be due to the higher energy diet, increased weights for early-weaned cattle, or possibly because early-weaned steers had been implanted for 35 d while steers that were weaned at 202 d of age had not been implanted. However, Bruns et al. (2001) demonstrated that for cattle placed in the feedlot at approximately 250 d of age, timing of the final implant had no effect on fat thickness, longissimus muscle area, and percent carcass fat and protein at 94 d prior to harvest. Cattle were either implanted early (150 d prior to harvest) or were implanted late (94 d prior to harvest). In the present trial, fat thickness and longissimus area for cattle placed in the feedlot at 202 and 371 d of age did not differ (0.25 cm and 47.7 cm2, respectively) at 202 d of age and had changed very little since 111 d of age. Even though cattle were all harvested at a similar fat thickness (1.32 ± 0.69; P > 0.13) as was planned, early-weaned calves had the smallest (P < 0.01) longissimus area, followed by cattle that entered the feedlot at 202 d of age; yearlings had the largest longissimus muscle area due, in part, to their heavier carcass weights. However, when longissimus muscle area was expressed as cm2/100 kg BW, calves that were placed in the feedlot at 111 and 202 d of age had a higher value (24.4, 24.0 cm2/100 kg BW for calves that entered the feedlot at 111 and 202 d of age, respectively) compared to yearlings (21.4 cm2/100 kg BW), indicating that cattle placed on feed at 111 or 202 d of age may have a higher lean cutout percentage than yearlings. Early introduction (111 d of age) to a high-energy diet results in rapid fat deposition; thus, if production of a carcass with a fat thickness of less than 1.27 cm is desired, carcass weight may be compromised. In contrast, delayed (371 d of age) introduction to a high-energy diet results in slower fat deposition; thus, if production of a carcass with a fat thickness of approximately 1.27 cm is desired, carcass weight may be excessive. Dressing percent was similar among age groups (P > 0.25). Cattle that were placed in the feedlot at 111 or 202 d of age had lower yield grades (P < 0.04), higher quality grades (P < 0.01), and fewer Select grade carcasses than yearlings (P < 0.01), suggesting that earlier feedlot placement results in a greater intramuscular fat deposition despite the lower carcass fat. This occurred even though early-weaned steers had fewer days from last implant to slaughter than both steers that entered the feedlot at 202 d of age and steers that entered the feedlot at 371 d of age (69, 100, and 150 d from final implant to slaughter, respectively). Bruns et al. (2001) reported that in cattle that entered the feedlot at approximately 250 d of age delayed Revalor-S exposure (94 d prior to harvest) increased carcass weights and tended to increase the rate of intramuscular fat development (% basis) but not subjective marbling score, compared to early implant exposure (150 d prior to harvest). This also indicates that intramuscular fat development is easily influenced in younger steers. Roeber et al. (2000) demonstrated that there were no significant differences in carcass weight, yield grade, and marbling score between cattle that were exposed to Synovex-plus (28 mg estradiol benzoate, 200 mg trenbolone acetate) early in the feeding period (140 d prior to harvest) or later in the feeding period (81 d prior to harvest). A feedlot entry age x implant timing interaction has not previously been investigated, and it is unknown whether one exists. In the present trial, choice quality grade distribution did not differ (P > 0.15) among age groups even though cattle that were placed in the feedlot at 111 and 202 d of age had a higher (P < 0.03) percentage of carcasses that graded low choice or greater. Larger longissimus muscle area is thought to dilute apparent marbling (Duckett et al., 1999; Schoonmaker et al., 2001). However, in the current trial, marbling score did not differ between cattle that were placed in the feedlot at 111 and 202 d of age, despite the fact that cattle that entered the feedlot at 202 d of age had larger longissimus muscle areas than early-weaned cattle. Exogenous hormones from steroid implants (Buttery and Sinnett-Smith, 1984) and endogenous hormones from testicles (Anderson et al., 1988) promote growth primarily through increasing the rate of protein deposition, with minimal effects on lipid deposition. Therefore, an increased longissimus muscle area associated with heavier muscled cattle (Duckett et al., 1999) may proportionately reduce intramuscular fat compared to lighter muscled cattle. Marbling is thought to be a later maturing fat depot in cattle weaned at approximately 205 d of age (Anderson, 1991); however, intramuscular fat can be deposited at a young age in cattle placed on a high-energy diet (Williams et al., 1975; Fluharty et al., 2000; Schoonmaker et al., 2002). There was no difference (P > 0.45) in carcass composition due to age at feedlot entry, which is in contrast to Gill et al. (1993a,b), who reported that body composition at harvest was markedly altered by the pre-feedlot management system. However, in the present trial, yearling cattle had longissimus muscles with the highest percent protein, and calves that were placed in the feedlot at 111 and 202 d of age had longissimus muscles with the lowest percent protein (P < 0.10). Percent ash in the longissimus muscle was greatest for yearlings and lowest for cattle placed in the feedlot at 111 and 202 d of age (P < 0.02).
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Implications |
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TopAbstractIntroductionExperimental ProceduresResults and DiscussionImplicationsLiterature Cited |
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Footnotes |
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Received for publication February 13, 2002. Accepted for publication May 27, 2002.
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Literature Cited |
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TopAbstractIntroductionExperimental ProceduresResults and DiscussionImplicationsLiterature Cited |
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