Ninety-six steers (average BW = 335 ± 36 kg) were allotted in a completely randomized design with a 2 × 2 factorial arrangement of treatments. Factors were 1) implant on d 1 or no implant and 2) ad libitum access to feed on d 1 or programmed-fed for a target BW gain of 1.4 kg/d during the first 62 d of the feeding period. On d 63, all steers were implanted with Revalor-S® (Hoechst Roussel Vet, Overland Park, KS) and provided ad libitum access to feed until harvest. From d 63 to 116, ADG and gain efficiency (g gain/kg DMI) for steers programmed-fed to gain slowly were greater (P
(Key Words: Steers, Implants, Programmed Feeding, Ultrasound, Carcass.)
Anabolic growth agents are routinely used in beef cattle to increase growth efficiency and decrease production costs (Montgomery et al., 2001). Managing feed intake by restricted or programmed feeding for specific rates of gain may yield performance advantages to beef cattle feeders (Galyean, 1998). Restricted or programmed feeding has the potential to decrease costs by simplifying bunk management, avoiding over-consumption of feed when starting cattle, improving feed efficiency, and decreasing manure loads. Ultrasound technology can be used as an objective method to estimate carcass attributes of the live animal, which has potential to increase income, shorten the length of the finishing period, and avoid wasting feed resources (Brethour, 2000).
Managing rate of gain of steers during the initial days in the feedlot, as well as delaying the first implant application, can improve performance (ADG and gain efficiency) and carcass quality and composition (Samber et al., 1996; Drager et al., 2001; Duckett and Andrae, 2001). The overall objective of this study was to determine the effect of management strategies (time of implant and programmed feeding) on feedlot performance and carcass characteristics at harvest. Specific objectives were 1) to determine the impact of programmed feeding and implant strategies during the finishing period on ADG and gain efficiency of feedlot steers, 2) to determine the impact of programmed feeding and implant strategies on marbling and other carcass characteristics and carcass composition of feedlot steers, and 3) to evaluate the use of ultrasound technology as a tool to measure the evolution of marbling and fat thickness and its potential use as a selection criteria for marketing cattle.
Materials and Methods
The experiment was conducted at the Texas Agricultural Experiment Station/United States Department of Agriculture-Agricultural Research Service Experimental Feedlot at Bushland, Texas. The animal use protocol was approved by the Cooperative Research, Education, and Extension Triangle Animal Use and Care Committee.
Ninety-six British times; Continental steers (average BW = 335 ± 36 kg; 14 mo of age) were selected from a group of steers that had previously grazed sorghum × Sudan hybrid forage for 62 d and a short-grass prairie range for 72 d. At the beginning of the grazing period, steers had been implanted with Synovex-S® (20 mg estradiol benzoate, 200 mg progesterone; Fort Dodge Animal Health, Overland Park, KS). Steers were fed a diet consisting of 49% steam-rolled corn, 41% cottonseed hulls, and 10% supplement (DM basis). The percentage of these ingredients was gradually changed to a high concentrate diet presented in Table 1. The chemical composition of the diet was analyzed at the Dairy Herd Improvement Forage Testing Laboratory (Ithaca, NY; Table 1).
Steers were allotted to 12 pens (8 steers per pen; 3 pens per treatment) in a completely randomized design. Initial average pen BW of steers was similar. Steers were assigned to implant and rate of gain treatments in a 2 × 2 factorial arrangement. Factors were 1) implant of SynovexS® on d 1 of the finishing period or no implant and 2) ad libitum access to feed (rapid rate of gain) or restricted feeding for a rate of gain of 1.4 kg/d (slow rate of gain). Every week, the amount of feed offered was adjusted for the cattle fed for a slow rate of gain to maintain an estimated ADG of 1.4 kg (NRC, 1996), assuming non-implanted steers were fed an ionophore. Steers assigned to the rapid rate of gain treatment group were expected to gain approximately 1.9 kg/d. Steers were weighed at 0800 h before being fed on d 1, 63, and 117. Steers were not weighed at the experimental feedlot immediately before they were sent to harvest to prevent stress and bruising. Hence, final BW of each steer was calculated as the product of hot carcass weight and average dressing percentage of the load being marketed. On d 63, all steers were implanted with Revalor-S® (Hoechst Roussel Vet, Overland Park, KS) and provided ad libitum access to feed until harvest.
Ultrasound measurement (SSD-500V; Aloka Co., Wallingford, CT) of external fat thickness and intramuscular fat (marbling) was obtained on d 1, 62, and 116 (53 d after reimplantation). The measurement obtained on d 116 was used as a criterion to determine the date of harvest following equations developed by Brethour (1994). An average external fat thickness of 12 mm for the pen was deemed to be the time of harvest. Ultrasound measurements were taken caudal to the last rib and approximately 8 cm distal to the centerline of the steer's back. Both external fat thickness and marbling score were estimated with procedures that incorporate image analysis software (Brethour, 1994).
On the date of harvest for each group, steers were transported to a commercial packing plant located 34 km from the experimental feedlot. After a 36-h chilling period, carcasses were evaluated by trained personnel for Longissimus area at the 12th rib; subcutaneous fat thickness; estimated kidney, pelvic, and heart fat as a percentage of carcass weight; marbling score at the 12th rib (USDA, 1989), and lean color score (Herschler et al., 1995).
A 6th to 13th rib section was obtained from the left side of the carcass of each steer and transported (28 km) to the West Texas A&M University Meat Laboratory in Canyon, Texas. The following day, the rib section was divided into three sections from 6th to 8th ribs, 9th to 11th ribs, and 12th rib, according to the methodology described by Hankins and Howe (1946). The 9th to 11th and the 12th rib sections were wrapped in aluminum foil then in poly-coated freezer paper, vacuum-packaged, frozen at -20°C, and stored. The 9th to the 11th rib sections were thawed and dissected (Hankins and Howe, 1946), and weight of bone, lean, Longissimus, and fat tissue were recorded. Lean, fat, and bone weights were used to estimate percentage of these variables for the whole carcass (Hankins and Howe, 1946). The lean and fat portion from each rib of individual steers were mixed and ground three times. The first pass through the grinder was with a 0.64-cm plate followed two times with a 0.48-cm plate. Two, 200-g sub-samples were obtained, vacuum-packed, and frozen for later analyses of moisture, ash, protein, and ether extract (AOAC, 1995). Moisture content of the sample was determined after thawing and drying in a forced-air oven for 24 h at 105°C. Ether extraction of the dried samples was performed in a Soxhlet apparatus over a 24-h extraction period using petroleum ether at a drip rate of 2 drops/s. Crude protein percentage was determined by sample combustion to release gaseous N in a LECO FP2000 Protein Analyzer (St. Joseph, MO). The values for protein and ether extract were used to estimate the relationship between them and the amount of lean and fat obtained in the rib dissection.
All data were analyzed by the GLM procedures of SAS (2001) as a completely randomized design with a 2 × 2 factorial arrangement of treatments. Pen was used as experimental unit for all analyses. Mean separation was performed using LSD ([alpha] = 0.05) when an interaction between rate of gain and implant treatment occurred (P
Results and Discussion
Growth Performance. The ADG, feed intake, and gain efficiency (g gain/kg DMI) of steers implanted or not implanted on d 1 and programmed-fed to gain slowly or rapidly from d 1 to 62 of the finishing period are presented in Table 2. There tended (P=0.179) to be an interaction between main effects for days on feed. This interaction occurred because steers fed to gain rapidly from d 1 to 62 and implanted on d 1 tended to require less days on feed than any other treatment combination.
During the first 62 d, steers implanted on d 1 tended to have a 13.9% greater (P=0.065) ADG than those not implanted on d 1. Mader et al. (1999) and Bartle et al. (1990) reported similar results. However, Foutz et al. (1997) found no differences in ADG or gain-to-feed ratio between steers implanted with Synovex-S® on d 1 (1.58 kg/d and 0.188, respectively) and non-implanted steers (1.68 kg/d and 0.186, respectively).
As by experimental design, steers programmed-fed to gain slowly gained 35.2% less (P=0.0001) than did steers fed to gain rapidly (1.27 and 1.96 kg/d, respectively). During d 1 to 62, implant treatment did not affect feed intake (P=0.639) but did improve (P=0.064) gain efficiency (194 and 169 g/kg DMI for implanted and non-implanted steers on d 1, respectively). As designed by the experimental procedure, feed intake was affected (P=0.0001) by programmed feeding level for slow and rapid rates of BW gain. However, feeding level did not affect gain efficiency from d 1 to 62 (P=0.468). Hicks et al. (1988), Loerch (1990), and Murphy and Loerch (1994) found similar results when evaluating restricted feeding of feedlot steers.
Implant treatment during the first 62 d did not affect ADG or feed intake from d 63 to 116 (P=0.535 and 0.171, respectively) or from d 63 to harvest (P=0.845 and 0.314, respectively). However, delayed-implanted steers gained more efficiently from d 63 to 116 (P=0.020) than did steers receiving the implant on d 1 but not from d 63 to harvest (P=0.204). However, Bartle et al. (1990) reported that from d 64 to 91 (on d 63 steers were implanted for the second time with trenbolone acetate) of the feeding period, steers initially implanted on d 1 with Synovex-S® gained faster than did steers that were not implanted on d 1, but no differences in DMI or gain efficiency were detected. Milton et al. (2000) also detected a positive effect of delaying the initial implant (Synovex-Plus®; Fort Dodge) on ADG and gain efficiency in the second period of their study (d 70 to 152). Mader et al. (1999) reported that from d 67 to harvest, steers that were not implanted and those implanted with Synovex-S® on d 0 gained similarly, but non-implanted steers were more efficient than implanted steers. Foutz et al. (1997) found a difference in ADG (from d 59 to harvest) between steers implanted with Synovex-S® on d 1 compared with those implanted with Revalor-S® on d 1. No difference was detected between the same treatments for gain efficiency.
Steers programmed-fed to gain slowly from d 1 to 62 had greater ADG (P=0.0002) from d 63 to 116 than did steers that were fed to gain rapidly. This result reflects a compensatory gain response by restricted-fed steers. From d 63 to 116, feed intake was similar (P=0.473), resulting in an improved (P=0.0001) gain efficiency for steers fed to gain slowly compared with those fed to gain rapidly from d 1 to 62.
From d 117 until harvest, there was no effect of implant or rate of gain treatments imposed during the first 62 d on ADG, DMI, and gain efficiency. Overall, from d 1 to harvest, no difference was detected for ADG (P=0.354), DMI (P=0.296), and gain efficiency (P=0.339) for the implant treatments. Milton et al. (2000) found similar results on performance when delaying the initial implant (Synovex-Plus® at d 0, 35, or 70). Samber et al. (1996) observed that delaying the administration of the first implant for 30 d resulted in ADG and gain efficiency values that were similar to the treatments with an implant on d 0. Mader et al. (1999) reported that steers implanted on d 0 gained more rapidly (1.55 kg/d) and were more efficient (146 g/kg DMI) than non-implanted steers (1.42 kg/d and 139 g/kg DMI, respectively).
Carcass Characteristics. Carcass characteristics are presented in Table 3. No interactions between main effects were detected in any of the carcass parameters measured. Because cattle were harvested at a constant fat thickness, implant and rate of gain treatments did not affect fat thickness. Implant and rate of gain treatments during d 1 to 62 did not affect hot carcass weight (P=0.836 and 0.553, respectively), marbling score (P=0.126 and 0.317, respectively), Longissimus area (P=0.757 and 0.841, respectively), or USDA yield grade (P=0.935 and 0.833, respectively). A factor that may be affecting these results is the fact that all steers were implanted with Synovex-S® at the beginning of a grazing experiment conducted when the steers were 8 mo of age. Foutz et al. (1997) and Kerth et al. (1995) found similar results with no effect of implant strategy on carcass characteristics. Mader et al. (1999) reported that marbling scores were lowered when Synovex-S® was used as the initial implant vs neverimplanted steers. However, in the same study, carcasses from steers that were implanted with Synovex-S® on d 0 and those that were not implanted on d 0 but were implanted with Synovex-S® on d 70 had similar marbling scores. In the present experiment, delaying the first implant until d 63 did not affect marbling scores, even though this implant was Revalor-S®. Revalor-S® is considered a higher potency implant than Synovex-S® (Duckett and Andrae, 2001). Apple et al. (1991) and Johnson et al. (1996) reported that the use of a combination implant (estrogenic and androgenic hormones) did not affect marbling. Duckett and Andrae (2001) reported that a single use of an estrogenic hormone or a combination of estrogenic and androgenic hormones decreased marbling score and increased Longissimus area. In their study, a second implant (estrogenic or a combination of estrogen and androgen) increased Longissimus area and decreased marbling score. Bruns et al. (2001) indicated that the percentage of intramuscular fat of the Longissimus was reduced by early implanting and was unaffected by a delayed implant. When DMI of a steer is restricted, usually both protein and lipid deposition rate are decreased (NRC, 1996). Pritchard (1997) reported that when DMI was restricted by >10%, marbling scores were lowered. However, in the present study, this was not observed, probably because the feed restriction was not long enough to cause a negative effect. Longissimus area was not affected by any of the treatments applied (P=0.818).
Percentage kidney, pelvic, and heart fat was affected by implant application on d 1. Steers implanted on d 1 had more (P=0.039) kidney, pelvic, and heart fat than did non-implanted steers. However, Bruns et al. (2001) reported that implants decreased the estimated percentage of kidney, pelvic, and heart fat compared with non-implanted cattle. Milton et al. (2000) reported no effect of delaying implant on estimated percentage of kidney pelvic, and heart fat.
Lean color score was affected by rate of gain (P=0.036) during the first 62 d of the feeding period. Lean color scores of carcasses from program-fed steers was darker than those from steers that had ad libitum access to feed.
The 9th to 11th rib section lean, fat, and bone weight (using rib weight as a covariate in the analysis) is presented in Table 4. Implant or rate of gain treatments during d 1 to 62 of the feeding period did not affect the weight of the Longissimus (P=0.497 and 0.676, respectively), lean weight (P=0.200 and 0.418, respectively), and bone weight (P=0.289 and 0.568, respectively). An interaction (P=0.024) occurred between implant and rate of gain treatments during d 1 to 62 for fat weight. The interaction existed because rib fat weight was less when implanted steers were fed to gain rapidly compared with those fed to gain slowly, but, for non-implanted steers, program feeding for rapid gain increased fat weight in the rib section compared with the slow rate of gain.
Using equations developed by Hankins and Howe (1946), percentage of ether extract from the Longissimus of the 9th to 11th rib were estimated as were percentage of CP of the carcass and proportion of lean, fat, and bone in carcass (Table 5). No differences (P>0.17) were detected because of implant and rate of gain treatments imposed during d 1 to 62 of the feeding period. Averaged across treatments, the percentages of protein and fat in the carcass were 15.3 and 33.4%, respectively. These data are similar to those of Nour and Thonney (1994), Myers et al. (1999), and Hutcheson et al. (1997).
The carcass grade data are presented in Table 6. Eighty-six percent of the cattle graded USDA Prime or Choice. The proportion of carcasses grading Prime and Choice was similar (P=0.789) among treatments. The use of implants is considered as one of the reasons for decreasing quality grades (Duckett and Andrae, 2001). In the present study, implanting on d 1 had no effect on final USDA quality grades.
The data obtained from the ultrasound conducted at different times across the experimental period are presented in Figure 1. In general, the nearer the day of harvest, the better the estimation of final fat thickness and marbling score. The simple coefficient of determination (r^sup 2^) for fat thickness varied from 0.17 (when estimated on d 1; an average of 172 d before harvest) to 0.55 (when estimated 56 d before harvest). The same tendency was observed for marbling score, with an r^sup 2^ that varied from 0.14 to 0.30. Brethour (2000) reported similar r^sup 2^ when marbling scores were estimated 159 d before harvest (r^sup 2^ = 0.18), 122 d before harvest (r^sup 2^ = 0.28), and 83 d before harvest (r^sup 2^= 0.24). In the present study, it was not advantageous to classify cattle according to external fat thickness at the beginning of the feeding period; the second implant (d 62 on feed) was a more appropriate time.
Steers program-fed to gain slowly from d 1 to 62 tended to be (P=0.097) more efficient from d 1 to harvest and also tended (P=0.067) to consume less feed than steers that were fed to gain rapidly. Given these data, steers programmed-fed to gain slowly were fed less feed, making production more economical. Although in this experiment no effect was noted on marbling and other carcass characteristics, effects were observed in terms of the amount of fat and lean deposited. Using ultrasound technology to predict the number of days that cattle should be fed to produce the most valuable carcass will be a useful technology for the industry.
1 This research was funded by Texas Agricultural Experiment Station and The Beef Carcass Research Center, West Texas A&M University (Canyon). Reference to a company or trade name does not imply endorsement or approval by the Texas Agricultural Experiment Station, West Texas A&M University, or USDA-ARS.
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G. SCAGLIA*, PAS, L. W. GREENE2,[dagger],[double dagger] PAS, F. T. MCCOLLUM[dagger], PAS, N. A. COLE§, PAS, and T. H. MONTGOMERY[double dagger], PAS
* Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061; [dagger] Department of Animal Science, Texas A&M University, College Station 77843-2471 and Texas A&M University Agricultural Research and Extension Center, Amarillo 79106; [double dagger] Division of Agriculture, West Texas A&M University, Canyon 79106; and § Conservation and Production Research Laboratory, ARS, USDA, Bushland 79012
2 To whom correspondence should be addressed: email@example.com
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