Robbi H. Pritchard¹
Department of Animal and Range Sciences

Yearling steers (n = 400) were used to evaluate relative payout periods for implants when feeding high grain content diets. Implant treatments included (1) control (nonimplanted), (2) Synovex Plus, (3) revalor-S, and (4) Ralgro-revalor-S. The Synovex Plus (2), revalor-S (3) and Ralgro (4) were administered on day 1. The reimplant with revalor-S (4) was admnistered after 56 d on feed. Steers were managed in two groups. Initial BW and days fed were 782 lb, 131 d (Group I), and 661 lb, 145 d (Group II). Implants increased production rates and efficiencies, increased carcass size and reduced marbling when compared to nonimplanted controls. Production rates and efficiencies and carcass sizes were similar among steers that received implants. Marbling scores and percentage choice carcasses were affected by implants. In general, the delayed use of an estradiol-trenbolone acetate implant improved marbling over d 1 implanting even though there were 56 fewer days on feed after implanting. The energy density of live weight gain was calculated over the course of the feeding period based upon interim period BW and DMI determinations. Higher energy content of gain early in the feeding period for treatments 1 and 4 were related to marbling, while the energy content of gain late in feeding period was not. These data showed no differences in the relative effective duration of Synovex Plus and revalor-S implants. The influence of implants on carcass quality grades was affected by factors other than elapsed time from implanting to harvest.

Optimizing implant strategies requires striking a balance between implant payout, production costs, and carcass specifications.

The influence of implants on cost of gain erodes over time. This encourages shortening the expected payout of the implant. However, it is generally considered that carcass marbling is increased as the elapsed time from implanting to slaughter is increased. The label associated with implant clearances generally does not stipulate how prolonged exposure to the implant relates to these conflicting variables.

Presently there are two estradiol-trenbolone acetate implant formulations available for use in steers being fed for slaughter. One product provides 24 mg estradiol and 120 mg trenbolone acetate. A second available product provides the equivalent of 20 mg estradiol as estradiol benzoate and 200 mg trenbolone acetate. These are potent tools for improving production rates in steers. The concentration and proportion of active ingredients in these implants differ as does the carrier matrix. This experiment was designed to compare the relative payout of these products. The information is intended to help producers define the time exposure appropriate for their production constraints.

To evaluate effective payout, it is advantageous to have a nonimplanted control to use as a reference point during growth. It would also be advantageous to have a positive control that provides high levels of implant payout during the same time frame associated with the expected depletion of the test implant(s). This could be accomplished by administering implants in a staggered time schedule in the positive control treatment. Coincidentally, this would also allow consideration of a re-implant program.

The implant strategies used included (1) Control (nonimplanted); (2) Synovex Plus; (3) revalor-S; and (4) Ralgro-revalor-S. The Synovex Plus (2), revalor-S (3), and Ralgro (4) were administered on day 1. The re-implant with revalor-S (4) was administered after 56 d on feed.

Forty pens of 10 steers were assigned to the experiment. Steers were purchased as two major groups. Group I consisted primarily of black hided steers and Group II was predominately continental crosses. Each group provided enough steers to fill 20 pens (5 pens per implant treatment per group). The groups were fed and managed as distinctive lots of cattle to accommodate differences in implant response and marketing needs that could occur between differing biological types. The nutrition, processing and implant management was common across groups. Days on feed were 131 for Group I and 145 for Group II.

The 200 steers used in Group I were selected from a group of 223 steers. The Group II steers (n=200) were drawn from a pool of 234 steers. At arrival cattle were observed for thriftiness, structural soundness and type characteristics. Any unacceptable subjects were deleted. Within a source group, cattle were ranked by arrival BW and outliers were deleted. Once the pool was reduced to 200 subjects, treatment was assigned (1 to 4) using a random sequence of treatment codes. Data were resorted by treatment and BW and assigned a random sequence of replicate codes. The treatment-replicate combinations were then assigned pen numbers such that treatment was randomly distributed throughout the 20 pens allocated to the group. This allotment system distributed BW ranges similarly in all pens.

Starting dates were May 1, 1996, for Group I and May 23, 1996, for Group II.

Incoming cattle were eartagged and then vaccinated for IBR, BVD, PI₃, BRSV, H. somnus and 7 clostridial SP using Ultrabac 7 and Resvac 4/somubac⁵. Parasite control was provided by administering Expar⁴ (external) and Panacur² (internal) according to label instructions. During processing, ears were palpated for evidence of viable implants. None were found. During the receiving period long hay and the step 1 diet (Table 1) were fed. The milled feed delivery was limited to 1.5% BW during receiving (3 or 4 d).

Initial and final individual BW each were recorded on two consecutive days. Initial implants were administered during the second initial BW processing. Re-implanting with revalor-S was done during the d 56 BW processing. Implant integrity was evaluated at the next weigh day following implanting. Interim BW was determined as noted in Table 2. All BW were collected with no prior restriction of feed or water.

Cattle were fed twice daily. A five step program was used to adapt cattle to the finishing diet (Table 1). Feed calls were made daily at 0700 based on bunk and cattle condition. A clean bunk management system was used. Rations were mixed using a stationary mixer. A single batch of feed was distributed within replicate so that implant treatment and feed batch were not confounded. Samples of feed ingredients were collected once each week. The analyses of these samples were combined with batching records to reconstruct the composition of diets fed. While on step 5, these diets contained DM 75.5% ± .5, CP 12.4% ± .07, ADF 5.6% ± .15, NDF 12.6% ± .8, and ash 2.7% ± .04. The estimated final diet energy density was NE_m 94.8 Mcal/cwt ± .12 and NE_g 63.7 Mcal/cwt ± .10. All pens were fed the final diet (5) within 15 d on trial. These weekly assays and feed delivery records were used to produce DMI summaries each week or more frequently when necessary.

Initial and interim BW reported in Table 3 were not corrected for fill. The final BW referred to in Table 3 included a 3% shrink. This shrunk BW was used to calculate cumulative ADG and dressing percentage. To evaluate the performance response to the re-implant

treatment (4), performance variables were summarized for the periods prior to (EARLY) and following re-implanting (LATE). The Group I cattle were fed for 131 d and the Group II were fed for 145 d. This caused the LATE performance windows to be 57 to 131 and 57 to 145 d, for Group I and II, respectively.

Two steers were removed from the study; one for lameness and one suffered apparent metabolic disorders. These individuals had been individually hospitalized prior to deleting them from the study. Their BW contribution to the pen mean was deleted from the onset of the experiment. Feed records were corrected for the days the subjects were hospitalized. It was assumed that these individuals consumed pen average DMI up to the point of hospitalization.

On the evening following the final BW, steers were transported 75 miles to the beef packing plant at Luverne, MN. They stood overnight and were processed at 0700 the following day. Individual carcass identity was maintained. Hot carcass weight was recorded the day of slaughter. Longissimus area, ribfat thickness, marbling score, bone maturity, lean maturity, KPH (omitted in Group I), and masculinity were collected 24 h after exsanguination. Data were collected by SDSU personnel trained in carcass evaluation.

One steer was mishandled during transit and was not slaughtered as part of this experiment. Consequently, carcass data were available for 397 subjects.

All performance variables were evaluated in a statistical model that included treatment, group, and the treatment x group interaction using the GLM package of SAS. The experimental unit in these analyses was the pen. Orthogonal contrasts were used to separate treatments. The contrasts included (a) control vs. implants; (b) Synovex Plus and revalor-S vs. Ralgro/revalor-S, and (c) Synovex Plus vs. revalor-S. Carcass data were handled similarly except that the individual steer was considered to be the experimental unit.

The initial BW for Groups I and II were 782 lb ± 5.5 and 661 lb ± 4.2, respectively. The predominately Angus x Hereford steers in Group I were large framed and had never been implanted prior to entering our feedlot. The Continental cross steers used in this study were smaller framed than the Angus x Hereford steers. Initial body condition was not quantified. Flesh was considered typical for yearlings entering our feedlot.

Implants increased (P<.001) ADG and DMI and reduced feed/gain (P<.001). These responses were evident during most interim measures of performance (Table 3). In the latter stages of feeding period, interactions developed between cattle group and implant treatment for ADG and feed/gain. The nonimplanted steers in Group II were growing more rapidly and more efficiently than Group I contemporaries during 113 to 130 d on feed. These (Group II) steers started on feed at a lighter weight and were not as close to finish at 130 d. In contrast, the Group II steers implanted with Synovex Plus had lower ADG at 112 d (3.63 vs. 3.09) and 130 d (3.47 vs. 2.44) than Group I contemporaries. The DMI of these steers also tended to be lower during these interim periods.

Short intervals between BW measurements can be misleading. To average responses overtime ADG from 90 to 130 d was calculated (Table 3). This approach showed that cattle were becoming less efficient as they approached harvest BW. A response to implanting was still in effect as feed/gain was 15% lower in steers initially receiving Synovex Plus or revalor-S than in nonimplanted steers. There was an additional 11% improvement (P<.01) in feed/gain during this period in re-implanted steers.

To evaluate the merits of re-implanting, data were calculated for 1 to 56 (early) and 57 to final (late) feeding periods. During the early phase, combination implants caused better ADG and feed/gain than Ralgro implants (P<.001). Synovex Plus tended (P=.095) to cause higher ADG than revalor-S. During the late phase, re-implanted steers grew faster (P=.012) and more efficiently (P=.006) than single implant steers. Interactions existed because the magnitude of response to implants differed between groups. This may be an artifact of this experiment or that cattle respond differently to these implants based upon their relative size when implants are administered.

Longissimus area was increased (P<.001) by implants. There was no appreciable influence on ribfat thickness. Bone maturity and masculinity were increased by implants. Bone and lean maturity were greater for re-implanted than single implanted cattle, but the magnitude of difference is probably inconsequential as regards carcass value.

Influences on marbling were more distinctive. Implants reduced marbling scores and percentage Choice carcasses (Table 5). Marbling scores were lower (P<.05) for single implant strategies (Synovex Plus and revalor-S) than the re-implant strategy. These influences were more pronounced in the lighter cattle of Group II (Table 6). As was noted earlier regarding late gain responses, cattle may be responding differently to implants based on their relative size when implants are administered.

Actual payout optimums for implants were not defined by this research. In Group I it appeared that Synovex Plus was more potent at 130 d than was revalor-S. This observation was reversed in the Group II replication.