Effects of estradiol or an estradiol-trenbolone acetate reimplant Effects of estradiol or an estradiol-trenbolone acetate reimplant scheme and time on feed on performance and carcass traits of scheme and time on feed on performance and carcass traits of finishing steers finishing steers

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Summary
Two hundred eighty-eight predominantly British and British crossbred steers (702 lb) were used in a 2×3 factorially arranged experiment.Main effect factors were reimplant scheme [estradiol (E ) vs estradiol plus 2 trenbolone acetate (E + TBA)] and time on 2 feed (111, 125 or 139 days).The initial slaughter occurred when 65 to 70% of all steers were estimated to grade low Choice.No interactions occurred for any variable measured.Reimplanting 57 days after the initial implant with E + TBA increased overall daily 2 gain 6.9% (P<.003) and feed efficiency 4.9% (P<.005).Feeding steers for an additional 14 or 28 days resulted in linear decreases in overall daily gain (P<.005) and feed efficiency (P<.0004).Reimplanting with E + 2 TBA increased (P<.001) carcass weight but did not reduce marbling score or percent Choice carcasses.Feeding steers for an additional 14 or 28 days resulted in linear increases (P<.0001) in hot carcass weight, ribeye area, adjusted backfat (P<.004), and skeletal maturity (P<.0005).Additional days on feed increased dressing percentage (P<.002)and marbling score (P<.05) curvilinearly and tended (P=.25) to increase the percentage of carcasses grading Choice and Prime.Incidence of dark cutters was higher (P<.05) for E + TBA carcasses, and was very high at the 2

Introduction
The improvement in rate and efficiency of gain in feedlot steers from combined use of estradiol (E ) and trenbolone acetate (TBA) 2 implants is well documented.It appears that 1) implanting more than once with TBA is not effective and 2) the response to TBA is greater when it's used at reimplant time rather than at the beginning of the feeding period.However, research has shown this may result in an average of 8 to 10% fewer Choice carcasses, particularly if reimplanting with TBA in the form of Finaplix® occurs less than 60 to 70 days before slaughter.The effective payout period of TBA from Finaplix implants is considered to be 60 to 65 days.Because most implant studies have utilized a time-constant slaughter endpoint, whether an additional 14 to 28 days on feed would overcome TBA's observed quality grade reduction, yet maintain a feedlot performance advantage, is largely unknown.Therefore, we evaluated the main effects and potential interaction of reimplant scheme (E vs E + TBA) and additional days

Experimental Procedures
Three hundred eighty-eight predominantly British and British crossbred steers that had been pastured together in an early-intensive stocking program on Flint Hills range were delivered to the KSU Beef Research Unit on July 17, 1991.Steers were individually weighed; ear-tagged; dewormed (Ivomec®); and vaccinated against IBR, PI , BVD (modi-3 fied live vaccine), and seven clostridial organisms.Steers were adapted to a high grain finishing diet over 14 days by stepwise increases in concentrate and reductions in roughage.The final diet, based on rolled corn and sorghum silage (10% of DM), was formulated to contain (DM basis) 12% CP, .70%Ca, .35%P, .36%salt, .70%K, 75 ppm Zn, 1,800 IU Vitamin A/lb, and 27.7 plus 11 g/ton of Rumensin® plus Tylan®, respectively.Following the step-up period, individual, early morning weights were obtained on 2 consecutive days.Steers were selected on weight and breed type uniformity, implanted with E (Synovex-S®; 20 mg estradiol benzoate 2 plus 200 mg progesterone), and allotted to one of four weight replicates.Within each replicate, steers were randomly allotted to one of six pens, with the exception that Angus and Angus-crossed cattle were stratified equally across pens.Each pen was randomly assigned to one of six treatments in a 2×3 factorially arranged experiment.Main effect factors were reimplant type (E or E + TBA) and addi-2 2 tional time on feed (0, 14, or 28 days).
Trenbolone acetate was supplied in the form of Finaplix-S (140 mg TBA).The initial slaughter was when 65 to 70% of all steers in the study were estimated to grade Choice, based on projected performance and visual appraisal.Thus, slaughter dates were 54, 68, and 82 days following reimplantation.Final weights were the average of early morning weights on 2 consecutive days.Weighing, shipping, and slaughter procedures were identical for each slaughter group.Carcass measurements were made following a 24-hour chill.

Results and Discussion
There was no effect of interaction between reimplant scheme and time on feed on any performance or carcass variables.Therefore, results are presented for main effects of reimplant scheme and additional days on feed.There was no difference in steers' performance during the initial implant period (Table 1), indicating that all groups responded similarly to the initial E implant.However, use of E + particular, enhance rate and efficiency of growth by extending the physiological growth curve (same composition at a heavier weight), rather than by any "nutrient partitioning" activity that favors lean tissue deposition at the expense of fat deposition.
Although feeding for an additional 14 or 28 days beyond a 65 to 70% Choice endpoint resulted in a linear increase (P<.0001) in final liveweight (Table 2), performance during the extra periods was expectedly low.As fed cattle approach finish weight, composition of gain changes to an increased proportion of fat deposition, resulting in poorer feed conversion.Analysis of data for the entire feeding period revealed linear reductions in daily gain (P<.005) and feed efficiency (P<.0004) with days on feed.Using a ration cost of $95/ton (as fed) and non-feed costs (interest, yardage, etc) of $.35/hd daily, the cost of gain was $.487, .507,and .522per lb for steers fed for 111, 125 and 139 days, respectively.The added gain realized from feeding for an additional 14 or 28 days cost $77.03or $74.62 per cwt, respectively, to produce.These results emphasize the fact that carrying cattle beyond normal finish weight can result in negative feeding margins, which reduce profitability or increase losses.
Feeding for an additional 14 or 28 days resulted in linear increases in hot carcass weight, ribeye area (all P<.001), adjusted backfat thickness (P<.004), and skeletal maturity (P<.0005).Dressing percentage (P<.002)and marbling score (P<.05) increased curvilinearly with additional days fed.The lower average marbling score for steers fed for an additional 28 vs 14 days may be partially explained by the fact that carcasses from that slaughter date apparently were not as well chilled as those from the two previous dates.Standard deviations for marbling score were 60, 69, and 66 degrees for steers fed for 111, 125, and 139 days, respectively, suggesting that variation within a slaughter group was relatively constant, and that the statistical distribution of marbling level was not affected by feeding for 14 or 28 additional days.
Chi-square statistics were used to evaluate the frequency of Choice grading and darkcutting carcasses as affected by reimplant scheme and additional days on feed (Table 3).Additional days on feed tended (P=.25) to increase percentage Choice in this study, whereas reimplant scheme had no effect.Steers implanted with E + TBA had a higher 2 (P<.05) incidence of dark cutting carcasses, which was particularly pronounced in the first slaughter group (54 days after reimplanting).Pooled across reimplant scheme, the overall incidence was higher (P<.03) in the initial slaughter group than in subsequent slaughter groups.There were no differences in weighing, shipping, or slaughter procedures nor any discernible changes in environment between slaughter dates to account for these differences.Weights pencil shrunk 4%.Probability values for linear (L) or quadratic (Q) effect of additional days on feed.

Table 3 . Effect of Reimplant Scheme and Additional Days on Feed on Carcass Quality Grade, Yield Grade, and Incidence of Dark Cutting
a Chi-square probabilities for effect of reimplant scheme (RS) and DOF, respectively.b