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Effect of live weight gain of steers during winter grazing: III. Blood metabolites and hormones during feedlot finishing^1,2

M. J. Hersom³, R. P. Wettemann^*, C. R. Krehbiel^*, G. W. Horn^*,4 and D. H. Keisler

^* Department of Animal Science, Oklahoma State University, Stillwater 74078 and and Department of Animal Sciences, University of Missouri, Columbia 65211

	Abstract

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Two experiments were conducted using 48 Angus x Angus-Hereford steers in each experiment to determine the effect of previous winter grazing BW gain on jugular concentrations of metabolites and hormones during feedlot finishing. In each experiment, steers were randomly assigned to one of three treatments: 1) high rate of BW gain grazing winter wheat (HGW), 2) low rate of BW gain grazing winter wheat (LGW), or 3) grazing dormant tallgrass native range (NR) with 0.91 kg/d of a 41% CP (DM basis) supplement. Steers grazed for 120 or 144 d in Exp. 1 and 2, respectively. Plasma and serum were collected from all steers before placement into a feedlot, and six or seven times during finishing in Exp. 1 and 2, respectively. In Exp. 1, before steers entered the feedlot, concentrations of insulin, triiodothyronine (T₃), and thyroxine (T₄) were greater (P < 0.05) in HGW than in LGW or NR steers, and concentrations of IGF-I and plasma urea-N were greater (P < 0.05) in steers that grazed wheat pasture than in NR steers. In Exp. 2, concentrations of glucose, T₃, T₄, and IGF-I were greater (P < 0.05) in steers that grazed wheat pasture than NR steers. In Exp. 1 (P < 0.19) and 2 (P < 0.86), glucose concentration did not differ among treatments during finishing. In Exp. 1, insulin concentration across days on feed was greater for HGW than LGW steers, which were greater than for NR steers (treatment x day interaction, P < 0.03). In Exp. 2, insulin concentration increased (P < 0.001) as days on feed increased. Concentrations of IGF-I were greater in steers that had grazed wheat pasture, whereas the increase in IGF-I with increasing days on feed was greater for NR steers (treatment x day interaction, P < 0.003). Concentrations of T₃ and T₄ during finishing were greater (P < 0.001) in HGW and LGW than in NR steers in Exp. 1. In Exp. 2, T₄ concentration also differed (P < 0.009) among treatments (HGW > LGW > NR). In Exp. 2, final concentration of glucose was greater (P < 0.01) in NR than in HGW and LGW steers, and serum insulin concentration was greater (P < 0.04) in NR than LGW steers. Final concentrations of T₃ (P < 0.01) and T₄ (P < 0.004) were greater in NR than in HGW steers. Our data show that previous BW gain can affect blood metabolites and hormones in steers entering the feedlot. However, lower concentrations of T₃, T₄, and IGF-I in steers when they entered the feedlot did not inhibit the growth response of previously restricted steers.

Key Words: Cattle Finishing • Grazing • Hormones • Metabolites

	Introduction

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The growth rate of cattle may be influenced by alterations in anabolic hormones (Hayden et al., 1993). For example, steer rate of gain is correlated with plasma concentrations of IGF-I and insulin (Ellenberger et al., 1989), and nutritional restriction decreases plasma concentrations of triiodothyronine (T₃), thyroxine (T₄), IGF-I, (Ellenberger et al., 1989; Yambayamba et al., 1996) and insulin (Yelich et al., 1995). Animals become more sensitive to anabolic hormones, especially IGF-I, after a period of nutritional restriction (Van den Brande, 1986), and Yambayamba et al. (1996) showed that the concentration of metabolites in blood changed slowly when animals were realimented after restriction. The lag in time before blood metabolites increase during realimentation might allow increased efficiency of energy use by decreasing maintenance energy requirements. Decreased concentrations of thyroid hormone have been shown to decrease maintenance energy requirements (Murphy and Loerch, 1994) and decrease protein degradation (Ellenberger et al., 1989). Therefore, decreased secretion of thyroid hormones and increased sensitivity and/or responsiveness to anabolic hormones are probably associated with increased energy utilization for growth.

In this experiment, we examined endocrine and metabolite responses by steers during feedlot finishing. Steers had similar genetics but differed in BW gains and body fat, resulting from their different winter grazing programs before placement into the feedlot (Hersom et al., 2004). Our hypothesis was that differenes in the growth rate of grazing steers would influence endocrine function and body fat deposition, which would alter growth rate and efficiency during subsequent consumption of a high-energy diet.

	Materials and Methods

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Animals and Management

Procedures used to produce steers with different body composition for this study were described by Hersom et al. (2004). Briefly, in each of two experiments in consecutive years, 48 fall-weaned Angus x Angus-Hereford steers (244 ± 23 kg, Exp. 1; 231 ± 25 kg, Exp. 2) from the same herd were randomly allotted to one of three winter grazing programs. Grazing programs were grazing hard red winter wheat pasture (Triticum aestivum, variety 2,174) to achieve either a high (HGW; 1.31 kg/d Exp. 1 and 1.10 kg/d Exp. 2) or low (LGW; 0.54 kg/d Exp. 1 and 0.68 kg/d Exp. 2) rate of BW gain, or grazing dormant tallgrass native range (NR; 0.16 kg/d Exp. 1 and 0.15 kg/d Exp. 2). Low-gain wheat steers (2.45 steers/ha) were placed in one pasture, and HGW steers grazed an adjacent wheat pasture of 14.6 ha (1.10 steers/ha). Stocking densities were adjusted throughout the wheat pasture grazing period by varying the size of each pasture. The steers grazed each pasture continuously, and height of the available forage for HGW steers was always in excess (15 to 20 cm), whereas it was limited and often less than 5 cm in height for LGW steers (Hersom et al., 2004). Steers grazing NR were fed 0.91 kg/steer^–1•d^–1 of a cottonseed meal supplement (41% CP, DM basis). Steers were not implanted during winter grazing period. At the end of the grazing phase, four steers from each treatment were slaughtered to determine body composition before finishing on a high-grain diet as previously reported (Hersom et al., 2004). Steers were stratified by BW within winter grazing program and assigned to feedlot pens to minimize the range of BW within a pen. In Exp. 1, steers were fed in 12.2- x 30.5-m open pens at the Willard Sparks Beef Research Center, Stillwater, OK (three pens per treatment, with four steers per pen). Steers were fed two times daily at 0800 and 1300. In Exp. 2, steers were fed individually once per day at 0800 by use of a Calan Broadbent Feeding System (American Calan, Northwood, NH) in 4.57-m² pens in an open-fronted building. In both experiments, steers were adapted to a high-grain finishing diet over 4 wk; final feedlot diets were 13.4% CP, and 2.13 and 1.37 Mcal/kg of NE_m and NE_g (DM basis), respectively. Steers from all treatments were slaughtered at approximately the same backfat end point (1.27 x 0.06 cm). The Oklahoma State University Institutional Animal Care and Use Committee approved the use of animals and research protocols before the initiation of the experiments.

Blood Collection

Three days before placement in the feedlot (d –3), steers were removed from pastures and water withheld for 5 to 6 h; then blood was collected via jugular venipuncture beginning at 1400 and completed within 1.5 h. Blood for plasma was collected into tubes containing sodium heparin, placed on ice, and centrifuged (3,000 x g for 20 min at 4°C) within 1 h after collection. Blood was also collected into tubes and allowed to clot for 16 h at 4°C, and then serum was harvested (3,000 x g for 20 min). Plasma and serum were stored at –20°C until analyzed.

In Exp. 1, blood samples for plasma and serum were also collected beginning at 1000, approximately 3 h after steers received half their daily feed allotment on d 14, 21, 28, 35, 42, and 49 of the feedlot period. In Exp. 2, blood samples were collected beginning at 1000, 3 h after steers received their total feed allotment for the day on d 26, 46, 67, 86 (HGW, LGW, and NR), 111 (LGW and NR), and 145 (NR). In the feedlot, all cattle were removed from their pens at the same time and moved to the processing facility to minimize potential differences in the time allowed for feed consumption. Because of the number of steers sampled, blood was collected across treatment groups to eliminate any time after eating bias among treatment groups. In the feedlot, all blood collection was completed within 1 h.

Metabolite and Hormone Assays

Plasma concentrations of glucose and urea-N (PUN) were determined using a Cobas Mira analyzer (Roche Diagnostic Corp., Indianapolis, IN). Glucose intra- and interassay CV (assays, n = 6) were 1.5 and 3.7%, respectively; PUN intra- and interassay CV (assays, n = 6) were 2.3 and 3.9%, respectively. Serum concentrations of NEFA were determined by an enzymatic colorimetric procedure (Wako-NEFA C; Wako Chemicals U.S.A., Dallas, TX) with modifications described by McCutheon and Banman (1986). Intra- and interassay CV (assays, n = 6) were 9 and 18%, respectively. Serum concentrations of T₃ and T₄ were quantified with solid-phase RIA (Coat-A-Count, Diagnostic Products Corp., Los Angeles, CA). Assay sensitivity was 10 ng/mL of serum, and 96% of the 48 ng of T₄ added to 25 µL of serum was recovered. When 5, 10, 15, 20, and 25 µL of bovine serum were assayed, concentrations of T₄ were parallel to the standard curve. Intra- and interassay CV (n = 6) were 12 and 18%, respectively. Assay sensitivity of T₃ was 0.4 ng/mL of serum, and 91% of the 15 ng of T₃ added to 100 µL of serum was recovered. When 4, 6, 8, and 10 µL of bovine serum were assayed, concentrations of T₃ were parallel to the standard curve. Intra- and interassay CV (assays, n = 6) were 11 and 16%, respectively. Concentrations of insulin in serum were quantified by solid-phase RIA as described by Bossis et al. (1999). Serum concentrations of IGF-I were determined using RIA with acid-ethanol extraction (Echternkamp et al., 1990). Recombinant human IGF-I (R&D Systems, Minneapolis, MN) was used for standards. Intra- and interassay CV (assays, n = 6) were 19 and 18%, respectively. Plasma concentrations of leptin were quantified in a single RIA (Delavaud et al., 2000) using purified recombinant ovine leptin (Gertler et al., 1998) for standards with an intraassay CV of 5%.

Statistical Analyses

For both experiments, plasma constituents on d –3 were analyzed as a completely random design using the mixed procedure of SAS (SAS Inst. Inc., Cary, NC). The statistical model included grazing program as the fixed effect and steer within grazing program as a random effect; steer was the experimental unit. Treatment least squares means were compared using an F-protected LSD (P < 0.05). Plasma constituents during finishing were analyzed as a completely random design with the mixed procedure with days as a repeated measure; the model included terms for treatment, day, and the treatment x day interaction. Pen within treatment or steer within treatment served as a random variable for Exp. 1 or 2, respectively. Pen was the experimental unit in Exp. 1, whereas steer was the experimental unit in Exp. 2. The covariance structure used was autoregressive Order 1 (Littell et al., 1996). In Exp. 1, all sampling dates were included; however, in Exp. 2, because steers in different treatments were slaughtered at different dates, only samples collected through d 86 were analyzed. If metabolites or hormones had significant treatment x day interactions (P < 0.10), best-fit polynomial response curves were tested for heterogeneity of regression to determine differences among treatments (Yelich et al., 1995). Response curves for treatments were compared using LSD pairwise comparisons HGW vs. LGW, HGW vs. NR, and LGW vs. NR. In Exp. 2, blood samples collected for steers on all treatments on the last day before slaughter were analyzed similarly to samples on d –3. Simple Pearson correlations (SAS) were determined between leptin concentration and BW, and final leptin concentration and final carcass fat (kilograms).

	Results

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Performance data were presented and discussed in a companion manuscript (Hersom et al., 2004). Briefly, winter grazing program ADG were 1.31, 0.54, and 0.16 kg/d and 1.10, 0.68, and 0.15 kg/d for HGW, LGW, and NR steers in Exp. 1 and 2, respectively. During finishing in Exp. 1, live BW gains (average = 1.79 kg/d) were not different (P = 0.43) among treatments, nor did live gain efficiency differ (P = 0.41) among treatments (Hersom et al., 2004). Similarly, in Exp. 2, live BW gains did not differ (P = 0.24; 1.68 kg/d) among treatments, and live gain efficiency was not different (P = 0.58) among treatments (Hersom et al., 2004).

Blood Metabolite and Hormone Concentrations Before Finishing

Experiment 1. Before steers were placed into the feedlot (d –3), plasma concentration of glucose did not differ (P < 0.12) among treatments (Table 1); however, glucose concentrations in steers that had grazed winter wheat were 20% greater than the glucose concentration in NR steers. Similarly, concentrations of PUN (P < 0.009) and IGF-I (P < 0.04) were greater in HGW and LGW than NR steers before entering the feedlot. Nonesterified fatty acid concentration was lower (P < 0.001) in HGW compared with LGW and NR steers. In contrast, concentrations of insulin (P < 0.04), T₃ (P < 0.10), and T₄ (P < 0.01) were greater in HGW than in LGW or NR steers.

View larger version (13K): [in this window] [in a new window]   Figure 1. Relationship between preslaughter jugular plasma concentrations of leptin with slaughter carcass fat (kg) in feedlot steers in Exp. 2 (n = 6).

Experiment 1. No treatment x sampling day interactions were observed for glucose, T₃, or T₄; therefore, main effects least squares means for main effects are presented in Table 2. During the 49-d sampling period, glucose concentrations did not differ (P < 0.19) among treatments or across increasing days on feed (P < 0.29). Concentrations of T₃ and T₄ were greater (P < 0.001) for HGW and LGW than for NR steers. Both T₃ (11.3%) and T₄ (30.8%) concentrations increased (P < 0.001) as days on feed increased from d 14 to 49.

View larger version (21K): [in this window] [in a new window]   Figure 2. Relationship between jugular plasma concentrations of leptin with BW in feedlot steers in Exp. 2 (HGW, n = 11; LGW and NR, n = 12).

In Exp. 2, blood was collected on the morning of slaughter for comparisons among treatments at a similar backfat end point (Hersom et al., 2004; Table 6). Plasma concentration of glucose in NR steers on the day of slaughter (d 145) was greater (P < 0.01) than in HGW (d 86) or LGW steers (d 111). Plasma concentration of PUN did not differ (P < 0.65) among treatments, whereas serum NEFA concentration tended (P < 0.06) to be greater in NR than in LGW steers, and in LGW steers greater than in HGW at slaughter. On the day of slaughter, NR steers had greater (P < 0.04) concentrations of insulin than LGW; HGW steers were intermediate. Serum concentrations of IGF-I (P < 0.77) and plasma concentrations of leptin (P < 0.58) did not differ among treatments. Serum concentration of T₃ was greater for NR than for HGW steers (P < 0.01); LGW steers were intermediate. Serum T₄ was greater (P < 0.004) in LGW and NR steers than in HGW steers.

	Discussion

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Concentrations of PUN in HGW and LGW steers before entering the feedlot were most likely an effect of the wheat-forage diet; wheat forage has a large amount of soluble N (Horn, 1983; Vogel et al., 1989) that is highly degradable in the rumen and absorbed into blood. Reasons for lower PUN in HGW steers in Exp. 2 are not clear. In both Exp. 1 and 2, the increased PUN concentrations in LGW steers before placement into the feedlot might have resulted from a combination of the highly soluble N in wheat forage and mobilization of body tissues to meet energy demands. Ellenberger et al. (1989) found elevated blood urea N after a 189-d restriction period characterized by gains of 0.4 kg/d in steers.

In NR steers, decreased plasma concentration of glucose corresponded with a decreased concentration of insulin, which at low concentration is associated with lipolysis (Hayden et al., 1993). The decrease in plasma glucose most likely elicited the increase in serum NEFA concentrations that served as an alternative energy source for NR steers. The lower NEFA concentration in both LGW and NR steers by d 14 (Exp. 1) of finishing indicates that, upon entering the feedlot, steers returned to a more positive energy balance. In addition, the change in NEFA concentration in NR steers (Exp. 1) after placement into the feedlot was greater than in HGW steers that were rapidly gaining BW when they entered the feedlot. The response observed in NEFA concentrations of LGW and NR steers after placement into the feedlot suggests that these steers were consuming adequate energy rather than mobilizing body energy reserves.

Serum insulin concentration increased for all treatments when steers entered the feedlot. Insulin concentrations in LGW and NR steers exhibited an overall increase but were never greater than in HGW steers during the first 49 d in Exp. 1 and through d 86 in Exp. 2. Sampling time relative to feeding might have influenced insulin concentrations measured, particularly if differences in the consumption pattern of the diet by steers occurred across the day. Additionally, the absence of a compensatory growth response by NR and LGW steers could be partially explained by the lack of increased insulin concentration in response to the feedlot diet. However, insulin concentration was greater for NR (d 145) than for LGW (d 111) steers at slaughter. Hayden et al. (1993) reported that previously restricted steers exhibited linear increases in peripheral concentrations of insulin during realimentation and had greater peripheral concentrations of insulin compared with unrestricted, full-fed steers after 60 d of realimentation. Perhaps an increased sensitivity to insulin through an increase in insulin receptor affinity or up-regulation of receptor number occurred in both LGW and NR steers in the present experiment, thereby decreasing the amount of circulating insulin required for homeostasis but not enough to elicit compensatory growth. Eisemann et al. (1997) examined insulin responsiveness and sensitivity in beef steers of different ages and BW (275 vs. 490 kg) and determined that the metabolism of glucose by the hindquarters decreased in sensitivity and responsiveness to insulin at heavier BW, which also corresponded with increased age and body fat content. The results of Eisemann et al. (1997) would indicate insulin resistance by peripheral tissues of beef steers as BW, age, and body fat content increases. In the present experiments, steers started the feedlot phase at the same age; however, HGW steers had greater initial body fat content (Hersom et al., 2004) than in LGW and NR steers. High-gain wheat steers would have had greater fat content relative to days on feed compared with LGW and NR steers. Our results from Exp. 2 agree with Eisemann et al. (1997) in that insulin concentration increased with increasing age and BW. In addition, body fat content and accretion rates are most likely factors that affect the insulin response in finishing cattle.

Serum concentrations of IGF-I before entering the feedlot were less for NR steers compared with steers that grazed wheat forage in both Exp. 1 and 2. Breier et al. (1988a) suggested that regulation of circulating IGF-I might be mediated through high-affinity hepatic GH receptors that are subject to nutritional manipulation. After a 92-d energy restriction, peripheral concentrations of GH in restricted steers had increased 45%, whereas IGF-I concentrations were decreased by 43% compared with steers offered adequate energy for feedlot growth (Hayden et al., 1993). In addition, a restriction of metabolizable protein or energy decreases liver mass and peripheral concentrations of IGF-I in cattle (Drouillard et al., 1991) and sheep (Wester et al., 1995). Decreased liver mass would then result in a decreased total number of hepatocytes and GH receptors for stimulation of hepatic IGF-I synthesis. In addition to the decrease in hepatocytes, protein restriction decreases peripheral concentrations of IGF-I through GH dependent postreceptor events (Thissen et al., 1990). The uncoupling of the GH/IGF-I axis when animals are on restricted diets was also documented because only steers on a high plain of nutrition responded to boluses of GH with increased peripheral concentrations of IGF-I (Breier et al., 1988b).

In the present experiments, once steers were in the feedlot, peripheral concentrations of IGF-I in NR steers were less than in steers that grazed wheat forage (HGW and LGW) until d 49 (Exp. 1) or 86 (Exp. 2). Breier et al. (1986) and Yambayamba et al. (1996), using intake-restricted steers and heifers, respectively, found that IGF-I concentrations in previously energy-restricted animals were similar to that of ad libitum fed animals after d 10 of realimentation. Steers that had been restricted to 0.35 kg/d from 242 to 310 kg of BW exhibited as similar rapid return of IGF-I concentrations to those similar to ad libitum-fed steers when realimented (Ellenberger et al., 1989). Previously energy-restricted steers also tended to have greater peripheral concentrations of IGF-I compared with adequate energy, ad libitum-fed steers during the later finishing period (Ellenberger et al., 1989), whereas Yambayamba et al. (1996) reported that realimented heifers and ad libitum-fed heifers had similar IGF-I concentrations. Although our results generally agree with that of Hayden et al. (1993), in that IGF-I in previously energy-restricted steers was nearly equal to that of adequate energy-fed steers by 49 (Exp. 1) or 86 d on feed (Exp. 2), reasons for the slower return of IGF-I concentrations to levels of steers fed adequate energy (wheat forage) are unclear. In Exp. 1 and 2, ADG by LGW and NR steers and gain efficiency did not differ from those of HGW steers during finishing (Hersom et al., 2004). The lower IGF-I concentrations in NR steers (Exp. 1 and 2) and LGW steers (Exp. 2 only) until d 49 (Exp. 1) or 86 (Exp. 2) corresponded with a lack of compensatory growth (Hersom et al., 2004) compared with HGW steers. Although no compensatory growth was observed, the fact that concentrations of IGF-I were increasing with increasing days on feed may have been adequate to stimulate growth rate and efficiency similar to HGW steers. Stick et al. (1998) found that an increase of 1 ng/mL of serum IGF-I was associated with an increase in ADG of 0.00135 kg/d and an improvement in efficiency of 0.0001 kg of gain per kilogram of feed across three levels of feed intake.

Leptin concentrations were measured in Exp. 2 because the blood collection protocol extended to slaughter. The level of energy intake and subsequent BW gains of the grazing steers affected leptin concentrations before placement into the feedlot. Leptin concentration responsiveness to energy intake has been demonstrated (Daniel et al., 2002; Delavaud et al., 2002). Before placement into the feedlot in Exp. 2, HGW steers had 60.8% greater leptin concentrations than LGW and NR steers. Delavaud et al. (2000) found a 56% decrease in plasma concentration of leptin in ewes that were restricted to 39% of their estimated energy requirement for 65 d and incurred a 3.5% reduction in body fat and –0.21 kg/d BW change. Steers in the present Exp. 2 did not lose BW; however, the relative differences in concentrations of leptin in restricted steers (NR) and adequately fed steers (HGW) are comparable with leptin concentrations of restricted and well-fed ewes found by Delavaud et al. (2000). Additionally, plasma concentrations of leptin and body fat content are positively related (Houseknecht et al., 1998; Delavaud et al., 2000). In Exp. 2, steers with greater fat content (HGW) had greater peripheral concentrations of leptin. Steers on all treatments had increased leptin concentrations with increasing days on feed. Our objective was to slaughter all steers at a similar backfat end point and BW. The similarity in final backfat and BW resulted in similar concentrations of leptin at slaughter among steers on all treatments. Whereas leptin exhibited numeric increases in concentrations, IGF-I concentrations appeared to plateau in all treatments. Interestingly, the plateau in IGF-I and steady increases in leptin concentrations might correspond with the decreasing accretion of body protein and continued increased accretion of body fat in maturing animals. The significant correlation between carcass fat and leptin concentrations in steers is similar to the positive relationships between leptin and beef carcass fat (McFadin et al., 2002) and sheep backfat thickness (Daniel et al., 2002).

Concentrations of T₃ in serum of steers were influenced by winter grazing treatment. In Exp. 1, concentration of T₃ in HGW steers was greatest, followed by LGW, and was lowest in NR steers, which had the lowest BW gains. However, in Exp. 2 concentrations of T₃ were not different in HGW and LGW steers and lower for NR steers. Inconsistency of thyroid hormone results between Exp. 1 and 2 might be due in part to different patterns of growth near the end of the grazing period in LGW and NR steers in Exp. 1 compared with Exp. 2 (Hersom et al., 2004). The results of Exp. 1 and difference between HGW and NR steers in Exp. 2 agrees with Hayden et al. (1993), who reported that T₃ concentrations are indicative of energy balance, and Murphy and Loerch (1994), who related T₃ concentration to level of intake of a finishing diet. The effect of grazing treatment on serum T₄ before placement into the feedlot followed the same pattern as T₃. There was a positive relationship between energy intake and T₄ concentration. Hayden et al. (1993) stated that T₄ appears to be positively associated with energy consumption. High-gain wheat steers had access to abundant forage and had the greatest peripheral concentrations of T₄ entering the feedlot, whereas LGW (Exp. 1) and NR (Exp. 1 and 2) steers had limited forage intake or consumed low-quality forage, and had lower peripheral concentrations of T₄. The shifts in T₄ concentration in LGW and NR steers demonstrate the increased access to energy that these steers had when allowed to consume ad libitum quantities of feed.

Variations in metabolite and hormone concentrations are apparent between Exp. 1 and 2. Possible explanations for the variation are described in Hersom et al. (2004) and include differences in diet, finishing feeding protocol, and timing of placement into the feedlot. In particular, feeding twice a day (Exp. 1) vs. once a day (Exp. 2) likely increased the observed variation between experiments. Nonetheless, data presented herein offer additional insight into the complex nature of compensatory growth and the effect of previous nutrition on physiological responses in growing and finishing beef cattle.

	Implications

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Growth of previously energy-restricted steers during feedlot realimentation depends on many factors. In the present experiments, the potential decreases in metabolic rate as indicated by thyroid hormone concentrations and adequate stimulation of insulin-like growth factor-I production may have occurred to achieve similar body weight gains by restricted compared with unrestricted steers. The complex interaction of previous nutrition, refeeding strategies, and hormonal regulation influence growth during realimentation. Interactions of body composition and hormone concentrations might also signal a decrease in responsiveness to anabolic hormones as cattle approach slaughter.

	Footnotes

 
¹ Approved for publication by the director of the Oklahoma Agric. Exp. Stn. This research was supported by the Oklahoma Agric. Exp. Stn. under projects H-2397, H-2457, and H-2438, and the Cooperative State Research, Education, and Extension Service, USDA, under Agreement No. 99-34198-7481 and 2001-34198-10403.

² The authors thank D. Perry, C. Lunsford, and L. Mackey for their help in analyses of samples; K. Poling for animal care and sample preparation; Willard Sparks Beef Research Center animal caretakers; and USDA-ARS Grazinglands Research Center animal caretakers.

³ Current address: 231 E. Animal Sciences, Bldg. 459, Dept. of Anim. Sci., Univ. of Florida, Gainesville 32611.

⁴ Correspondence: 208 Animal Science Bldg. (phone: 405-744-6621; fax: 405-744-7390; e-mail: horngw{at}okstate.edu).

Received for publication July 8, 2003. Accepted for publication April 5, 2004.

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