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Effect of live weight gain of steers during winter grazing: II. Visceral organ mass, cellularity, and oxygen consumption^1,2

Department of Animal Science, Oklahoma State University, Stillwater 74078

Abstract

Two experiments were conducted to examine the effect of BW gain during winter grazing on mass, cellularity, and oxygen consumption of splanchnic tissues before and after the feedlot finishing phase. In each experiment, 48 fall-weaned Angus x Angus-Hereford steer calves were assigned randomly 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 supplemented with 0.91 kg/d of a 41% CP supplement (NR). At the end of winter grazing, four steers were selected randomly from each treatment for initial slaughter to measure organ mass, cellularity, and oxygen consumption. All remaining steers were placed into a feedlot and fed to the same backfat end point (1.27 cm). Six steers were selected randomly from each treatment for final organ mass, cellularity, and oxygen consumption. Initial empty BW (EBW) was greatest (P < 0.001) for HGW, intermediate for LGW, and least for NR steers in both Exp. 1 and 2 (355 > 263 > 207 ± 6.5 kg and 337 > 274 > 205 ± 8.7 kg, respectively). For both experiments, the initial total gastrointestinal tract (GIT; g/kg of EBW) proportional weight was greater (P < 0.05) in NR steers than in LGW, and LGW steers had greater (P < 0.05) initial GIT proportional weight than HGW steers. Proportional weight of total splanchnic tissues (TST; g/kg of EBW) did not differ (P < 0.19) among treatments. Initial duodenal RNA concentration and RNA:protein were greater (P < 0.02) in LGW than in HGW steers, and NR steers were intermediate. Initial in vitro liver O₂ consumption was greater (P < 0.09) in HGW and LGW than in NR steers (34.5 > 16.9 mL/min), whereas initial small intestinal oxygen consumption was greater (P < 0.01) in LGW than in HGW and NR steers (12.1 > 5.2 mL/min). Ruminal papillae oxygen consumption did not differ (P < 0.55) among treatments. The rate of decrease of GIT (g•g EBW^-1•d^-1) during finishing was greater in NR than in HGW and LGW steers in both Exp. 1 and 2, but mesenteric fat (g•g EBW^-1•d^-1) increased for NR steers, resulting in a similar (P < 0.75) increase in TST across the finishing period for all treatments. Similar rates of increase in TST across the finishing phase corresponded with similar rates of live and carcass weight gain among treatments. Our data support the hypothesis that increased visceral organ mass increases maintenance energy requirements of growing cattle.

Key Words: Cattle Finishing • Cells • Organs • Oxygen Consumption

Introduction

A strong positive relationship exists between fasting heat production and visceral organ weight in response to plane of nutrition in sheep (Ferrell et al., 1986; Burrin et al., 1989, 1990) and cattle (Sainz and Bentley, 1997; Ferrell and Jenkins, 1998). This relationship most likely occurs because the portal-drained viscera (PDV) and liver are responsible for 45 to 50% of total body O₂ consumption (McBride and Kelly, 1990; Reynolds et al., 1991) while making up only 8 to 14% of an animal’s live BW (Burrin et al., 1990; Kelly et al., 1993; Seal and Reynolds, 1993). Therefore, oxidative metabolism by total splanchnic tissue (TST) represents a much greater proportion of whole-body metabolism relative to the mass of TST as a proportion of whole-body mass. McBride and Kelly (1990) demonstrated that this greater energy consumption-to-mass ratio when they showed that total liver oxygen consumption was 1.2 to 1.6 times greater than that of skeletal muscle.

Whereas energy use by tissues that livestock production favors (i.e., lean skeletal muscle) is necessary, energy use by TST has been considered a "tax on production" (Reynolds, 2002). Therefore, a change in splanchnic organ mass and, by convention, energy use, most likely alters the amount of energy and protein available to the animal for growth (Fluharty and McClure, 1997). We hypothesized that differences in performance by feeder cattle with respect to subsequent growth and gain efficiency in the feedlot might be partially explained by differences in splanchnic organ mass and associated energy expenditure. Our objective was to determine the effect of different winter grazing programs for growing cattle on splanchnic organ mass, cellularity, and O₂ consumption in relation to subsequent feedlot performance.

Materials and Methods

Animals and Management
A detailed description of the grazing programs and cattle management for these experiments has been reported (Hersom et al., 2004) and is briefly summarized here. In each of two experiments, 48 fall-weaned Angus x Angus-Hereford steers (initial BW = 244 ± 23 kg, Exp. 1 and 231 ± 25 kg, Exp. 2) from the same herd were randomly allotted to one of three winter grazing treatments. Treatments were as follows: 1) grazing winter wheat pasture to achieve a high rate of BW gain (HGW); 2) grazing winter wheat pasture and adjusting stocking density to maintain a low rate of BW gain (LGW); or 3) grazing dormant tallgrass native range and fed 0.91 kg•steer^-1•d^-1 of a cottonseed meal-based, 41% CP supplement (NR). At the end of the grazing phase, steers were slaughtered (n = 4/treatment) or placed in a feedlot and fed a high-grain finishing diet (Hersom et al., 2004) to a common end point of 1.27 cm of backfat between the 12th and 13th rib as determined by ultrasound (Model 210, probe model UST-5021; Aloka Co. Ltd., Wallingford, CT). When the backfat threshold was achieved, all steers in each treatment were slaughtered within 9 d. The Oklahoma State University Institutional Animal Care and Use Committee approved the use of animals for these experiments.

Organ Mass and Tissue Collection
In each experiment, four steers were randomly selected from each treatment to estimate empty-body weight (EBW) and organ mass after grazing and before placement in the feedlot. Steers were removed from their respective grazing treatments the morning of slaughter and were transported to the Oklahoma Food and Agricultural Products Research and Technology Center abattoir, stunned with a captive bolt, and exsanguinated. Weights of blood, feet and ears, hide, head, heart, lungs, kidneys, gastrointestinal tract (GIT; reticulorumen, omasum, abomasum, small and large intestine, and cecum), GIT contents, mesenteric fat trimmed from GIT organs, pancreas, spleen, liver, and hot carcass were recorded. Total offal mass was calculated as the sum of blood, feet and ears, hide, trim (tail, spinal cord, and carcass trim), all organs, and mesenteric fat. The reticulorumen and omasum were cut open, contents removed, and organs were rinsed free of remaining feed particles before weighing. Abomasal and intestinal contents were removed by gently squeezing contents through the length of the organ. Empty-body weight was calculated as hot carcass mass plus total offal mass, and TST mass was calculated as GIT plus liver, spleen, pancreas, and mesenteric fat.

At final slaughter, six steers from each treatment were randomly selected to estimate final EBW and organ mass. On the morning of slaughter, steers were removed from their pens before feeding and transported to the abattoir. The slaughter procedure was the same as the initial slaughter procedure.

Tissue Cellularity and In Vitro O₂ Consumption
In Exp. 2, samples of the liver (central lobe), ruminal epithelium (ventral sac), and duodenum (15 cm distal to the pylorus) were collected at initial and final slaughters for determination of DNA, RNA, and protein concentrations (liver and duodenum) and in vitro O₂ consumption (liver, ruminal epithelium, and duodenum). All tissue samples were collected within approximately 45 min of exsanguination. Samples of liver (10 g) and duodenum (45 g) were weighed, snap-frozen in liquid nitrogen, and subsequently stored at -80°C for later analysis of DNA, RNA, and protein. Adjacent liver (15 g) and duodenal (45 g) tissue samples were collected and placed in containers with ice-cold Krebs-Hensleit buffered saline (KHS; Kelly et al., 1993) for transport to the laboratory. Immediately after weighing, the ventral rumen was identified and an approximately 50-g sample excised. Ruminal epithelial tissue was transported to the laboratory in KHS containing 25 mM HEPES (Harmon et al., 1991).

In the laboratory, tissue samples for the determination of in vitro O₂ consumption were transferred to containers containing fresh, oxygenated (95% O₂ and 5% CO₂) KHS maintained at 37°C. Krebs-Hensleit buffer for ruminal tissue contained 90 mM acetate, 60 mM propionate, and 30 mM butyrate (Harmon et al., 1991). Individual ruminal papillae were cut free from the ruminal epithelial tissue and 50 mg was used for analysis (Burrin et al., 1990; Harmon et al., 1991). All visible adipose tissue was removed from the small intestine, which was then cut longitudinally to open the lumen. Small cross-sections were excised and weighed (50 mg) for analysis. Similarly, a 50-mg subsample of liver tissue was excised using a scalpel and was lightly scored before analysis (Burrin et al., 1990). Rates of O₂ consumption were measured polargraphically using a Clark-style electrode (YSI model 5300, Yellow Springs Instruments, Yellow Springs, OH) positioned within a thermostatically controlled (37°C) cell chamber (Yellow Springs Instruments). Triplicate tissue samples were placed in unoxygenated KHS solution in the O₂ electrode chamber and allowed to acclimate to the chamber for 1 min, after which O₂ consumption was measured over 5 min (Kelly et al., 1993). To quantify the contribution of Na⁺,K⁺-ATPase and protein synthesis to in vitro O₂ consumption, triplicate tissue samples were also used to estimate ouabain- and cyclohexamide-sensitive O₂ consumption, respectively (Kelly et al., 1993). Samples were placed in unoxygenated KHS solution containing 1 x 10^-4 M of either ouabain or cyclohexamide in the O₂ electrode chamber and allowed to acclimate to the chamber for 1 min, after which O₂ consumption was measured over 5 min.

A 0.50-g subsample of frozen duodenal tissue, taken as a cross section of the duodenum (included all tissue layers), was sliced from the previously frozen sample for analyses of DNA and RNA. Similarly, a 0.25-g subsample of liver tissue was sliced from the frozen tissue. Samples were incubated at room temperature in TRIzol reagent (Invitrogen, Carlsbad, CA) for 10 min. The sample was then homogenized with a Virtishear homogenizer (Vir Tis Co., Gardiner, NY) for 1 min. Concentrations of liver and duodenal DNA and RNA were analyzed using TRIzol reagent. Liver and duodenal tissue protein concentrations were determined by analyzing for Kjeldahl N (AOAC, 1990), and converted to protein using the factor of 6.25.

Calculations and Statistical Analysis
Mass of initial and final organs was expressed as kilograms and grams per kilogram of EBW. The rate of change in organ mass across the feedlot period (kg/d and g•g EBW^-1•d^-1) for each steer in each treatment was calculated as ([final organ mass - average initial organ mass]/days on feed). The percentage of the inhibition of O₂ consumption by ouabain or cyclohexamide in liver, duodenum, and ruminal epithelium was calculated as follows: 1 - (O₂ consumption [µL•min^-1•g^-1] with inhibitor ÷ O₂ consumption [µL•min^-1•g^-1] without inhibitor) x 100. Whole-organ O₂ consumption was calculated using the following equation: O₂ consumption [µL•min^-1•g^-1] x organ mass. All data were analyzed using the MIXED procedure of SAS (SAS Inst. Inc., Cary NC) with steer as the experimental unit. The statistical model included the fixed effect of previous winter grazing treatment. Least squares means were calculated and tested against residual error. Least squares means were compared using LSD when protected by a (P < 0.10) F-value.

Results

Winter grazing BW gain and subsequent feedlot performance and carcass characteristics, and carcass, offal, and empty-body composition data are reported in the companion article (Hersom et al., 2004). Days on feed were 89, 116, and 163 in Exp. 1 and were 85, 111, and 158 in Exp. 2 for HGW, LGW, and NR steers, respectively.

Experiment 1
Initial Slaughter.
Initial slaughter EBW was 92 kg greater (P < 0.001) for HGW than for LGW steers, and LGW steers had 56 kg greater EBW than NR steers (Table 1). In addition, carcass mass was 64 kg greater (P < 0.001) for HGW than LGW steers, and LGW steers had 36 kg greater carcass mass than NR steers. In contrast, carcass proportional weight (g/kg of EBW) did not differ (P < 0.41) among treatments. Mass (kilograms) of total offal, feet and ears, hide, trim, and heart were greater (P < 0.001) for HGW than LGW steers, and LGW were greater than NR steers. Mass of the head was greater (P < 0.10) for HGW than NR steers; LGW steers were intermediate. Blood (P < 0.001) and kidney (P < 0.004) masses were greater for HGW than LGW or NR steers. Proportional weights (g/kg of EBW) of total offal, blood, head, and kidney were not different (P < 0.41) among treatments; however, feet and ear proportional weight was greatest (P < 0.001) for NR, intermediate for LGW, and lowest for HGW steers. Hide proportional weight was greater (P < 0.07) for LGW than HGW steers, and NR steers were intermediate. Heart mass tended (P < 0.10) to be greater in NR compared with LGW steers. Lung mass did not differ (P < 0.21) among treatments, although lung proportional weight was 3.6 g/kg of EBW greater (P < 0.09) for NR than HGW steers, and LGW steers were intermediate. Reticuloruminal mass was 1.6 kg greater (P < 0.001) for HGW than LGW, and LGW steers had 1.3-kg greater reticuloruminal mass than NR steers. Omasal mass did not differ (P < 0.20) among treatments. Native range and LGW steers had greater (P < 0.004) reticuloruminal proportional weight than HGW steers, and the omasum of NR steers was 6.4 and 7.3 g/kg of EBW greater (P < 0.009) compared with the omasum of LGW and HGW steers, respectively. Abomasal mass (kg) was greater (P < 0.04) for HGW than NR steers, whereas LGW steers were intermediate. However, abomasal proportional weight was greater (P < 0.05) for NR than HGW steers, and LGW steers were intermediate. Small intestinal mass (kg) was greater (P < 0.003) for HGW than LGW or NR steers, cecal mass did not differ (P < 0.22) among treatments, and large intestinal mass was greater (P < 0.003) for HGW than LGW steers, whereas LGW had greater large intestinal mass than NR steers. In contrast, no differences (P < 0.11) were observed among treatments for proportional weights of the small intestine, cecum, or large intestine. Total GIT mass was 4.3 kg greater (P < 0.001) for HGW than LGW steers, which had 2.5 kg greater total GIT than NR steers. However, because of the greater proportional weight of stomach organs, NR steers had 11.2% greater (P < 0.002) total GIT than LGW steers, and LGW steers had 9.4% greater GIT than HGW steers. The mass (kg) of mesenteric fat was greater (P < 0.001) for HGW than LGW steers, which were greater than NR steers. Similarly, HGW steers had 12.0 g/kg of EBW greater (P < 0.001) mesenteric fat compared with LGW steers, and LGW steers had 8.6 g/kg of EBW greater mesenteric fat than NR steers. Mass of the pancreas was 0.2 kg greater (P < 0.001) for HGW compared with LGW or NR steers, but mass of the spleen did not differ (P < 0.19) among treatments. Proportional weights of pancreas and spleen were not different (P < 0.19) among treatments. Liver mass of HGW was 2.5 kg greater (P < 0.001) than LGW steers, and LGW steers had 0.9 kg greater liver mass than NR steers. Likewise, proportional weight of the liver in HGW steers was approximately 15.5% greater (P < 0.008) compared with livers of LGW and NR steers (19.4 vs. 16.8 g/kg of EBW, respectively). Because of differences in initial total GIT, mesenteric fat and liver, initial TST mass of HGW was 13 kg greater (P < 0.001) than LGW steers, and LGW steers had 6.1 kg greater TST mass than NR steers. However, initial proportional weight of TST (g/kg of EBW) was not different (P < 0.19) among treatments.

Change in Mass.
Empty BW gain of feedlot steers was not different (P < 0.20) among treatments (Table 1; Hersom et al., 2004). Similarly, carcass gain (kg/d) and carcass gain as a proportion of EBW (g•g EBW^-1•d^-1) were not different (P < 0.24) among treatments. Although total offal and blood became a smaller proportion of EBW across the finishing period, the rate of change of proportional weight (g•g EBW^-1•d^-1) was not different (P < 0.20) among treatments. Due to initial differences in proportional weight, LGW and NR steers had a decrease in rate of change of proportion of feet and ears that was 2.3-fold greater (P < 0.001) than HGW steers, and LGW steers had a decrease in proportion of head that was greater (P < 0.001) than HGW and NR steers. Lungs of HGW steers became a greater (P < 0.03) proportion of EBW across the feeding period, whereas lungs of NR steers became a smaller proportion; lungs of LGW steers were intermediate. Low-gain wheat and NR steers gained (P < 0.001) greater kidney mass during finishing than HGW steers. In contrast, decrease in change of proportional weight of kidneys was greatest (P < 0.001) for HGW, intermediate for NR, and lowest for LGW steers. Change in proportional weight of heart across the finishing period was not different (P < 0.17) among treatments. The reticulorumen (P < 0.01) and omasum (P < 0.003) of HGW and the omasum of LGW steers became a greater proportion of EBW across the feeding period, whereas the reticulorumen and omasum of NR steers became a lower proportion. Change in proportional weight of the abomasum and small and large intestine was not different (P < 0.22) among treatments, whereas change in cecal proportion was greater (P < 0.002) for HGW and NR than LGW steers. Across the feeding period, total GIT became a smaller proportion of EBW for all treatments; however, the decrease in proportional change of GIT mass in NR steers was threefold greater (P < 0.004) compared with steers that had grazed winter wheat (average = -36.6 g•g EBW^-1•d^-1). In contrast, change in mesenteric fat mass increased for all treatments. Interestingly, the increase in proportional change of mesenteric fat was 48% greater (P < 0.09) in NR compared with HGW and LGW steers (average = 117.7 g•g EBW^-1•d^-1). Pancreas mass increased (P < 0.001) across the finishing period in LGW and NR compared with HGW steers. The pancreas became a smaller (P < 0.001) proportion of EBW across the finishing period in HGW compared with LGW and NR steers. Change in proportional weight of the spleen was not different (P < 0.43) among treatments. Liver mass increase in LGW and NR steers during finishing was 12.8 and 13.9 g/d greater (P < 0.001) than HGW steers. This resulted in a nearly fivefold greater (P < 0.001) decrease in rate of proportional change in liver mass for HGW compared with livers of NR and LGW steers (average = -7.7 g•g EBW^-1•d^-1). Numeric differences in total GIT and mesenteric fat mass change offset differences in liver mass change during the finishing period to produce TST mass increases that were not different (P < 0.93; average = 221.8 g/d) among treatments. Likewise, decreased proportional changes in total GIT and increased changes in mesenteric fat in NR steers resulted in similar (P < 0.93; average = 53.7 g•g EBW^-1•d^-1) proportional increases of TST among treatments.

Experiment 2
Initial Slaughter.
Similar to Exp. 1, the EBW of HGW steers entering the feedlot was 63 kg greater (P < 0.001) compared with LGW steers, and LGW had 69 kg greater EBW than NR steers (Table 2). Carcass mass of HGW steers was 48 kg greater (P < 0.001) than LGW steers, and LGW had 46 kg greater carcass mass than NR steers; however, carcass proportional weight of NR steers was 23 g/kg of EBW greater (P < 0.05) compared with HGW or LGW (average = 646 g/kg EBW). Total offal mass was 16 kg greater (P < 0.001) for HGW than LGW steers, and LGW steers had 23-kg greater total offal mass than NR steers. Total offal was approximately 23 g/kg of EBW greater (P < 0.05) in LGW and NR steers than HGW steers (average = 354 vs. 331 g/kg of EBW, respectively). Blood mass did not differ among treatments, but proportional weight of blood tended (P < 0.10) to be greater for LGW compared with HGW steers. Mass of the feet and ears and head were greater (P < 0.001) for HGW and LGW than NR steers, whereas proportional weight of feet and ears and head was greatest (P < 0.001) for NR, intermediate for LGW, and lowest for HGW steers. Hide mass was greatest (P < 0.001) for HGW, intermediate for LGW, and lowest for NR steers, but hide proportional weight was not different (P < 0.87) among treatments. Heart mass was greater (P < 0.001) for HGW and LGW than NR steers, whereas lung mass was greater (P < 0.01) for HGW than LGW steers, which had greater lung mass than NR steers. Heart and lung proportional weights were not different (P < 0.21) among treatments. Kidney mass (kg) did not differ (P < 0.19) among treatments, but proportional weight of kidneys were greater (P < 0.004) for NR than LGW or HGW steers. Similar to Exp. 1, ruminal mass was 1.2 kg greater (P < 0.002) for HGW than LGW steers, and LGW steers had 1.4 kg greater reticuloruminal mass than NR steers. In contrast to Exp. 1, reticuloruminal proportional weight was not different (P < 0.11) among treatments, although numerical trends were similar. Similar to Exp. 1, omasal mass did not differ (P < 0.52) among treatments, but proportional weight of the omasum was greater (P < 0.001) for NR than LGW or HGW steers. Abomasal, small intestinal, and large intestinal mass of HGW and LGW steers were greater (P < 0.09, 0.02, and 0.002, respectively) than NR steers. Proportional weight of the abomasum and small intestine were greater (P < 0.02) for NR and LGW compared with HGW steers. Mass of the cecum and proportional weights of the cecum and large intestine were not different (P < 0.45) among treatments. Greater mass of the stomach and intestinal organs resulted in HGW and LGW steers having 22.5 and 16.4% greater (P < 0.005) total GIT mass than NR steers. However, greater proportional weights of stomach organs and small intestine in NR steers resulted in 11.8% greater (P < 0.001) total GIT (g/kg of EBW) compared with LGW steers, and LGW steers had 13.9% greater total GIT compared with HGW steers. Initial mesenteric fat in Exp. 2 followed a pattern similar to that in Exp. 1; mesenteric fat in HGW steers was 3.7 kg greater (P < 0.001) than LGW steers, and LGW steers had 3.6 kg greater mesenteric fat mass than NR steers. As a proportion of EBW, mesenteric fat in HGW steers was 7.4 g/kg of EBW greater (P < 0.001) than LGW steers, and LGW steers had 10.7 g/kg of EBW greater mesenteric fat than NR steers. Similar to Exp. 1, liver mass of HGW was 0.8 kg greater (P < 0.001) than LGW steers, which had 1.3 kg greater liver mass than NR steers. In contrast to Exp. 1, the proportion of EBW made up by the liver was not different (P < 0.40; average = 17.0 g/kg of EBW) among treatments. No differences (P < 0.23) in pancreas or spleen mass or proportional weight were observed. Initial TST mass was 5.9 kg greater (P < 0.001) in HGW than LGW steers and LGW steers had 8.3 kg greater TST mass than NR steers. Because of divergent patterns in total GIT and mesenteric fat, initial TST mass as a proportion of EBW was not different (P < 0.77; average = 108 g/kg EBW) among treatments, similar to Exp. 1.

Change in Mass.
Empty-body weight gain (P < 0.70), carcass mass (P < 0.60), and proportional weight gain (P < 0.16) did not differ among treatments (Table 2). In general, response of total offal, blood, hide, and feet and ears were similar to Exp. 1. Across the feeding period, decrease in the proportion of EBW as head was 1.6-fold greater (P < 0.001) for LGW and NR than HGW steers. Change in lung proportional weight was not different (P < 0.68) among treatments, but heart of LGW steers became a greater (P < 0.02) proportion of EBW across the feeding period compared with HGW and NR steers. Kidney mass of HGW and NR steers increased at a greater (P < 0.03) rate than LGW steers. Kidneys became a lower (P < 0.01) proportion of EBW at a twofold faster rate for LGW and NR compared with HGW steers. Increase in change of reticuloruminal proportional weight was greater (P < 0.02) for steers that previously grazed winter wheat than NR steers. Similar to Exp. 1, the omasum of HGW and LGW steers became a greater (P < 0.001) proportion of EBW, whereas the omasum of NR steers became a lower proportion of EBW across the feeding period. Small intestinal mass of HGW and NR steers increased at a greater (P < 0.003) rate than LGW steers. The decrease in small intestinal proportional weight during finishing was greater (P < 0.001) in LGW than NR steers, and NR steers decrease in small intestinal proportional weight was greater than HGW steers. Change in proportional weight of the abomasum, cecum, and large intestine was not different (P < 0.55) among treatments. As a proportion of EBW, change in total GIT weight increased in HGW steers and decreased in LGW and NR steers; change in total GIT weight decreased at a greater rate (P < 0.001) in NR compared with LGW and HGW steers. In contrast to Exp. 1, mesenteric fat mass increased in LGW and NR steers at a greater (P < 0.03) rate than in HGW steers. However, mesenteric fat in LGW and NR steers (average = 192.6 g•g EBW^-1•d^-1) increased in proportion to EBW at a nearly 2.5-fold greater (P < 0.001) rate than HGW steers, similar to Exp. 1. The pancreas became a smaller (P < 0.02) proportion of EBW across the finishing period in LGW compared with HGW and NR steers. Change in proportional weight of the spleen was greatest (P < 0.001) in LGW followed by NR and then HGW steers. Across the feeding period, daily decrease in liver proportion of EBW was greatest (P < 0.05) for LGW, intermediate for HGW, and lowest for NR steers. Similar to Exp. 1, TST proportion of EBW increased, but was not different (P < 0.75; average = 151.8 g•g EBW^-1•d^-1) among treatments.

Tissue Cellularity
Initial.
Concentration of duodenal RNA before placement in the feedlot was 2.31 mg/g greater (P < 0.01) in LGW than in HGW steers, whereas NR steers were intermediate (Table 3). Initial duodenal DNA and protein concentrations and protein:DNA were not different (P < 0.20) among treatments. Duodenal RNA:protein was greater (P < 0.004) in LGW compared with NR steers, and NR steers had a greater RNA:protein than HGW steers. Concentration of liver RNA was greater (P < 0.06) for NR than LGW steers; HGW steers were intermediate. Similarly, NR steers had greater (P < 0.07) liver protein concentration than HGW or LGW steers. Initial liver DNA concentration, RNA:protein, and protein:DNA were not different (P < 0.12) among treatments.

Oxygen Consumption
For steers entering the feedlot, ruminal papillae O₂ consumption was not different (P < 0.55; average = 5.99 µL•min^-1•g^-1 of wet tissue) among treatments (Table 4). In addition, the percentage of the inhibition of O₂ consumption by either ouabain or cyclohexamide did not differ (P < 0.90; average = 34.5 and 21.0%, respectively). In contrast, initial duodenal O₂ consumption was twofold greater (P < 0.01) in LGW than in HGW or NR steers. Similarly, duodenal whole-organ O₂ consumption was 2.3-fold greater (P < 0.01) in LGW steers compared with HGW and NR steers (average = 5.19 mL/min). Inhibition of duodenal O₂ consumption was not different (P < 0.11) among treatments for either ouabain or cyclohexamide (54.1 and 47.1%, respectively). Oxygen consumption by liver tissue was not different (P < 0.33; average = 6.02 µL•min^-1•g^-1 of wet tissue) among treatments. However, O₂ consumption by the whole liver of HGW and LGW steers (average = 34.5 mL/min) was greater (P < 0.09) than NR steers. Similar to the other tissues, inhibition by ouabain and cyclochexamide was not different (P < 0.50; average = 32.7 and 39.9%, respectively) among treatments.

Discussion

It is well established that the GIT and liver have considerable impact on the partitioning of ME between maintenance (i.e., fasting heat production) and tissue NE_g (Webster, 1980; Reynolds and Maltby, 1994). However, much of the previous work examining splanchnic tissue mass and related energy expenditure has utilized different levels of intake of concentrate or forage-based diets (Sainz and Bentley, 1997; McLeod and Baldwin, 2000) during the growing phase rather than grazing programs. Therefore, little information is available with grazing cattle under different grazing pressures and/or different forage qualities. In response to winter grazing treatments, steers entered the feedlot with different EBW and body composition in the present experiments (Hersom et al., 2004). High-gain wheat steers entered the feedlot with an average of 28.6% greater EBW than LGW steers, and LGW steers had 30.6% greater EBW than NR steers. In addition, across both experiments empty-body fat averaged 19.7, 13.1, and 5.5% for HGW, LGW, and NR steers, respectively, when steers were placed in the feedlot. Interestingly, subsequent empty-body accretion rates were not different among treatments (Hersom et al., 2004). However, differences in the proportion of EBW made up by total offal (Exp. 2) and several metabolically active organs (Exp. 1 and 2) were observed. In response to increased BW, total offal mass of HGW steers was greater than LGW steers, and LGW steers had greater offal than NR steers. However, when expressed per kilogram of EBW, offal weight at the initiation of finishing was not different (Exp. 1) or was greater for LGW and NR steers than for HGW steers (Exp. 2).

Although they represent only 8 to 14% of BW, heat produced by the GIT and liver account for approximately one-half of a ruminant’s maintenance energy requirement (Seal and Reynolds, 1993). Several authors have observed that when ME intake was restricted, mass of the reticulorumen was similar or decreased compared with animals fed above maintenance or ad libitum (Wester et al., 1995; Fluharty and McClure, 1997; Nozière et al., 1999). Although DMI was not measured during grazing for the present experiments, NR and LGW steers in Exp. 1 and NR steers in Exp. 2 exhibited an increased proportion of EBW as reticulorumen before entering the feedlot, and proportional weights of omasum and abomasum were greater for NR than HGW steers in both experiments. Based on rates of EBW gain during grazing (Hersom et al., 2004), our data seems to conflict with the aforementioned results. However, Sainz and Bentley (1997) reported that steers limit-fed a high-concentrate diet had similar stomach (reticulorumen, omasum, and abomasum) weights as steers fed a high-concentrate diet ad libitum, and, in the same experiment, steers fed a high-roughage diet had greater stomach weights compared with steers fed the concentrate diet at either intake level. Similarly, McLeod and Baldwin (2000) found increased weights of the omasum and abomasum in sheep fed 75% forage compared with 75% concentrate diets at equal ME intake. Interestingly, Rompala et al. (1988) reported increased mass of the stomach complex in sheep consuming 10% greater DM, but isoenergetic diets. The 10% increase in DMI was due to the addition of polyethylene powder to the diet. Similar research (Sun et al., 1994; Goetsch et al., 1997; Kouakou et al., 1997) has shown that diets high in moderate- to low-quality forage decreased energy absorbed and facilitated high GIT mass or oxygen use relative to DE intake. Increased GIT mass (% of EBW) in animals fed low-quality forage compared with high-quality forage is consistent with the present results.

Rates of in vitro O₂ consumption by ruminal papillae have also been shown to be influenced by forage type and quality (Kelly et al., 1993). Kelly et al. (1993) found that ruminal papillae biopsied from steers fed alfalfa hay had greater in vitro rates of O₂ consumption than ruminal papillae biopsied from steers fed bromegrass hay. However, results were dependent on the time after feeding at which the biopsy was taken. Harmon et al. (1991) found a 27% increase in ruminal epithelial tissue oxygen consumption between steers fed forage diets at 2 x maintenance vs. maintenance. In the present Exp. 2, the rate of in vitro O₂ consumption by ruminal papillae was not affected by treatment, although HGW steers had numerically greater ruminal papillae O₂ consumption compared with LGW and NR steers. Ouabain-sensitive O₂ consumption accounted for 27.4 to 39.5% of total O₂ consumption, which is greater than reported by Kelly et al. (1993), and cyclohexamide-sensitive O₂ consumption accounted for 12.1 to 25.6% of total O₂ consumption, which is similar to Kelly et al. (1993). Our results compare favorably with several experiments that have shown that rates of O₂ consumption by ruminal papillae (or ruminal tissue) are not affected by energy or protein restriction (Drouillard et al., 1991; McLeod and Baldwin, 2000).

Differences in the initial mass of small intestine varied between the present Exp. 1 and 2. In Exp. 1, both restricted treatments (LGW and NR) had lower small intestinal mass than HGW steers, but proportional weight was not different among treatments. In Exp. 2, steers that had previously grazed wheat pasture had greater small intestinal mass; however, when expressed on a proportion of EBW basis, LGW and NR steers had greater small intestinal weight than HGW steers. Wester et al. (1995) found no difference in small intestinal mass between lambs fed diets adequate in energy and protein vs. energy- or protein-restricted lambs, and Fluharty and McClure (1997) reported no difference in small intestinal weight between steers fed at 100 or 85% of ad libitum intake. In contrast, work by Drouillard et al. (1991) with energy- and-protein restricted lambs, and Burrin et al. (1990) and Nozière et al. (1999) using different multiples of maintenance energy intake showed decreased small intestinal mass as a proportion of EBW with nutrient restriction. Jejunal mass was greater in sheep fed 75% roughage vs. 75% concentrate diets (McLeod and Baldwin, 2000), and weights of intestines were greater in steers fed a high-roughage growing diet compared with steers fed high-concentrate at restricted or ad libitum intake (Sainz and Bentley, 1997). These results are consistent with the present Exp. 2 if one assumes greater dietary bulk in the intestine of steers consuming low-quality winter native range. Synergistic interactions between DMI, digesta characteristics, and absorbable nutrients presented to the small intestine may ultimately dictate the proportion of EBW contributed by the small intestine.

The concentration of DNA has previously been used to estimate organ cell number (hyperplasia; Burrin et al., 1992; Sainz and Bentley, 1997; Nozière et al., 1999), whereas the concentration of RNA has been reported as an indicator of protein synthetic capacity (Sainz and Bentley, 1997; Nozière et al., 1999), and protein:DNA and RNA:protein have been used to estimate cell size (hypertrophy; Burrin et al., 1992; Sainz and Bentley, 1997; Nozière et al., 1999) and ribosomal capacity, respectively, the later of which provides an indication of in vivo fractional synthesis rate of tissue protein (Burrin et al., 1992; Nozière et al., 1999). Variable effects of nutrition and intake have been observed in cellularity of the small intestine. Nozière et al. (1999), using jejunum from underfed (restricted to 41 and 47% of energy and protein requirements, respectively) vs. maintenance-fed ewes, observed no difference in DNA or protein:DNA, which is consistent with the present results. Similar results were reported by McLeod and Baldwin (2000) for duodenal, jejunal, and ileal tissue from sheep fed low vs. high ME intakes of predominately roughage vs. predominately concentrate diets. No significant change in DNA or protein:DNA is consistent with intestinal masses of 4.48, 4.66, and 3.59 kg for HGW, LGW, and NR steers in the present Exp. 2, and this suggests that intestinal growth increases with age regardless of forage quality and empty-body growth rate. Interestingly, we observed greater duodenal tissue protein synthetic capacity (RNA) and fractional synthesis rate (RNA:protein) in LGW and NR compared with HGW steers in the present Exp. 2. In the companion paper (Hersom et al., 2004), we calculated increased heat production for LGW and NR steers during finishing compared with HGW steers. This might have resulted in part from the increased energy cost associated with the high fractional synthesis and protein turnover rate of visceral tissues as suggested by Tess et al. (1984). Oxygen consumption by duodenal tissue was less consistent. In vitro O₂ consumption was twofold greater in LGW, but not NR, compared with HGW steers. In addition, cyclohexamide-senitive O₂ consumption, used to quantify the contribution of protein synthesis to duodenal O₂ consumption, was not different among treatments, although the percentage of inhibition was numerically lower (39.7%) for NR steers.

In both Exp. 1 and 2, liver mass followed the same pattern as BW gain. In contrast, liver proportional weight (g/kg of EBW) appeared to be less responsive to grazing than GIT proportional weight. In Exp. 1, HGW steers had unlimited access to high-quality wheat forage and had increased liver size relative to their EBW compared with LGW and NR steers, which had limited DMI or forage of lower nutritive value, respectively. However, a similar pattern in liver mass as a proportion of EBW was not observed in Exp. 2. A decrease in proportion of liver with decreasing DMI has previously been reported for sheep (Burrin et al., 1990) and steers (Sainz and Bentley, 1997). Other dietary effects on liver mass have been reported. For example, increases in dietary protein concentration increased (Fluharty and McClure, 1997), whereas restrictions in dietary protein concentration decreased (Drouillard et al. 1991; Wester et al., 1995) liver mass as a percentage of EBW. Interestingly, McLeod and Baldwin (2000), using diets with different forage:concentrate but similar CP:ME ratios, reported sheep had similar liver mass as a percentage of EBW. Our data are in agreement with those of Drouillard et al. (1991) and Sainz and Bentley (1997) in that the liver appears to respond to changes in protein and energy supply from portal-drained viscera, but may be less responsive to changes in diet type and/or energy source (Rompala et al., 1988; McLeod and Baldwin, 2000).

Cellularity of the liver is an important response to level of nutrition in previously restricted animals. Nozière et al. (1999) reported a 17% increase in cell number (DNA, mg/g of tissue) in ewes that had been underfed for 78 d compared with maintenance-fed ewes. Similarly, Burrin et al. (1992) reported an increase in liver DNA in maintenance-fed lambs compared with ad libitum-fed lambs. In the present Exp. 2, no differences in cell number or size were observed, which is consistent with the similar liver mass expressed per unit of EBW. Our data compares favorably to Sainz and Bentley (1997), who reported no difference in liver DNA content in steers previously limit- or ad libitum-fed concentrate diets or fed roughage diets ad libitum. However, in their experiment, liver cell size was greater for steers fed high-concentrate ad libitum compared with livers from steers fed roughage ad libitum or limit-fed concentrate. In the present Exp. 2, NR steers generally had greater liver RNA and protein concentrations compared with steers that had grazed winter wheat pasture, whereas RNA:protein was not different among treatments. In contrast to our results, Nozière et al. (1999) and Burrin et al. (1992) reported that RNA concentrations were not different between underfed and maintenance-fed ewes and maintenance and ad libitum-fed lambs, respectively.

Rate of in vitro O₂ consumption per gram of liver was similar among treatments; however, liver O₂ consumption, expressed on a whole-organ basis, was greater for steers that grazed wheat pasture than for NR steers. Burrin et al. (1990) reported that increased liver O₂ consumption by sheep was the result of an increased organ size rather than metabolic activity. However, numerically greater inhibition (8.5 percentage units) of liver O₂ consumption by cyclohexamide in our study is in agreement with the increased total RNA in livers of NR steers. Although total liver O₂ consumption was lower, our data suggests that a greater proportion of O₂ consumption is being partitioned toward protein synthesis in NR steers, compared with faster-gaining steers on wheat pasture. The 24 to 46% energy expenditure in support of Na⁺,K⁺-ATPase and protein synthesis is in close agreement with data in isolated hepatocytes from sheep (McBride and Early, 1989).

Our data shows that steers grazing forage that resulted in restriction of BW gain had lower GIT mass but a greater proportion of their EBW as GIT when they entered the feedlot. This suggests that growth of visceral organs continues during periods of restricted carcass growth when steers are grazing low-quality forage or are restricted in intake of high-quality winter wheat forage. Relative to EBW gain, growth of visceral organs at the expense of carcass tissue may be a survival mechanism. Accretion of tissues that aid in the consumption and extraction of nutrients would logically be of greater priority than accretion of tissues that act as storage depots for protein and energy. Given this scenario, when previously restricted steers are placed on a high-grain finishing diet with adequate surplus protein and energy, visceral organ size is large and should be capable of extracting nutrients in excess of maintenance requirements for carcass growth. In support of this, during the finishing phase, increase in GIT mass was not different among treatments, but the decrease of total GIT as a proportion of EBW in NR steers was greater than LGW and HGW steers. Because of the greater GIT mass after grazing and the decreasing proportion of EBW as GIT mass across the finishing period, one might anticipate greater performance by NR steers compared with LGW and HGW steers. However, in our companion experiments (Hersom et al., 2004), no differences in rate of carcass accretion were observed, and whole empty-body accretion was greater for NR steers in Exp. 2, but not in Exp. 1. Reynolds et al. (1991) demonstrated that partial efficiency of ME use for tissue gain was lower in heifers fed 75% roughage than in heifers fed 75% concentrate at equal ME intake, primarily due to greater heat production by the portal-drained viscera. Using similar diets, McLeod and Baldwin (2000) showed that the digestive tract was a greater proportion of EBW in sheep fed 75% roughage compared with 75% concentrate, and that at high ME intake, the GIT became a greater proportion of EBW when sheep consumed forage compared with equal ME from concentrate. Although in vitro O₂ consumption by the ruminal papillae and duodenum was not greater for NR steers in the present Exp. 2, our data support the hypothesis that increased visceral organ mass per unit of EBW increases the maintenance energy requirements of finishing steers (Hersom et al., 2004).

After feeding steers to the same estimated 12th-rib fat (Hersom et al., 2004), proportions of EBW comprised by various organs, the concentrations of DNA, RNA, and protein, and in vitro O₂ consumption were remarkably similar among treatments. Similar organ masses and weight per unit of EBW after realimentation agrees with Drouillard et al. (1991) and Nozière et al. (1999). In contrast, work by Sainz and Bentley (1997), in which steers were slaughtered at 481 kg EBW, demonstrated continued differences in liver, stomach, and intestinal mass and cellularity in steers with different BW gain before finishing. The important exception in our data was mesenteric fat. In Exp. 1, final proportional weight of mesenteric fat was greater in HGW compared with LGW steers, and NR steers were intermediate. During Exp. 2, LGW and NR steers finished with a greater proportion of mesenteric fat compared with HGW steers. The difference in mesenteric fat deposition did not appear to affect live- or empty-body gain among treatments. However, with less energy being deposited into mesenteric fat, one would expect more energy to be available for the synthesis of lean tissue in previously restricted steers. Total mesenteric fat deposition (initial plus final mesenteric fat, kg) showed that in Exp.1, HGW steers deposited a greater amount of mesenteric fat during grazing and finishing compared with LGW and NR steers (30.1 vs. 18.9 and 20.2 kg). In contrast, total mesenteric fat deposition by HGW and LGW steers was similar in Exp. 2, and greater than NR (25.1 and 24.3 vs. 21.3 kg, respectively). McLeod et al. (2002) reported that total energy intake did not affect empty-body fat accretion rates. However, the authors did report differences in visceral fat accretion due to differences in energy source infused into the abomasum (glucose > hydrolyzed cornstarch). The importance of the timing of mesenteric fat deposition in the present experiments is unclear. However, increased accretion of mesenteric fat following a period of energy restriction would conceivably reduce energy available for growth of peripheral tissues, and decrease dressing percentage.

Although only two slaughter points were used in the present experiments, it should be considered that rates of change in absolute organ mass and organ mass as a proportion of EBW across the finishing period were most likely not linear. During both experiments, the mass and proportional mass of TST increased at a similar rate for all treatments; however, change in components of the TST varied among treatments. Across the finishing phase, decreased rate of GIT mass as a proportion of EBW was negated by increased mesenteric fat, which appeared to be the moderating factor, causing no change in TST among treatments despite differences in other organ proportions. Increases in TST proportion would indicate an overall increase in the maintenance energy requirement of steers during the finishing period (Huntington and Reynolds, 1987). Whereas this is physiologically unavoidable, our data suggests that the timing of the increase in TST and relative proportions of different organs are potentially subject to manipulation through grazing management practices. This opportunity for manipulating organ and/or tissue accretion appears to be especially true for the stomach and mesenteric fat. However, in the present experiments, greater initial GIT mass and similar increases in TST mass across the finishing phase for steers restricted in forage intake or quality resulted in similar feedlot performance compared with high-gaining wheat pasture steers.

Implications

Grazing programs can result in important differences in the proportion of empty body that is comprised of metabolically active organs when steers enter the feedlot. Greater gastrointestinal organ mass per unit of body weight was found in steers restricted in wheat-forage intake or grazing low-quality dormant native range compared with high-gaining wheat pasture steers. During finishing, the gastrointestinal tract of previously restricted steers became a smaller proportion of empty-body weight, but mesenteric fat increased at a greater rate, so that no differences were observed in rate of change of total splanchnic tissues. Similar increases in total splanchnic tissues across the finishing phase are consistent with the similar rate of live and carcass gain observed. Greater partitioning of energy toward protein synthesis in visceral tissues may be part of the mechanism that increases maintenance energy requirements of steers restricted in energy intake during grazing.

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 project H-2438 and the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. 99-34198-7481 and 2001-34198-10403.

² The authors thank D. Perry, C. Lunsford, and J. Summers for their help in collection and analysis of samples; K. Poling and J. Kountz for animal care and sample preparation; the Willard Sparks Beef Research Center animal caretakers; the USDA-ARS Grazinglands Research Lab animal caretakers; and numerous other graduate students who assisted with cattle harvest.

³ Correspondence: 208 Anim. Sci. Bldg., (phone: 405-744-8857; fax: 405/744-7390; e-mail: kclinto{at}okstate.edu).

Received for publication April 2, 2003. Accepted for publication August 13, 2003.

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