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,1
* University of Georgia, Athens 30602; and
and
Clemson University, Clemson, SC 29634-0311
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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0.01) than 1, with a trend (P = 0.09) for a greater substitution rate in HI than in MED. In vivo DM, OM, and NDF digestibility were linearly decreased (P < 0.01) by corn oil supplementation. Average daily gain and final BW tended (P = 0.09) to increase linearly in response to oil level. Oil conversion (0.36 kg of BW gain/kg of corn oil) was greater (P 0.05) than zero and did not differ (P = 0.15) between MED and HI. Dressing percent (P = 0.09), carcass weight (P = 0.01), and carcass backfat thickness (P = 0.01) increased linearly with oil supplementation. No treatment effect was observed for carcass LM area, KPH percentage, marbling score, or yield grade (P > 0.10). Oil supplementation to grazing steers linearly reduced forage DMI intake; however, animal performance was maintained and tended to be greater for oil-supplemented cattle. Oil supplementation increased carcass fat thickness and weight without altering other carcass quality parameters.
Key Words: beef • carcass • corn oil supplementation • digestibility • forage
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Madron et al., 2002; Gillis et al., 2004). Research (Duckett et al., 2002; Sackmann et al., 2003) has shown that plant oil supplementation to high-concentrate diets favors a greater predominance of the trans-10 biohydrogenation pathway and increases ruminal outflow of trans-10 octadecenoic acid. Beef and milk produced from cattle grazing forages have greater concentrations of TVA and CLA (Dhiman et al., 1999; Scollan et al., 2001; Realini et al., 2004). Sackmann et al. (2003) reported a linear increase in duodenal outflow of TVA and linear decrease in trans-10 octadecenoic acid when dietary forage level increased from 12 to 32% in finishing cattle diets. Thus, oil supplementation to grazing animals has the potential to increase CLA and TVA to a greater extent than in grain-fed cattle.
Research has shown that oil supplementation can impact fiber digestion and potentially reduce animal performance. Effects of oil supplementation on fiber digestion depend on the oil source and fatty acid composition, quantity of lipid supplemented, and proportion of forage in the diet (Palmquist, 1984; Jenkins, 1993). For high-concentrate diets, optimal growth performance is often obtained at total dietary lipid levels less than 1.6 g/kg of BW (Zinn, 1994). Limited information is available on the supplementation of unsaturated plant oils to grazing steers in a forage finishing system.
Therefore, the objective of this study was to determine the effect of corn oil supplementation level on performance, in vivo digestibility, and carcass quality in steers grazing endophyte-free tall fescue.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The steers had ad libitum access to the paddock, to fresh water, and to a vitamin-mineral premix (MAG-O-PHOS Beef Mineral, Southern States Coop., Richmond, VA; 10.5 to 12.5% Ca; 
5% P; 11.0 to 13.0% NaCl; 14.0% Mg; 0.35% K; 100 ppm I; 250 ppm Cu; 40 ppm Co; 52 ppm Se; 4,000 ppm Zn; 3,700 ppm Mn; 200,000 IU of vitamin A/lb; 15,000 IU of vitamin D/lb; and 50 IU/0.45 kg of vitamin E), except during the 2 h (0800 to 1000) during which the daily supplements were offered. Initial and final forage availabilities were estimated by harvesting 10 random samples (0.09 m2 frame) at a height of 1 cm. The 10 samples from each cutting were weighed and pooled, and a subsample was frozen at – 20 ° C for subsequent chemical analysis. An additional subsample was oven dried at 60 ° C for 48 h to estimate the DM content.
Animals
Eighteen Angus steers (438 ± 4 kg of BW; 16 mo of age), obtained from the Northwest Georgia Experiment Station in Calhoun, GA, were randomly assigned to 1 of 3 supplementation treatments. Steers were treated for internal and external parasites (Unimectrin, Universal Coop., Eagan, MI) on d 29, 21, and 61. The 3 treatments consisted of 3 corn oil supplementation levels: 0 g/kg of BW (none), 0.75 g/kg of BW (MED), or 1.5 g/kg of BW (HI). Pelleted cottonseed hulls were used as a carrier for the oil supplement and were fed at equal amounts to all steers throughout the experiment regardless of treatment. Levels of cottonseed hulls fed to the steers (0.7 to 1% of BW) were adjusted across treatments during the experiment according to forage availability. Samples of cottonseed hulls and corn oil were taken on a monthly basis and frozen at – 20 ° C for subsequent analyses.
The steers were trained to use Calan gate feeders (American Calan Inc., Northwood, NH) and then adapted to supplement treatments for 29 d before the beginning of the experiment. During adaptation, the steers were allowed to graze the same pasture that was used for the experimental period. At the beginning (d 0) and end (d 116) of the experimental period, steer BW (unshrunk) were recorded on 2 consecutive days at 0800 and averaged. Live BW were recorded before supplementation at d 21, 42, 69, 85, and 104 and were used to determine BW gain and to adjust the amount of supplement to the appropriate BW level. Real-time ultrasound measures of s.c. fat thickness and LM area were collected on d 116 using an Aloka 500-V ultrasonograph (Corometrics Medical Systems, Wallingford, CT) equipped with a 17-cm, 3.5-MHz linear probe. The images were interpreted using Beef Information Manager software, Version 3.0 (Critical Vision Inc., Atlanta, GA).
Intake and Digestibility
Forage DM intake and apparent, in vivo, total digestibility of DM were estimated using chromium sesquioxide as an external marker and indigestible NDF (INDF) as an internal marker of the digesta (Lippke et al., 1986). On d 49, a controlled-release fecal marker capsule [CAPTEC, Cattle Chrome MCM for 300 to 700-kg cattle (active constituent, chromium sesquioxide, 65% wt/wt; release rate 1.7 g of Cr2O3 daily) NUFARM Ltd., Auckland, NZ] was given as a bolus to each steer. Twelve days later, the steers were moved to a new paddock, and fecal collection began 1 d later.
Fecal samples were collected from each steer at 0700 and 1700 for 7 d. Fecal samples (25 g of wet weight) were pooled for each animal and frozen at – 20 ° C for subsequent analyses. Supplement refusals were recorded daily, and samples were collected for DM determination. Pre- and postgrazing pasture samples from each paddock were collected and stored at – 20 ° C for subsequent analyses of DM. Samples were used to determine DM composition of consumed forage in each paddock during fecal collection period by herbage mass difference (Burns et al., 1994), assuming that treatment did not affect forage selection.
Forage, cottonseed hulls, and fecal samples were lyophilized, ground through a Wiley mill (model 4, Thomas Scientific, Swedesboro, NJ) equipped with a 1-mm screen, and stored at – 20 ° C for subsequent determination of OM, NDF, ADF, CP, total fatty acid percentage, and fatty acid profile. Organic matter was measured as the weight loss after combustion for 8 h at 500 ° C. Neutral detergent fiber and ADF were sequentially determined using an Ankom 200 fiber extractor (Ankom Technologies, Fairport, NY) according to the method of Van Soest et al. (1991). Crude protein concentration was determined by the combustion method using a Leco FP-2000 N analyzer (Leco Corp., St. Joseph, MI). Total fatty acid percentage and fatty acid profile were also determined for the corn oil samples. Fatty acids were methylated according to the method described by Park and Goins (1994) and were separated by GLC according to Duckett et al. (2002). Nutrient and fatty acid composition of forage, cottonseed hulls, and corn oil is shown in Table 1.
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) in the rumen of 2 Holstein steers that had ad libitum access to tall fescue hay. Residual samples were analyzed for the NDF concentration as described previously. Chromium concentration was measured in the fecal samples via spectrophotometry (Fenton and Fenton, 1979). Forage DMI was calculated based on fecal output and indigestibility (Reis and Combs, 2000, with modifications). Fecal output was calculated based on chromium concentration in the feces, whereas indigestibility was determined based on forage, cottonseed hulls, and feces INDF concentrations. Total forage INDF excreted was estimated as the difference between total INDF excreted and total INDF in the cottonseed hulls. Forage DMI was then calculated as the ratio between total forage INDF excreted and INDF concentration in the forage sample. The DMI for each component of the diet was multiplied by the corresponding concentration of OM, CP, FA, NDF, and ADF, which then were added to obtain total DMI and total OM, CP, FA, NDF, and ADF intakes. Total tract DM, OM, FA, NDF, and ADF digestibilities were estimated as the coefficient of the total tract disappearance (intake minus excretion) and intake. The proportion of calcium soaps in the total fecal lipid excreted was estimated as the ratio of the difference in lipid content between the acidified and nonacidified fecal samples, and lipid content in the acidified samples. The acidified samples were pretreated with 0.5 N HCl for 8 h (Bohman and Lesperance, 1962) before the lipid extraction to account for any calcium soap present in the fecal sample. Lipid extraction was performed using the chloroform:methanol procedure (Folch et al., 1957) instead of ether, as suggested by Andrae et al. (2000).
Carcass Traits
On d 117, the steers were transported 45 km to the University of Georgia Meat Science and Technology Center in Athens, and fasted live animal BW were obtained after an overnight feed withdrawal. After slaughter, HCW was recorded, and the carcasses were chilled at – 1 ° C for 24 h. At 24 h postmortem, adjusted s.c. fat thickness, LM area, marbling score, percentage of KPH, and USDA skeletal maturity class were determined on the left side of each carcass. Carcass price and value were calculated (AMS, 2004).
Supplement Conversions
Total diet G:F was calculated as the ratio between total BW gain (final BW minus initial BW) and total DMI throughout the experiment. Total DMI throughout the experiment was calculated by adding daily cottonseed hulls and corn oil intake per animal and the total forage DMI, which was estimated from the average BW of each animal during the supplementation period and the forage DMI as a percentage of BW estimated using chromium sesquioxide and INDF. Corn oil G:F was estimated as the slope of the linear regression between the total BW gain and the total corn oil intake during the 116 d of supplementation. Similarly, the substitution rate of forage DMI with corn oil supplementation was estimated as the slope of the linear regression between forage DMI (kg/d) and corn oil intake (kg/d). The relative stocking density was defined as the approximate number of animals needed in the MED and HI treatments to utilize equal amounts of forage relative to the none treatment. Relative stocking density was calculated as the ratio between the average forage DMI for the none treatment and the actual forage DMI. The relationship between relative stocking rate and corn oil intake (kg/d) was estimated by linear regression.
Diet NE Estimation
Expected dietary NE content for maintenance and gain was calculated based on tabular values (NRC, 2000) for individual feed ingredients. In addition, the observed dietary NE for maintenance and gain were calculated according to Zinn and Plascencia (1996), using the growth performance measurements (BW during the period of forage DMI determination and overall ADG). The NEm and NEg values for the corn oil were calculated according to Zinn and Plascencia (1996), as follows: NEm = [(Observed NEm for MED or HI – Observed NEm none)/Dietary proportion of corn oil] + (Expected NEm value for MED or HI without the corn oil); NEg = [(Observed NEg for MED or HI – Observed NEg none)/Dietary proportion of corn oil] + (Expected NEg value for MED or HI without the corn oil).
Statistical Analysis
The fecal chromium concentration for 1 steer was 3 SD below the overall mean, resulting in an extremely high DMI (28 kg) compared with the overall mean (13.4 kg and SD 0.97). Therefore, data from this animal were removed from the data set due to an apparent malfunction of the controlled release chromium sesquioxide capsule. Intake, digestibility, and carcass variables were analyzed as a completely randomized design using the MIXED procedure (SAS Inst. Inc., Cary, NC), with animal as the experimental unit and dietary treatment as a fixed effect. Initial BW (d 0) was used as a covariate when P 0.05. Differences in corn oil level were compared using orthogonal polynomial contrasts for linear and quadratic effects. When significant (P < 0.05), linear or quadratic equations were predicted using the REG procedure of SAS. The relationship between carcass and real-time ultrasound measurements taken just before slaughter was determined using the REG procedure of SAS.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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0.12) or as percentage of BW (0.73% of BW, P 0.39). Corn oil intake increased linearly (P < 0.01) with greater levels of corn oil supplementation (Table 2). As corn oil intake increased, there was a linear decrease in forage (– 2.42 kg of DMI x g of corn oil/kg of BW; P < 0.01) and total DMI (– 1.76 kg of DMI x g of corn oil/kg of BW; P < 0.01). Total fatty acid intake increased (P < 0.01) with corn oil supplementation level. The proportion of total lipid excreted as calcium salts was not altered by oil supplementation (16.2 ± 4.6%, linear effect, P = 0.99; quadratic effect, P = 0.88). These changes in DMI with oil supplementation resulted in greater proportions of supplement contributing to total dietary DMI [supplement, % of DMI = 24.6 + (10.5 x g of oil/kg of BW); P < 0.01]. The proportion of OM and ADF in the diet increased linearly (P < 0.01) with oil supplementation. In contrast, the calculated dietary proportion of CP decreased linearly (P < 0.01) with oil supplementation. Dietary NDF concentration was not altered with oil supplementation. Gross energy concentration of the diet increased linearly (P < 0.01); however, DE concentration was not changed (P = 0.42) by oil level. Thus in accordance with total DMI, total DE and total GE intake linearly decreased (P < 0.01) with oil supplementation.
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). Oil supplementation did not alter the apparent in vivo digestibility of dietary ADF or total fatty acids. The proportion of total lipid excreted as calcium salts was not affected by the level of corn oil supplemented, averaging 16.3% (P > 0.87).
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). Thus there was a trend (P = 0.08) for total BW gain to increase with corn oil supplementation. During the first 61 d, ADG tended (P = 0.10) to increase linearly with oil supplementation level. From d 61 to 116, there was a trend for a quadratic response (P = 0.07) in ADG with MED having the greatest rate of gain. Overall, ADG tended to increase (P = 0.09) 0.12 ± 0.07 kg per each unit (g/kg of BW) of added corn oil. Similarly, final BW tended (P = 0.09) to increase linearly with corn oil supplementation. Ultrasound, s.c. fat thickness tended to increase linearly (P = 0.08; 0.10 ± 0.05 cm) with oil supplementation. Oil supplementation did not alter LM area as measured by real-time ultrasound. The standard errors of prediction for fat thickness, and LM area with ultrasound (0.11 cm and 6.0 cm2, respectively) were within the range required for certification by the Beef Improvement Federation (0.30 cm and 7.74 cm2). Oil supplementation increased HCW and fat thickness linearly (P < 0.01), whereas no differences in carcass LM area were observed among treatments. For LM area and fat thickness, ultrasound measures overestimated actual carcass measures of fat thickness [carcass fat thickness cm = 0.002 + (0.699 x ultrasound fat thickness); R2 = 0.54] and LM area [carcass LM area cm2 = 5.43 + (0.84 x ultrasound LM area; R2 = 0.38)]. Oil supplementation did not alter other carcass traits of KPH percentage, marbling score, quality grade, or yield grade. Carcass price ($/kg) did not differ among treatments; however, carcass value was greater (P = 0.05) with oil supplementation because of heavier carcass weights.
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. Corn oil supplementation linearly increased (P < 0.001) NEm and NEg regardless of the calculation methodology. The actual diet NEm and NEg calculated from growth-performance were increased 32 and 54%, respectively, when the greatest level of corn oil was supplemented (HI) with respect to the control diet (none). The estimated NEm and NEg for the corn oil did not differ (P = 0.44) with the level of corn oil-supplemented (6.42 and 5.23 Mcal/kg, respectively).
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0.001, R2 = 0.69, SE = 1.08; Figure 1a). These results show that each kilogram of corn oil supplement substituted for 5.68 kg of forage DM. This substitution rate obtained with corn oil supplementation was reflected in a linear increase of the relative stocking density when corn oil intake increased (y = 0.95x + 0.96, R2 = 0.60, P < 0.001, SE = 0.22; Figure 1b). Thus, 1.55 animals could be stocked when supplemented with corn oil at 1.5 g/kg of BW to obtain the same forage utilization as with 1 nonsupplemented steer. In addition, BW gain increased by 0.31 kg per kg of corn oil intake (R2 = 0.17, P = 0.09, SE = 19.70; Figure 1c).
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; 1.8 to 2.7% of BW, McCracken et al., 1993; 2.5% of BW, Judkins et al., 1997), low-endophyte (2.4 to 2.7% of BW, Hitchcock et al., 1990), or endophyte-infected (3.2%, Elizalde et al., 1998) tall fescue without supplementation or with supplementation (3.4% of BW, Elizalde et al., 1998; 2.8 to 3.0% of BW, Judkins et al., 1997). To our knowledge, this is the first work evaluating effects of high levels of supplemental corn oil supplementation to grazing cattle in a forage-finishing system. Recently, Schroeder et al. (2004) reviewed the literature on fat supplementation to grazing dairy cattle. In this review, effects of fat supplementation on DMI were nonsignificant; however, only 8 of 25 comparisons evaluated the effect of supplementation on DMI. In the reviewed studies, lipid sources supplemented were of calcium salts of fatty acids or hydrogenated oils. According to Jenkins (1993), supplementation of calcium salts or hydrogenated oils would have a lower negative effect on ruminal fermentation than unsaturated plant oils. Brokaw et al. (2001) observed no changes in DMI when heifers were supplemented with low levels (0.375 g/kg of BW) of vegetable oil while grazing a summer pasture (75% Bromus biebersteinii). Others (Hardin et al., 1989; Hall et al., 1990; Patil et al., 1993) reported reductions in DMI when lipids were supplemented to high-forage diets when hay was used as the forage source.
In this study, oil supplementation linearly reduced NDF in vivo digestibility. Jenkins (1993) also reported reductions in OM and NDF digestibilities with oil supplementation in a review of the literature; however limited research is available on effects of lipid supplementation to grazing cattle. Brokaw et al. (2001) evaluated the effect of lipid supplementation on in vivo digestibility of grazing beef cattle; however, the level of soybean oil-supplemented was relatively low (0.35 g/kg of BW) and no effect was observed. Others have evaluated effects of lipid supplementation on in vivo digestibility for high-forage ( 50%) diets using Bermudagrass hay (Hardin et al., 1989; Hall et al., 1990; Patil et al., 1993) or bromegrass hay (Scholljegerdes et al., 2004) as the forage source. Hardin et al. (1989) and Hall et al. (1990) observed a reduction in NDF digestibility of Bermudagrass hay with lipid supplementation compared with nonsupplementated. Total tract NDF digestibility of bromegrass hay was reduced 17% when 5% high-linoleate or high-oleate safflower cracked seeds were supplemented (Scholljegerdes et al., 2004). In contrast, Patil et al. (1993) reported no differences in NDF digestibility of Bermudagrass hay with the addition of partially hydrogenated tallow. In our study, corn oil supplementation decreased NDF digestibility by 6 and 12%, respectively, for 0.75 g/kg of BW and 1.5 g/kg of BW.
The OM and NDF digestibilities were lower than those reported by Judkins et al. (1997) and Elizalde et al. (1998) in steers grazing tall fescue without supplementation. This lower digestibility could, in part, be due to the supplementation of cottonseed hulls to all steers in this study. Lippke et al. (2000) reported reduced OM digestibility in steers grazing a high-quality wheat pasture (Triticum aestivum) supplemented with cottonseed hulls. These authors estimated the digestibility of cottonseed hulls to be 21.5%. In addition, the increased proportion of CSH in the diet as forage DMI declined with oil supplementation could, as suggested by Scholljegerdes et al. (2004), confound the direct effect of corn oil level on NDF digestion. In our experiment, cottonseed hulls INDF was 43.5%, whereas forage INDF was 31.6%. The reduction in forage intake with increasing level of corn oil supplementation also decreased dietary CP content. However, according to Satter and Roffler (1975), dietary CP content with oil supplementation in this study should still be adequate for ruminal function and would not be responsible for the observed depression in NDF digestibility. In addition, an excess of ruminal degradable protein would not increase NDF degradability (Hristov et al., 2004).
Research has shown that lipid supplementation levels of 10.7% of dietary DM for steers using yellow grease (Plascencia et al., 2003) or 12.5% of dietary DM with soybean oil for lambs (Kucuk et al., 2004) in high-concentrate diets linearly reduced postruminal fatty acid digestion. Palmquist (1991) observed that apparent digestibility of fatty acid remained constant when fatty acid intake increased up to 5% of dietary DM but that it was reduced when fatty acid intake was further increased to 8% of dietary DM. The reduction in fatty acid digestion in these experiments was largely explained by a reduction in C18:0 digestion (Plascencia et al., 2003; Kucuk et al., 2004). Apparent fatty acid digestibility increased when Wu et al. (1991) increased dietary fatty acid content from 2.5% of dietary DM to 4.5 or 5.2% using animal-vegetable blend fat or calcium soaps, respectively, but declined when dietary content was further increased to 6.5 or 7.9%, respectively. In contrast, others have not observed any change in apparent fatty acid digestibility with supplementation of different fat sources to dairy (Palmquist, 1991; Palmquist et al., 1993; Kalscheur et al., 1997) or beef cattle (Zinn and Plascencia, 1993, 1996; Scholljegerdes et al., 2004), which is in agreement with our study.
The ADG observed in this study (0.81 ± 0.07 kg) is similar to those reported by Thompson et al. (1993) for steers grazing low endophyte infected ( < 5%) tall fescue in southeastern United States during spring (0.84 ± 0.06 kg). Others (Hess et al., 1996; Judkins et al., 1997; Elizalde et al., 1998) have also reported similar gains (0.82, 0.84, and 0.69 kg/d) for steers grazing endophyte-free tall fescue without supplementation. In all 3 studies, energy (corn grain) or protein (wheat bran or corn gluten meal) supplementation increased ADG to 1.04, 1.11, and 0.74 kg, respectively. Similarly, oil supplementation in this study tended to increase ADG. In contrast, others (Hardin et al., 1989; Patil et al., 1993) reported no change in ADG with either soybean oil or partially hydrogenated tallow supplementation to grazing steers. Whitney et al. (2000) observed a quadratic increase in ADG with soybean oil supplementation to heifers consuming hay; however, no differences where observed in a companion study.
Corn oil supplementation to grazing steers increased diet GE concentration, but the negative effect of oil supplementation on DM intake and digestibility resulted in a reduction of DE intake. Similar results were observed by Scholljegerdes et al. (2004) and McGinn et al. (2004) who found that the decrease in NDF digestibility offset the additional DE supplied by the lipid supplement. Corn oil supplementation increased the efficiency of energy utilization as fat deposition and ADG increased when DE intake was reduced. According to NRC (2000), the relationship between DE and ME as well as that of ME and NE can vary considerably among diets with different composition (fiber, starch, fat). One of the forms of energy lost from DE to ME is as methane produced during ruminal fermentation; Zinn and Plascencia (1996) and McGinn et al. (2004), either indirectly or by direct determination, observed reductions in methane production with lipid supplementation. Because methane production is greater in high-fiber than high-concentrate diets, the reduction in methane production with lipid supplementation would have a greater impact on a high-fiber diet (Zinn and Plascencia, 1996). Another factor that may have been involved in an increase in the efficiency of energy utilization by oil supplementation could have been by its effect on DMI. Caton and Dhuyvetter (1997) suggested that energy supplementation would reduce maintenance energy cost by reducing grazing and ruminating energy expenditures if forage intake is reduced. In addition, the reduction in forage intake could reduce maintenance energy costs by reducing gastrointestinal tract mass (Sainz et al., 1995; Sainz and Bentley, 1997). The trend for a linear increase in dressing percent with oil supplementation in our study supports this theory.
Corn oil supplementation did not affect the DE content of the diet; however, corn oil supplementation linearly increased NEm and NEg content up to 32% and 55%, respectively, with the greatest level of corn oil supplementation (HI). The average NEm and NEg values of corn oil obtained in the current study are greater than those reported by Zinn and Plascencia (1996) for yellow grease when supplemented to high-concentrate diets. In their study, Zinn and Plascencia (1996) observed that when forage content of the diet was increased from 10% to 30% of total dietary DM content, the NEm and NEg values of the yellow grease increased from 3.33 and 2.65 Mcal/kg to 5.71 and 4.65 Mcal/kg, respectively. Thus, a kg of gain could have been obtained per kg of corn oil-supplemented if the added calories from corn oil are supplemented above maintenance level required for steers with less than 500 kg of BW (NRC, 2000). This might be feasible especially if the additional gain is not all fat.
Lipid supplementation increased external fat deposition but did not alter marbling deposition in our study. Patil et al. (1993) observed a numerical increase in fat thickness (1.4 mm) and an increase in marbling deposition with lipid supplementation. The lack of oil effect on marbling score with corn oil supplementation observed in this study may be the result of the relatively low amount of reserves in that depot. Duckett et al. (1993) observed in a serial slaughter study that a significant increase in marbling score for steers in a high-concentrate diet occurs after fat thickness reached 6.8 mm. In our study, the maximum fat thickness at slaughter was 5.1 mm for HI whereas the fat supplemented treatment of Patil et al. (1993) reached 6.6 mm. The trend for a linear increase in ADG was reflected in the linear increase of HCW with oil supplementation. Patil et al. (1993) observed an increase in HCW with supplementation of grazing steers; however, supplemental fat addition did not further increase HCW.
Animal response to supplementation is subject to the forage substitution rate (Bargo et al., 2003). In grazing dairy cattle, the substitution rate for fiber- or starch-rich supplements typically ranges between 0 and 1 (Bargo et al., 2003; Doyle et al., 2005). However when 0.5 or 1 kg of partially hydrogenated oil was added to the concentrate of grazing dairy cattle, the substitution rate was 7 and 4.1 kg/kg, respectively (Schroeder et al., 2002). Thus our results and those of Schroeder et al. (2002) suggest that the substitution rate with oil supplementation could be considerably greater than 1. According to our results with nonlimiting forage availability, oil-supplemented steers consumed less forage and produced heavier carcasses than unsupplemented steers. Supplement conversion improved when forage substitution rates were reduced by lower forage availability (Beretta et al., 2006). In our study, oil conversion was within the range (0.12 to 0.77) observed for fiber-and starch-rich supplements when offered to grazing beef cattle (Grigsby et al., 1991; Horn et al., 1995; Bodine et al., 2001). According to our data when supplementing with 1.5 g/kg of BW of corn oil, stocking density could be increased by 62% relative to the stocking density without oil supplementation. Additional research is warranted to further examine how oil supplementation influences forage intake depending on oil source, forage availability and quality, and physiological stage (growing vs. mature; pregnant vs. lactating mature cows).
1 Corresponding author: sducket{at}clemson.edu
Received for publication September 11, 2006. Accepted for publication January 4, 2007.
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