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ANIMAL PRODUCTION |
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,5,6
* USDA-ARS Conservation and Production Research Laboratory, Bushland, TX 79012;
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New Mexico State University Clayton Livestock Research Center, Clayton 88415; and
and
Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Key Words: beef cattle • crude protein concentration • feedlot • phase feeding • serum urea nitrogen
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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). Thirty to fifty percent of feed N may be lost via volatilization, primarily as ammonia (Bierman et al., 1999; Todd et al., 2005). The quantity of ammonia lost can be affected by dietary CP concentration and source (Smits et al., 1995; Cole et al., 2005; Todd et al., 2006). These ammonia losses may adversely affect air quality and also decrease the fertilizer value of manure (Paul et al., 1998). Thus, it is necessary to develop management regimens that decrease N losses to the environment without adversely affecting animal performance or economics.
Protein requirements of cattle, as a percentage of dietary DM, decrease as they mature (NRC, 2000). Thus, by decreasing the concentration of CP in the diet as animals grow, the inputs of N, and subsequent loss of N to the environment, could potentially be decreased without adversely affecting animal performance. With dry-rolled corn-based diets, Erickson et al. (1999), Cooper et al. (2000), and Trenkle (2002) noted no adverse effects on cattle performance when dietary CP concentrations were decreased during the later stages of the feeding period. However, the requirement for degraded intake protein (DIP) in diets containing steam-flaked grains is estimated to be greater than in less fermentable dry-rolled corn-based diets (Galyean, 1996; NRC, 2000; Cooper et al., 2002). Thus, decreasing the dietary CP concentration could potentially cause a DIP deficiency in diets based on steam-flaked corn.
The objectives of this study were to determine the effects of phase-feeding protein to finishing beef cattle fed a steam-flaked, corn-based diet on animal performance, carcass characteristics, N utilization, and potential N volatilization losses.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Cattle
Three hundred eighteen, crossbred, medium- to large-framed (British x Continental; initial BW 315 ± 5 kg) beef steers, previously used in receiving and growing programs at the Clayton Livestock Research Center, were used in the study. Steers had been processed on arrival, including 1) individual identification, branding, and horn-tipping as needed; 2) vaccination with a Clostridial antigen (Ultra Choice, Pfizer Animal Health, Exton, PA); 3) vaccination with an IBR-PI3-BVD-BRSV vaccine (Bovishield 4, Pfizer Animal Health); and 4) treatment for internal and external parasites (Ivermectin, Merck, Rahway, NJ).
Steers were initially fed a 70% concentrate diet and were subsequently adapted over a 19-d period to a 90% concentrate, finishing diet. Specifically, the 70% concentrate diet was fed for 5 d, a 75% concentrate diet was fed for 5 d, an 80% concentrate diet was fed for 5 d, and an 85% concentrate diet was fed for 4 d before beginning the study. All steers received a Component-ES implant (14.4 mg of estradiol and 200 mg of progesterone; Vet-Life, Overland Park, KS) 1 d before the experiment began. Steers were reimplanted with Revalor-S (24 mg of estradiol and 120 mg of trenbolone acetate: Intervet; Millsboro, DE) on d 56 of the experiment.
Treatment Assignment and Experimental Diets
All steers were weighed (unshrunk) 1 d before initiation of the study (November 23, 2004) and were stratified from the heaviest to the lightest BW. The lightest 48 steers were designated as block 1, and the next lightest 48 steers were designated as block 2. The next 3 blocks (block 3, 4, and 5) contained 54 steers per block. The heaviest 60 steers were designated as block 6. The average weights of block 1 to 6 were 269, 297, 310, 321, 335, and 358 ± 29 kg, respectively.
Thirty-six pens were used, with 6 weight blocks and 6 dietary treatments. Each soil-surfaced pen measured 12.2 x 34.8 m, with a 10.4-m fence-line feed bunk and an individual water trough. Pens contained 8 (block 1 and 2), 9 (block 3, 4, and 5), or 10 (block 6) steers. The 6 treatments consisted of the following: 1) 11.5% CP diet fed throughout the study (11.5); 2) 13.0% CP diet fed throughout the study (13.0); 3) initially fed an 11.5% CP diet and switched to a 10% CP diet with 56 d remaining on feed (11 to 10); 4) initially fed a 13.0% CP diet and switched to an 11.5% CP diet with 56 d remaining on feed (13 to 11); 5) initially fed a 13.0% CP diet and switched to a 10% CP diet with 56 d remaining on feed (13 to 10); and 6) initially fed an 13.0% CP diet and switched to an 11.5% CP diet with 28 d remaining on feed (13 to 11/28).
Compositions of the isocaloric diets are presented in Table 1. The 13% CP diet was considered a positive control diet because it is similar to diets commonly used in the cattle feeding industry, and the 11.5% diet was considered a negative control because the CP concentration was below that used in the industry (Galyean and Gleghorn, 2001). Diets were changed so that the phase-feeding stage was 28 or 56 d within all weight blocks; therefore, the number of days that the cattle were on their initial diet varied from 105 (block 6) to 133 (block 1, 2, and 3) in those phase-fed the last 56 d and from 133 (block 6) to 161 d (block 1, 2, and 3) in those phase-fed the last 28 d.
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). Weighing and Feeding Procedures
Each steer was weighed individually before feeding on d 0, 56, 84, 112, and 140 of the experiment. Scales (Silencer Single-Animal Squeeze Chute suspended from 2 load cells; Moly Manufacturing Inc., Lorraine, KS) were calibrated before use with 454 kg of certified weights. Average daily gains were calculated using a 4% pencil shrink for animal weights. Final slaughter weights were calculated as carcass weight divided by the average dressing percent of that slaughter group.
On weigh days, feed bunks for each pen were swept, and unconsumed feed was weighed and analyzed for DM content to allow for calculation of DMI for the period. Feed bunks were visually examined each day at approximately 0730. The estimated quantity of feed remaining in the bunk was used to determine a suggested feed delivery. The bunk-reading process was designed to allow for little or no accumulation of feed within the bunk from day to day; however, cattle were challenged regularly (typically at 3-d intervals) to ensure that consumption was ad libitum. When feed was left in the bunk, the pen was held at the previous level of feeding or slightly restricted until the total quantity of feed delivered was consumed. Fresh feed was milled once daily, and the quantity to be delivered to each pen was weighed to the nearest 0.45 kg and delivered to the pens by a feed truck fitted with 6 individual bins, each with an auger for dispensing feed.
Carcass Evaluation
Blocks of steers were slaughtered (March to April, 2005) when approximately 60% of the steers within the block had a visually estimated 12th rib fat thickness of 10 mm. Cattle were individually weighed and shipped approximately 195 km to the Tyson Fresh Meats Inc., facility in Amarillo, TX, for slaughter, and carcass data were collected by experienced personnel from the Beef Carcass Research Center at West Texas A&M University, Canyon. Cattle in block 6 were fed for 161 d; those in blocks 4 and 5 were fed 183 d; and cattle in blocks 1, 2, and 3 were fed for 189 d (average days on feed = 182.3 d). Individual carcass measurements included HCW, 12th rib fat thickness, LM area, percentage of KPH, marbling score, and USDA quality grade. Carcass measurements were used to calculate USDA yield grade.
Empty body fat percent was estimated using the following equation (Perry and Fox, 1997):
 | [1] |
where EBF = empty body fat as a percentage of BW, and EBW = empty BW (i.e., shrunk BW x 0.89) in kg.
Blood Collection and Serum Urea Nitrogen Analysis
Blood was collected in the morning and before feeding from randomly selected steers in each pen the day before the diets were changed and again the day before slaughter. Three steers per pen were sampled via jugular venipuncture (10-mL Corvac tubes, Sherwood Medical Co., St. Louis, MO). The same steers within each pen were used for blood collection at both blood samplings. Blood was allowed to clot at room temperature for 1 h, was centrifuged at 1,000 x g for 20 min (Beckman, Model J2-21, Irvine, CA), and then separated serum was frozen in individual storage vials at –20°C. Serum urea nitrogen (SUN) was analyzed with a commercial kit (Sigma 535-B, Sigma Chemical Co., St. Louis, MO) using a spectrophotometer (Beckman DU-5, Beckman Instruments, Irvine, CA; 
= 540 nm).
Manure Characteristics and Apparent Digestion
To calculate apparent DM, N, and P absorption, diet and fresh fecal samples (3 steers/pen; approximately 150 g/steer obtained approximately 2 h after feeding) were obtained from each pen 56 d before slaughter and again the day on which the animals went to slaughter. Apparent DM, N, and P digestibility, and N and P fecal excretion were determined for individual steers using indigestible ADF as an internal marker (Schneider and Flatt, 1975). Nitrogen and P retention were estimated from individual animal performance using the following equations (NRC, 2000):
 | [2] |
 | [3] |
 | [4] |
 | [5] |
 | [6] |
where SWG = shrunk weight gain (kg/d); EBG = empty body gain (kg/d); RE = retained energy (Mcal/d); EQEBW = equivalent empty BW (kg); ProtRe = protein retention (g/d); PhosRe = P retention (g/d); and NRe = N retention (g/d). Urinary N and P excretion were estimated as the difference between nutrient intake and fecal + retained nutrients.
To estimate N volatilization losses, 5 samples of air-dried manure were collected from the concrete feed bunk pads and composited within each pen the day before the diets were changed and again the day after the cattle were slaughtered. Fresh fecal and urine spots were avoided during sampling because they are the major sources of ammonia emissions and represent the manure composition before most ammonia is lost (Mason, 2004). Apparent N volatilization losses were estimated based on the change in the N:P ratios of diets and air-dried pen manure using the following equation (Todd et al., 2005):
 | [7] |
Laboratory Analyses
Fresh feces, air-dried manure, and diet samples were analyzed for DM by drying to a constant weight at 60°C in a forced-draft oven. After block digestion of undried samples, N and P were determined colorimetrically using a flow injection analyzer (Quick Chem FIA+8000, Lachet Instruments, Milwaukee, WI; Methods 10-107-06-2-E and 15-115-01-4-A, respectively; AOAC, 1990). Fresh feces and diet samples were analyzed for indigestible ADF (Vogel et al., 1999) after a 96-h in vitro digestion in an Ankom Daisy System (Ankom Tech Corp, Fair-port, NY).
The pH and electrical conductivity of air-dried manure samples were determined by mixing manure with deionized water (1 g of DM/5 mL of water), shaking for 10 min, centrifuging for 20 min at 3,900 x g, and then reading the pH with a combination electrode (Model 125, Corning, Medfield, MA) and electrical conductivity with an EC meter (inoLab Cond Level 2 P, WTW GmbH, Weilheim, Germany).
Metabolizable Protein Calculations
Metabolizable protein requirements (g/d for maintenance, for actual gain, and total) were estimated using the following equations (NRC, 2000):
 | [8] |
 | [9] |
 | [10] |
where MPmnt = MP required for maintenance (g/d); MPgain = MP required for gain (g/d); and MPreq = total MP requirement (g/d).
Metabolizable protein intake (g/d) was calculated as follows (NRC, 2000):
 | [11] |
 | [12] |
 | [13] |
 | [14] |
where BCP = bacterial CP synthesis (g/d), adjusted for dietary NDF; TDN intake = intake of TDN (g/d); BMP = metabolizable protein from bacterial CP (g/d); UIPMP = metabolizable protein from undegraded intake protein (g/d); and MPintake = total calculated MP intake. The NRC (2000) uses a constant of 13 g of BCP/kg of TDN to calculate BCP synthesis; however, Russell et al. (1992) reported that BCP yield decreased 2.5% for every 1% decrease in diet NDF when dietary NDF was less than 20%. Thus, based on an average calculated NDF of 11.5 for these diets, the calculated BCP yield used was 10.24 g/100 g of TDN intake (Eq. 11).
Nutrient compositions of the ingredients used in these calculations were based on a combination of NRC (2000) values and historic values of feeds used in previous trials. Values used in the calculations were as follows: corn: CP = 8.5% of DM, DIP = 43% of CP, TDN = 93% of DM, and eNDF = 4.3% of DM; alfalfa hay: CP = 16% of DM, DIP = 84% of CP, TDN = 58% of DM, and eNDF = 37.7% of DM; cottonseed meal: CP = 45.2% of DM, DIP = 57% of CP, TDN = 76% of DM, and eNDF = 10.4% of DM.
Data and Statistical Analyses
Data were analyzed as a randomized block design with pen as the experimental unit using the MIXED procedure (SAS Inst. Inc., Cary, NC). The model statement for overall performance data, carcass data, and N volatilization losses included CP treatment combination as a fixed effect, and block was included in the RANDOM statement. All treatment least squares means were compared with the continuous 11.5 and 13.0% CP treatments using the PDIFF procedure if the F-test was significant. Values were considered significant at P < 0.05.
Because the number of days fed before the diet change varied with the weight block, performance for the first 56 and 112 d on feed was used to standardize the prephase-feeding period across all weight blocks. The model statement for 56- and 112-d performance, 112-d N metabolism, and 112-d N volatilization losses included initial diet CP. The random statement included block and block x diet interaction.
Two different ANOVA were performed on data for the last 56 d on feed—one that only included the final diet being fed (10, 11.5, or 13% CP) and the second that included the 6 dietary regimens. In the first, the model statement for performance, N metabolism, and N volatilization losses during the last 56 d included the final diet CP as a fixed effect and the random statement included block and the block x final diet interaction. Dietary treatment least squares means were compared using the PDIFF procedure if a significant F-test was obtained. Values were considered significant at P < 0.05. In the second ANOVA, the model statement for performance, nutrient metabolism, and N volatilization losses included the 6 CP treatment combinations as fixed effects, and block was included in the random statement. All treatment least squares means were compared with the continuous 11.5 and 13.0% CP treatments using the PDIFF procedure.
Serum urea nitrogen data were analyzed as repeated measures in a randomized complete block design using the MIXED procedure of SAS. Fixed effects tested were dietary CP concentration, sample day, and their interactions. Block was included in the random statement, and the repeated measure was defined as the individual steer nested within block x CP treatment.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Animal performance during the first 56 and 112 d of the feeding period are presented in Table 2. During the first 56 d on feed, steers fed the 13% CP diet had greater (P < 0.10) ADG and DMI (P < 0.05) than steers fed the 11.5% CP diet; however, G:F was not affected by CP concentration. During the first 112 d on feed, steers fed the 13% CP diet had greater (P < 0.05) DMI than steers fed the 11.5% CP diet; however, ADG and G:F were not affected by CP concentration. These results tend to agree with those of Gleghorn et al. (2004), who noted a linear increase (P < 0.05) in ADG and DMI by finishing beef steers as dietary CP concentrations increased from 11.5 to 14.5% during the first 56 d on feed and a quadratic effect during the first 112 d on feed.
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Performance—Final 56 d on Feed
With steam-flaked corn-based diets, Gleghorn et al. (2004) suggested that dietary CP concentration requirements of beef cattle for maximum rate of gain were approximately 11.5% of diet DM during the later stages of feeding. However, in the current study ADG, DMI, and G:F during the last 56 d on feed increased (P < 0.05) with increasing dietary CP concentration (Table 3). Steers continuously fed the 11.5% CP diet had performance that did not differ from that of cattle continuously fed the 13.0% CP diet; however, steers switched from the 13.0% CP diet to the 11.5% CP diet had lower (P < 0.05) ADG than those continuously fed the 11.5 and 13% CP diets, and lower (P < 0.05) DMI than steers continuously fed the 13% CP diet. However, G:F did not differ for steers continuously fed the 11.5 and 13% CP diets. Switching steers from the 13 to 10% CP diet had a dramatic (P < 0.05) adverse effect on ADG, DMI, and G:F. Switching from 13.0 to 11.5% CP during the last 28 d on feed seemed to have a greater effect on performance than switching diets with 56 d remaining on feed, suggesting there is an adaptation period when the dietary CP concentration of the diet is changed. In agreement with our findings, Trenkle and Barrett (2005) reported that short-term adjustments in dietary CP concentration and source of CP resulted in decreased ADG for approximately 21 d. However, in contrast, we previously reported that changing dietary CP concentrations at 48-h intervals did not adversely affect cattle performance (Cole et al., 2003).
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Animal performance data for the entire feeding period are presented in Table 4. Overall, performance by cattle continuously fed the 11.5 and 13.0% CP diets was not significantly different. In a larger study (27 pens per main treatment) with steers fed very similar diets, Gleghorn et al. (2004) reported that steers continuously fed an 11.5% CP diet had lower ADG (3.93%; P < 0.04) and DMI (3.55%; P < 0.06) after 112 d on feed than steers continuously fed a 13.0% CP diet. Average daily gains and DMI in this trial were considerably less than values reported by Gleghorn et al. (2004; means = 1.86 and 8.7 kg, respectively) in cattle fed similar diets. With steam-flaked sorghum based-diets, Thomson et al. (1995) reported that increasing dietary CP from 10 to 13% CP increased DMI, ADG, and G:F. The optimal CP concentration in their steam-flaked sorghum-based finishing diets was determined to be between 12 and 13%, which was the point at which performance was maximized and protein wastage, as determined from plasma urea N concentrations, was minimized.
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). Decreasing the dietary CP concentration from 13.0 to 10% with 56 d remaining on feed or from 13.0 to 11.5% with 28 d remaining on feed decreased (P < 0.05) ADG and DMI. Gain:feed of steers continuously fed the 11.5% CP diet was greater (P < 0.05) and G:F of steers continuously fed the 13% CP diet tended (P < 0.10) to be greater than steers in which the dietary CP concentration was decreased from 13.0 to 10% with 56 d remaining on feed. These results tend to contrast with several studies that reported phase-feeding did not adversely affect performance by cattle fed dry-rolled corn-based diets (Erickson, et al., 1999; Cooper et al., 2000; Trenkle, 2002) or steam flaked corn-based diets (Vasconcelos et al., 2006). Starch availability or unit weight values were not obtained for the steam flaked corn in the studies of Vasconcelos et al. (2006). In the current study corn was steam flaked to a unit weight of 360 kg/m3 (28 lb/bu). Based on flake appearance, the steam-flaked corn used in the current study was more extensively processed than the corn used by Vasconcelos et al. (2006; N. A. Cole, personal observations). In addition, Vasconcelos et al. (2006) had a shorter feeding period and did not reimplant the steers in their study. Preston et al. (1975) noted variable results when phase-feeding steers a corn silage-coarsely crimped corn-based diet and attributed the variation to differences in the CP content of the corn fed and to differences in dietary mineral content caused by supplement removal. Differences in results between previous research and the current study may be attributable in part to differences in corn processing methods and implanting strategies. A proper blend of DIP and UIP is necessary to maximize animal performance (Sindt et al., 1993; NRC, 2000). Optimal DIP concentrations (% of DM) in dry-rolled corn-based diets are approximately 6.6% of diet DM (NRC, 2000; Cooper et al., 2002), whereas in steam-flaked corn-based diets, the optimal DIP concentration is approximately 8.2% (Cooper et al., 2002; Gleghorn et al., 2004) of dietary DM. This greater DIP requirement is probably due to greater ruminal fermentation of steam-flaked corn, compared with dry-rolled corn (Galyean, 1996). Gleghorn et al. (2004) reported that ADG and G:F were consistently greater for cattle supplemented with 100% urea than in cattle supplemented with cottonseed meal or a blend of urea and cottonseed meal. In contrast, Milton et al. (1997) reported that supplementation of dry-rolled corn-based diets with CP sources that had a higher percentage of UIP (soybean meal or cottonseed meal) increased ADG compared with equivalent concentrations of CP provided by urea alone: the response appeared to be greater with soybean meal than with cottonseed meal. The calculated DIP values for the experimental diets in the present trial based on NRC (2000) values were 5.08, 6.63, and 8.21% of DM for the 10, 11.5, and 13.0% CP diets, respectively. Thus, the calculated DIP concentrations of the 10 and 11.5% CP diets were below predicted DIP requirements. These calculations assume N metabolism, N recycling, and other N utilization mechanisms are relatively constant. However, cattle, ruminal microbes, or both may adapt to dietary CP concentrations via altering metabolic processes, urea recycling, or other mechanisms. Metabolizable Protein Required and Intake
Based on tabular values (NRC, 2000) the average NEm and NEg concentrations for all diets were approximately 2.14 and 1.46 Mcal/kg of DM, respectively, and the average eNDF was 8.19%. The calculated UIP content of the diets was 5.22, 5.20, and 5.55% for 10, 11.5, and 13% CP diets, respectively, which is well above the concentration reportedly needed to meet MP requirements of cattle fed high-concentrate diets based on dry-rolled corn (4.6%; Sindt et al., 1993; Shain et al., 1998) and steam-flaked corn (4.9% or less; Gleghorn et al., 2004).
Calculated MP requirements for maintenance and gain, and calculated MP intake are presented in Tables 5 and 6. During the first 112 d on feed, calculated MP requirements (738 and 758 g/d for 11.5 and 13% CP, respectively; P < 0.01) were approximately equal to the calculated MP intake (718 and 756 g/d for 11.5 and 13% CP, respectively; P < 0.01; Table 5). Metabolizable protein-allowable gains were less than ME-allowable gains, suggesting that early in the feeding period, MP, rather than energy, was the first-limiting nutrient for growth in both diets. Gleghorn (2003) reported that during the first 56 d on feed the first-limiting nutrient for gain was MP in calves fed an 11.5% CP diet, whereas energy was first-limiting for calves fed a 14.5% CP diet: for calves fed 13.0% CP diets MP-allowable and ME-allowable shrunk weight gains were similar.
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) calculated MP intakes for each diet were greater than the calculated MP requirements for observed weight gains. Metabolizable energy-allowable gains were less than MP--allowable gains, suggesting that treatment differences in performance resulted from differences in ME intake, rather than MP intake.
Over the entire feeding period (Table 6), calculated MP intake was approximately 106% of calculated MP required. Metabolizable energy-allowable and MP-allowable gain were greater than actual ADG. Thus, based on the model used, it seems that neither MP nor ME were limiting and that genetic or other environmental factors limited animal performance in this study.
Carcass Characteristics
Cattle continuously fed the 13.0% CP diet had heavier (P < 0.05) HCW (369 kg) than steers switched from 13.0 to 10% (351 kg) or switched from 13.0 to 11.5% with 28 d remaining on feed (355 kg; data not shown). The HCW of the remaining treatments were not affected by diet (overall mean = 359 ± 2.7 kg). Dietary treatment did not affect yield grade (2.9 ± 0.05), LM area (88.2 ± 0.73 cm2), 12th rib fat thickness (1.39 ± 0.03 cm), marbling score (542 ± 7.3), dressing percent (61.8 ± 0.15%), or calculated percentage of carcass fat (32.9 ± 0.21%). A greater (P < 0.05) percentage of steers continuously fed the 13.0% CP diet graded Choice than steers switched from 13.0 to 11.5% with 28 d remaining on feed (72 vs. 50%). There were no differences in percentage of Choice carcasses for the remaining treatments (overall mean = 65.3 ± 3.2%). Gleghorn et al. (2004) reported that HCW responded quadratically (P = 0.02) to increasing dietary CP concentration, with heaviest HCW observed with 13% CP compared with 11.5 and 14.5% CP; however, those differences were attributable to greater ADG by cattle fed the 13% CP diet. Erickson et al. (1999), Trenkle (2002), and Vasconcelos et al. (2006) noted no effect of phase-feeding on carcass characteristics of steers.
Serum Urea Nitrogen Concentrations
On the day that dietary CP concentrations were changed, steers fed the 13.0% CP diet had greater (P < 0.01) SUN concentrations than steers fed the 11.5% CP diet (13.5 vs. 9.62 mg/100 mL; SEM = 0.32). Similarly, the day before slaughter, SUN concentrations increased (P < 0.04) with increasing dietary CP concentrations (6.18, 8.52, and 12.60 ± 0.32 mg/100 mL for 10, 11.5, and 13.0% CP diets, respectively). In cattle continuously fed the 11.5 and 13.0% CP diets, SUN concentrations 56 d before slaughter were similar to values at slaughter. This finding contrasts with the results of other studies in which SUN concentrations of cattle fed diets with constant CP concentrations increased with days on feed (Johnson and Preston, 1995; Thompson et al., 1995; Cole et al., 2003; McBride, 2003) possibly because of a decreased rate of protein accretion as animals mature, decreased urea excretion, decreased urea recycling, or greater urea production. Nonetheless, with diets very similar to those fed in the current study, Gleghorn et al. (2004) reported that SUN concentrations were similar at 56, 84, and 112 d on feed. Discrepancies in results among studies might be attributable to differences in postprandial time of sampling or to differences in daily CP or DIP intake.
Previous research suggests that SUN concentrations greater than approximately 5 to 9 mg/100 mL indicate excessive N intake and N wastage (Johnson and Preston, 1995; Thompson et al., 1995; Cole et al., 2003). At both collection periods, SUN concentrations for cattle fed the 11.5% diet were near or above the 9 mg/100 mL threshold; however, during the first 56 d of the feeding period, ADG and DMI tended to be greater in steers fed the 13% CP diet with mean SUN values greater than 13 mg/100 mL. During the last 56 d of the feeding period, ADG and DMI were greater for steers with mean SUN concentrations of 12.6 (13% CP diet) than for steers with mean SUN of 8.5 mg/100 mL (11.5% CP diet).
Dry Matter, N, and P Metabolism and Volatilization Losses
Apparent DM digestibilities were not affected by diet CP concentration and averaged 76.7 ± 1.2% during the first 112 d on feed and 80.1 ± 1.5% during the last 56 d on feed. At 112 d on feed, apparent N digestion, N retention (calculated from ADG and equations of NRC, 2000), and fecal N excretion were not affected by dietary CP concentration (Table 7). However, as a result in part of greater (P < 0.01) N intakes, steers fed the 13.0% CP diet had greater (P < 0.02) quantity of N absorbed, greater urinary N excretion, and lower N retention as a percentage of N intake (P < 0.02), and N retention as a percentage of N absorbed (P < 0.08) than steers fed the 11.5% diet. Using similar diets, McBride (2003) measured nutrient metabolism by steers at approximately 35, 90, and 150 d on feed. At 90 d on feed McBride (2003) noted values for N digestion, N excretion, and N retention that were very similar to calculated values in the current study.
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); however, N intake, N absorbed, fecal N excretion, and urinary N excretion increased (P < 0.04) with increasing dietary CP concentration. Nitrogen retention, as a percentage of N intake (P < 0.01) and as a percentage of N absorbed (P < 0.06), decreased with increasing dietary CP concentration. These values are similar to those reported by McBride (2003) in steers on feed for 150 d.
Apparent P absorption and retention during the first 112 d and the last 56 d on feed were not affected by diet CP concentration (Table 8). Phosphorus metabolism values in this study agreed well with results of McBride (2003) and Geisert et al. (2005). Geisert et al. (2005) reported that apparent P absorption averaged about 43% of P intake, that fecal P excretion accounted for 89% of total P excretion, and that urinary P excretion averaged 2.1 g/d. At 90 d on feed, McBride (2003) reported similar values for P absorption (37% of P intake), fecal P excretion (81% of total P excretion), and urinary P excretion (2.01 g/d). At 150 d on feed, P absorption was similar (35% of P intake); however, fecal P excretion (65% of total P excretion) was lower, and urinary P excretion (3.68 g/d) was greater than at 90 d on feed (McBride, 2003). Similarly, in the current study fecal P accounted for 87% of excreted P during the first 112 d on feed and 71% of excreted P during the last 56 d on feed, the result of an apparent increase in urinary P excretion between the first 112 d and last 56 d on feed (2.08 vs. 6.04 g/d, respectively).
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). Nutrient metabolism values for d 112 were used to calculate total nutrient intakes and excretions during the first 124 d on feed and nutrient metabolism values obtained on the day of slaughter were used to calculate values for the last 56 d on feed. Steers continuously fed the 13% CP diet had greater (P < 0.01) N intakes than all other treatments; however, calculated N and P retention were not affected by dietary regimen. In general, phase-feeding decreased estimated N intake by approximately 1.4 to 3.8 kg/steer and decreased estimated N excretion by approximately 0.1 to 2.9 kg/steer. Because cottonseed meal, a source of natural protein and P, was removed from the diets, phase-feeding decreased calculated P intake by 0 to 205 g and estimated P excretion by 0 to 210 g. With steers fed dry-rolled corn-based diets Cooper et al. (2000) noted that phase-feeding (6 steps at 45-kg intervals) decreased N intake and N excretion by approximately 3 kg per steer and did not affect N retention or ADG.
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). As a percentage of N intake and as grams per day, estimated N volatilization losses of steers continuously fed the 11.5 and 13% CP diets were greater during the last 56 d on feed than during the first 112 d on feed. This finding agrees with our previous results (Cole et al., 2005) and suggests that N volatilization losses, as a percentage of N intake, increase with days on feed, most likely as a result of greater urinary N excretion. However, environmental factors may have also been involved because ambient temperatures during the early part of the study were lower than during the last 56 d on feed. Todd et al. (2005) noted that ammonia emissions from a commercial feedyard during summer were approximately twice the emissions during winter. Estimated N volatilization losses over the entire feeding period (Table 11) were affected by dietary CP regimen. Phase-feeding decreased estimated N volatilization losses by 3 to 5 kg/steer (approximately 20 to 25%). Similarly, feeding the 11.5% CP diet throughout the feeding period decreased calculated N volatilization losses by approximately 4.4 kg/steer, or 22% compared with continuous feeding of the 13.0% CP diet. Similar decreases in estimated N volatilization losses have been reported in beef (Erickson et al., 1999; Cole et al., 2005; Todd et al., 2006) and dairy cattle (Smits et al., 1995; Paul et al., 1998) when dietary protein concentrations are decreased.
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reported a 6.72-kg decrease in N volatilization losses of yearlings phase-fed during the summer but no decrease in N volatilization losses of calves phase-fed during the winter and spring. In their study N volatilization losses were 58% of N intake in the summer and 35% in the winter, values that are similar to ours. Using micrometeorology and N balance methods, Todd et al. (2005) noted similar ammonia-N losses (as a % of N intake) from a 50,000-animal commercial feedyard in Texas.
The pH of the air-dried pen manure on the day diets were switched was greater (P < 0.05) in pens fed the 13.0% CP diet than in pens fed the 11.5% CP diet (8.05 vs. 7.68 ± 0.06). At slaughter, the pH of pen manure tended (P < 0.12) to increase with increasing dietary CP concentration (8.37, 8.41, and 8.60 ± 0.05 for 10, 11.5, and 13.0%, respectively). The average pH of the air-dried pen manure was greater (P < 0.05) at slaughter than at 56 d before slaughter. These differences in pH may be related to differences in ammonia + ammonium-N concentration in the manure (Mason, 2004; N. A. Cole, unpublished data). The electrical conductivity of pen surface manure was not affected by diet but was greater (P < 0.05) 56 d before slaughter than at slaughter (10.33 vs. 6.12 ± 0.28 mS/cm). In contrast, Mason (2004) noted an increase in pen surface electrical conductivity as time on feed increased, which was evidently a result of an accumulation of electrolytes in the manure.
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2 The mention of trade or manufacturer names is made for information only and does not imply an endorsement, recommendation, or exclusion by USDA-ARS, Texas Tech Univ., or New Mexico State Univ.
4 Present address: Cactus Cattle Feeders, Amarillo, TX 79116.
5 Present address: Dep. of Anim. Sci, Univ. of Arizona, Tucson 85721.
6 Present address: VetLife, Amarillo, TX 79124.
3 Corresponding author: nacole{at}cprl.ars.usda.gov
Received for publication March 16, 2006. Accepted for publication June 28, 2006.
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