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Effects of supplemental energy and/or degradable intake protein on performance, grazing behavior, intake, digestibility, and fecal and blood indices by beef steers grazed on dormant native tallgrass prairie^1,2

Department of Animal Science, Oklahoma Agricultural Experiment Station, Stillwater 74078-6051

³ Correspondence:
Phone: 405-744-6621; fax: 405-744-7390; E-mail:
zepper{at}okstate.edu.

	Abstract

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To evaluate the effects of balancing total diet degradable intake protein with dietary total digestible nutrients (TDN), we conducted two studies during 2 yr with 100 (302 ± 8 kg initial BW) mixed-breed yearling steers and 12 ruminally cannulated steers (526 ± 28 kg). Steers individually received one of four supplements 5 d/wk while grazing dormant native tallgrass prairie. Supplements included: 1) corn and soybean meal, balanced for total diet degradable intake protein in relation to total diet TDN (CRSBM), 2) corn and soybean hulls, equal in supplemental TDN to CRSBM (CORN), 3) soybean meal, equal in supplemental degradable intake protein to CRSBM (SBM), or 4) a cottonseed hull-based control supplement (CONT). At each feeding (5 d/wk), steers consumed 13.6, 13.6, or 4.2 g of dry matter/kg of body weight, or 178 g of DM, respectively, of supplement. Steers fed CRSBM had greater (P < 0.01) average daily gain than cattle fed CORN or SBM. Feeding soybean meal (CRSBM, SBM) resulted in improved (P < 0.01) efficiency of supplement. Grazing time, intensity, and harvesting efficiency were reduced (P < 0.05) by corn supplementation (CRSBM and CORN), whereas the number of grazing bouts per day was increased (P < 0.08). Intake and digestibility of forage organic matter were reduced (P < 0.01) for steers supplemented with corn (CORN and CRSBM) vs cattle not fed corn (SBM and CONT). Total diet digestibility (P < 0.12) and digestible organic matter intake (P < 0.01) were greater for CRSBM-fed steers than for cattle fed either CORN or SBM. Steers fed CRSBM had greater (P < 0.01) fecal nitrogen and serum insulin than cattle fed CORN or SBM. Corn-fed cattle had lesser (P < 0.01) fecal pH and ADF concentrations than steers not consuming grain. Cattle fed supplements with soybean meal (CRSBM and SBM) had greater (P < 0.01) serum urea nitrogen than steers fed supplements without soybean meal (CORN, CONT). Supplemented steers grazing dormant tallgrass prairie had a greater rate of gain, with the greatest response in animal performance occurring when grain supplements were balanced for total diet degradable intake protein in relation to total diet TDN. These results lead us to suggest that grain-supplemented cattle grazing dormant tallgrass prairie require a balance of total diet degradable intake protein in relation to total diet TDN to optimize animal performance.

Key Words: Beef Cattle • Blood Analysis • Corn • Foraging • Soyabean Oilmeal • Supplement Feeding Programs

	Introduction

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Cattle grazing native tallgrass prairie during the dormant season may encounter many nutrient deficiencies. Supplementation is necessary to allow stocker cattle to gain weight during this period. Protein is typically considered the primary limiting nutrient; however, increasing forage intake with protein supplements might not result in adequate increases in energy intake for animal performance to achieve a desired rate of gain (Bowman and Sanson, 1996). Summaries of previous research have indicated that forage intake and digestibility will decrease when high-starch, low-protein grains are used as a supplement for cattle consuming low-quality (protein-deficient) forages (Horn and McCollum, 1987; Bowman and Sanson, 1996; Caton and Dhuyvetter, 1997). These decreases in forage intake and digestibility have resulted in lower than expected energy intake when corn was supplemented to cattle consuming low-quality forages (Chase and Hibberd, 1987). However, more recent research (Hibberd et al., 1987; Olson et al., 1999; Bodine et al., 2000b; 2001) has indicated that the addition of degradable intake protein (DIP) to grain supplements will increase the utilization of low-quality forages. This increased utilization will result in greater energy intake for grain plus DIP-supplemented cattle grazing low-quality forage. Therefore, our objective was to determine the effects of energy and/or protein supplementation using the current NRC (1996) model to balance DIP in relation to TDN for steers grazing low-quality forage.

	Materials and Methods

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Study Site, Vegetation, and Stocking Rate.

We conducted two experiments during 2 yr in which 100 crossbred stocker steers grazed 130 ha of native tallgrass prairie located at the Oklahoma State University Bluestem Range, 11 km southwest of Stillwater (long 36°03'N, lat 97°12'W; elevation, 331 m; Payne County). Climate at the experimental location is continental with an average frost-free growing period of 204 d, extending from April to October. Annual precipitation averages 831 mm with 65% falling as rain from May to October. The mean annual temperature is 15°C, and ranges from an average daily low of -4.3°C in January to an average high of 34°C in August (Myers, 1982;NOAA, 1999). Range sites at the study location include shallow savannah, sandy savannah, eroded sandy savannah, loamy prairie, shallow prairie, eroded prairie, shallow claypan, and eroded clay sites. The majority of slopes range from 0 to 8%, with stream drainages having steep slopes (8 to 45%). Livestock had ad libitum access to drinking water from free-flowing streams, ponds, and improved water sources.

Bulldozers mechanically cleared the pasture to reduce cover of dominant woody plant species approximately 10 yr prior to the trial, and the Oklahoma State University Research Range Fire Crew burned the pasture to control woody plant species 18 mo prior to the initiation of the trial. Woody plant species currently present at the experimental site include eastern red cedar (Juniperus virginiana L.), post oak (Quercus stellata Wangenh.), and blackjack oak (Quercus marilandica Muenchh. Herbaceous vegetation is typical of tallgrass prairie in a late seral state. Dominant forage grass species are big bluestem (Andropogon gerardii Vitman), little bluestem (Schizachyrium scoparium [Michx.] Nash), and indiangrass (Sorghastrum nutans[L.] Nash). Subdominant grass species include switchgrass (Panicum virgatum L.), tall dropseed (Sporobolus asper [Michx.] Kunth), sideoats grama (Bouteloua curtipendula [Michx.] Nash), and Scribner’s dicanthelium (Dicanthelium oligosanthes [J. A. Schultes] Gould). Forbs include lespedeza species (Lespedeza spp.) and western ragweed (Ambrosia psilostachya DC).

Stocking rates were 33.5 and 22.5 animal-unit-days (AUD)/ha for yr 1 and 2, respectively. This corresponds to traditional stocking rates of 2.5 and 2.7 ha/steer (0.4 and 0.37 steers/ha) for the winter grazing season (96 and 70 d) for yr 1 (52 steers) and 2 (48 steers), respectively. We calculated animal units as BW kg^0.75 divided by 454^0.75 (Vallentine, 1990), using the initial shrunk BW of the steers in each experiment. In the second year, four fewer steers grazed the pasture, resulting in a decreased stocking density. In addition to this reduction in stocking density, the length of the experimental grazing period was shortened by 26 d because of earlier than expected spring growth of forage grasses in yr 2. The combination of fewer steers and a reduced number of days on study resulted in the reduced stocking rate in the second year.

Animals.

All experimental protocols were approved by the Oklahoma State University Animal Care and Use Committee. In both years, fall-born calves from the same two herds grazed 65 ha of native tallgrass prairie during the late summer, and we trained the steers to enter individual feeding stalls by offering each steer 0.9 kg of a 20% CP cubed supplement two times/wk for a period of 4 wk prior to the initiation of the trials. Steers were not implanted prior to weaning (verified by palpation of ears), nor were they implanted while on trial during either year. We weighed steers approximately 3 wk prior to the initiation of each study (d -21), at trial initiation (d 1), at the completion of each trial (~d 83), and approximately 1 mo (~d 116) after the completion of the studies. These weights were taken following an overnight (14 h) removal of access to feed and water and used to calculate weight gain and rate of gain to determine animal performance responses to treatments. We weighed cattle without removal of access to feed and water 1 d prior to the initiation of the trial (d -1), adjusted the weights for a 4% mathematical shrink, and used these shrunk weights to calculate preliminarily supplement intake (g/kg of BW) and to randomly allot cattle to treatments. On d 1, steers were ear tagged, randomly allotted to treatment, treated for parasites according to label directions (Ivomec, Merial Limited, London, U.K.), and allowed to graze for 4 h prior to the feeding (start of adaptation) of supplements. We also weighed steers without removal of access to feed and water at approximately 28 and 56 d of each study to recalculate supplement intake (g/kg of BW; using a 4% mathematical shrink) to adjust for increased BW as a result of gain. In both years, we weighed cattle approximately 3 wk (~d -21) prior to the initiation of the study to determine a pretrial rate of gain. Steers grazed together as a group prior to trial initiation and no differences were noted between the subsequent treatment groups prior to trial initiation. In an attempt to determine the effects of our supplementation treatments on subsequent animal performance, we weighed steers approximately 1 mo (~33 d) after the completion of each trial to determine post-trial animal performance. During this period, steers grazed 25 ha of standing dormant and early-spring growth of Old World bluestem (Bothriochola ischaemum) and were fed 1.2 kg of DM/(steer•d) of a 34% CP (as-is) range cube prorated for feeding four times weekly. Twelve ruminally cannulated (10 cm i.d.) steers (526 ± 28 kg; Angus and Angus x Hereford; 3 to 5 yr old) were used for masticate sample collection and to validate Cr recovery from total fecal collection using fecal collection bags. Cannulates were randomly allotted to supplement treatments, grazed the experimental site along with the intact steers, and were included in the calculations of stocking rate expressed as AUD/ha.

Experimental Diets and Feeding.

Treatments (Table 1) consisted of: 1) 7.5 g of dry-rolled corn DM/(kg of BW•d) and added an adequate amount of soybean meal to balance total diet DIP requirements (CRSBM), 2) 7.5 g of dry-rolled corn DM/(kg of BW•d), and added soybean hulls to achieve an equal amount of supplemental TDN, g/(kg of BW•d) as CRSBM (CORN), 3) soybean meal to supply an equal amount of supplemental DIP g/(kg of BW•d) to CRSBM (SBM), or 4) a cottonseed hull-based control supplement (CONT). This level of supplemental corn feeding is based on our previous metabolism studies (Bodine et al., 2000b; 2001) and is similar to quantities of corn fed by Chase and Hibberd (1987) and Sanson and Clanton (1989), and provides 41 g of supplemental DM/kg of BW^0.75, which is above the level of 30 g of DM/kg of BW^0.75 suggested by Horn and McCollum (1987) to depress forage intake. Requirements for DIP were determined using the NRC (1996) level 1 model software with estimated forage intake [(18 g of DM/(kg of BW•d)] and estimated forage chemical composition: 93% OM; 7.4% CP; DIP 70% of CP; 75% NDF, and 60% TDN (estimated from in vitro organic matter disappearance) from historical masticate samples (1993 to 1998) previously collected from the experimental pasture (Basurto et al., 2000). Other model inputs included measured steer BW and supplemental corn intake, and assumed an average value of 10.25% microbial protein yield from TDN. This value of microbial protein yield from TDN agrees with data we collected previously (our unpublished results) and is at the upper end of the range described in the text of the NRC (1996) for low-quality forage diets with 50 to 60% digestibility, and is similar to the value suggested by Cochran et al. (1998). The decision to use a value at the upper end was made in order to ensure that DIP requirements were met. We added soybean meal to the CRSBM diet until DIP requirements were met and added pelleted soybean hulls (SBH) to the CORN supplement to achieve equal TDN intake g/(kg of BW•d) to CRSBM. Steers consumed supplements in quantities based on the mean BW measured during the previous experimental period of all noncannulated steers on CRSBM, CORN, and SBM; cannulated steers received those supplements based on the mean initial BW of all cannulates on each treatment. The CONT supplement was fed to all steers on that treatment at 128 g of DM/d. Steers individually received their supplements 5 d/wk in individual stalls at 0800. We calculated daily supplement intake (g of DM/kg of BW), multiplied it by seven to determine weekly intake, and divided weekly intake by five to determine the quantity offered at each feeding. This resulted in quantities of supplement (CRSBM, CORN, SBM, and CONT) fed of 4.4, 4.4, 1.4, and 0.178 kg of DM per feeding, respectively. Cattle were adapted to supplements during the first 6 d of the trial. Adaptation was accomplished by feeding the total quantity of CONT and the total quantities of soybean meal and soybean hulls, along with 50% of the corn, until most steers consumed all supplement fed. Steers received increasing quantities (454 g/d) of corn until the target quantity of corn intake was achieved. Beginning on approximately d 50, we top-dressed supplements with 100 g of a 7.5% chromic oxide, 92.5% dried molasses supplement [7.5 g of chromic oxide/(steer•d)], and continued top-dressing supplements for the 5 d prior to (~d 50 to 54) and during the 5-d fecal collection period (~d 55 to 59). During this period, steers consumed all supplement and feeders were clean at the end of the 1-h feeding period. Steers had ad libitum access to covered mineral feeders containing a trace-mineralized salt mix while on trial both years.

We estimated forage mass from 30 clipped 0.1-m² quadrats taken at the initiation and completion of the trial, and at approximately d 28 and 56 of each trial. Clipped samples were dried in a forced-air oven at 55°C for 48 h and weighed. We collected masticate samples as described by Lesperance et al. (1960) from two unsupplemented, cannulated steers at the initiation and completion of the trial, and from the supplemented, cannulated steers on approximately d 28 and 60 of each trial. During each trial, feed ingredients were sampled weekly, and because all feeds in each year were from a single source, they were composited within each year at the completion of the trial. Ingredients were ground to pass a 2-mm screen in a Wiley mill (Thomas Scientific, Philadelphia, PA) for determination of DM, OM, NDF, ADF, acid detergent insoluble ash (ADIA), starch, CP, and DIP.

We estimated grazing time during the 5-d fecal collection periods that occurred starting at about d 55 of each year by using 12 grazing collars with vibracorders. Three steers per treatment wore grazing collars 1 d prior to (~d 54) the initiation of grazing time measurements and for five consecutive days (~d 55 to 59), during which time supplements were fed.

We collected feces at 0800 via rectal grab samples 1 d prior to the initiation of each study to provide a pretreatment baseline for fecal indices, and again 1 mo after the completion of the studies, to provide a post-treatment measurement of fecal indices. Fecal samples were also collected at about d 28, and for five consecutive days starting at about d 55 of each trial from each steer to evaluate the responses of fecal indices to our treatments. We placed fecal samples on ice, transported them to the lab, and directly determined fecal pH using a portable combination electrode pH meter (Corning 314i pH/mV/temperature portable pH meter with an ion-selective field effect transistor electrode, Corning, NY). Fecal samples were dried (55°C, 72 h), ground to pass a 2-mm screen in a Wiley mill (Thomas Scientific), and stored for later analyses. Because fecal output was estimated from Cr concentration in the feces, we performed total fecal collection on the cannulated steers in an attempt to measure Cr recovery to validate fecal output estimates obtained from Cr concentration. Recovery of Cr was calculated as grams of Cr recovered in feces per day divided by grams of Cr fed per day multiplied by 100. Cannulated steers wore fecal collection bags that were changed twice daily (0800/1700), starting at about d 55 of each trial for 5 d (d 55 to 59), during which time supplements were fed. Contents of the fecal bags were weighed, mixed thoroughly, and subsampled. The subsamples were weighed, dried (55°C, 72 h), reweighed, ground (2-mm screen), and reserved for later analyses.

We collected blood samples at 0800 1 d prior to the initiation and 1 mo after the completion of the studies to provide pre- and post-trial measures of serum urea nitrogen and serum insulin. Blood samples were also collected within 1 h of feeding on the fifth day of five consecutive days of feeding supplements at approximately d 28 and 59 of both years to measure responses to treatment. All blood samples were collected via tail venipuncture, placed ice, and transported to the lab where they were stored at 4°C overnight (~20 h) prior to centrifugation, at which time we harvested serum and stored it frozen (-20°C) for further analyses.

Laboratory Analyses.

Dry matter and ash content were determined by oven drying at 105°C for 24 h, followed by ashing at 500°C for 6 h in a muffle furnace. A combustion method (Leco NS2000, St. Joseph, MI) was used in accordance with AOAC (1996) to determine N content. Degradable intake protein concentrations of masticate and supplement samples were estimated from an enzymatic in vitro degradation technique (Roe et al., 1991). Supplement and masticate sample NDF (procedure A, without sodium sulfite) and supplement, masticate, and fecal sample ADF and ADIA concentrations were determined as described by Van Soest et al. (1991). We used ADIA as an internal marker to estimate forage and total-diet OM digestibility (Van Soest et al., 1991; Van Soest, 1994). Starch content of feeds was estimated enzymatically from -linked glucose by a colorimetric procedure (Galyean, 1997). We ashed and digested composite fecal samples in a solution of phosphoric acid, manganese sulfate, and potassium bromate with heat, according to the procedure outlined by Williams et al. (1962), and quantified Cr concentration in fecal composites using inductively coupled argon plasma optical emission spectroscopy (SpectroFlame, Spectro Analytical Instruments Inc., Fitchburg, MA). Serum urea N was determined using an enzymatic (urease/Berthelot) technique (Sigma Procedure No. 640, Sigma Diagnostics, St. Louis, MO) that uses phenol-hypochlorite as the colorimeteric agent. Serum insulin was determined using RIA kits (Coat-A-Count Insulin, DPC, Los Angeles, CA) using bovine insulin for standards. A 48-h in vitro procedure similar to the method of Goering and Van Soest (1970) was used to determine in vitro OM disappearance. Masticate samples (0.5 g) were incubated in buffered (casein added as a N source) ruminal fluid (4:1) for 48 h. Samples were frozen immediately following the 48-h incubation to stop microbial activity. Samples were thawed and an NDF extraction was performed on the residue, which was then dried and ashed.

Calculations.

To improve brevity and clarity, the calculations performed in this study are listed in Table 2. Details regarding these calculations and the basis for their use can be found in Owens and Goetsch (1993), Van Soest (1994), and Galyean (1997). Many of the values reported in this study are based on markers and the associated assumptions that accompany marker-derived data. We attempted to validate Cr recovery under experimental conditions and observed 73% recovery. In addition, we used ADIA as an indigestible internal marker for forage digestibility. The use of 100 minus supplemental TDN value to calculate indigestible supplement is supported by previous work that has shown little or no effect of supplemental treatment on starch digestibility (Vanzant et al., 1990; Chan, 1992; Bodine et al., 2000a,b). However, changes in the digestibility of the supplement would alter estimates of forage digestibility, especially given the quantities of supplement fed in this study. Estimates of forage intake are dependent on estimates of fecal output from Cr and forage digestibility from ADIA. Because steers were fed for 10 consecutive days (5 d of adaptation to Cr plus 5 d of Cr feeding and fecal sampling), the observed intake and digestibility values are reflective of the quantity of supplement fed per feeding. Therefore, fecal output and ADIA values were adjusted based on this quantity of supplement. However, it might be expected that given 2 d without supplementation, forage intake, digestibility, fecal measures, and grazing behavior would be slightly different from the values we observed. Based on the assumptions made, some caution should be taken when interpreting absolute values; however, relative differences between treatments are still valid.

Because this trial was based on the NRC (1996) level 1 model using the metabolizable protein (MP) system, we evaluated how well the model predicted animal performance, intake, digestibility, ruminal pH, and DIP and MP balances. The model was evaluated using observed environmental measures with the grazing option turned off, steer age, type, condition, and breed, mean steer BW, no feed additives or implants, observed forage chemical composition, intake, digestibility, and supplement amount and chemical composition.

Experimental Design and Statistical Analyses.

Experimental design for both years was a completely randomized design. We included year, supplemental dietary treatment, and their interaction in the model as fixed effects (Littell et al., 1996). Because steers individually received supplements and they grazed a common pasture, individual steer was considered the experimental unit (Adams et al., 2000). We analyzed all response variables using PROC MIXED (SAS Inst., Inc., Cary, NC), calculated means using least squares means (LSMEANS option), and separated the means using least significant differences methods only when the overall F-value <0.05. Observed significance levels were adjusted with the Tukey procedure to account for the number of comparisons made. Interactions among years and treatments did not occur (P > 0.24) for any of the response variables. The order of ranking of treatments was similar for all variables among years, and as a result, data were pooled across years and mean responses for all variables are reported.

	Results and Discussion

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Forage Mass, Forage Allowance, and Diet Quality.

Forage mass and allowance (Table 3) were similar among years and were relatively similar among times, and never seemed to be a limiting factor for animal performance. However, diet quality decreased as time progressed. Based on chemical composition of masticate samples, forage quality was such that response to supplementation would be expected by steers (McCollum and Horn, 1990). Chemical composition of masticate samples collected from cannulated steers on different supplementation treatments did not differ (P > 0.62). Because of these similarities, diet quality was pooled within time period across treatments and years, and means are reported by time periods.

Cattle had similar (P > 0.88) ADG (Table 4) for the 3 wk before the initiation of the trials [0.11 kg/(steer•d)], as well as similar (P > 0.49) initial BW at the start of the experiments. Steers fed CRSBM had greater (P < 0.01) ADG and final BW (P < 0.10) than either CORN- or SBM-fed cattle. All treatments gained more (P < 0.01) weight per day and had heavier (P < 0.01) BW at trial completion than the CONT cattle. Cattle responded to both energy and protein, as demonstrated by increased animal performance resulting from the addition of either corn or soybean meal to the diets. However, the greatest response in animal performance occurred when soybean meal was fed with corn to adequately balance DIP for total diet TDN. Steers fed three times the supplemental TDN with half the protein (CORN vs SBM) and one-fifth the DIP did not have different ADG (P < 0.16). Cattle fed equal supplemental TDN with twice the protein (CRSBM vs CORN) had three times the ADG. When steers were fed similar supplemental protein (CRSBM vs SBM) with half the DIP but three times the TDN, they had twice the ADG. Grain supplements fed with DIP have previously improved animal performance of forage-fed beef cattle compared with those not fed added protein (DelCurto et al., 1990; Garcés-Yépez et al., 1997; Bodine et al., 2001). Feeding either energy (Horn and McCollum, 1987; Bowman and Sanson, 1996; Caton and Dhuyvetter, 1997) or protein (McCollum and Horn, 1990; Owens et al., 1991; Moore et al., 1999) supplements to cattle consuming low-quality forages in our study resulted in similar observations of improved animal performance, as many other researchers have noted. However, the combination of energy and protein in a single supplement resulted in the greatest response in animal performance, which also agrees with previous findings (Sanson et al., 1990; Beaty et al., 1994; Heldt et al. 1998).

Supplement Efficiency.

Added gain (gain greater than that observed for the CONT-fed steers) per unit of added supplement (supplement fed greater than the CONT steers were fed) was greatest (P < 0.01) for SBM, least (P < 0.01) for CORN, and intermediate (P < 0.01) for CRSBM (Table 4). If these efficiencies are transformed into supplement conversions, SBM, CRSBM, and CORN treatments converted 1.5, 3.5, and 7.2 kg of added supplement into a kilogram of added gain. These values are similar to those suggested in a review paper by McCollum and Horn (1990) for either protein- or energy-supplemented grazing livestock. Supplement conversions of less than 3:1 are symptomatic of a N deficiency (McCollum and Horn, 1990), and indicate that a response greater than could be attributed to the energy supplied by the supplement alone (positive associative effect) was occurring, very similar to our results for the SBM-fed cattle. They also suggest that conversions of 8:1 or greater were typical of energy supplementation and might be a result of substitution or inefficient utilization of the supplemental nutrients (negative associative effect), which agrees with our observations of the CORN-supplemented steers. However, when we fed grazing steers CRSBM, we observed an improvement in supplement conversion compared with CORN-fed cattle. The steers converted the CRSBM supplement into added gain at a rate that was at the high end of typical protein supplements and at the low end of typical energy supplements, indicating that N was not deficient in relation to the supplemental energy (no associative effects, only an additive effect). The responses in ADG and supplement conversions are indicative of a situation where animals are deficient in both protein and energy, and the greatest response occurs when both deficiencies are addressed in the supplement.

Grazing Behavior.

Steers fed supplements with corn (CRSBM and CORN) had reduced (P < 0.01) grazing time and intensity (Table 4) and an increased (P < 0.08) number of grazing bouts vs cattle not receiving supplemental grain (SBM and CONT). Cattle fed corn (CRSBM and CORN) were similar (P > 0.35) in grazing time, bouts, and intensity, and steers that did not receive grain (SBM and CONT) were similar (P < 0.26). This agrees with Krysl and Hess (1993), who reported that supplementation seemed to decrease grazing time and intensity while increasing grazing bouts. It is possible, as suggested by Adams (1985), that feeding large quantities of an energy supplement in the middle of the morning grazing period also contributed to the reduced grazing time of corn-fed (CRSBM and CORN) steers. Time spent foraging was reduced by the quantity of grain supplementation fed in our study, which might result in decreased energy expenditure from grazing, as suggested by Caton and Dhuyvetter (1997). However, decreases in grazing time did not always result in improved ADG, since CORN-fed cattle grazed less time (possibly reducing energy cost) and had lower gains than SBM-fed steers. Grain supplementation decreased (P < 0.05) harvesting efficiency vs steers fed SBM, which does not agree with the general conclusion drawn by Krysl and Hess (1993), who suggested supplementation had little effect on harvesting efficiency. We believe that the decrease in harvesting efficiency in our study is because the large quantities of corn supplementation fed in our study decreased forage OM intake. Harvesting efficiency of steers fed supplements with grain (CRSM and CORN) were similar (P > 0.71), CRSBM and CONT treatments were similar (P > 0.21), and supplements without grain (SBM and CONT) were similar (P > 0.76). The addition of soybean meal (CRSBM vs CORN, SBM vs CONT) numerically increased harvesting efficiency in our study, which agrees with previous reports of increased harvesting efficiency from protein supplementation (Barton et al., 1992; Krysl and Hess, 1993).

Forage Intake.

Cattle fed corn (CRSBM and CORN) had reduced (P < 0.01) forage intake (Table 4) vs those not supplemented with corn (SBM and CONT). The decrease in forage OM intake is supported by the decreases in grazing time, intensity, and harvest efficiency that were observed due to feeding these relatively high levels of corn-based supplements. The decreased forage OM intake as a result of corn supplementation is in agreement with the findings of Sanson and Clanton (1989), Sanson et al. (1990), and Garcés-Yépez et al. (1997), whose corn supplements decreased intake of low-quality forages. Forage OM intake for CRSBM-fed cattle was not different (P > 0.22) vs CORN supplemented steers, which does not agree with our previous findings that adding soybean meal to grain supplements will increase forage intake of low-quality prairie hay (Bodine, et al., 2000b; 2001). However, CRSBM supplemented steers did have numerically greater forage intake and the lack of differences may be a result of the variation in the data due to the use of markers. We believe that the lack of a protein response by CRSBM- vs CORN-fed cattle in our study might be a result of the relatively high quantity of supplemental TDN fed (Moore et al., 1999), supplementation-caused alterations in grazing behavior (Krysl and Hess, 1993), the opportunity for diet selection by grazing cattle (Vavra and Ganskopp, 1998), the high variation associated with marker-derived data, or some combination of these factors. Protein supplementation (CRSBM and SBM) did not increase (P > 0.50) forage OM intake vs CORN- and CONT-fed cattle. Previous work (McCollum and Horn, 1990; Owens et al., 1991; Bodine et al., 2000b) has suggested that a response would be expected from a forage with this ratio of digestible OM to crude protein (Hogan, 1981; McCollum and Horn, 1990; Moore and Kunkle, 1995) or DIP as a percentage of TDN (Cochran et al., 1998). However, numerical increases were observed in forage intake for SBM vs CONT treatments, which is in agreement with previous work. The fact that added soybean meal did not increase forage intake (CRSBM vs CORN, or SBM vs CONT) might be as a result of diet selection (Vavra and Ganskopp, 1998), grazing behavior (Krysl and Hess, 1993), energy expenditure of foraging (Krysl and Hess, 1993; Caton and Dhuyvetter, 1997), environmental exposure (Fox et al., 1988), the added variation caused by markers, or some combination of these factors. An additional explanation for the lack of differences may be a result of the method that was used to collect the data. Forage intake may have been further depressed by the feeding of this amount of corn for the 10 consecutive days required to obtain the necessary fecal samples. Because of the use of markers, their associated assumptions, and the methods used to collect the data, some caution should be used in the interpretation of these intake values as absolute.

Forage Digestibility.

Forage OM digestibility (Table 4) was decreased (P < 0.01) for steers fed corn-based supplements (CRSBM and CORN) vs steers not fed supplemental grain (SBM and CONT), with the least value for steers fed CORN. This agrees with our findings of reduced grazing time, intensity, harvesting efficiency, and forage OM intake as a result of supplementing with large quantities of corn. Similar decreases in digestibility of low-quality forages have been reported when corn has been used as a supplement (Chase and Hibberd, 1987; Sanson et al., 1990). However, steers that were supplemented with corn and adequate DIP (CRSBM) had greater (P < 0.01) forage OM digestibility than cattle fed supplemental grain that were deficient in DIP (CORN). Other researchers (Hibberd et al., 1987; Olson et al., 1999; Bodine et al., 2000b) have noted a similar increase in forage digestibility when DIP has been added to grain supplements. Yet the increased forage OM digestibility for CRSBM- vs CORN-fed steers in our current study did not result in a statistically significant increase in forage OM intake for cattle supplemented with CRSBM, which is what we would have expected based on our previous research (Bodine et al., 2000b; 2001). The increased forage OM digestibility among CRSBM- vs CORN-fed steers might help to partially explain the greater ADG of CRSBM-steers. Similarities in forage OM digestibility for CONT- and SBM-fed steers agree with previous research that has reported no effect of protein supplementation on the extent of forage digestibility (Owens et al., 1991; Bodine et al., 2000b; 2001). However, although similarities in digestibility are typically accompanied by increases in passage rate and intake, no statistically significant increase in intake occurred in our study. This may be reflective of the differences in grazing vs pen-fed animals, or as a result of variation associated with marker-derived data. The observed value of 14% forage OM digestibility by steers on the CORN treatment is lower than would be expected. However, it is similar to the value of 18% reported by Chase and Hibberd (1987), and a forage OM digestibility of 21% is required for the NRC (1996) model to predict an ADG similar to our observations of ADG by CORN-fed cattle. An additional explanation for the extremely low values observed by CORN- and CRSBM-fed steers may be a result of the method that was used to collect the data. Forage digestibility may have been further depressed by feeding this relatively large amount of corn for 10 consecutive days. The usage of markers in this study also requires caution in the interpretation of digestibility values as absolute.

Total Diet OM Intake and Digestibility, and Digestible OM Intake.

Feeding corn with added protein (CRSBM) tended to increase (P > 0.17) total-diet OM intake on a per-day basis (Table 4). Feeding CORN or SBM supplements resulted in similar (P > 0.99) total-diet OM intakes, whereas steers fed greater supplemental TDN but similar DIP (CRSBM) tended to have greater (P < 0.09) total OM intake than SBM-fed cattle. Steers fed CORN and SBM had similar (P > 0.33) total OM digestibility and digestible OM intake. This similarity can be explained by the differences in supplemental feeding amount, forage intake, and digestibility. Cattle fed CRSBM tended (P < 0.12) to have greater total-diet OM digestibility and had greater (P < 0.01) intake of digestible OM than SBM-fed treatments. This was a result of the greater quantity of highly digestible supplement fed, even though forage intake and digestibility were greater for cattle fed SBM. Steers fed CRSBM had greater (P < 0.01) total-diet OM digestibility and digestible OM intake (P < 0.01) than CORN-fed cattle. These increases were a result of improved forage digestibility by CRSBM-fed steers vs CORN-supplemented cattle as a result of the addition of DIP to a diet deficient in ruminally degraded protein and numerically greater forage intake, because supplement intakes (DM and TDN) were similar. The increased forage OM digestibility as a result of greater protein intake for CRSBM- vs CORN-fed cattle aids in partially explaining the greater ADG of CRSBM-supplemented steers. Forage and total OM intake and grazing time were similar between these two treatments; however, forage and total-diet OM digestibility, and digestible OM intake were greater for cattle fed CRSBM supplements. When steers fed adequate quantities of DIP (SBM) were given additional energy in the form of corn (CRSBM), total-diet OM intake and digestible OM intake were increased. These increases in intake and digestibility agree with the observed increases in ADG by steers fed CRSBM vs SBM-supplemented cattle in our study. These results support the assertion of Chase and Hibberd (1987), who observed that supplemental corn resulted in decreased total-diet OM intake, digestibility, and digestible OM intake when compared with protein-supplemented cattle, that their findings were the result of a deficiency of DIP. No differences were detected (P > 0.21) in ADF digestibility.

The addition of supplemental feeds improved the total-diet digestible OM:CP ratio and DIP as a percentage of TDN ratios. The greatest improvement was when soybean meal was added without grain. Adding soybean meal to grain supplements resulted in the total-diet digestible OM:CP ratio being lesser than the value that Moore and Kunkle (1995) suggested as a threshold for protein adequacy in relation to energy, which agrees with our findings. Even though DIP as a percentage of TDN was not adequate according to Cochran et al. (1998), it was improved vs CORN-supplemented steers, which also aids in explaining our findings. These results emphasize the importance of adequate DIP when grain supplements are fed with low-quality forages. When DIP is not balanced for total-diet TDN of grain supplemented steers, animal performance will be reduced because the basal forage does not supply as much energy as would be expected as a result of decreased forage digestibility.

Evaluation of Experimental Data Using the NRC (1996) Model.

Using the previously listed model inputs, we evaluated the NRC (1996) model 1 predictions of ADG, intake, digestibility, ruminal pH, and DIP and MP balances (Table 5). Observed daily gains (kg/d) for CRSBM, CORN, SBM, and CONT treatments were 0.73, 0.24, 0.39, and -0.17. Predicted ME allowable daily gains were 1.15, 0, 1.34, and 0.58, and MP allowable daily gains were 1.37, not estimable (ERR), 0.95, and 0.18, respectively. Total intakes (kg/d) observed for CRSBM, CORN, SBM, and CONT treatments were 7.96, 6.99, 7.29, and 5.55, and predicted total intakes were 7.49, 5.39, 7.45, and 6.99. Because supplement intake was fixed, observed forage intakes were calculated as a percentage of total intakes, and predicted forage intakes were calculated by multiplying that percentage by predicted total intakes. Observed forage intakes (kg/d) for CRSBM, CORN, SBM, and CONT treatments were 4.76, 3.94, 6.33, and 5.42, and predicted forage intakes were 4.48, 3.04, 6.47, and 6.83. It appears that when supplements contain added DIP, predictions of total and forage intake are similar to the observed values. However, when CORN and CONT supplements were evaluated, predictions of intake did not agree with observed values.

Observed ruminal pH values for CRSBM, CORN, SBM, and CONT were 6.11, 6.12, 6.35, and 6.44; predicted values were 6.25, 6.21, 6.46, and 6.46, respectively, indicating good agreement between observed and predicted values.

Using the observed data, the NRC (1996) evaluated the DIP balances for CRSBM, CORN, SBM, and CONT at -113, -114, 56, and -155 g/d, respectively. Metabolizable protein balances were 65, 172, 116, and -122 g/d for CRSBM, CORN, SBM, and CONT, respectively. Degradable intake and MP balances were highly subject to the low DIP concentrations measured using the enzymatic in vitro procedure for CRSBM (36% of CP), CORN (30% of CP), and masticate samples (51% of CP), and the extremely high value for SBM (86% of CP). Using more realistic estimates of DIP concentration in corn (50% of CP), soybean hull pellets (67% of CP), and soybean meal (50 to 65% of CP), the CRSBM supplement would have been expected to supply 50 to 60% of the CP as DIP, and the CORN supplement 54%. Using these values, the NRC (1996) predicts DIP balances of -11, -41, and 19 g/d for CRSBM, CORN, and SBM, respectively. These balances would change based on the forage DIP and digestibility values used, but appear to be much more in line with what was observed in the experiment than the values derived using the enzymatic method of DIP determination.

Using the predicted forage TDN values, the forage consumed by steers on CRSBM, CORN, SBM, and CONT supplied 22, 0, 95, and 96.5%, respectively, of the NE_m requirement of these cattle, and 13.2, 0, 69, and 96.5% of the total NE_m intake.

Fecal Output and Indices.

Fecal OM output (Table 6) was greater (P < 0.02) for diets with corn (CRSBM and CORN) than for diets without corn (SBM and CONT). Feeding corn (CRSBM and CORN) reduced (P < 0.01) fecal ADF output and concentration compared with supplements without corn (SBM and CONT). Increased fecal OM output, reduced ADF output, and reduced ADF concentration by corn-supplemented steers (CRSBM and CORN) might be related to decreased digestibility of forage, lesser forage intake, increased total OM intake, greater digestibility of the supplements than the basal forage, or some combination of these factors.

Fecal N (Table 6) was similar (P > 0.64) among steers on all dietary treatments 1 d prior to and 1 mo after the completion of the study. Steers fed supplements had increased (P < 0.01) fecal N concentration after treatment initiation than were observed prior to the initiation of the studies because of their greater N intake during the studies. This agrees with the observations of Wofford et al. (1985), who reported a strong relationship between fecal N and dietary N intake. However, the majority of previous research conducted has correlated fecal N with forage quality (Raymond, 1948; Ward et al., 1982; Lyons and Stuth, 1992) or animal performance (Erasmus et al., 1978; Holechek et al., 1982; McCollum, 1990), and not with supplemental feeding. Little research exists on the effect that supplementation of grazing animals would have on correlations among plane of nutrition and fecal indices (McCollum, 1990; Lyons et al., 1993). The use of fecal N to predict dietary N intake and animal performance might be more complicated in supplemented animals than in livestock not offered supplemental feed. Steers fed CRSBM had the greatest increase (P < 0.01) in fecal N after the initiation of the experiments, which is also related to increased N intake, similar to the relation between dietary N intake and fecal N reported by Holechek et al. (1982). While on study, steers supplemented with corn and DIP (CRSBM) had greater (P < 0.01) fecal N than steers supplemented with similar quantities of either TDN (CORN) or protein (SBM) alone. Feeding SBM resulted in similar (P > 0.90) fecal N than steers supplemented with CORN, even though N intake was considerably different. Along with the decreased fecal pH and digestibility, the increased fecal N in steers supplemented with corn (CRSBM, CORN) appears to be related to increased large intestinal fermentation. The range of fecal N values in our study agrees with ranges reported by Holechek et al. (1982), Wofford et al. (1985), and McCollum (1990). Fecal N did not accurately describe treatment ranking of N intake, animal performance, or serum urea N. Leite and Stuth (1990) also reported that no single fecal measure could be highly correlated with observed animal dietary variables.

Blood Indices.

Concentrations of serum urea N (Table 6) were similar (P > 0.49) among all treatments 1 d before and 33 d after studies ended, indicating similar N and plane of nutrition status of experimental animals when dietary treatments were similar, which would be expected according to Hammond (1996). In comparison with pretreatment values, serum urea N increased (P < 0.01) after the initiation of the study for all treatments, with SBM-fed steers having the greatest increase (P < 0.01), followed by steers fed CRSBM supplements, and steers receiving CORN tending (P < 0.12) to have a greater increase in serum urea N than CONT cattle. During the trials, steers fed SBM supplements had the greatest (P < 0.01) serum urea N, CRSBM-fed steers had the second greatest, and CORN-supplemented cattle were greater (P < 0.05) than CONT steers. Feeding soybean meal has previously been found to increase serum urea N (Barton et al., 1992; Marston et al., 1995). Feeding similar amounts of DIP with different quantities of energy (CRSBM vs SBM) resulted in lower serum urea N, similar to the results observed by Chase et al. (1993). Serum urea N concentrations of cattle fed low DIP supplements (CORN and CONT) were below quantities (7 mg/dL) of blood urea N suggested to respond positively to protein supplementation by Hammond (1996). The serum urea N concentrations of steers with the greatest rates of gain (CRSBM, SBM) were similar to serum urea N values previously reported for optimal gain by growing steers (Byers and Moxon, 1980). After trial completion, serum urea N concentrations of CONT steers did not change (P > 0.56), whereas serum urea N of CORN-fed steers increased (P < 0.01), and steers that had consumed soybean meal (CRSBM and SBM) showed a decrease (P < 0.01). The reduction in serum urea N of protein-supplemented steers was a result of a decreased N intake when experimental supplements were no longer fed. A possible explanation for the increase in serum urea N of CORN-fed steers after the cessation of energy supplementation is a decreased demand for N necessary for ruminal fermentation, which is supported by the findings of Chase et al. (1993), or the feeding of a protein supplement, or the combination of the two.

Even though fecal indices are extremely easy to collect in a production setting, the current supplementation strategy in use will impact the fecal N concentrations of cattle and might result in erroneous conclusions if used to predict N status of grazing animals. This has been suggested previously (Lyons et al., 1993) and is supported by the lack of a relationship between fecal N and ADG in our study. Cattle that were fed CORN supplements had greater fecal N but lesser serum urea N and N intake. Particular caution should be taken when using fecal N concentrations as an indicator of N status for cattle grazing low-quality forages and receiving energy-based supplements without added DIP. Even though blood urea N is more difficult to collect, it seems to be a better measure of N status across a variety of supplement types for grazing cattle.

Serum insulin concentrations (Table 6) were similar (P > 0.55) among all treatments 1 d prior to the initiation, and one month after the completion of trials. After the initiation of the studies, CORN- and CONT-fed steers had similar (P > 0.84) serum insulin concentrations, which were similar (P > 0.21) to pretrial values. However, serum insulin increased (P < 0.01) for both CRSBM- and SBM-supplemented steers compared with pretrial concentrations. This agrees with previous work reporting insulin responses to increased energy or protein intake (Barton et al., 1992; Marston et al., 1995). While on study, steers fed corn plus soybean meal (CRSBM) had greater (P < 0.01) serum insulin than was observed in steers fed either similar supplemental TDN (CORN) or DIP (SBM). Supplemental energy (CORN) or protein (SBM) had similar (P > 0.18) serum insulin values while on trial. Steers fed supplements (CRSBM, CORN, SBM) had increased (P < 0.01) serum insulin vs animals not offered supplement (CONT). These results are in agreement with Wettemann et al. (1987) and Yelich et al. (1995; 1996), who reported that nutrient intake influenced insulin concentrations. The increased insulin concentrations for CRSBM- vs CORN-supplemented cattle might have been a result of greater digestible OM and N intake, greater total-diet digestibility (as a result of the added DIP) resulting in greater VFA production, greater protein flow to the small intestine, or a combination of these factors (Trenkle, 1978; Harmon, 1992). The increased energy and protein intake by CRSBM-fed steers would provide a greater supply of gluconeogenic precursors and insulinotropic factors (Trenkle, 1978; Harmon, 1992; Marston et al., 1995). This suggestion is supported by the observation that steers on the CRSBM treatment had the greatest measured ADG.

	Implications

Top Abstract Introduction Materials and Methods Results and Discussion Implications Literature Cited

 
Grain-based supplements fed in large quantities can be efficaciously utilized on low-quality forages when adequate degradable intake protein is included in the supplements. Corn fed alone with no added ruminally degradable protein will exacerbate the preexisting deficiency of degradable intake protein, resulting in the potential for reduced intake and digestibility of low-quality forages. When fed with adequate degradable intake protein, corn-based supplements can have acceptable efficiencies and conversions and will increase performance of cattle grazing low-quality forages because of greater forage digestibility than when corn is fed alone. Feeding large amounts of a supplement that provides both energy and adequate degradable intake protein to digest not only the supplement but also the basal forage diet, allows cattle to achieve greater rates of gain while grazing low-quality forages than either similar levels of grain alone or low-level protein supplementation.

	Footnotes

 
¹ Approved for publication by the Director, Oklahoma Agric. Exp. Stn. This research was funded under project H-OKL02365.

² The authors would like to acknowledge R. Basurto, D. Cox, P. Kircher, C. Krehbiel, and A. La Manna, Dept. of Anim. Sci., and C. Goad, Dept. of Statistics, for their assistance in this experiment, and J. Moore and F. Owens for their comments and suggestions on a previous version.

Received for publication December 28, 2001. Accepted for publication September 16, 2002.

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