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ANIMAL PRODUCTION |
Department of Animal Sciences and Industry, Kansas State University, Manhattan 66505
Abstract
Effect of supplementation frequency and supplemental urea level on forage use (Exp. 1) and performance (Exp. 2 and 3) of beef cattle consuming low-quality tallgrass-prairie were evaluated. For Exp. 1 and 2, a 2 x 2 factorial treatment structure was used, such that two supplements (30% CP) containing 0 or 30% of supplemental degradable intake protein (DIP) from urea were fed daily or on alternate days. In Exp. 1 and 2, supplement was fed at 0.41% BW daily or at 0.83% BW (DM basis) on alternate days. For Exp. 3, a 2 x 4 factorial treatment structure was used, such that four supplements (40% CP) containing 0, 15, 30, or 45% of supplemental DIP from urea were fed daily or 3 d/wk. Supplements were group-fed at 0.32% BW daily or at 0.73% BW (DM basis) 3 d/wk. In Exp. 1, 16 Angus x Hereford steers (initial BW = 252 kg) were blocked by BW and assigned to treatment. Urea level x supplementation frequency interactions were not evident for forage intake, digestion, or rate of passage. Forage OM intake (OMI) and total digestible OMI (TDOMI) were not significantly affected by treatment. Total-tract digestion of OM (P = 0.03) and NDF (P = 0.06) were greater for steers supplemented daily. In Exp. 2, 48 Angus x Hereford cows (initial BW = 490 kg) grazing winter tallgrass prairie were used. Significant frequency x urea interactions were not evident for BW and body condition (BC) change; similarly, the main effects were not substantive for these variables. In Exp. 3, 160 Angus x Hereford cows (initial BW = 525 kg) grazing dormant, tallgrass prairie were used. Supplement refusal occurred for cows fed the highest urea levels, particularly for cows fed the supplement with 45% of the DIP from urea 3 d/wk, and supplement refusal increased closer to calving. A frequency x urea interaction (P = 0.02) was observed for prepartum BW changes. As supplemental urea level increased, prepartum BW loss increased quadratically (P = 0.02); however, a greater magnitude of loss occurred when feeding supplements containing 
30% of DIP from urea 3 d/wk. Cumulative BC change followed a similar trend. In conclusion, moderate protein (
30% CP) supplements with 30% of supplemental DIP from urea can be fed on alternate days without a substantive performance penalty. However, infrequent feeding of higher protein (>30% CP) supplements with significant urea levels (>15% of DIP from urea) may result in decreased performance compared with lower urea levels.
Key Words: Beef Cattle • Forage Frequency • Supplementation • Urea
Introduction
Providing supplemental degradable intake protein (DIP) in the form of true protein to low-quality, nitrogen-deficient forage improves intake and digestion of forage (McCollum and Galyean, 1985
; Bandyk et al., 2001) and prepartum maintenance of body condition and/or BW (Clanton and Zimmerman, 1970; Mathis et al., 1999) of beef cattle. Because urea is ruminally degraded to ammonia, which is the preferential nitrogen source for fibrolytic bacteria (Russell et al., 1992), replacing a portion of the DIP supplied by true protein with urea seems viable. Recent research with 30% CP supplements suggests that low-level urea inclusion (45% of the supplemental DIP) does not greatly depress prepartum performance of beef cows grazing dormant, tallgrass prairie when supplemental DIP was fed in amounts sufficient to maximize total digestible organic matter intake (TDOMI; Köster et al., 2002). Additionally, reducing the frequency with which supplements are fed may further reduce supplementation costs. Beaty et al. (1994) and Farmer et al. (2001) demonstrated supplements high in DIP, without urea, can be fed as infrequently as 2 or 3 d/wk without dramatically affecting prepartum performance of beef cows grazing winter, tallgrass prairie. Finally, low-quality forage use was not different when urea was drenched daily or on alternate days for beef cattle (Romero et al., 1976) or sheep (Tudor and Morris, 1971). Even so, limited research is available evaluating the frequency with which urea-containing supplements can be hand fed, especially with regard to effects on cow weight or body condition (BC) change. Therefore, the objective of these experiments was to evaluate the effect of varying supplementation frequency of high-protein (30 and 40% CP) supplements containing various urea levels on intake, digestibility, and performance of beef cattle consuming low-quality, tallgrass prairie forage.
Materials and Methods
Three experiments were conducted to evaluate the impact of replacing a portion of the supplemental DIP with urea and altering frequency of supplementation on intake, digestion, and performance of cattle consuming dormant, tallgrass-prairie forage. Experiments 1 and 2 were designed as a 2 x 2 factorial arrangement. The factors evaluated were supplementation frequency (daily and alternate-day) and supplemental urea level (0 or 30% of DIP from urea; 0 or 22% of CP from urea [DM basis]). In Exp. 3, treatments were arranged in a 2 x 4 factorial structure with two supplementation frequencies (daily and 3 d/wk) and four supplemental urea levels (0, 15, 30, or 45% of DIP from urea; 0, 10, 22, or 34% of CP from urea [DM basis]). The amount of supplemental DIP provided (supplemental DIP determined with NRC [1996] constituent feedstuff values) was estimated to be sufficient to maximize projected TDOMI of a low-quality, tallgrass prairie diet (Köster et al., 1996). Supplements were formulated to contain approximately 30% CP (Exp. 1 and 2) or 40% CP (Exp. 3) and, when consumed in conjunction with the forage, to deliver a nitrogen:sulfur ratio of approximately 10:1 in the total diet (Tables 1 and 2). Supplemental CP levels used in these experiments were chosen to evaluate supplemental CP levels typically used in less frequently supplemented feeding strategies. The supplements in each experiment were not pelleted, and urea was added during the mixing of all supplemental ingredients. Several batches of supplement were made, and subsamples were taken from each batch over time and composited for chemical analysis. The experimental protocol and surgery procedures were approved by the Institutional Animal Care and Use Committee at Kansas State University.
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guidelines. The minerals provided by the supplement ingredients were estimated from NRC (1984) tables and, for the prairie hay, from the data of Umoh et al. (1982).
The 24-d trial was comprised of a 14-d adaptation period followed by a 6-d voluntary intake measurement and total fecal collection period, 2-d ruminal evacuation period, and a 2-d fermentation profile. Ruminal evacuations and fermentation measurements were conducted on days when only the daily group received supplement and on days when both groups received supplement. Fecal bags were placed on the steers for collection of feces, which was used for determination of digestion values. Fecal bags were emptied, feces weighed, and subsamples collected (approximately 5% wet weight) daily at 0630 for all steers. On each day of the 2-d evacuation period, ruminal contents from each steer were removed just before feeding and 4 h after feeding; contents were weighed, sampled (450 to 750 g wet weight) in triplicate, and immediately returned to each steer. Samples of feed, feces and ruminal contents were dried at 50°C in a forced-air oven and stored for later analyses. On each day of the 2-d fermentation profile, steers were given a pulse dose of 1.5 g of Co (as CoEDTA; Uden et al., 1980) and solubilized in 250 mL of deionized water for determination of fluid dilution rate. The dose was administered into various areas within the rumen. Ruminal fluid samples were taken using a suction strainer at 0 (before dosing), 3, 6, 9, 12, and 24 h after dosing on both days for determination of ruminal pH, VFA, ammonia N, and Co concentrations. Two milliliters of ruminal fluid were acidified with 8 mL of 0.1 N HCl for ruminal ammonia analysis, and 8 mL of ruminal fluid was mixed with 2 mL of 25% (wt/vol) metaphosphoric acid for VFA analyses. A 16 mL sample of ruminal fluid, to be used for Co analysis, was frozen (-20°C).
Laboratory Analyses
The dry feed, orts, fecal, and ruminal contents samples were ground to pass a 1-mm screen with a Wiley mill (Thomas Scientific, Swedesboro, NY). Forage and supplement samples were composited across days into a single sample for each steer. Orts and fecal samples were composited by steer across days, and ruminal contents were composited within evacuation time for each steer for each day. Feed, orts, fecal, and ruminal contents samples were analyzed by standard procedures for DM, ash, and (excluding ruminal contents) Kjeldahl N (AOAC, 1990). Feed, orts, ruminal contents, and fecal samples were analyzed for NDF and acid detergent insoluble ash (ADIA; excluding fecal samples) by the procedures described by Van Soest et al. (1991). Fifty microliters of amylase (Sigma Chemical Co., St. Louis, MO) was added during NDF analysis of supplements to aid filtering. Forage DIP content was estimated by the procedure described by Mathis et al. (2001). Supplement DIP was calculated using constituent feedstuff values of the supplement from NRC (1996).
At each sampling, pH of ruminal fluid samples was measured immediately using a portable pH meter with a combination electrode (Orion Research, Boston, MA). Upon thawing, samples for ammonia, VFA, and Co analyses were centrifuged at 39,000 x g for 20 min. Ammonia concentration was determined by the phenol hypochlorite procedure (Broderick and Kang, 1980), and ruminal VFA concentration was measured using a gas chromatograph as described by Vanzant and Cochran (1994). Cobalt concentration was determined by atomic absorption spectroscopy with an air-acetylene flame. Fluid dilution rate was calculated by regressing the natural logarithm of Co concentration against sampling time (Warner and Stacy, 1968). Acid detergent insoluble ash was used as an internal marker and ADIA passage was calculated by dividing the rate of internal marker consumption by the amount of ADIA fill at 0 h and 4 h after feeding.
Intake and digestibility data were analyzed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC) with a model appropriate for a randomized complete block design. The terms in the model were supplement type, frequency, supplement type (frequency, and weight block. Fill, passage, and fermentation data were analyzed as a randomized complete block, split-split-plot design. The terms in the model were supplement type, frequency, supplement type x frequency, block, supplement type x frequency x block, sampling day, sampling day x supplement type, sampling day x frequency, sampling day x supplement type x frequency, sampling day x supplement type x frequency x block, sampling hour, sampling hour x supplement type, sampling hour x frequency, sampling hour x sampling day, sampling hour x supplement type x frequency, sampling day x supplement type x sampling hour, sampling day x frequency x sampling hour, and sampling day x supplement type x frequency x sampling hour. Supplement type, frequency, and supplement type x frequency were tested using the supplement type x frequency x block interaction. Sampling day, sampling day x supplement type, sampling day x frequency, sampling day x supplement type x frequency were tested using the sampling day x supplement type x frequency x block interaction. The remaining effects were tested using residual error. Because of a supplement type x frequency x sampling day x sampling hour interaction, data were sorted and evaluated by the day on which the measurement occurred. The terms in this model were supplement type, frequency, supplement type x frequency, block, supplement type x frequency x block, sampling hour, supplement type x sampling hour, frequency x sampling hour, and supplement type x frequency x sampling hour. Supplement type, frequency, and their interaction were tested using the supplement type x frequency x block interaction. All other effects were tested using residual error.
Experiment 2: Cow Performance Trial
Forty-eight Angus x Hereford cows (final 3 to 5 mo of gestation; average initial BW = 490 kg, average initial BC = 5.2) were sorted by BW and BC and grouped into three blocks of 16 animals representing low, medium, and high BW and BC. All animals within each of the blocks were randomly assigned according to BW and BC strata within each block to receive supplements either daily or on alternate days and to be fed supplements that contained either 0 or 30% of supplemental DIP in the form of urea. This resulted in four groups of four cows each within each block. Cows were group-fed their supplement and therefore, groups of four cows were the experimental unit. Thus, for analytical purposes, we had three blocks with four mean observations in each block. Each group received approximately 8 kg of supplement DM/d (0.46% BW/d, as-fed basis) or 16 kg of supplement DM on alternate days (0.92% BW/feeding, as-fed basis). Cows grazed tallgrass-prairie pasture. The native range was comprised primarily of big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), and indiangrass (Sorghastrum nutants; Olson, 1998).
All cows were gathered daily (typically beginning at approximately 0800), sorted into treatment groups (three groups per treatment), and group-fed their respective supplements. On days when only the daily supplemented groups were supplemented, the alternate-day groups were held in an adjacent alley. All of the cows were returned to pasture at the same time.
After calving, cows were removed from the supplementation treatments and group-fed 4.04 kg of alfalfa hay (DM basis) daily (17.3% CP; DM basis), while grazing dormant tallgrass-prairie until sufficient new growth was available (late April). Cow BW and BC were measured after standing overnight without feed or water on trial initiation (December 2) and within 48 h after calving (average date = March 1). Body condition scores (1 to 9 scale; NRC, 1996) were the average of evaluations from four trained individuals. Weights of calves were recorded within 48 h after calving and at initiation of breeding (May 8).
Statistical analysis was performed using the groups of four cows as the experimental unit. Data were analyzed as a randomized complete block using the GLM procedure of SAS with effects for treatment and block. Treatment sums of squares were partitioned using contrasts for urea inclusion, frequency of supplementation, and their interaction.
Experiment 3: Cow Performance Trial
One hundred sixty Hereford x Angus cows were weighed and their BC scored (1 to 9 scale; NRC, 1996) and assigned randomly to receive supplements either daily or 3 d/wk and to be fed supplements that contained 0, 15, 30, or 45% of their DIP in the form of urea. The two supplementation frequency treatments were assigned randomly to two of four pastures of similar size (approximately 130 ha each). Two pastures were designated to contain cows supplemented daily. The other two pastures were designated to contain cows supplemented Mondays, Wednesdays, and Fridays (3 d/wk). Pastures were typical of tallgrass prairie range in eastern Kansas, and clipped forage samples (clipped with 2.54 cm left standing) collected during mid-February indicated that the standing forage (across all pastures) contained 4.1% CP and 73% NDF (DM basis). The native range was comprised primarily of big bluestem, little bluestem, and indiangrass (Olson, 1998). Pastures were used to represent frequencies of supplementation so that potential effects of treatment on behavior could be expressed if existent. Cows were stratified by BCS and BW and assigned randomly to one of the four pastures. Finally, within each pasture, cows were assigned randomly to receive one of the four different supplemental urea levels previously mentioned. Supplements were primarily comprised of soybean meal and ground sorghum grain (Table 1
) and were group-fed at 1.64 kg/animal daily (DM basis) to cows that received supplement daily. Cows fed 3 d/wk were offered the same amount of supplement per week (i.e., 11.5 kg/animal weekly on a DM basis), but supplement was evenly split among the three days (3.84 kg per supplementation event on a DM basis). On their supplementation days, cows were gathered (typically beginning at approximately 0800) and sorted into their supplement treatment groups and fed their supplement. Supplement refusals were measured throughout the trial. Prairie hay (5.8% CP; 70% NDF on a DM basis) was fed (4 kg/animal daily on a DM basis) because of significant snow coverage from December 23 through January 3. Hay was fed as weighed square bales and group-fed to cows in their respective pastures during this period. Cows were weighed and BC was scored at the initiation of the trial (November 27) and within 48 h after calving (average date = March 9). Body condition and BW were measured after an overnight (16 h) fast. Treatment application began on December 6.
Supplementation treatments ended at calving for each cow, and all cows were fed alfalfa hay (21.6% CP; DM basis) at 4 kg/d (DM basis) after parturition until there was significant green grass available (mid- to late-April). Cows were pregnancy tested on August 14 by rectal palpation. Calves were weighed within 48 h after birth and on August 14 (ending weight).
Cow BW and BC change, as well as changes in calf birth weight and performance, was analyzed using a split-plot ANOVA with the GLM procedure of SAS. Treatment groups within each pasture served as the experimental unit. As a result, each treatment had two replications. Frequency of supplementation served as the whole-plot factor. Supplemental urea level and frequency of supplementation x supplemental urea level were the subplot. Frequency of supplementation effect was tested with pasture within frequency as the error term. Supplemental urea level and frequency of supplementation x supplemental urea level effects were tested with pasture within frequency of supplementation x supplemental urea level as the error term. The LSMEANS option was used to determine treatment means. Supplemental urea level sums of squares were partitioned using linear, quadratic, and cubic contrasts. The LOGISTIC and FREQ procedures of SAS were used to analyze subsequent pregnancy rate of cows. The LSMEANS option of the MIXED procedure of SAS was used to determine the standard error associated with each of the eight treatment groups.
Results
Experiment 1: Digestion Trial
Varying the frequency of supplementation did not interact with the inclusion of urea for forage OM intake (OMI), total OM intake (TOMI), OM digestibility, or TDOMI (Table 3). Forage OMI, TOMI, and TDOMI (Table 3) were not significantly affected by supplementation frequency or the inclusion of urea. In general, changing the frequency of supplementation exerted a greater effect (P 0.06) on OMD and NDFD than did inclusion of urea in the supplement. Organic matter (P = 0.03) and NDF digestion (P = 0.06) were reduced when steers were supplemented on alternate days. However, the trends toward interactions were the result of larger magnitudes of response when the supplementation frequency was changed for the supplements with urea. The numerical trends in TDOMI for a frequency effect were largely due to the differences noted for OMD.
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0.05) treatment x frequency x sampling hour interactions occurred for a number of fermentation and passage variables (Tables 3 and 4). However, the interactions largely were a result of differences in the magnitude of response at different sampling hours. Therefore, data were averaged over sampling hours and are presented according to the day on which the measurement was taken (i.e., days when only cattle that were supplemented daily received supplement and those days when both groups were supplemented; Tables 3 and 4). There was no significant interaction observed between supplementation frequency and supplemental urea for ADIA passage rate (Table 3). Passage rate of ADIA was greater (P 0.04) for groups fed the supplement without urea and for the alternate-day group (P = 0.05) when measured on the day when only the daily group was supplemented. However, the magnitude of changes in passage was not large.
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). In contrast, on the day when both groups were supplemented, the alternate-day groups had higher ruminal ammonia concentrations, particularly when fed the supplement with urea (urea x frequency interaction; P < 0.01).
Ruminal pH was not significantly altered by supplementation frequency or urea inclusion (Table 4). On the day when only the daily group was supplemented, the daily group exhibited increased total VFA concentration (P = 0.02). In contrast, no treatment differences were evident on the day when both groups were supplemented. Changes in the proportion of acetate were relatively minor, although statistically significant in some cases. Supplementation frequency and urea inclusion interacted (P 0.01) for molar proportion of propionate on both days. When only the daily group was supplemented, the treatment differences were small and the direction of the difference due to supplementation frequency depended on whether urea was included in the supplement. In contrast, when both groups were supplemented, the alternate-day groups exhibited higher (P < 0.01) propionate concentrations, although the magnitude of difference was greater when urea was in the supplement. Butyrate proportions were greater (P = 0.07) in the alternate-day groups on the day both groups received supplement, but were not affected by the inclusion of urea. Although statistical differences were evident for the proportions of branched-chain VFA and valerate, most of the differences were relatively minor in magnitude.
Experiment 2: Cow Performance Trial
Initial cow BW and BC were similar among treatment groups (Table 5). Urea level x supplementation frequency interactions were not evident for the traits monitored in this experiment. Urea inclusion did not substantively impact cumulative BW (P = 0.15) and BC (P = 0.13) change through calving, although the reader’s attention is drawn to the consistent direction of change in this instance. Frequency of supplementation did not affect either BW (P = 0.90) or BC (P = 0.83) change in this study. The birth weight of calves and calf ADG were not significantly affected by supplement feeding frequency or urea inclusion during the prepartum period. The average calving date for this experiment was March 20.
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For BW changes during the winter supplementation period (December 6 to calving), there was a frequency of supplementation x supplemental urea level interaction (P = 0.02; Table 6). In general, as supplemental urea level increased, there was greater loss in BW, particularly at the highest urea level (quadratic, P = 0.02). However, the effect of increasing urea level was most dramatic when cows were supplemented only 3 d/wk. There was no significant frequency of supplementation x supplemental urea level interaction (P = 0.35) from December 6 through calving for BC changes. Body condition loss increased (quadratic, P = 0.05) with an increase in supplemental urea level during the winter supplementation period (December 6 calving), whereas no significant difference was observed due to different frequencies of supplementation.
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) pregnancy rate. Recognizing the challenge of statistically discriminating treatment effects in pregnancy rate with limited replication, the results from our analysis of the linear response (P = 0.15) to increasing urea level during the prepartum period warrants careful consideration. Calf birth weights and calf performance during the nursing period were not significantly affected by treatments imposed during gestation (Table 6). Discussion
Experiment 1: Digestion Trial
In general, all steers readily consumed the supplements provided. In some cases, after the steers had fully adapted to the treatments (>2 wk), there was some reluctance to consume the entire supplement containing urea. However, this amount was seldom greater than 0.18 kg (DM basis). The lack of treatment effects on forage OMI or TDOMI agrees with previous work conducted by Köster et al. (1997) on urea inclusion. In contrast with this experiment, Beaty et al. (1994) and Farmer et al. (2001) observed an increase in forage intake and digestible OMI of the diet with increasing frequency of supplementation.
The ability of urea to increase the digestion of low-quality, nitrogen-deficient forages occurs via the contribution of urea to the ruminal ammonia pool, which is considered to be the primary source of nitrogen for fibrolytic bacteria (Petersen, 1987; Russell et al., 1992). Reasonably high levels of OM and NDF digestion for this forage were supported by both supplements in our study. This concurs with the observations of Köster et al. (1997). However, we also observed a tendency for digestion to be poorer for the supplements with urea when they were fed less frequently. Because supplemental urea supplies no intact AA or peptides, precursors for branched-chain VFA can be limited and, depending on diet, could impose limitations in the cellulolytic capacity of the ruminal microflora (Maeng and Baldwin, 1976; Hoover, 1986). In our trial, urea was included at a relatively low level (approximately 30% of the DIP). In addition, other ruminally degradable proteins were present in the supplement and basal forage. Thus, it seems unlikely that this mechanism would be responsible for any observed changes in digestion. Asynchronous release of nitrogen from urea (relative to the carbohydrate fermentation in forages) is another attribute that is often suggested as a potential limitation to the use of urea-based supplements (NRC, 1996). Whereas this does not seem to present much of a problem when supplements with low to moderate levels of urea are fed daily (Köster et al., 1997), it may be more of a concern if supplements are fed infrequently. The time-series changes in ruminal ammonia concentration in our study were more dramatic in response to changing supplementation frequencies when the supplement contained urea (data not shown). Although the effect of changing supplementation frequency tended to depend on whether a supplement contained urea, supplementation frequency exerted a more dramatic effect on digestion than did urea inclusion. Similar to observations from Farmer et al. (2001), daily supplementation in our study improved OM and NDF digestion. This may have been due, at least in part, to large amounts of readily fermentable carbohydrate offered to steers that were supplemented on alternate days. On the day when only those groups supplemented daily received supplement, alternate-day-supplemented steers had faster ADIA passage rates. However, the magnitude of change was relatively small and seems unlikely to explain the lower digestion coefficient for this group.
A reduction of the acetate:propionate ratio is often associated with a shift from cellulolytic to amylolytic fermentation (Owens and Goetsch, 1988). In our study, the propionate proportions were typically greater when cattle were supplemented on alternate days. The large influx of grain resulting from supplementation on alternate days likely caused this shift.
Ruminal pH was similar across treatments with average values within a range considered to be conducive to fiber digestion (Hoover, 1986), even when providing up to 0.83% of BW (DM basis) of high-protein supplement on alternate days. This response concurs with Farmer et al. (2001), in that ruminal pH stayed above 6.2 when steers were fed as much as 1.26% BW of high-protein supplement as a single meal. Thus, the observed effects on NDF digestion occurred in spite of the favorable pH conditions. This implies that mechanisms beyond the effect of pH per se on cellulolysis were at work in this instance. For instance, perhaps cellulase activity was inhibited by large amounts of readily available carbohydrate (starch) in these supplements (Mould et al., 1983). The differences noted in total VFA concentrations reflect the response to provision of highly fermentable substrate. On those days when both groups were supplemented, total VFA concentrations were similar.
Offering supplements on alternate days not only increased the molar proportions of propionate but also increased the butyrate proportions. This observation concurs with the findings of Chase and Hibberd (1987), who found that butyrate proportions were increased by increasing the level of grain supplemented, which is similar to the alternate-day supplementation pattern. Although some minor shifts in fermentation characteristics and digestion were evident in our study, the overall effect (as represented by lack of effect on TDOMI) of these treatments was negligible. That is, the overall effect on forage utilization of low level inclusion of urea in high-protein (30% CP) supplements, regardless of daily vs. alternate-day feeding, was not great. Similarly, Romero et al. (1976) observed no difference in TDOMI when beef steers were drenched at different frequencies (daily with 50 g or with 100 g delivered on alternate days) with urea as the sole nitrogen supplement for low-quality forage.
Experiment 2 and 3: Cow Performance Trials
In Exp. 2, all cows readily consumed the 30% CP supplement with 30% of DIP from urea. This is in agreement with the findings of Köster et al. (2002) who fed supplements that contained 30% CP with up to 45% of DIP from urea to prepartum beef cows grazing dormant, tallgrass-prairie. Conversely, in Exp. 3, supplement refusal was observed for cows fed supplements with higher urea levels, especially when supplemented 3 d/wk. The difference in response for these studies likely relates to the use of higher protein levels in the supplement in Exp. 3 (40% CP), thereby changing the urea content of the supplement. For example, the supplement with 30% of DIP from urea in Exp. 2 (30% CP) had 2.1% urea (DM basis) as opposed to 3.1% urea (DM basis) for the supplement with 30% of DIP from urea in Exp. 3 (40% CP). It appears that daily hand feeding of high-protein supplements (30 to 40% CP) with up to 3.1% urea (DM basis) can be done with low likelihood of experiencing supplement refusal. This concurs with observations from Köster et al. (2002). However, based on the results from Exp. 2 and 3, if one is going to hand feed a high-protein supplement on a less frequent supplementation schedule, use of lower urea levels seems desirable. Based on conditions encountered in our study, less than 2% of supplement DM from urea would likely ensure complete consumption of a "hand-fed" high-protein supplement fed to prepartum cows consuming low-quality forage.
In Exp. 2, urea inclusion and supplementation frequency did not interact with regard to their effect on BW and BC changes. Supplementation frequency also did not impact BW or BC change. With regard to supplementation frequency, this agrees with the observations of Melton and Riggs (1964), Huston et al. (1999), and Farmer et al. (2001), who reported similar changes in BW and BC for cows that were supplemented daily or 3 d/wk on dormant pasture. In this experiment, cows fed the supplement with urea had the greatest magnitude of BW and BC loss through calving; although, the magnitudes of differences in cumulative losses were relatively minimal, which is consistent with the limited difference in TDOMI in Exp. 1. In Exp. 3, the three treatment groups that performed the poorest during the winter supplementation period were the three treatment groups that refused at least some portion of their supplement over the course of the experiment. Clearly, these cows were not receiving the same amount of supplemental nutrients as cows in groups that consumed their entire supplement allotment. The reduction in supplement intake may also have acted to limit potential increases in forage intake by reducing the amount of supplemental DIP consumed (Köster et al., 1996). Because of the supplement refusal, it is difficult to determine how much of the trend toward decreased performance with increasing urea level was due to urea inclusion per se. Regardless of the proportion of the performance reduction that is due to urea inclusion versus supplement refusal, clearly the use of high levels of urea in supplements that are hand-fed infrequently can have significant negative impacts on performance relative to isonitrogenous supplements that do not contain urea (or that have low levels of urea inclusion).
Supplementation of low-quality, nitrogen-deficient forage with moderate levels of urea (25 to 150 g/d) to nonlactating cows increased forage intake, digestion, and nitrogen retention compared with unsupplemented cows (Campling et al., 1962). However, when urea-containing supplements are compared with true DIP supplements on an isonitrogenous basis, forage intake and digestion may be similar, but nitrogen retention is often decreased with urea inclusion (Ammerman et al., 1972; Swingle et al., 1977; Petersen et al., 1985). Therefore, factors other than forage intake and digestion may be significant contributors at times to the performance reduction reported for cattle fed supplements that contain urea.
Lack of treatment effect on calf birth weights and growth agrees with previous research evaluating urea- vs. true protein-containing supplements (Köster et al., 2002) and supplementation frequency (Farmer et al., 2001; Bohnert et al., 2002) for cows consuming low-quality forage. Moreover, such information suggests that carryover effects were minimal with the magnitude of response elicited during the supplementation period. Although carryover did not seem significant with regard to calf performance, there appeared to be a trend for reduced pregnancy in the subsequent breeding season when urea was fed at high levels during the prepartum period. Similarly, for at least one of their experiments with prepartum cows, Köster et al. (2002) reported a tendency for a decrease in pregnancy rate when hand feeding a supplement with a relatively high level of supplemental urea (40% of DIP from urea) during the prepartum period. Furthermore, hand feeding of protein supplements with a high urea level (62.5% of CP from urea or slow release urea) during the postpartum period resulted in dramatically lower pregnancy rates compared with true CP supplementation (Forero et al., 1980). Additional research with a larger number of cows is warranted for elucidation of the effect of supplemental urea on beef cow reproduction, especially with attention given to the physiological state of the cows (lactation versus gestation) during the urea supplementation period.
Implications
For hand-fed supplements with moderately high crude protein levels (approximately 30% crude protein), including urea at 30% of the supplemental degradable intake protein or less and supplementing on alternate days offers the possibility of decreasing supplement costs without sacrificing overall forage intake and digestion or the performance of prepartum beef cows. However, caution should be used in hand feeding supplements that contain higher protein concentrations (>30% crude protein) in combination with higher levels of urea (>15% of degradable intake protein from urea) on an infrequent basis (e.g., 3 d/wk). Extrapolation of these results to periods of postpartum supplementation or to situations with low forage availability should be avoided.
Footnotes
1 Article No. 03-231-J from the Kansas Agric. Exp. Stn. 
2 The authors thank G. Ritter and W. Adolph for their assistance.
3 Current address: Performix High Plains, L.L.C., 1650 N. Sherlock Rd., Garden City, KS 67846.
5 Current address: Land O’ Lakes Farmland Feed, Gering, NE 69341.
6 Current address: Extension Anim. Resources Dept., New Mexico State Univ., Las Cruces 88003-8003.
7 Current address: Dept. of Anim. Sci., Univ. of Missouri, Columbia 65211.
4 Correspondence—e-mail: rcochran{at}oznet.ksu.edu.
Received for publication January 29, 2003. Accepted for publication November 10, 2003.
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