J. Dairy Sci. 90:978-986
© American Dairy Science Association, 2007.
Manipulation of Soluble and Rumen-Undegradable Protein in Diets Fed to Postpubertal Dairy Heifers1
G. I. Zanton,
M. T. Gabler2 and
A. J. Heinrichs3
Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802
3 Corresponding author: ajh{at}psu.edu
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ABSTRACT
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Eight postpubertal Holstein heifers (455 ± 4.0), fit with rumen cannulas, were used in 2 experiments to investigate the effects of altering dietary protein type on nutrient digestibility, rumen fermentation, and nitrogen utilization. Heifers were fed diets containing low or high levels of soluble (SP) and low or high levels of rumen-undegradable protein (RUP) in a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. The treatment rations in experiment 1 were formulated with corn silage composing the majority of the forage fraction, whereas in experiment 2, grass hay composed the highest proportion of ration DM. Blood and rumen samples were collected over 2 d and total fecal and urine collections were conducted for 4 d. Dry matter, organic matter, and neutral detergent fiber digestibility were not different in either experiment 1 or 2. Increasing the proportion of dietary crude protein that was SP increased mean daily rumen ammonia concentrations in each experiment, although no other rumen parameter differed. Excretion of urinary nitrogen in experiment 1 was highest for diets with low SP and low RUP and with high SP and high RUP, which resulted in these rations being the least efficient in retention of apparently digested nitrogen. The proportion of consumed or absorbed nitrogen retained in experiment 2 was not significantly different between treatments. Responses to alterations in crude protein degradability are observable in postpubertal heifers; however, the level of response may depend on the diet in which protein degradability is altered.
Key Words: heifer • rumen fermentation • nitrogen utilization • forage
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INTRODUCTION
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Growing heifers require AA to support maintenance, growth, and pregnancy. Providing these nutrients as efficiently as possible requires a coordinated effort between dietary provision, availability, and preabsorptive and postabsorptive processes. Ruminants have the capacity to utilize NPN sources alone or in combination with protein N to meet their AA needs (Loosli et al., 1949; Virtanen, 1966; Satter and Roffler, 1975). However, the efficiency with which dietary protein can be utilized for tissue growth is relatively low (Lobley, 1992). Ammonia is the preferred source of N for cellulolytic bacteria (Bryant and Robinson, 1961). Furthermore, fermentation has been shown to be most efficient if NH3 is present at a concentration of at least 5 mg/dL in continuous culture (Satter and Slyter, 1974), although AA and peptides are also important (Argyle and Baldwin, 1989). Dairy heifers are typically fed rations with a high quantity of forage and a high concentration of NDF. Therefore, feeding growing heifers a protein source with higher levels of solubility may improve nutrient efficiency. However, the suggestion has been made that greater proportions of slowly degraded CP also may improve fiber digestion by enabling a more consistent supply of nitrogenous compounds to the rumen microbial population (Veen, 1986).
Although much of the AA requirement for the ruminant is met by microbial sources (Allison, 1969; Merchen and Titgemeyer, 1992), there has been considerable interest in supplying a significant portion of dietary CP to dairy heifers in the form of RUP (Casper et al., 1994; Bethard et al., 1997; Tomlinson et al., 1997). Amos (1986) and Tomlinson et al. (1997) found that increasing the proportion of RUP led to greater gains and gain efficiencies. Moallem et al. (2004) found few statistical differences in slaughtered heifers fed diets varying in RUP concentration. Gabler and Heinrichs (2003a) indicated in a digestibility experiment that, although some rumen parameters were altered by changing the protein fractions, digestibility and N utilization were not affected. Each of the preceding experiments used heifers between 3 and 10 mo of age, whereas the majority of time, growth, and feed consumed throughout the life of a heifer occurs after this time frame. Because nutrient requirements and DMI differ with increasing BW of older heifers, it may not be possible to extrapolate results from experiments with younger animals.
Because of the effects that soluble protein (SP) and RUP may have on N efficiency and nutrient utilization, 2 experiments were conducted to examine the effects of altering dietary protein fractions fed to postpubertal dairy heifers. The experimental objectives were to evaluate nutrient digestibility, rumen fermentation, and N utilization in postpubertal dairy heifers given diets containing 2 levels of forage differing in the proportions of SP and RUP.
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MATERIALS AND METHODS
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Animals, Treatments, and Experimental Design
All procedures involving the use of animals in these experiments were approved by the Pennsylvania State University Institutional Animal Care and Use Committee. These experiments were conducted over the course of 2 yr, using 8 cannulated heifers in 2 separate 4 x 4 Latin squares. The 4 experimental periods consisted of 20 or 21 d for experiments 1 and 2, respectively: The first 10 d were used for adaptation to the treatment rations, and samples were collected over the remaining days.
Within each experiment, heifers were fed a diet with the forage component based on grass hay and corn silage, with corn silage composing a greater proportion of DM in experiment 1 and grass hay composing a greater proportion of DM in experiment 2. Experimental diets and their composition are shown in Tables 1
and 2 for experiments 1 and 2, respectively. Treatments were in a 2 x 2 factorial arrangement with the main effects being level of SP, level of RUP, and their interaction. Every 10 d, BW was recorded for 2 consecutive days; measurements were conducted 3 h after feeding and DMI was adjusted for the following 10 d. Treatment rations were provided to each heifer at 2% of BW per day once daily at 0930 h (experiment 1) and at 0900 h (experiment 2), and any refusals were collected immediately prior to feeding. Samples of forages and rations were collected 3 times weekly and concentrate samples were collected once per week and composited by period for chemical analysis.
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Table 1. Ingredient and nutrient composition of treatment rations containing low and high levels of soluble protein (SP) and low and high levels of RUP in experiment 1
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Table 2. Ingredient and nutrient composition of treatment rations containing low and high levels of soluble protein (SP) and low and high levels of RUP in experiment 2
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Each of the 8 heifers that was used in these experiments was surgically modified for a previous experiment with a rumen cannula (3.81 cm i.d.; Bar Diamond, Parma, ID) under local anesthesia. Heifers were housed in individual tie stalls in a mechanically ventilated barn for the duration of the experiment, except on days during which samples were not collected. For approximately 2 h on these days, heifers were allowed access to an exercise lot.
Sample Collection and Analysis
Feed and ration samples were composited by period, dried in a forced-air oven at 55°C for 48 h, and ground through a 1-mm screen (Wiley mill; Arthur Thomas, Philadelphia, PA). Period-composited ration samples (experiment 1) or feed samples (experiment 2) were analyzed for DM, ash, and CP (AOAC, 1990), SP (Krishnamoorthy et al., 1983), NDF and ADF (Van Soest et al., 1991), and total nonstructural carbohydrates (Smith, 1981). Analysis of NDF included the use of heat-stable 
-amylase and sodium sulfite (Van Soest et al., 1991). Metabolizable energy and RUP concentration were calculated according to procedures of the NRC (2001) using the actual ingredient chemical composition and actual heifer BW and DMI.
On d 11 of each period for each experiment, rumen fluid samples were collected from the ventral sac using a hand pump at 0, 1, 2, 3, 4, 6, 8, 12, 16, and 23.5 h after feeding. Rumen samples were immediately analyzed for pH (pH meter, model M90; Corning Inc., Corning, NY) and a 15-mL subsample was stored at –20°C with 3 mL of 25% metaphosphoric acid and with 3 mL of 0.6% 2-ethylbutyric acid as an internal standard. Immediately prior to analysis for VFA (Yang and Varga, 1989) and NH3 (Broderick and Kang, 1980), thawed samples were centrifuged 3 times at 4,000 x g for 30 min at 4°C and the clear supernatant was retained.
On d 12 of each period for each experiment, approximately 6 h after feeding, heifers were catheterized in the right jugular vein. Blood samples were taken at 0, 1, 2, 3, 4, 6, 8, 12, 16, and 23.5 h after feeding on d 13 into 10-mL tubes containing sodium heparin as an anticoagulant. Samples were immediately centrifuged at 4,000 x g at 4°C for 15 min. Plasma was aspirated and stored at –20°C until analysis for urea N (procedure no. 0580; Stanbio Laboratory Inc., San Antonio, TX).
On d 15 of each period for each experiment, heifers were urinary catheterized (#14 French balloons; Rusch Inc., Duluth, GA) prior to feeding for a 4-d total collection of urine and feces. Urine was collected continuously into a 25-L container, which was maintained at pH <3 by the addition of 12 N HCl. Urine and feces were weighed daily immediately after the morning feeding, and a representative subsample was removed for subsequent analysis. Feces were dried, ground, and analyzed for ash, N, and NDF in a manner consistent with feed samples. Urine samples were stored at –20°C until thawed and analyzed for N (AOAC, 1990).
Statistical Analysis and Calculations
The 2 experiments were analyzed and are reported separately. All statistical analyses were conducted in SAS using the mixed procedure (SAS Institute, 1999). All dependent variables were analyzed as a 4 x 4 Latin square design with a 2 x 2 combination of treatments including sources of variation associated with fixed design effects of period and heifer and fixed treatment effects of levels of SP and RUP and their interaction. Because of unequally spaced rumen and blood sampling, mean daily pH, NH3, VFA, and BUN concentrations were determined by calculating the area under the response curve according to the trapezoidal rule (Shipley and Clark, 1972). All denominator degrees of freedom for F-tests were calculated according to Kenward and Roger (1997) and repeated measurements for rumen samples were analyzed by including the first-order autoregressive covariance structure (Littell et al., 1998). Where mean separation procedures were conducted, a Tukey adjustment was included to account for multiple comparisons. Residual variances were assumed to be normally distributed. All data are presented as least squares means, and treatment effects were considered significant when P < 0.05 and a tendency toward significance at P < 0.10.
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RESULTS
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Mean BW of heifers, daily intakes, and digestibilities of several components are shown in Table 3. Intakes of DM, OM, and NDF did not differ in experiment 1, but the intake of dietary CP was significantly greater for heifers consuming high SP. In experiment 2, DM and OM intakes were not affected by the treatment ration. There was a tendency (P < 0.10) for heifers fed rations with increased RUP to consume greater quantities of OM. Heifers fed diets containing high SP consumed less NDF in experiment 2 (P < 0.05). Crude protein intake was significantly different in experiment 2, and all contrasts were significant. However, most of this significance was accounted for by the high SP and high RUP ration, because no other treatments differed from each other (P > 0.05). Dry matter, OM, and NDF digestibilities were not different in either experiment 1 or 2.
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Table 3. Daily nutrient intake and component digestibility of treatment rations differing in rumen-soluble protein (SP) and RUP delivered to heifers 16 to 18 mo of age
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Mean daily responses in rumen fermentation parameters and BUN are shown in Table 4. Increasing the proportion of dietary SP increased the mean daily rumen NH3 concentrations in each experiment (P < 0.05), although no other rumen parameter was found to differ. Blood urea N was not affected by dietary alterations in experiment 1, although heifers fed high RUP rations responded with elevated BUN in experiment 2 (P < 0.05). Differences in the mean daily rumen NH3 concentration occurred in both experiments (Figures 1 and 2), largely because of higher peak concentrations. Differences in and actual rumen NH3 concentrations 6 h after feeding were low and not affected by dietary treatment (P > 0.10).
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Table 4. Least squares means of rumen fermentation parameters and BUN of heifers fed treatment rations differing in rumen soluble protein (SP) and RUP
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The distribution of N intake is shown in Table 5. In experiment 1, as the intake of SP increased, the N intake, digested N, and N digestibility increased. The increase in these responses did not translate into a significant increase in retained N because of an interaction affecting urinary N and the high variability associated with this response. However, there was a tendency for heifers fed the high SP rations to retain more N. In experiment 2, N intake was greatest for heifers fed high SP and high RUP in combination. Nitrogen digestibility was also greater for this treatment, leading to increased N digestion for heifers receiving this treatment. Urinary N was increased on diets that contained a greater proportion of CP from RUP.
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Table 5. Nitrogen distribution of treatment rations differing in rumen soluble (SP) and RUP delivered to heifers 16 to 18 mo of age
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DISCUSSION
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The hypotheses of these experiments were that the utilization of nutrients, and in particular CP, would be affected by altering the solubility and degradability of CP provided. Results from these 2 experiments provided insufficient statistical evidence to conclude that varying the dietary solubility or degradability, when similar levels of CP are consumed, can affect the utilization of nonnitrogenous dietary compounds. These results concur with the findings of Gabler and Heinrichs (2003a), who reported that apparently digested DM was unaffected by altering the solubility and degradability of CP provided to prepubertal Holstein heifers (147 kg initially). Likewise, when the degradability of dietary CP was altered in diets fed to Friesian crossbred heifers in a high-concentrate diet (Devant et al., 2000; 101 kg initially), apparent total tract digestibilities of both DM and OM were unaffected by the treatments used. However, these results contrast with the results of Amos (1986), in which rations that contained a higher proportion of soluble CP resulted in the greatest total tract digestibility of DM, energy, and NDF, although starch digestibility was not different.
In an attempt to isolate the effects of CP degradability on ruminal feed digestion, in situ degradation of DM, OM, and NDF was conducted on various feedstuffs that were incubated in the rumen of beef steers (Hussein et al., 1995). The 24-h extent of degradation for all chemical components analyzed was not affected by the level of ruminal protein degradation. Furthermore, to obtain precise control over the site of CP degradation, several experiments have been conducted in which varying proportions of CP (CN) were infused either directly into the rumen, into the abomasum, or into both sites, with effects on various aspects of digestion monitored. In lambs fed low-quality bromegrass hay (Swanson et al., 2004b) or a high-concentrate, corn-based ration (Swanson et al., 2004a), DM, OM, energy, and NDF total tract digestibilities were not affected by infusion of a constant amount of CN in varying proportions into the rumen or the abomasum. This conclusion also was obtained with beef steers fed a low-quality forage (Bandyk et al., 2001) and dairy cows fed red clover silage ad libitum (Khalili and Huhtanen, 2002).
Of the rumen parameters analyzed in the present study, only the concentration of NH3 was affected by dietary changes. The microbial efficiency was determined to be maximized in continuous-culture fermenters when the NH3 concentration exceeded 5 mg/dL (Satter and Slyter, 1974). The mean daily concentration of rumen NH3 was only above this level for the high SP diets in experiment 1, whereas the diets delivered in experiment 2 produced a maximum mean daily rumen NH3 concentration of 
3.7 mg/dL. Even in heifers receiving the high-SP treatments in experiment 1, the concentration of rumen NH3 was above 5 mg/dL for only about 6 h/d, although peak concentrations exceeded 20 mg/dL. It should be noted that mean daily NH3 concentrations were above 2 mg/dL, which was stated to be closer to the limiting NH3 concentration without allowance for a safety margin of excess (Satter and Slyter, 1974) and above the concentration required to obtain 95% of the maximal specific growth rate (1.4 mg/dL; Schaefer et al., 1980). Given the NH3 requirements of cellulolytic bacteria (Bryant and Robinson, 1961) and the advantages obtainable by synchronizing carbohydrate and N availability (Hoover and Stokes, 1991), we hypothesized that formulating these treatment rations containing protein sources of altered degradability would allow for improved utilization of nonnitrogenous nutrients. Results from this experiment and those cited previously fail to support this hypothesis and indicate that rumen N availability may not have been limiting under these dietary situations, that all of the diets had insufficient or temporally disconnected levels of rumen available N, or perhaps that physiological adaptations exist to mitigate against the effects of limited N availability in the rumen.
Hoffman et al. (2001) indicated that the utilization of CP for growth and N retention was optimized at 13% of DM for postpubertal Holstein heifers. The CP levels fed in this experiment were below this recommendation, although improving the site and composition of absorbed N through alterations in degradability may allow for a reduced level of CP to be provided. Thus, it was hypothesized that manipulating the degradability of dietary CP would elucidate a level of degradability that improved N efficiency. These results are mixed and in some cases may be masked by differences in N intake or in the basal diet. The apparent digestion of N in experiment 1 was improved by including higher levels of SP. However, a significant interaction indicates that differences due to the level of SP were driven by effects of the diet with low levels of SP and high levels of RUP because all other treatments were not statistically different from each other (P > 0.05). The reason for these differences is difficult to resolve, but it may result from negative associative effects produced by the various sources of N in this ration. The authors are unaware of research that would support or reject this hypothesis for these feedstuffs. Total-tract N digestibility also was greater in the high SP, high RUP ration fed in experiment 2, but this enhanced digestibility was likely attributable to the greater consumption of N (Hoffman et al., 2001; Gabler and Heinrichs, 2003b).
The excretion of urinary N in experiment 1 was highest and was equivalent for the diets that had low SP and low RUP and high SP and high RUP, which indicated that these rations were the least efficient in terms of retaining apparently digested N. These results are in contrast to those reported by Gabler and Heinrichs (2003a) in which prepubertal heifers were delivered very similar rations and had no differences in utilization of N provided from sources of differing degradability. The reasons for this discrepancy may relate to the age of the heifers used in these 2 experiments; Hayashi et al. (2006) indicated that the capacity to recycle N to the rumen continued to change as animals grew from 105 to 190 kg. Alternatively, this discrepancy in response also may derive from a relatively higher provision of dietary CP in the experiment of Gabler and Heinrichs (2003a; 1.92 g of N/kg of BW0.75 vs. 1.75 g of N/kg of BW0.75), possibly making efficiency less amenable to manipulation by altering degradability of the CP source. Blood urea N concentrations also tended to be lower (P < 0.10) for these rations, with lower levels of urinary N excretion (P < 0.01), which may indicate that N recycling to the digestive tract was enhanced under these scenarios or that less NH3 entered the portal blood pool and therefore converted to urea. These responses were not measured in the current experiments. In experiment 1, providing N with low solubility and high rumen undegradability or high solubility and low rumen undegradability resulted in the maximum efficiency for retention of the dietary CP provided.
The N distribution for experiment 2 showed differences in the mass of N apparently digested and that which appeared in the urine, the majority of the difference precipitated by the high SP and high RUP treatment. This ration also provided a greater amount of dietary CP (P < 0.01), which was likely driving the responses observed in this treatment. Indeed, when retained N was scaled to that which was consumed or apparently digested, no alterations in N retention were detected. Although experiments 1 and 2 are not directly comparable because of differences in feedstuffs, time of completion, and dietary composition, responses within the experiments to the provision of diets differing in CP degradability do not lead to similar conclusions. The role of forage source and its level of inclusion affecting the response to CP degradability cannot be evaluated from these data but may contribute to the discrepancy. Alterations in passage rate, carbohydrate fermentability, or other factors that may alter ruminal responses to CP degradability were not evaluated in these experiments.
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CONCLUSIONS
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The results from these experiments indicate that improvements in nutrient utilization obtained by altering the degradability of dietary CP are minimal. Nitrogen excretion and efficiency may be improved by providing diets differing in CP degradability and solubility under certain dietary conditions (experiment 1). We conclude from the results of these experiments that the level of response may be dependent on the diet in which CP degradability is altered.
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ACKNOWLEDGEMENTS
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The authors would like to acknowledge the invaluable contributions of Maria Long for laboratory and technical assistance.
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FOOTNOTES
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1 This research was a component of NC-1119: Management Systems to Improve the Economic and Environmental Sustainability of Dairy Enterprises. 
2 Current address: ADM Alliance Nutrition, Box 44037, Madison WI 53744.
Received for publication June 21, 2006.
Accepted for publication October 9, 2006.
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ABSTRACT
INTRODUCTION
) low SP, low RUP; (
) low SP, high RUP; (
) high SP, low RUP; and (x) high SP, high RUP (SEM ± 1.2008).
