J. Anim Sci. 2007. 85:3331-3339. doi:10.2527/jas.2006-418
© 2007 American Society of Animal Science
Effects of supplemental ruminally degradable protein versus increasing amounts of supplemental ruminally undegradable protein on nitrogen retention, apparent digestibility, and nutrient flux across visceral tissues in lambs fed low-quality forage1
R. L. Atkinson2,
C. D. Toone*,
T. J. Robinson,
D. L. Harmon
and
P. A. Ludden*,3
* Department of Animal Science, University of Wyoming, Laramie 82071; and and
Department of Animal Science, University of Kentucky, Lexington 40546
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Abstract
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Two experiments were conducted to determine effects of supplemental ruminally degradable protein (RDP) vs. increasing amounts of supplemental ruminally undegradable protein (RUP) on intake, apparent digestibility, N retention, and nutrient flux across visceral tissues in lambs fed low-quality forage. Lambs were fed a basal diet of crested wheatgrass hay (4.2% CP) for ad libitum consumption, plus 1 of 4 protein supplements: isolated soy protein (RDP source) fed to meet estimated RDP requirements (CON), or corn gluten meal (RUP source) fed at 50, 100, or 150% of the supplemental N provided by CON (C50, C100, and C150, respectively). In Exp. 1, 12 lambs (29.9 ± 2.7 kg) were used. Forage OM intake was not affected (P = 0.46) by protein degradability or by increasing RUP (P 
0.31). Apparent total tract OM digestibility was not affected (P = 0.10) by protein degradability, but increased (P 
0.004) with increasing RUP. Urinary N excretion was not affected (P = 0.20) by protein degradability, but increased (P 0.006) with increasing RUP. Similarly, N retention (g/d) was not affected (P = 0.69) by protein degradability, but increased (P = 0.001) as RUP increased. However, N retention (% of digested N) was not affected (P 0.40) by protein degradability or level of RUP. In Exp. 2, 16 catheterized lambs (32 ± 5 kg) were used. Net release of ammonia-N from the portal-drained viscera (PDV) was greater (P = 0.02) for CON than for C100 and increased linearly (P = 0.002) as RUP increased. Net uptake of ammonia-N by liver was not affected (P = 0.23) by protein degradability, but increased linearly (P = 0.04) as RUP increased. Net urea-N release from liver was not affected (P 0.49) by protein degradability or level of RUP. Net uptake of urea-N by PDV was greater (P = 0.02) for C100 compared with CON and increased (P = 0.04) with increasing RUP. Neither net release from PDV nor hepatic uptake of 
-amino N were affected (P 0.12) by protein degradability or level of RUP. Hepatic ammonia-N uptake accounted for 82, 38, 98, and 79% of net urea-N release from the liver for CON, C50, C100, and C150, respectively. Hepatic -amino N uptake for all treatments greatly exceeded that required for the remaining urea-N release by the liver, suggesting that -amino N may serve as a temporary means of storing excess N by liver between supplementation events. The pattern of net release or uptake of N metabolites between supplementation events requires further investigation.
Key Words: growing lamb • nitrogen retention • nutrient flux • ruminally undegradable protein
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INTRODUCTION
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Because low-quality forages (<7% CP) are often limiting in protein, a positive relationship exists between ruminally degradable protein (RDP) supplementation and forage utilization (Koster et al., 1996
). When dietary RDP is inadequate, the animal can sustain an adequate ruminal N supply through recycling of blood urea-N. The NRC (1985) suggests that if recycled N makes up a large proportion of the total supply of RDP, the long-term protein needs of the animal may be underestimated, resulting in decreased production. To counterbalance this effect, our hypothesis was that providing the animal with additional ruminally undegradable protein (RUP) will not only provide additional MP for tissue deposition, but a portion of that RUP will serve as a source of N for endogenous recycling. Ureagenesis from the N produced during deamination of AA supplied by supplemental RUP may occur more slowly than from excess ruminal ammonia. Thus, the greater hepatic ure-agenesis and subsequent increase in blood urea concentrations at times further removed from supplementation, when the dietary supply of RDP has been depleted and the demand for recycled N has increased, could result in an increase in efficiency of N utilization by the animal, concomitant with a potential reduction in the need for supplemental protein. Alternatively, because urinary excretion and ruminal recycling of blood urea are competing processes, the great reliance upon the N recycling process in this manner may decrease the overall efficiency of protein utilization, resulting in an increased demand for supplemental protein. Therefore, the objectives of this experiment were to examine the effects of supplemental RDP vs. increasing amounts of RUP on intake, apparent digestion of nutrients, N retention, and net flux of nutrients across visceral tissues in growing lambs fed low quality forage.
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MATERIALS AND METHODS
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Experiment 1
Animals and Diets.
Twelve Suffolk wether lambs (29.9 ± 2.7 kg initial BW) were used in a replicated completely randomized designed experiment to determine intake, apparent nutrient digestion, and N retention. Wethers were maintained in individual metabolism crates (1.4 x 0.6 m) at a constant temperature of 20°C under continuous lighting. Wethers had ad libitum access to fresh water and a trace mineralized salt block [Iofix T-M, Morton Salt, Chicago, IL; guaranteed analysis (% of DM) 97.1% NaCl, and 0.35% Zn, 0.28% Mn, 0.175% Fe, 0.035% Cu, 0.007% I, and 0.007% Co]. All animal care protocols were approved by the University of Wyoming Animal Care and Use Committee.
Wethers were fed a basal diet of mature crested wheatgrass hay (4.2% CP, 59% NDF, 42% ADF) for ad libitum consumption in 2 equal portions at 0630 and 1600 daily. Forage refusals were collected and weighed daily, and amount of forage offered adjusted to a minimum of a 10% refusal rate. Wethers were supplemented at 0600 daily with 1 of 4 supplemental protein treatments (Table 1). The control supplement (CON) was based upon isolated soy protein (ARDEX AF, Archer Daniels Midland Company, Decatur, IL) fed to meet estimated RDP requirements. The isolated soy protein contained 82% CP (DM basis), of which 100% of the CP was soluble in water. The RDP supplied by the forage (61.6% of CP) was determined by protein fractionation as described by Sniffen et al. (1992), and forage TDN (56.2% of DM) was estimated from the ADF value of the forage (Linn and Martin, 1989). Forage DMI was assumed to be 1,200 g/d (based upon average intakes during a 2-wk pretrial feeding period), and microbial efficiency was assumed to be 11% of TDN (Russell et al., 1992; Koster et al., 1996). Based upon these assumptions, the forage alone did not contain sufficient RDP (<2.6% of DM), and supplementation was necessary to meet RDP requirements. Consequently, an unsupplemented negative control treatment was not used. Three RUP supplements were based upon corn gluten meal and fed to supply 50, 100, or 150% of the supplemental CP provided by CON (C50, C100, and C150, respectively). The corn gluten meal contained 74.4% CP (DM basis) and was assumed to contain 59% RUP (% of CP; NRC, 1996). Supplements were fed at the rate of 0.236, 0.169, 0.285, and 0.402% of BW daily for the CON, C50, C100, and C150 treatments, respectively, throughout the experiment.
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Table 1. Ingredient and chemical composition of supplements1 fed to lambs consuming low-quality crested wheatgrass hay (Exp. 1 and 2)
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Sample Collection and Analysis.
Each of 2 experimental periods were 20 d in duration, with 14 d for diet adaptation followed by 6 d of sample collection. Wethers were removed from the metabolism crates and allowed to exercise in a drylot for 2 d between experimental periods. Wethers were reassigned to treatments in the second period such that no wether received the same treatment as in the first period (n = 6 observations per treatment). Total fecal and urinary output was collected on d 15 through 20 of each period. Urine was collected into plastic collection vessels containing 100 mL of 2 M phosphoric acid to inhibit ammonia volatilization. For each lamb, a 10% aliquot of the daily fecal output was composited within lamb, dried in a 55°C oven, and ground through a 1-mm screen (Wiley mill, Arthur H. Thomas Co., Philadelphia, PA). A 10% aliquot of the daily urinary output was composited within lamb and frozen at –20°C for later laboratory analysis. Feed and refusals were sampled on d 15 to 20 of each collection period and ground (1-mm screen) for later laboratory analysis. Feed, refusals, and feces were analyzed for DM and ash (AOAC, 1990) and for NDF and ADF content (Ankom 200 fiber analyzer, Ankom Technology, Fairport, NY). Feed, refusals, feces and urine were analyzed for N content (Leco model FP-528 Nitrogen Analyzer, Leco Corp., St. Joseph, MI).
Statistical Analyses.
Data were analyzed using the MIXED procedures of SAS (SAS Inst. Inc., Cary, NC) for a completely randomized design. The model included the effect of treatment and period. There were no period effects (P > 0.05); thus, data was pooled across period and only treatment effects are reported. Single df contrasts (Steel and Torrie, 1980) were used to compare the effects of protein degradability on an isonitrogenous basis (CON vs. C100) as well as to determine linear and quadratic effects within RUP supplemented treatments with significance set at P 0.05.
Experiment 2
Animals, Sample Collection, and Analysis.
Sixteen wether lambs (32 ± 5 kg initial BW) were used in a completely randomized designed experiment to examine nutrient flux across visceral tissues. Lambs were surgically fitted with chronic indwelling catheters in a hepatic vein, the hepatic portal vein, a mesenteric vein, and a mesenteric artery (McLeod et al., 1997). Catheters were prepared and maintained as described by Huntington et al. (1989). Lambs were fed the same basal diet (crested wheatgrass hay) and supplements described for Exp. 1. Lambs were assigned randomly and adapted to their respective diet prior to surgery and then were given a 1-wk recovery period from surgeries before sampling. At 0500 on day of sampling, a 15-mL priming dose of 1.5% (wt/vol) p-amino hippurate (PAH, pH = 7.4) was administered through a 0.45-µm filter (Whatman, Sanford, ME) into the mesenteric vein catheter followed by continuous infusion of 1.5% PAH (0.8 mL/min; model 22 syringe pump, Harvard Apparatus, Holliston, MA). Sixty minutes later, after blood PAH concentration had equilibrated (Huntington et al., 1989), simultaneous arterial, portal, and hepatic blood samples (5 mL) were collected immediately before feeding supplement (0600) and every hour thereafter for 6 h. This protocol was repeated 1 wk later from 12 to 18 h after supplementation. Blood was collected into heparinized syringes, transferred to EDTA blood collection tubes (Kendal Monoject, Mansfield, MA), centrifuged (1,300 x g, 10 min), and the resulting plasma was transferred to polypropylene tubes, placed on ice, and transported to the laboratory.
In the laboratory, plasma samples were analyzed immediately for ammonia-N by the L-glutamate dehydrogenase enzyme assay (Da Fonseca-Wollheim, 1973) and urea-N by the diacetylmonoxime method (Marsh et al., 1965). Plasma (500 µL) was deproteinized with an equal volume of 0.6 M HClO4 and centrifuged (13,000 x g, 15 min), and the supernatant was analyzed for -amino N (AAN; Palmer and Peters, 1969) and PAH concentrations (Harvey and Brothers, 1962). Feed and refusals were sampled daily throughout the experiment and were analyzed for DM, ash, N, NDF, and ADF content as described for Exp. 1.
Computations and Statistical Analysis.
Plasma flows (PF) through the portal-drained viscera (PDV) and liver were calculated based on the Fick principle (Katz and Bergman, 1969): PF = IRPAH/(CvPAH – CaPAH), where PF is in L/h, IRPAH is PAH infusion rate (mg/h), and CvPAH and CaPAH are PAH concentrations (mg/L) in portal venous or hepatic venous and arterial blood, respectively. Hepatic arterial plasma flow (APF) was calculated by difference between portal and hepatic venous flows. Net flux of nutrients across the PDV, hepatic, and total splanchnic (TS) vascular beds were computed using the following equations: PDV flux = PPF x (Cp – Ca); TS flux = HPF x (Ch – Ca); and hepatic flux = TS flux – PDV flux, where PPF and HPF are portal and hepatic venous plasma flow (L/h), and Ca, Cp, and Ch are nutrient concentrations in arterial, portal, and hepatic plasma, respectively. A positive net flux denotes absorption or release of a nutrient and a negative net flux denotes uptake or utilization of that nutrient. Hepatic extraction ratios (HR) were calculated using the equation: HR = {(HPF x Ch)/[(PPF x Cp) + (APF x Ca)]} – 1. A positive ratio indicates production, and a negative ratio indicates extraction or uptake by the liver.
Means were computed within lamb for arterial, portal, and hepatic concentrations of ammonia-N, urea-N, AAN, and PAH. Individual plasma flows deviating more than 2 SD from the mean were removed, and the mean was recalculated. All data were analyzed using the MIXED procedure of SAS for a completely randomized design. The model included the effects of treatment, time (0 to 6 h vs. 12 to 18 h), and the interaction. The random effect of lamb within treatment (specified in the RANDOM statement) accounted for the correlations among repeated observations on the same lamb. There were no treatment x time interactions (P 0.23); thus, only treatment means are reported. Single df contrasts (Steel and Torrie, 1980) were used to compare the effects of protein degradability on an isonitrogenous basis (CON vs. C100) as well as to determine linear and quadratic effects within RUP-supplemented treatments with significance set at P 0.10.
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RESULTS AND DISCUSSION
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Experiment 1
Forage OM intake was not affected by protein degradability (P = 0.46) or by increasing RUP (P 0.31; Table 2). By design, supplement intake increased with increasing RUP, resulting in an increase (P = 0.03) in total OM intake with increasing RUP. However, total OM intake did not differ (P = 0.76) between CON and C100. Fiber intake (NDF or ADF) was not affected (P 0.60) by protein degradability or increasing RUP (P 0.29). Swanson et al. (2000) reported that forage intake did not increase in mature ewes fed low-quality grass hay in response to increasing levels of supplemental RUP. Likewise, Salisbury et al. (2004) observed no difference in forage intake in wethers consuming low-quality blue grama and lovegrass hay supplemented with low or high RUP. However, those authors suggested that RDP from forage might have been adequate to maintain ruminal fermentation. In the current study, the forage was of low digestibility and supplied limited CP (4.2% CP), which necessitated supplementation with RDP to meet requirements. Although, lambs fed C100 were fed approximately 69% less supplemental RDP (31% total RDP) than CON lambs, they were able to maintain forage intake and digestion. The lack of response in forage intake suggests that lambs supplemented with RUP were recycling sufficient N to compensate for the RDP deficiency, potentially utilizing the increased supply of AA reaching the small intestine as a source of recyclable N. This suggestion is supported in a companion study (Atkinson et al., 2007), wherein neither apparent ruminal OM nor NDF digestion in lambs fed diets similar to the current study were affected by protein degradability or increasing RUP. Bandyk et al. (2001) observed an increase in forage intake in steers fed low-quality tallgrass-prairie hay and infused with casein either ruminally or postruminally. Those authors suggested that increased intake by steers given postruminal infusion of casein was dependent on recycling of postruminally infused N to the rumen as urea because the N from forage alone was inadequate to support rumen microbial metabolism.
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Table 2. Effects of supplemental ruminally degradable protein vs. increasing level of supplemental ruminally undegradable protein on intake and total tract digestibility of OM, NDF, and ADF in lambs consuming low quality forage (Exp. 1)
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Apparent total tract OM digestibility was not affected (P = 0.10) by protein degradability, but increased (P 0.004) with increasing levels of RUP. Similarly, apparent total tract NDF and ADF digestibilities were not affected (P 0.47) by protein degradability. However, increasing levels of RUP increased (P < 0.007) apparent total tract ADF digestibility linearly, but apparent total tract NDF digestibility was affected quadratically (P 0.02) by level of RUP. This is in contrast to others (Swanson et al., 2000, 2004; Bandyk et al., 2001; Salisbury et al., 2004) who observed no effect on apparent total tract digestion with increasing supply of RUP. These data suggest that total tract digestibility is not compromised as site of protein degradation is shifted from the rumen to the small intestine.
Forage N intake did not differ (P 0.31) due to protein degradability or level of RUP (Table 3). By experimental design, supplemental N intake increased with increasing RUP, but was less for C100 compared with CON. Consequently, total N intake was greater (P = 0.001) for CON compared with C100 and increased (P = 0.001) with increasing levels of RUP. Apparent total tract N digestibility was not affected (P = 0.70) by protein degradability, but increased (P = 0.001) with increasing levels of RUP. The increase in N digestion as RUP increased would be expected due to the increase in N intake from supplement, which was likely more digestible than the forage. However, Swanson et al. (2004) observed no difference in apparent N digestion as site of protein digestion shifted from the rumen to the small intestine. Salisbury et al. (2004) also observed no effect on total tract N digestion when low RUP or high RUP supplements were fed to wethers consuming low-quality forage. Galyean and Owens (1991) suggested that source of supplemental N has little effect on site of digestion of low-quality forage, possibly due to increased N recycling when supplements high in RUP are fed. In contrast, Swanson et al. (2000) observed an increase in N digestion as supplemental RUP increased in wethers fed low-quality grass hay when RDP was held constant within the supplements. The observation of Swanson et al. (2000) and data from the current study would suggest that an increased supply of AA to the small intestine enhanced total tract N digestion because increased protein flow postruminally has been shown to increase animal performance if protein is a limiting nutrient (Donaldson et al., 1991).
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Table 3. Effects of supplemental ruminally degradable protein vs. increasing level of supplemental ruminally undegradable protein on intake, apparent digestibility, and retention of N in lambs consuming low-quality forage (Exp. 1)
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Fecal N output was greater (P = 0.05) for CON compared with C100 lambs and increased (P 0.008) with increasing RUP. The increase in fecal N output by CON lambs contributed to greater (P = 0.009) total N output compared with C100 lambs, and total N output increased linearly (P = 0.001) as supplemental RUP increased. However, urinary N excretion (g/d) was not affected (P = 0.20) by protein degradability but increased (P 0.006) with increasing RUP. Similarly, urinary N excretion (% of N intake) was not affected (P = 0.46) by protein degradability but increased linearly (P = 0.03) with increasing RUP. Additionally, when expressed as a percentage of N digested, urinary N excretion was not affected (P 0.33) by protein degradability or level of RUP. Urinary urea-N excretion (g/d) was greater (P = 0.001) for C150 and CON lambs than C100 lambs and increased (P = 0.001) as RUP increased. However, urea-N excretion accounted for only 28, 11, 19, and 26% of urinary N output for CON, C50, C100, and C150, respectively. Because sheep typically excrete 25 to 60% of urinary N as urea (Sarraseca et al., 1998), the low percentages of urinary N as urea suggest that urea-N was efficiently recycled to the rumen rather than being excreted, especially for the C50 and C100 lambs. In the face of low urinary urea excretion, other constituents (i.e., nucleic acids) would make up a larger proportion of the N in the urine. Swanson et al. (2000) observed that urinary N output increased as RUP increased in wethers fed low-quality grass hay. Salisbury et al. (2004) observed no difference in fecal N or urinary N output between wethers supplemented with a low RUP vs. a high RUP when consuming a low-quality grass hay mixture. In contrast, as site of protein digestion was shifted from the rumen to the small intestine (Swanson et al., 2004), urinary and total N output decreased. This suggests that when utilizing a natural protein source for supplementation, N intake may have a greater influence on urinary N output than ruminal protein degradability.
Nitrogen balance was positive for all treatments. Overall, N retention (g/d or % of N intake) increased (P 0.002) with increasing RUP, but was similar (P 0.34) between CON and C100. However, N retention (% of N digested) did not differ (P 0.33) across treatments. Salisbury et al. (2004) observed no difference in N retention in wethers supplemented with low RUP vs. high RUP and consuming low-quality forage. The authors suggested that the lack of response may have resulted from similar total N flow to the duodenum and similar apparent postruminal N digestibility. Swanson et al. (2000) also observed an increase in N retention with increasing level of supplemental RUP in wethers fed low-quality grass hay. More recently, Swanson et al. (2004) observed that N retention (g/d and % of N intake) increased as casein infusion was shifted from 100% ruminal to 67% abomasal, but then decreased with 100% abomasal infusion in lambs fed low-quality (6.2% CP) bromegrass hay. Based on regression analysis, maximal N retention was predicted to occur with 68% of casein infused into the abomasum, but changing the percentage of postruminal casein infusion within the range of 33 to 100% resulted in minimal differences in N retention. The corn gluten meal used in the current study contained approximately 59% RUP (NRC, 1996), which is close to the recommendations of Swanson et al. (2004). This suggests that a mixture of RDP and RUP may increase the propensity for efficient N utilization. To determine the appropriate replacement value of our RUP supplement relative to CON, we calculated both linear and quadratic regressions of N retention (g/d) on the quantity of RUP provided (% of CON). Based upon this analysis, our RUP supplement would need to be provided at 99.6 or 93.7% of the N provided by CON, respectively, to provide the same level of N retention as CON. Although these estimates are close to 100%, and there was no difference in N retention between CON and C100, it may be possible to feed slightly less supplemental N as RUP without having detrimental effects on intake, digestion, or N retention. Nonetheless, any such decrease in supplemental N would need to be substantiated with further investigation.
Experiment 2
Similar to Exp. 1, forage and total OM intake did not differ (P 0.47) due to protein degradability or increasing RUP (data not shown), and averaged 595 and 651 g/d, respectively. Additionally, NDF intake was not affected (P 0.36) by protein degradability or level of RUP and averaged 539 g/d. However, total N intake increased (P 0.01) with increasing RUP due to the increase in supplemental N provided, with no difference (P = 1.00) between CON and C100. In general, the intakes achieved in Exp. 2 were less than those in Exp. 1.
Neither protein degradability nor level of RUP was a significant (P 0.19) source of variation in portal venous plasma flow (Table 4). However, hepatic arterial (P = 0.09) and hepatic venous (P = 0.07) plasma flows increased linearly with increasing RUP. Because ad libitum intake should minimize differences in blood flow with time after feeding (Goetsch et al., 1994), this increase in plasma flow can be attributed to the increased supplemental RUP.
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Table 4. Effects of supplemental ruminally degradable protein vs. increasing level of supplemental ruminally undegradable protein on plasma flow and metabolite concentration in lambs fed low-quality forage (Exp. 2)
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Arterial ammonia-N concentrations were greater (P = 0.05) for C100 than CON, but were not affected (P = 0.18) by increasing RUP. However, CON lambs had greater (P = 0.03) portal vein ammonia-N concentrations than C100 lambs. This would be expected due to the difference in ruminal degradability of the supplements. Moreover, as RUP increased, ammonia-N concentrations within the portal vein increased (P = 0.02). Although this may reflect an increase in supplemental RDP provided by our RUP supplement, it could also reflect a PDV N flux in excess of the demands of the microbes in the rumen. In contrast, ammonia-N concentrations within the hepatic vein were not affected (P 0.31) by protein degradability or increasing RUP. Arterial concentrations of AAN were greater (P = 0.09) for CON compared with C100; however, portal and hepatic venous concentrations of AAN were not affected (P 0.34) by protein degradability. Urea-N concentrations within the artery, portal vein, and hepatic vein were not affected (P 0.24) by protein degradability or level of RUP.
Release of ammonia-N from the PDV was greater (P = 0.02) for CON than C100 and increased (P 0.05) with increasing RUP (Table 5). Similarly, Ferrell et al. (1999) observed an increase in PDV release of ammonia-N when supplementing soybean meal vs. RUP to sheep consuming a low-quality forage. This pattern of PDV release reflects the solubility of the nitrogenous sources being supplemented and the rate of degradation and release of ammonia within the rumen. Hepatic uptake of ammonia-N mirrored PDV release, increasing (P = 0.04) with increased RUP, but was not affected (P = 0.23) by protein degradability. Net splanchnic uptake of ammonia-N was not affected (P 0.47) by protein degradability or level of RUP, suggesting that the liver had sufficient capacity to detoxify the ammonia-N presented.
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Table 5. Effects of supplemental ruminally degradable protein vs. increasing level of supplemental ruminally undegradable protein on net portal, hepatic, total splanchnic flux of metabolites,1 and hepatic ratio in lambs consuming low-quality forage (Exp. 2)
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There was no effect due to protein degradability or level of RUP on PDV release (P 0.17) or hepatic uptake (P 0.12) of AAN. Similarly, Ferrell et al. (1999) observed no difference in PDV release or hepatic uptake of AAN in sheep supplemented with soybean meal or RUP while consuming low-quality forage. There was a net release of AAN by the TS bed in lambs fed CON, whereas C100 lambs took up AAN (P = 0.04). Bohnert et al. (1999) observed that lambs supplemented with various RUP sources had greater AAN removed by the liver compared with lambs supplemented with soybean meal. Possible explanations for this apparent discrepancy include differences in AA composition of protein reaching the small intestine, the pattern of AA absorbed in relation to requirements, or AA absorbed as small peptides (Bohnert et al., 1999).
Uptake of urea-N by the PDV was greater (P = 0.02) for C100 compared with CON and increased linearly (P = 0.04) as RUP increased. Increased removal of urea-N by the PDV with the inclusion of RUP would suggest enhancement of N recycling to the gastrointestinal tract. Kennedy and Milligan (1980) suggested that transfer of blood urea into the rumen is affected by ruminal ammonia concentration and the amount of OM fermented in the rumen. In lambs fed similar diets in a companion study (Atkinson et al., 2007), ruminal ammonia concentrations were decreased with the inclusion of RUP, but OM fermentation was not affected. This suggests that the increased removal of urea-N by the PDV with the inclusion of RUP is influenced by ruminal ammonia concentrations. In contrast, Bohnert et al. (1999) and Ferrell et al. (1999) observed that protein degradability (soybean meal vs. RUP) did not influence PDV uptake of urea-N. Despite difference in hepatic uptake of ammonia-N, hepatic release of urea-N was not affected (P 0.49) by level of RUP or protein degradability. However, both Bohnert et al. (1999) and Ferrell et al. (1999) observed an increase in hepatic release of urea-N in lambs supplemented with soybean meal compared with lambs supplemented with RUP, but this difference can be attributed to differences in hepatic uptake of ammonia-N.
Total N uptake (NH3-N + AAN) by the liver accounted for 100.5% of urea-N synthesis in C50 lambs, but was 195, 146, and 156% of urea synthesis in CON, C100, and C150 lambs, respectively (Figure 1). Consequently, the quantity of N removed by the liver greatly exceeded that of hepatic ureagenesis for those treatments. Similarly, Bohnert et al. (1999) reported that 34 to 43% of the N removed was not converted to urea by the liver. This suggests that a portion of the AAN taken up by the liver was converted to a metabolite other than urea (Krehbiel et al., 1998), thereby decreasing the conversion of total N uptake into urea-N. We further believe that excess AAN may be utilized for protein synthesis, thereby permitting for short-term storage within the liver, which may be made available for utilization between supplementation events. Fluharty and McClure (1997) and Hersom et al. (2004) have shown that as dietary CP content increases, liver mass also increases in lambs and steers, respectively. Therefore, the increase in liver mass could partially be due to synthesis of labile proteins, resulting in short-term storage of protein within the liver. As such, the inclusion of RUP may delay the timing at which N is utilized for ureagenesis. The potential for prolonged deamination of the AA contained in supplemental RUP may provide a mechanism whereby ureagenesis is delayed within the liver. The specific mechanism responsible for the variation in the net flux of urea-N throughout the supplementation interval remains to be elucidated. However, we may have been able to observe this potential effect of RUP on ureagenesis had we chosen to utilize a longer sampling window (i.e., 0 to 12 h). Therefore, further investigation with an extended sampling window is needed to pattern this effect of supplementation frequency.
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Figure 1. Effects of supplemental ruminally degradable protein (RDP) versus increasing level of supplemental ruminally undegradable protein (RUP) on total hepatic N (ammonia-N + AAN) uptake and urea-N synthesis in lambs consuming low-quality forage (alpha-amino N SEM = 14.5, ammonia-N SEM = 5.8, and urea-N SEM = 22). Supplements were: CON = supplemental RDP fed to meet estimated RDP requirements; C50, C100, and C150 = supplemental RUP fed at 50, 100, or 150% of the supplemental N provided by CON, respectively.
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Conclusions
Decreasing the ruminal degradability of supplemental protein fed to ruminants consuming low-quality forages has the potential to enhance N recycling while maintaining a positive N balance and excreting less N into the environment. Furthermore, it may be possible to provide slightly less N as ruminally undegradable protein without having detrimental effects on overall animal performance. This greater reliance upon AA vs. ammonia-N for ureagenesis may permit the short-term storage of excess N by the liver between supplementation events, thereby enhancing the potential for recycling at times removed from supplementation.
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Footnotes
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1 This project was supported in part by the National Research Initiative Competitive Grant No. 2003-35206-12818 from the USDA Cooperative State Research, Education, and Extension Service. Appreciation is also extended to ADM Alliance Nutrition Inc., Decator, IN, for donation of the ARDEX AF used in this research. We would also like to thank M. J. Nijland and S. P. Ford for the use of their surgical equipment and D. True for analytical assistance associated with this research. 
2 Current address: Department of Animal Science, Food & Nutrition, Ag Bldg. – MC 4417, Southern Illinois University, Carbon-dale 62901.
3 Corresponding author: ludden{at}uwyo.edu
Received for publication June 29, 2006.
Accepted for publication August 16, 2007.
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LITERATURE CITED
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AOAC. 1990. Official Methods of Analysis. 15th ed. Assoc. Off. Anal. Chem., Arlington, VA.
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R. L. Atkinson, C. D. Toone, and P. A. Ludden
Effects of supplemental ruminally degradable protein versus increasing amounts of supplemental ruminally undegradable protein on site and extent of digestion and ruminal characteristics in lambs fed low-quality forage
J Anim Sci,
December 1, 2007;
85(12):
3322 - 3330.
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Abstract
INTRODUCTION
