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J. Anim Sci. 2007. 85:3322-3330. doi:10.2527/jas.2006-417
© 2007 American Society of Animal Science
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ANIMAL NUTRITION

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 forage1

R. L. Atkinson2, C. D. Toone and P. A. Ludden3

Department of Animal Science, University of Wyoming, Laramie 82071


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Four ruminally and duodenally cannulated Suffolk wether lambs (34.5 ± 2 kg initial BW) were used in a 4 x 4 Latin square designed experiment to compare effects of supplemental ruminally degradable protein (RDP) vs. increasing amounts of supplemental ruminally undegradable protein (RUP) on ruminal characteristics and site and extent of digestion in lambs. 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 assuming a microbial efficiency of 11% of TDN (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). Neither NDF nor ADF intake was affected (P ≥ 0.18) by protein degradability, but they increased or tended to increase (P ≤ 0.07) with increasing level of RUP. Total OM and N intakes were similar (P ≥ 0.26) for CON and C100, but increased (P ≤ 0.01) as level of RUP increased. True ruminal OM and ruminal digestibilities of NDF and ADF were not affected (P ≥ 0.33) by protein degradability. However, true ruminal N digestibility was greater (P = 0.03) for CON compared with C100. Ruminal ammonia concentrations were greater (P = 0.002) for CON compared with C100 lambs, and increased (P = 0.001) with increasing RUP. Microbial N flows were not affected (P ≥ 0.12) by protein degradability or increasing RUP. Likewise, neither ruminal urease activity (P ≥ 0.11) nor microbial efficiency (P ≥ 0.50) were affected by protein degradability or level of RUP. Total tract OM, NDF, and ADF digestibility was greater (P ≤ 0.05) for C100 compared with CON. Likewise, total tract N digestibility was greater (P = 0.03) for C100 than for CON, and increased linearly (P = 0.001) with increasing RUP. Lambs fed C100 consumed approximately 69% less supplemental RDP (31% less total RDP) than CON, but were able to maintain forage intake and digestion. This lack of response in forage intake would suggest that lambs supplemented with RUP were recycling sufficient N to compensate for an apparent RDP deficiency. Although ruminal degradability of protein has little effect on forage intake or ruminal digestion of nutrients, there is potential to enhance total tract digestion of nutrients by decreasing the ruminal degradability of supplemental protein.

Key Words: growing lamb • low-quality forage • nitrogen digestion • ruminally undegradable protein


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Because low-quality forages (<7% CP) are often limiting in the supply of protein available for ruminal degradation, a positive relationship exists between ruminally degradable protein (RDP) supplementation and forage utilization (Koster et al., 1996Go). When dietary RDP is inadequate, the animal can sustain an adequate ruminal N supply through recycling of blood urea N (Van Soest, 1994Go). However, the NRC (1985)Go 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. However, the level of supplemental RUP required to support this process remains to be elucidated. In addition to moderating ruminal ammonia levels immediately following supplementation (Bohnert et al., 2002Go), the prolonged deamination of AA contained in supplemental RUP may support a more stable ruminal environment by providing the animal with a sustained source of recyclable N. Thus, forage intake and utilization by the animal may be maintained concomitant with a potential reduction in the need for supplemental protein. Alternatively, greater reliance upon the N recycling process in this manner could decrease the overall efficiency of protein utilization, resulting in an additional demand for supplemental protein.

Therefore, the objectives of this experiment were to examine the effects of supplemental RDP vs. increasing amounts of RUP on site and extent of digestion and ruminal characteristics in growing lambs fed low quality forage.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 LITERATURE CITED
 
Animals, Diets, and Sampling

All animal care protocols were approved by the University of Wyoming Animal Care and Use Committee.

Four Suffolk wether lambs (34.5 ± 2.0 kg initial BW) were fitted with ruminal and T-type duodenal (inserted cranial to the common bile and pancreatic duct) cannulas and used in a 4 x 4 Latin square-designed experiment. Wethers were maintained in individual metabolism crates (1.4 x 0.6 m) at a constant room 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].

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 the amount of forage offered was adjusted to a minimum of a 10% refusal rate. Wethers were supplemented at 0600 daily with 1 of 4 supplemental protein treatments (Table 1Go).


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  		Table 1. Ingredient and chemical composition of supplements1 fed to lambs consuming low-quality crested wheatgrass hay
 
The control supplement (CON) was based on 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)Go, and forage TDN (56.2% of DM) was estimated from the ADF value of the forage (Linn and Martin, 1989Go). 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., 1992Go; Koster et al., 1996Go). Based on these assumptions, the forage alone did not contain sufficient RDP (<2.6% of DM), and supplementation was necessary to meet the 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, 1996Go). 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.

Four experimental periods were each 17 d in duration, with the first 14 d for adaptation followed by 3 d of sample collections. On d 7 through 16, wethers received intraruminal doses of 2.5 g of TiO2 via gelatin capsules (Torpac Inc., Fairfield, NJ) at each feeding as an indigestible digesta flow marker. On d 15 through 16 of each experimental period, duodenal (150 mL) and fresh fecal (10 g) samples were collected at 4-h intervals. Collection times were advanced by 2 h on d 16, such that samples represented every 2 h in a theoretical 24-h clock. Duodenal samples were composited within wether by collection period and frozen at –20°C until lyophilized (Genesis 25 freeze dryer, The VirTis Co., Gardiner, NY) at the end of the experiment. Fecal samples were composited within wether for each collection period and dried in a 55°C forced-air oven. Duodenal and fecal samples were ground through a 1-mm screen (Wiley mill, Arthur H. Thomas Co., Philadelphia, PA) before subsequent laboratory analyses. Feed and refusals were sampled daily throughout each collection period and ground through a 1-mm screen (Wiley mill) for subsequent laboratory analysis.

On d 17 of each collection period, 100 mL of whole ruminal contents were collected from each wether immediately before feeding (0 h sampling time) and at 3-h intervals for 24 h. Ruminal pH was determined immediately for each fresh ruminal sample using a combination electrode (Orion Research Inc., Boston, MA). Ruminal contents were then strained through 4 layers of cheesecloth, and 10 mL of the strained fluid was acidified with 0.1 mL of 3.6 M H2SO4 and frozen for later VFA and NH3 analysis. The remaining whole ruminal contents were placed in a blender (Hamilton Beach/ Proctor Silex, Washington, NC) with an equal volume of 0.9% NaCl (wt/vol) solution and homogenized for 1 min to dislodge particulate-associated bacteria. The homogenized solution was then strained through 8 layers of cheesecloth and frozen for later bacterial isolation by differential centrifugation (Merchen et al., 1986Go). The resulting bacterial isolate was lyophilized and ground with a mortar and pestle for subsequent laboratory analysis. On d 17 of the collection period, an additional sample (15 mL) of ruminal fluid was collected from the ventral sac with a suction strainer at 3 h after the morning feeding; this sample was placed on ice and transported to the laboratory for immediate determination of urease activity (Ludden et al., 2000Go).

Laboratory Analysis

Feed, refusals, bacterial isolates, duodenal digesta, and fecal samples were analyzed for DM and ash (AOAC, 1990Go) and for N content (Leco Model FP-528 Nitrogen analyzer, Leco Corp., St. Joseph, MI). The NDF and ADF contents of feed, refusals, duodenal digesta, and fecal samples were determined using an Ankom 200 fiber analyzer (Ankom Technology, Fairport, NY). Duodenal and fecal samples were analyzed for TiO2 concentrations according to the methods of Myers et al. (2004)Go. Duodenal and isolated bacteria samples were analyzed for purine concentration as described by Zinn and Owens (1986)Go and modified by Obispo and Dehority (1999)Go. Ruminal NH3 concentrations were determined by the phenol-hypochlorite procedure (Broderick and Kang, 1980Go). Ruminal fluid was analyzed for VFA concentrations (Goetsch and Galyean, 1983Go) using a Hewlett-Packard 5890 gas liquid chromatograph (Hewlett-Packard, Avondale, PA) equipped with a 15 m x 0.53 mm (i.d.) column (Nukol, Supelco, Bellefonte, PA) and with an initial oven temperature of 110°C to a final temperature of 150°C at 8°C/min. Helium was used as carrier gas, with a column flow rate of 20 mL/ min. Injector and flame ionization detector temperatures were 250°C.

Calculations and Statistical Analysis

Dry matter flow was calculated by dividing the amount of TiO2 dosed by the concentration of TiO2 (DM basis) in the sample (duodenal and fecal). Digesta flows of OM, N, NDF, and ADF were calculated by multiplying the nutrient concentration (DM basis) at each site by DM flow. True ruminal OM and N digestibilities were calculated by correcting duodenal OM and N flows for the contribution of bacteria to OM and N.

All site and extent of digestion data were analyzed using the MIXED procedure (SAS Inst. Inc., Cary, NC) using the model for a Latin square design. Ruminal fermentation data (pH, NH3, and VFA) were analyzed using the MIXED procedure of SAS for repeated measures. The model included period, treatment, animal ID, time, and the treatment x time interaction. The 3-way interaction of animal x period x treatment was used to specify variation among animals using the RANDOM statement of SAS. An autoregressive covariance structure (AR1 of the MIXED procedure) was determined to be most appropriate based on Akaike’s information criterion. Single df contrasts (Steel and Torrie, 1980Go) 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.


   RESULTS AND DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
LITERATURE CITED
 
OM Intake and Digestion

Forage OM intake was not affected (P = 0.17) by protein degradability but tended (P = 0.09) to increase as the level of RUP increased (Table 2Go). This tendency, along with the increase in quantity of supplement OM, resulted in a linear increase (P = 0.01) in total OM intake as level of RUP increased. However, total OM intake was similar (P = 0.26) between CON and C100. Bandyk et al. (2001)Go observed that steers consuming a low-quality tallgrass-prairie forage and receiving ruminal infusion of casein had greater forage and total OM intakes compared with steers receiving postruminal casein infusion. However, Salisbury et al. (2004)Go observed no difference in forage OM intake in wethers consuming blue grama and lovegrass hay (7.5% CP) and supplemented with low or high RUP. Moreover, Swanson et al. (2000)Go observed no difference in forage or total intake when ewes consuming low-quality forage (6.5 to 7.1% CP) were supplemented with low, medium, or high RUP. Swanson et al. (2000)Go and Salisbury et al. (2004)Go suggested that the lack of response to RUP supplementation may be because forage RDP was adequate for ruminal fermentation. However, the forage fed in the current experiment was of low digestibility and of limited protein (4.2% CP), which necessitated supplementation to meet requirements. Similar forage OM intake between CON and C100 would suggest 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.


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  		Table 2. Effects of supplemental ruminally degradable protein vs. increasing amounts of supplemental ruminally undegradable protein on OM intake and digestion by lambs fed low-quality forage
 
Duodenal OM flow increased linearly (P = 0.01) with increasing RUP, but was similar (P = 0.22) between CON and C100. Moreover, protein degradability had no effect (P = 0.30) on microbial OM flow, but tended (P = 0.09) to increase with increasing RUP. Despite differences in duodenal OM flow, apparent ruminal OM digestibility (% of intake) was not affected (P = 0.88) by protein degradability or by increasing levels of RUP (P ≥ 0.55). Conversely, Swanson et al. (2000)Go observed an increase in OM digestibility in ewes consuming a medium or high RUP supplement compared with ewes consuming a low RUP supplement, but this increase was likely from the greater digestibility of the supplement. Similar to our study, others have not observed differences in OM digestion when supplementing low or high RUP to wethers (Salisbury et al., 2004Go) or infusing casein ruminally or postruminally to steers (Bandyk et al., 2001Go). In our study, true ruminal OM digestibility was not affected (P = 0.97) by protein degradability or by level of RUP supplemented (P ≥ 0.45). This observation was unexpected because we replaced RDP with RUP, thereby creating an RDP deficiency, which should have depressed true ruminal OM digestibility, especially for the C50 lambs. Because true ruminal OM digestibility was not affected by treatment, this suggests that the provision of RUP may have enhanced N recycling in an effort to sustain microbial metabolism.

Lower tract OM digestibility tended (P = 0.07) to be greater for C100 compared with CON, but was not affected (P ≥ 0.15) by increasing RUP. This supports the suggestion of Galyean and Owens (1991)Go that source of supplemental N (nonprotein N, natural protein, RDP, or RUP) has little to no effect on site of digestion of low-quality forage. However, total tract OM digestibility was greater (P = 0.02) for C100 compared with CON, but only tended (P = 0.08) to increase as level of RUP increased. Others have not observed an effect of increasing level of RUP on total tract OM digestion in steers grazing annual ryegrass (Donaldson et al., 1991Go) or in wethers consuming low-quality forage (Salisbury et al., 2004Go).

Fiber Intake and Digestion

The intake of NDF and ADF from forage mimicked forage OM intake, with no difference (P ≥ 0.14) due to protein degradability but a trend (P ≥ 0.08) toward increased intake as level of RUP increased (Tables 3Go and 4Go). Additionally, total NDF intake tended (P = 0.07) to increase and ADF intake increased (P = 0.05) with increasing levels of RUP, with no difference (P ≥ 0.18) due to protein degradability.


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  		Table 3. Effects of supplemental ruminally degradable protein vs. increasing amounts of supplemental ruminally undegradable protein on NDF intake and digestion by lambs fed low-quality forage
 

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  		Table 4. Effects of supplemental ruminally degradable protein vs. increasing amounts of supplemental ruminally undegradable protein on ADF intake and digestion by lambs fed low-quality forage
 
Duodenal NDF flow tended (P = 0.06) to be greater for CON compared with C100, but apparent ruminal NDF digestibility (% of intake) was not affected by protein degradability (P = 0.33) or by increasing levels of RUP (P ≥ 0.48). Duodenal ADF flow was not affected (P = 0.21) by protein degradability, but tended (P = 0.07) to increase with increasing level of RUP. However, apparent ruminal ADF digestibility was not affected (P ≥ 0.27) by protein degradability or level of RUP. Similar to the current study, others have not observed differences in NDF digestion when supplementing increasing levels of RUP to ewes (Swanson et al., 2000Go), low vs. high RUP to wethers (Salisbury et al., 2004Go), or infusing casein ruminally or postruminally to steers (Bandyk et al., 2001Go).

Similar to apparent ruminal digestion, lower tract NDF and ADF digestibilities were not affected (P ≥ 0.15) by protein degradability or increasing level of RUP (P ≥ 0.14). This further supports the suggestion of Galyean and Owens (1991)Go that source of supplemental N (nonprotein N, natural protein, RDP, or RUP) has little to no effect on site of digestion of low-quality forage. Conversely, total tract NDF (P = 0.03) and ADF (P = 0.05) digestibilities were greater for C100 compared with CON. However, Salisbury et al. (2004)Go did not observe an effect of increasing level of RUP on total tract NDF digestion in wethers consuming low-quality forage, which may be attributed to adequate RDP for forage digestion. This suggests that restricting ruminal RDP supply with RUP supplementation may result in a shift in fiber digestion from the rumen to the lower tract. Because apparent ruminal NDF digestibility was not affected by protein degradability, but total tract NDF digestibility increased for C100 compared with CON, this suggests that more NDF was digested postruminally (cecum and large intestine) and could be due to the increased supply of AA postruminally.

Nitrogen Intake and Digestion

Forage N intake was not affected (P ≥ 0.11) by protein degradability or level of RUP (Table 5Go). However, total N intake increased linearly (P = 0.0001) with increasing RUP, which reflects the linear increase in quantity of supplemental N. Total duodenal N flow increased linearly (P = 0.0001) with increasing RUP, but duodenal N flow did not differ (P = 0.16) due to protein degradability. Conversely, microbial N flow was not affected (P ≥ 0.12) by protein degradability or increasing level of RUP. Therefore, the increased total N flow with increasing RUP reflected the increased (P = 0.0002) flow of nonmicrobial N from the supplemented RUP. The lack of differences in OM truly fermented and microbial N flow resulted in no difference (P ≥ 0.52) in microbial efficiency due to protein degradability or level of RUP. This lack of response in microbial efficiency supports the contention that provision of RUP enhanced N recycling, thereby allowing ruminal microbes to sustain metabolism. The ability of RUP supplementation to maintain digestion and microbial efficiency in the face of an apparent RDP deficiency suggests that the contribution of endogenous N recycling to ruminal N status is under-valued. Apparent ruminal N digestibility was greater (P = 0.004) for CON compared with C100 and tended to increase linearly (P = 0.07) with increasing RUP. True ruminal N digestibility (% of intake) was greater (P = 0.008) for CON vs. C100, but was not affected (P ≥ 0.18) by level of RUP, which reflects the inclusion of RUP in the supplement. Donaldson et al. (1991)Go and Salisbury et al. (2004)Go did not observe a decrease in ruminal N digestion in steers or wethers supplemented with increasing amounts of RUP. Donaldson et al. (1991)Go suggested that their results were confounded because of the increased forage intake that was observed for steers supplemented with both low and high RUP, thus RDP from the forage contributed to the lack of response. Additionally, Salisbury et al. (2004)Go held N intake constant in wethers supplemented low vs. high RUP and suggested that this was the reason for their lack of response in apparent ruminal N digestibility. In our study, N intake increased due to supplementation, whereas forage intake did not differ; thus the increased N digestibility observed can be attributed to the increased supply of supplemental N in the form of RUP or the increase in total N flow.


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  		Table 5. Effects of supplemental ruminally degradable protein vs. increasing amounts of supplemental ruminally undegradable protein on N intake and digestion by lambs fed low-quality forage
 
Lower tract N digestibility was greater (P = 0.001) for C100 compared with CON and increased linearly (P = 0.001) with increasing level of RUP. This suggests that the RUP provided by corn gluten meal was readily digestible in the small intestine. Similar to lower tract digestibility, total tract N digestibility tended to be greater (P = 0.14) for C100 compared with CON and increased linearly (P = 0.0007) with increasing level of RUP. Similarly, Swanson et al. (2000)Go observed an increase in CP digestion in ewes consuming low-quality forage and supplemented with high levels of RUP compared with ewes fed low levels of RUP. Salisbury et al. (2004)Go observed no effect of low RUP vs. high RUP on total duodenal N flow, but did observe a decrease in total tract N digestion in wethers fed high RUP vs. low RUP. Moreover, those authors observed a decrease in microbial N flow to the duodenum as level of RUP increased, but level of RUP did not affect microbial efficiency. Similar to our study, Caton et al. (1994)Go observed an increase in total tract N digestion as level of RUP increased in steers offered chopped grass hay, but there was no effect of level of RUP on microbial N flow or microbial efficiency. Additionally, as RUP replaced RDP in supplements fed to wethers consuming grass hay, Schloesser et al. (1993)Go observed no effect on total or microbial N flow to the duodenum, suggesting that ruminal ammonia-N and other growth factors were sufficient to meet the requirements for microbial protein synthesis.

These data suggest that it is possible to maintain intake and digestion of low-quality forage through the provision of RUP rather than RDP. However, if one were to use the current prediction model for endogenous N recycling, urea N recycled would have been underestimated. Estimated N recycled (based on dietary CP content; NRC, 1985Go) would have been 43.6% of intake, and thus both CON and C100 would have recycled 5.3 g/d. However, in a companion study (Atkinson et al., 2007Go), portal-drained visceral uptake of urea-N was 9.14 g/d greater for C100 compared with CON. Based upon the true ruminal N digestibility from this experiment, lambs fed C100 consumed 31% less total RDP, or approximately 69% less supplemental RDP (assuming the forage was 61.6% RDP as a percentage of CP) than lambs fed CON. This suggests that the current prediction model (NRC, 1985Go) would have underestimated endogenous N recycling. Moreover, C100 lambs were able to maintain forage intake and digestion in spite of this apparent RDP deficiency. This suggests that C100 lambs needed to recycle more urea N in order to sustain ruminal N status. Therefore, we suggest that future prediction equations for endogenous N recycling should include a protein degradability component to better predict overall ruminal N status.

Ruminal Characteristics

Neither protein degradability (P = 0.66) nor increasing level of RUP (P = 0.15) affected ruminal pH (Table 6Go). A treatment x hour interaction (P < 0.001; Figure 1Go) was observed for ruminal NH3 concentrations, which peaked by 3 h after supplementation for CON, then decreased and was similar to C100 by 12 h after supplementation. However, RUP-supplemented treatments peaked at lower concentrations and remained consistent through the remainder of the supplementation interval. These data are consistent with others (Schloesser et al., 1993Go; Bandyk et al., 2001Go) who observed decreases in ruminal NH3 concentrations as RUP replaced RDP. However, urease activity was not affected (P ≥ 0.11) by protein degradability or increasing level of RUP. We had expected urease activity to be greater in lambs fed RUP given the inverse relationship that exists between ruminal ammonia concentrations and the expression of urease activity in the rumen (Cheng and Wallace, 1979Go). Likewise, Kennedy and Milligan (1980)Go noted that ruminal urease activity was negatively cor-related with ruminal NH3 concentrations, suggesting a mechanism of feedback inhibition of NH3 on urease activity. Thus, the lack of treatment effects on urease activity may suggest a greater degree of recycling via the salivary route rather than diffusion across the ruminal wall, as would be typical of animals consuming greater amounts of forage (Huntington and Archibeque, 1999Go). Ludden et al. (2000)Go concluded that despite a 77% reduction in urease activity, sufficient urease activity remained to completely hydrolyze urea to ammonia in lambs fed up to 2% dietary urea. This would suggest that although urease facilitates N recycling by maintaining a positive concentration gradient for urea-N diffusion into the rumen, relative changes in urease activity may play only a minor role in the regulation of the N recycling process. Moreover, the adaptation of urease activity to the NH3 level in the ruminal fluid may have required a period longer than that studied in our experiment (Cheng and Wallace, 1979Go).


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  		Table 6. Effects of supplemental RDP vs. increasing amounts of supplemental RUP on ruminal characteristics of lambs fed low-quality forage
 

Figure 1
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  		Figure 1. Effects of supplemental ruminally degradable protein (RDP) vs. increasing amounts of supplemental ruminally undegradable protein (RUP) on ruminal ammonia concentrations over the 24-h supplementation interval (treatment x hour interaction, P < 0.001, SEM = 0.37). 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.

 
Total VFA concentrations were not affected (P ≥ 0.47) by treatment or increasing levels of RUP. However, a treatment x hour interaction (P ≤ 0.05) was observed for acetate, isobutyrate, isovalerate, and valerate; nonetheless, there was no definite pattern of biological importance (data not shown). Although mean isovalerate concentrations increased (P = 0.03) linearly as level of RUP increased, there was no difference (P ≥ 0.26) in the other individual VFA concentrations. The branched-chain AA leucine is metabolized to the branched-chain VFA isovalerate (Wallace et al., 1997Go), and corn gluten meal characteristically contains a high level of leucine (>16%; Ludden and Cecava, 1995Go; Ludden and Kerley, 1997Go); therefore, as level of RUP increases we would have expected isovalerate concentrations to increase. Similarly, Bandyk et al. (2001)Go observed no treatment effect on total VFA concentrations in steers receiving casein ruminally or postruminally, but ruminal casein infusion resulted in greater concentrations of branched-chain VFA. In contrast, Salisbury et al. (2004)Go observed that wethers supplemented with low RUP had greater total VFA concentrations compared with wethers supplemented with high RUP, with no difference in individual VFA concentrations.

Conclusions

Ruminal degradability of protein has little effect on forage intake or ruminal digestion of nutrients in lambs fed low-quality forage, and RUP may support greater production by supplying additional MP once microbial N requirements are met. However, decreasing the ruminal degradability of supplemental protein has the potential to enhance total tract digestion of nutrients, potentially by moderating ruminal ammonia concentrations and providing a mechanism for sustained recycling of endogenous N. Consequently, ruminal N status could be better estimated if prediction models included a ruminal protein degradability component rather than estimating the contribution of N recycling simply from dietary protein concentration.


   Footnotes
 
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., Decatur, IN, for donation of the ARDEX AF used in this research. Back

2 Current address: Department of Animal Science, Food & Nutrition, Ag. Bldg.—MC 4417, Southern Illinois University, Carbon-dale 62901. Back

3 Corresponding author: ludden{at}uwyo.edu

Received for publication June 29, 2006. Accepted for publication August 16, 2007.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 


AOAC. 1990. Official Methods of Analysis. 15th ed. Assoc. Off. Anal. Chem., Arlington, VA.

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Bohnert, D. W., C. S. Schauer, S. J. Falck, and T. DelCurto. 2002. Influence of rumen protein degradability and supplementation frequency on steers consuming low-quality forage: II. Ruminal fermentation characteristics. J. Anim. Sci. 80:2978–2988.[Abstract/Free Full Text]

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R. L. Atkinson, C. D. Toone, T. J. Robinson, D. L. Harmon, and P. A. Ludden
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 forage
J Anim Sci, December 1, 2007; 85(12): 3331 - 3339.
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