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Nitrogen balance in lambs fed a high-concentrate diet and infused with differing proportions of casein in the rumen and abomasum^1,2

USDA, ARS, U.S. Meat Animal Research Center, Clay Center, NE 68933

Abstract

Twenty-five wether lambs (34 ± 0.9 kg) fitted with ruminal and abomasal infusion catheters were used in a completely randomized design to determine the effects of differing proportions of ruminal and abomasal casein infusion on N balance in lambs fed a high-concentrate diet (85% corn grain, 1.6% N; DM basis) for ad libitum intake. Wethers were infused with 0 (control) or 10.4 g/d of N from casein with ruminal:abomasal infusion ratios of 100:0, 67:33, 33:67, or 0:100% over a 14-d period. Feed, orts, feces, and urine were collected over the last 5 d. Total N intake and excretion were greater (P < 0.01) in lambs infused with casein than in controls; however, N retention did not differ in lambs infused with casein compared with controls, suggesting that N requirements were met without casein supplementation. Total N intake and total N excretion did not differ among casein infusion treatments. Urinary N excretion decreased linearly (P = 0.07) with decreasing ruminal infusion of casein. Site of casein infusion quadratically (P = 0.06) influenced N retained (g/d), with the greatest retention observed in the 33:67 ruminal:abomasal infusion treatment. Dry matter intake from feed decreased from 1,183 to 945 g/d (P = 0.02) in lambs infused with casein compared with controls, but apparently digested DM did not differ among treatments. These data indicate that decreasing the ruminal degradability of supplemental protein above that required to maximize N retention results in decreased urinary excretion of N without greatly affecting apparent diet digestion.

Key Words: Nitrogen Metabolism • Nutrient Balance • Sheep

Introduction

An important goal among livestock producers is to increase (or maintain) livestock production, while decreasing nutrient losses. Unfortunately, the efficiency of conversion from feed to product is low (Lobley, 1992). Volatilization of N from the feedlot surface in the form of ammonia is a large concern because of the environmental consequences and the imbalance it creates in the N:P ratio when manure is used as fertilizer (Van Horn et al., 1996; Varel et al., 1999). Because the major source of ammonia is urea from urine, dietary regimens that decrease urinary N excretion or shift the site of N excretion from urine to feces may be valuable in reducing ammonia emissions from feedlots (Bierman et al., 1999).

Protein supplement sources differ in protein concentration, ruminal and postruminal degradability (site of digestion), and amino acid composition (NRC, 1996). Site of protein digestion can be an important factor determining metabolizable protein supply. However, when comparing sources of supplemental protein with differing protein degradability, it can be difficult to ascertain whether changes in the efficiency of N utilization are the result of changes in site of digestion or protein quality (digestibility and amino acid composition). By using a common protein source (casein) and infusing different proportions in the rumen and abomasum, we can examine the effect of site of protein digestion without changes in protein quality. Our goal was to examine the effect of changing the proportion of supplemental protein (casein) that is digested in the rumen and abomasum on N balance in wethers fed a high-concentrate diet.

Materials and Methods

Animals and Abomasal Infusion Treatment.

The experiment was approved by the U.S. Meat Animal Research Center Animal Care and Use Committee and complied with the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 1999). Twenty-five wether lambs (34 ± 0.9 kg; 15 polled Dorset x Suffolk, five polled Dorset x Rambouillet, and 5 polled Dorset) were housed in individual 1.17-m² pens before the experimental periods. Infusion catheters were placed in the rumen and abomasum to facilitate casein infusion ruminally and postruminally, respectively (Gross et al., 1990). Ruminal and abomasal infusion catheters were placed in lambs under general anesthesia. Feed and water were removed 48 and 24 h, respectively, before surgery. General anesthesia was induced with sodium pentothal and maintained with halothane. Wethers were allowed to recover at least 14 d before the initiation of experimental treatments.

After recovery, wethers were adapted to a corn-based, pelleted diet (4.8 mm die; Table 1) for at least 12 d before experimental periods. The basal diet was formulated to be deficient in CP (NRC, 1985) and was fed for ad libitum intake. Wethers were fed once daily at 0700. The amount of feed offered was approximately 125% of the previous 3-d average. Before feeding, the orts were removed, weighed, and discarded. Diets were pelleted to avoid sorting of dietary components. Wethers were placed in metabolism crates during experimental periods. There were three experimental periods in which 5, 10, and 10 wethers were used, respectively. Wethers were randomly assigned within breed to one of five experimental treatments. Wethers were infused with 0 (control) or 68.0 ± 0.7 g sodium caseinate/d (10.4 ± 0.1 g N/d; New Zealand Milk Products, Santa Rosa, CA) with ruminal:abomasal infusion ratios of 100:0 (100R:0A), 67:33 (67R:33A), 33:67 (33R:67A), and 0:100 (0R:100A), respectively. A total of 1,222 ± 6 g and 1,229 ± 6 g of casein solution (or water) were infused into the rumen and abomasum, respectively, per day. Continuous infusion of casein solution or water was accomplished with a peristaltic pump (Harvard Apparatus Model 1217, South Natick, MA) during the 14-d periods (described below). A 5.5% (DM basis) stock casein solution was made, stored at -30°C, and diluted as necessary, depending on treatment, before use.

Statistical Analyses.

All data, except plasma urea N concentrations, were analyzed as a completely randomized design using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC). The model included abomasal infusion treatment. Treatment means were compared using orthogonal contrast statements. Orthogonal contrasts were control vs. the mean of the casein infusion treatments and linear, quadratic, and cubic effects of site of casein infusion. Plasma urea N concentrations were analyzed as a split-split-plot design using GLM of SAS with treatment as the main plot, day as the subplot, and time (a.m. vs. p.m.) as the sub-subplot. Differences were considered significant when P < 0.10.

Results

Intake of N from feed was decreased (P = 0.02) in lambs receiving casein infusion compared to controls, but did not differ between casein infusion groups (Table 2). Total N supply was greater (P = 0.001) in lambs receiving casein infusion compared to controls, but did not differ between casein infusion groups. Urinary N excretion (g/d) was greater (P = 0.001) in lambs receiving casein infusion and linearly decreased (P = 0.07) as casein infusion was shifted to the abomasum. Fecal N excretion (g/d) decreased (P = 0.08) in lambs receiving casein infusion compared to controls, but did not differ between casein infusion groups. Urinary and fecal N excretion (percentage of N excreted) increased and decreased (P = 0.001), respectively, in lambs receiving casein infusion and tended to decrease and increase linearly (P = 0.11), respectively, as casein infusion was shifted to the abomasum. Urinary and fecal N excretion (percentage of N intake) increased and decreased (P = 0.001), respectively, in lambs receiving casein infusion. Urinary N excretion (percentage of N supply) decreased linearly (P = 0.06) as casein infusion was shifted to the abomasum, but fecal N excretion (percentage of N supply) was not influenced by site of casein infusion. Apparently digested N (g/d) increased (P = 0.001) in lambs that received the casein infusion and was quadratically affected (P = 0.04) by site of casein infusion with the greatest amount digested observed in lambs receiving the 33R:67A treatment. Apparently digested N (percentage of supply) increased (P = 0.001) in lambs receiving casein infusion but did not differ between casein infusion groups. Retention of N (g/d) did not differ between casein infusion groups and controls but was quadratically affected (P = 0.06) by site of casein infusion with the greatest retention observed in lambs receiving the 33R:67A treatment. Retention of N as a percentage of N supply and as a percentage of digested N also tended to have a quadratic effect (P < 0.15) due to the site of casein infusion, with the greatest retention observed in lambs receiving the 33R:67A treatment.

View larger version (64K): [in this window] [in a new window]   Figure 1. Influence of infusing different proportions of casein in the rumen and abomasum on plasma urea nitrogen concentration in lambs fed a high-concentrate diet. Wethers were infused with 0 (control) or 10.4 g/d of nitrogen from casein with ruminal:abomasal infusion ratios of 100:0 (100R:0A), 67:33 (67R:33A), 33:67 (33R:67A), or 0:100% (0R:100A) over a 14-d period. Data are the means ± SEM (n = 5). Abomasal infusion treatment (P < 0.001); day (P = 0.05); time (P < 0.001); treatment x day (P = 0.01); treatment x time (P = 0.01); day x time (P = 0.78); treatment x day x time (P = 0.97).

An increasingly important goal among livestock producers is to improve or maintain production while decreasing nutrient excretion. Protein supplementation is often necessary to optimize production. Site of protein digestion is an important component associated with metabolizable protein supplied to the animal. However, changes in protein quality among supplements make it difficult to ascertain whether differences in performance are the result of differences in site of protein digestion or the quality. Our objective was to specifically examine the effect of site of protein digestion on N balance in lambs fed a high-concentrate diet.

Increasing the intake of a highly digestible protein source through casein infusion had dramatic effects on N excretion by increasing urinary N excretion and decreasing fecal N excretion. This suggests that increasing digestible N intake above the requirement generally increases urinary N excretion. However, when shifting the casein infusion from the rumen to the abomasum, urinary N excretion decreased linearly and fecal N excretion numerically increased linearly, suggesting that shifting the digestion of supplemental CP postruminally changes the site of excretion from urine to feces. The reason for the shift in N excretion is likely not because of the fecal excretion of indigestible casein because abomasally infused casein is nearly 100% digested (Richards et al., 2002). These data emphasize the importance of not overfeeding ruminally degradable intake protein (DIP) because the excess N is largely excreted as urea in urine. Once reaching the feedlot surface, urea rapidly gets converted to ammonia by microbial urease and volatilizes, causing environmental concerns (Varel et al., 1999).

The basal diet in this experiment was formulated to be deficient in CP (NRC, 1985). Lambs on casein infusion treatments received approximately 15% of the diet DM as CP compared with 10% for the control lambs. However, that there were no differences in N retention between lambs infused with casein as compared with the controls indicates that the CP requirement was met by the basal diet alone. Our data may suggest that crude protein requirements are overestimated or that the contribution from urea N recycling is underestimated in current models (NRC, 1985; 1996). The reason that N retention was not increased by casein infusion may be partly because of a high proportion of urea N production recycled to the gastrointestinal (GI) tract for controls compared with casein-infused lambs and the importance of this recycled N in meeting the N requirements of the lambs. The proportion of urea production that is recycled to the GI tract can vary widely depending on diet (Harmeyer and Martens, 1980). Others have reported that, as protein intake decreases, a larger fraction of urea N production gets recycled to the GI tract (Harmeyer and Martens, 1980; Bunting et al., 1987; Huntington and Archibeque, 1999). Additionally, increasing the amount of fermentable OM increases the fraction of urea N production that gets recycled to the GI tract (Kennedy and Milligan, 1980; Norton et al., 1982). Although dietary CP intake for the control treatment was only moderately low, fermentable OM intake was high and may have resulted in a large proportion of urea N production recycled to the GI tract. Huntington (1989) and Ferrell et al. (2001) suggested that recycled urea N is a major source of N within the GI tract and that it greatly influences the N economy of ruminants fed high-concentrate diets.

Although casein infusion, compared with the control, did not influence N retention, and site of casein infusion did not influence intake or excretion of N, a quadratic effect on N retention was observed, with the highest retention in the 33R:67A treatment. Digested N (g/d) also was greatest in the 33R:67A treatment and did not have an effect on N digested as a percentage of intake. Because the basal diet contained primarily corn, which is relatively high in ruminally undegradable intake protein (UIP; NRC, 1996), it is probable that lambs fed the basal diet were deficient in DIP. Therefore, we expected N retention to increase in lambs receiving a portion of the casein infusion in the rumen. Our results suggest that the amount of protein supplied to the rumen in the 33R:67A treatment helped overcome a DIP deficiency or provided peptides or AA to support microbial requirements (Chalupa, 1972). However, additional DIP was not beneficial and may have reduced the efficiency of N utilization through increased urinary excretion. Shain et al. (1998) suggested that cattle performance is reduced when corn-based finishing diets contain less than 6.4% DIP (DM basis). Calculated DIP (NRC, 1996) values for this experiment were 4.5, 10.9, 8.6, 6.2, and 4.2%, respectively, for the control, 100R:0A, 67R:33A, 33R:67A, and 0R:100A treatments. Our data agree with those of Shain et al. (1998), suggesting an optimum level of approximately 6.2 to 6.4% DIP. Although N retained was not different between the control and the mean of the casein-infused lambs, N retained tended to be greater (P = 0.12) in lambs receiving the 33R:67A treatment compared with controls (contrast statement; data not shown).

Although the digestibility of the infused casein likely approached 100%, the efficiency of postabsorptive use was low, especially for the 100R:0A treatment. Intake of N increased by 5.8, 6.4, 8.0, and 6.2 g/d, whereas total N excretion increased by 7.2, 5.7, 5.9, and 5.9 g/d when compared with the control treatment for the 100R:0A, 67R:33A, 33R:67A, and 0R:100A treatments, respectively. This resulted in incremental efficiencies of -24, 11, 26, and 5% for the 100R:0A, 67R:33A, 33R:67A, and 0R:100A treatments, respectively. Although these calculations do not account for the substitution of casein protein for a portion of the basal diet protein, relative differences between treatments are evident. These data emphasize that increasing N intake decreases the efficiency of N retention and that rumen degradability of the added supplemental protein can influence the efficiency of N utilization.

The shift in excretion from urine to feces as casein infusion was shifted to the abomasum could be the result of greater recycling to the hindgut. Urea N can enter the GI tract at many sites (Kennedy and Milligan, 1980; Egan et al., 1986). Huntington (1989) reported that approximately 75% of recycled urea N enters the rumen in steers fed a high-concentrate diet. Although not measured, it is likely that ruminal ammonia concentrations were lowest in the control and greatest with the ruminal casein infusion treatment. Low ruminal ammonia in control lambs may have limited ruminal starch digestion and shifted more digestion postruminally (Milton et al., 1997). Increased fermentable substrate at the large intestine can increase urea transfer to the hindgut (Thornton et al., 1970). Additionally, diet can have a large effect on where urea N enters the GI tract (Egan et al., 1986; Huntington, 1989). Lambs receiving a casein infusion postruminally may have had greater amounts of urea N recycled to the hindgut because of potentially greater amounts of fermentable substrate in the hindgut. Increasing urea N recycling to the hindgut likely results in increased excretion of bacterial N in feces.

Intake of DM (and intake of other components except N) decreased in lambs receiving casein infusion compared with controls, but DM apparently digested (g/d) did not differ, suggesting that a metabolic (chemical) limit rather than a physical limit (digesta fill) depressed intake. The fact that apparently digested energy (Mcal/d) was not significantly different between treatments strengthens the argument for a metabolic limit on intake. In this experiment, the intake of DM may have been reduced in lambs receiving casein infusion because the increased digestibility of the total diet resulted in similar amounts of DE intake between treatments.

Although the quantity of digested DM (and other components except N) was not influenced by casein infusion, the percentage of intake digested increased with casein infusion. This may be partly because of decreased intake, but also could be because of improved ruminal digestion from improved ruminal N status. If we assume that infused casein is 100% digested, then feed DM digestibility increased by 2.8, 4.5, 3.7, and 0.8 percentage units compared with the control for the 100R:0A, 67R:33A, 33R:67A, and 0R:100A treatments, respectively. Digestibilities of DM, OM, and energy were not influenced by site of casein infusion, suggesting that site of casein infusion did not greatly affect total-tract digestion.

In summary, protein supplementation through casein infusion greatly increased N excretion, with only minor effects on N retention and nutrient digestion. Increasing the proportion of casein infused postruminally shifted the site of N excretion from urine to feces, possibly because of increased urea N recycling to the hindgut and excretion of bacterial protein.

Implications

These data emphasize the importance of meeting, but not exceeding, the ruminally degradable intake protein requirement in growing ruminants fed high-concentrate diets because use of excess ruminally degradable intake protein is limited. Additionally, supplementing ruminally undegradable intake protein to lambs fed corn-based diets is likely unnecessary. However, supplementing excess ruminally undegradable intake protein may have less deleterious environmental effects than supplementing excess ruminally degradable intake protein because of the shift in site of nitrogen excretion from urine to feces, which could result in a decrease in ammonia volatilization from the feedlot surface.

Footnotes

¹ The authors wish to thank B. Larsen, S. Whitcomb, C. Haussler, C. Felber, J. Waechter, J. Barkhoff, and J. Byrkit for their assistance.

² Mention of a trade name or manufacturer does not constitute a guarantee or warranty of the product by the USDA or an endorsement over products not mentioned.

³ Present address: University of Guelph, Dept. of Anim. and Poult. Sci., Guelph, Ontario N1G 2W1, Canada.

⁴ Correspondence: P.O. Box 166 (phone: 402-762-4202; fax: 402-762-4209; e-mail: freetly{at}email.marc.usda.gov).

Received for publication January 9, 2003. Accepted for publication December 2, 2003.

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