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Effects of dietary methionine and lysine sources on nutrient digestion, nitrogen utilization, and duodenal amino acid flow in growing goats¹

Z. H. Sun^*,, Z. L. Tan^*,2, S. M. Liu, G. O. Tayo^*,, B. Lin^*, B. Teng^||, S. X. Tang^*, W. J. Wang^*, Y. P. Liao^||, Y. F. Pan^*, J. R. Wang^*, X. G. Zhao^* and Y. Hu^*

^* Institute of Subtropical Agriculture, Chinese Academy of Sciences, Hunan 410125, China; and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China; and Commonwealth Scientific and Industrial Research Organisation Livestock Industries, Floreat, WA 6014, Australia; and Babcock University, Ikeja Lagos 21244, Nigeria; and and ^|| Guangzhou Tanke Bio-Tech Industry Co. Ltd., Guangzhou 510520, China

	Abstract

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This study investigated the effects of supplementation of various sources of Met and Lys on nutrient digestion, N utilization, and duodenal AA flows in growing goats. Four 4-mo-old Liuyang Black wether goats were used in a 4 x 4 Latin square experiment and were assigned to 4 dietary treatments: (1) control, (2) control + lipid-coated Met-Zn chelate and Lys-Mn chelate (PML), (3) control + Met-Zn chelate and Lys-Mn chelate (CML), and (4) control + DL-Met, L-Lys-HCl, ZnSO₄·7H₂O, and MnSO₄·H₂O (FML). Compared with control, PML reduced (P < 0.05) ruminal NH₃ concentration, urinary N excretion, and plasma urea N concentration and increased (P < 0.05) the activity of ruminal endo-1,4-ß-D-glucanase and ß-glucosidase, the duodenal flow of N, N retention (g/d as well as % of absorbed N), the duodenal flows of Met, Lys, His, Val, and total essential AA, and plasma concentrations of Lys, Val, Phe, and total essential AA. Supplementing Zn-Met and Mn-Lys chelates had similar (P > 0.05) but lesser effects on these measures compared with PML, and the effects on most of the measures were not statistically significant (P > 0.05) when compared with control. Supplementing free-form Met and Lys had no effects compared with control (P > 0.05). The results indicate that lipid coating and chelating of AA provide a protection, and to a lesser extent by only chelating, of the AA from microbial degradation in the rumen and possibly has effects on rumen fermentation, which increases MP supply. This technology could improve productive performance and be of potential benefit to ruminant production if cost-effective products are developed.

Key Words: goat • rumen-protected amino acid • methionine • lysine • nitrogen balance • rumen fermentation

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The importance of limiting AA to the growth and production of ruminants has been documented (Abe et al., 1998; Korhonen et al., 2002). The limiting AA and their orders under the conditions of specific species, breeds, and diets have been determined (Ragland-Gray et al., 1997; Shan and Tan, 2004). In these studies, Met and Lys were either first- and second-limiting AA or second- and first-limiting AA, respectively.

The majority of dietary nutrients are degraded in the rumen by microorganisms; limiting AA supplemented in diet must have the ability to bypass the rumen for better utilization by host ruminant, hence rumen-protected (RP) AA was developed. Two forms of RP AA were introduced. The first form was synthetic polymers, which were successful (Papas et al., 1984). The second form was AA coated with materials such as fats and processed fats. At present, this second form of RP AA is widely accepted in ruminant nutrition (Davenport et al., 1990; Overton et al., 1998).

Robinson (1996) pointed out that there are 2 issues about the use of RP AA. One is the need to quantitate AA requirements of animals for intestinally absorbable AA. The other is the ability to predict AA delivery to the intestine from both dietary sources as well as the rumen microbes. Knowing both allows us to balance the requirement of animals and the dietary supply. Shan and Tan (2004) proposed a model to quantify intestinal AA delivery and predict the quantity of Met and Lys for growing goats. This study was carried out to study the effects of supplementation of the predicted amounts of Met and Lys, which were protected from degradation in the rumen using a novel approach, on digestibility of DM, NDF, and ADF and N utilization in goats.

	MATERIALS AND METHODS

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Animal Use and Care
The experiment was conducted according to the animal care and use guidelines of the Institute of Subtropical Agriculture, the Chinese Academy of Sciences.

Four indigenous Liuyang Black goats about 4 mo old with an initial BW of 20.0 ± 0.5 kg were used in a 4 x 4 Latin square experiment. The goats were surgically fitted with ruminal, proximate duodenal, and terminal ileal fistulae. The fistulation procedures were as described by Stevens et al. (1985). The surgery was followed by a recovery period of 2 wk, during which the wound was cleaned with potassium iodide and then covered with an antiinflammatory agent. Each goat was administered with penicillin K (80,000 IU) twice daily, and close attention was paid to possible displacement or plugging of fistulae. The animals were held individually in stainless steel metabolic cages in a temperature-controlled (21°C) and lighted facility.

Experimental Diets
Goats were assigned to 4 dietary treatments: (1) control (basal diet, Table 1), (2) control diet with a supplementation of the RP Met and Lys (i.e., a mixture of Met-Zn chelate and Lys-Mn chelate coated with stearic acid, calcium stearate, and zeolite, which was manufactured by Tianke Technological Company, Guangzhou, China), (3) control diet with a supplementation of Met-Zn chelate and Lys-Mn chelate (manufactured by Tianke Technological Company), and (4) control diet with a supplementation of DL-Met (Degussa Company, Düsseldorf, Germany) and L-Lys-HCl (Samsung Company, Seoul, South Korea). These 3 supplements are called PML, CML, and FML, respectively. The amounts of Met and Lys supplemented in PML, CML, and FML treatments were 0.77 and 0.91 g/100 g of concentrate (DM basis), respectively. These supplemented amounts were determined according to the results of Sun et al. (2007), which were calculated from the intestinal digestibility of AA and the AA pattern in muscle protein of growing goat to balance the AA supply to the duodenum. To minimize the potential effect of the extra Zn and Mn from the AA chelates on the variables under study, control and FML groups were supplemented with 0.09 g of ZnSO₄·7H₂O/100 g of concentrate DM and 0.05 g of MnSO₄·H₂O/100 g of concentrate DM, so the 4 groups had the same level of intakes of Zn and Mn from the supplements.

Sampling
Each experimental period lasted for 14 d, the first 7 d for adaptation and another 7 d for sample collection. Samples of feed (concentrate and maize stover) were taken before feeding, and samples of the refusals were collected during the collection period. The samples were stored at –20°C for CP, NDF, and ADF analyses. During each experimental period, the quantity of feed consumed by the goats was recorded daily. From d 8 to 14, Cr₂O₃ (as a digesta marker) was administered via the rumen fistula at 0600, 1200, 1800, and 2400 to allow for a total dosage of 4 g/d. From d 8 to 11, total feces and urine were collected and stored at –20°C for chemical analysis. From d 12 to 14, ruminal, duodenal, and ileal digesta samples were taken at 6-h intervals. The sampling time for collecting digesta sample varied day to day to obtain representative digesta samples: 0100, 0700, 1300, and 1900 on d 12; 0300, 0900, 1500, and 2100 on d 13; and 0500, 1100, 1700, and 2300 on d 14. Digesta samples of 80, 50, and 30 mL were collected from the rumen, duodenum, and ileum, respectively, during each period and stored separately at –20°C. At the end of each collection period, the samples of feces, urine, and digesta of rumen, duodenum, and ileum were thawed, pooled, and mixed thoroughly. Urine (30 mL) was taken from each sample for analysis of N. Feces (50 g) and digesta (100 mL) of rumen, duodenum, and ileum were taken from each sample.

The samples were freeze-dried, ground in a laboratory mill (DF-2, Changsha Instrument Factory, Changsha, China) through a 1-mm screen, and mixed for analysis of DM, CP, NDF, ADF, and Cr. Mixed ruminal digesta (10 mL) was squeezed through 8 layers of cheesecloth, and the filtered fluid was acidified with an equal volume of 0.2 M HCl and centrifuged at 25,000 x g for 20 min at 4°C. The supernatant was harvested and then frozen at –20°C for analysis of NH₃-N. To determine the activity of endo-1,4-ß-D-glucanase, endo-1,4-D-glucanase, ß-glucosidase, and xylanase, 10 mL of mixed digesta of rumen was placed in 50-mL centrifuge tubes with 20 mL of 10 mM sodium phosphate buffer containing 20 µg of lysozyme/mL (Sigma Chemical, St. Louis, MO). The pH of the solution was adjusted to 6.8 by the addition of 50% (wt/vol) NaOH followed by 2.5 mL of CCl₄. Tubes were vortexed and incubated in a water bath at 37°C for 3 h and then centrifuged (29,000 x g) at 4°C for 20 min. The supernatant was collected and stored at –20°C until analysis. At the end of each experimental period, 10-mL blood samples were collected from the jugular vein of each goat. The blood samples were treated with 100 U of sodium heparin/ mL of blood and centrifuged at 3,000 x g for 20 min, and the resultant plasma was stored at –20°C for the analysis of plasma urea N (PUN) and plasma AA.

Chemical Analysis
Milled samples were analyzed for DM and Kjeldahl N content, and urine samples were analyzed for Kjeldahl N content (AOAC, 1990). Neutral detergent fiber and ADF were determined according to the methods of Van Soest et al. (1991). Chromium was determined as described by Williams et al. (1962) using atomic absorption spectroscopy (model 210 VDT AASpectr., Buck Scientific Inc., East Norwalk, CT) with an air-acetylene flame. For AA analyses, with the exception of the S-containing AA and Trp, samples were hydrolyzed with 6 M HCl at 110°C for 24 h, and AA were derivatized with ninhydrin and then detected colorimetrically according to the method of Mason et al. (1980) using an automatic AA analyzer (Beckman model 6300, Beckman Instruments Inc., Palo Alto, CA.). Methionine contents were determined as Met sulfone after oxidation with performic acid; the oxidation process was carried out according to AOAC (1984), and the oxidized samples were then hydrolyzed and analyzed in the same manner as for the other AA. Tryptophan analysis was carried out according to the procedures described by Jones et al. (1981). The concentration of NH₃ was determined by the procedure of Chaney and Marbach (1962). Plasma urea N was determined by colorimetric assay using a commercially available kit (procedure no. 640, Sigma Diagnostics, St. Louis, MO); the intra- and interassay variabilities were 5.0 and 2.6%, respectively.

Activities of the enzymes endo-1,4-ß-D-glucanase, endo-1,4-D-glucanase, ß-glucosidase, and xylanase were determined according to methods described by Bowman and Firkins (1993). Briefly, the supernatant fluid was thawed, and 1 mL was placed into 15-mL culture tubes along with 1.5 mL of prewarmed (39°C) 2% (wt/vol) substrate solution. Tubes were vortexed and incubated in a water bath at 39°C for 30 min. Upon removal from the water bath, 3 mL of 3,5-dinitrosalicylic acid reagent was added to each tube. Color was developed by placing the tubes in a boiling water bath for 5 min. After cooling in tap water for 5 min, absorbance at 560 nm was measured.

Calculations and Statistical Analysis
The apparent reticulorumen digestibility of DM, NDF, and ADF was determined according to Eq. [1]:

[1]

where D_i = the apparent reticulorumen digestibility of DM, NDF, and ADF; Int_i = the intakes of DM, NDF, and ADF each day from diet; Con_i = the concentration of DM, NDF, and ADF in duodenal digesta; and Con_Cr = the concentration of Cr in duodenum.

The whole-tract apparent digestibilities of DM, ADF, NDF, and N were determined according to Eq. [2]:

[2]

where D_i = the whole digestive tract apparent digestibility of DM, NDF, ADF, and N; Int_i = the intakes of DM, NDF, ADF, and N each day from a diet; Con_i = the concentration of DM, NDF, ADF, and N in feces; and Weight = weight of the feces.

The results were statistically analyzed as a 4 x 4 Latin square design. Analysis of variance was performed using the GLM procedure from the Minitab program to estimate variation among animals, among treatments, and among periods. Treatment means were then tested for differences (P < 0.05) according to Tukey’s multiple comparison test (Tukey, 1949).

	RESULTS

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Intakes of Concentrate and Maize Stover and Digestibility of Nutrients
The effects of supplementing Met and Lys on intakes and digestibility of nutrients are presented in Table 2. There were no differences in the intake of the concentrate (P = 0.234), maize stover (P = 0.287), total DM (P = 0.315), NDF (P = 0.285), and ADF (P = 0.273) among the 4 groups. No differences among the 4 groups were observed in the activity of ruminal endo-1,4-D-glucanase (P = 0.142) or ruminal xylanase (P = 0.638) or in the reticulorumen apparent digestibility of DM (P = 0.623) and ADF (P = 0.140) and whole digestive tract digestibility of DM (P = 0.402) and ADF (P = 0.218). Compared with the control and FML groups, supplementation of chelated Met-Zn and Lys-Mn, with or without the lipid coating (i.e., PML and CML groups), enhanced (P < 0.05) the activity of ruminal endo-1,4-ß-D-glucanase, whereas only lipid-coated Met-Zn and Lys-Mn chelates enhanced the activity of ß-glucosidase. Lipid-coated Met-Zn and Lys-Mn chelates increased NDF apparent digestibility of reticulorumen (P < 0.05) when compared with control and increased the NDF apparent digestibility of whole tract (P < 0.05) when compared with FML.

	DISCUSSION

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In this study, the activities of endo-1,4-ß-D-glucanase and ß-glucosidase were enhanced by supplementing Met-Zn and Lys-Mn chelates. The 2 enzymes are responsible for degradation plant fiber in cooperation with ruminal endo-1,4-D-glucanase. An increase of the enzyme activity could imply enhancement of NDF disappearance (Bowman and Firkins, 1993). Relative reports have not been found in previous studies. The mechanism by which the activity of endo-1,4-ß-D-glucanase and ß-glucosidase is influenced by Met and Lys is unknown. A possible explanation could be that endo-1,4-ß-D-glucanase and ß-glucosidase are basically proteins produced through a series of processes including DNA duplication, DNA transcription, and mRNA translation, whereas Met and Lys are necessary to promote these processes through complex mechanisms, including an increase in the activities of DNA and RNA polymerase as well as activation of the protein-synthetic pathway (Xu and Chen, 1998).

Supplementation of lipid-coated Zn-Met and Mn-Lys chelates increased the digestibility of NDF in the reticulorumen compared with control in this study, and related reports are as follows. Hall et al. (1990) found that providing Met to steers fed Bermudagrass hay and ground corn had a tendency to increase digestibility of total-tract NDF. Lodman et al. (1990) noted there were numerical increases in in situ rate of DM and NDF disappearance in the rumen of gestating cows as a result of Met supplementation. Wiley et al. (1991) found that the disappearance rate of DM and NDF in the rumen of cows was increased by supplementing Met plus urea. McCracken et al. (1993) also found that in situ rate of forage DM and NDF disappearance in the rumen of steers was increased by Met supplementation. Dinn et al. (1998) reported that supplementing RP Lys and RP Met in lactating dairy cows fed low-CP diets increased ADF apparent digestibility but not apparent digestibility of NDF and DM. In this study, we found that only supplementation of lipid-coated Met and Lys chelates increased the reticulorumen NDF digestibility in relation to the control. However, the amounts of digested NDF across the reticulorumen (i.e., NDF intake x digestibility) were the same at 95 g/d between the 2 groups, similar to 99 and 92 g/d for CML and FML groups. The differences between the groups were small in relevance to the NDF intakes (ranged from 186 to 197 g/d). This may be due to a relatively low feed intake determined by a low growth potential of this local breed of goats that have small mature size (about 35 to 40 kg) and a low growth rate.

N Utilization
Although the supplementation of lipid-coated Met-Zn and Lys-Mn chelates did not affect intake N in this study, the supplementation did have effects on N digestion in the rumen and utilization in the body. The increases in the duodenal flows of Met and Lys and their concentrations in plasma in response to the supplementation of the chelates, compared with the supplementation of both AA in free form, indicate both AA bypassed the rumen to some extent. This would certainly alter the AA profiles in the small intestine and in the circulation. However, the increase in the duodenal flow of N, which was greater than the N from supplemented Met and Lys; the increase in the duodenal flow of TEAA; elevated activity of ruminal ß-glucosidases; and decreased ruminal NH₃ concentration all imply that it was not only the duodenal supply and plasma concentrations of Met and Lys that were increased, but most or all other AA measured or MP supply were altered. Another reason may be the increase in NH₃-N utilization and production of microbial N in the rumen. Erasmus et al. (1992) noted that duodenal non-NH₃-N flow tended to be greater in cows supplemented with yeast culture because of greater bacterial N flow. Because this treatment did not change intakes, the altered MP supply is likely due to altered ruminal fermentation. Whether alteration is due to increased rate of rumen fermentation, which leads to more microbial protein synthesised, or due to increase outflow rate in the rumen by the AA chelates is not clear. Nevertheless, the potential effects of the AA chelates on rumen fermentation and the supply of MP from microbial protein synthesis warrants further investigations

The improvement of N utilization of goats by supplementation of the Met and Lys chelates is further indicated by the increases in N retention and increased utilization efficiency of absorbed N. Similar observations have been obtained by Lynch et al. (1991), who reported an improved N balance in lactating ewes fed RP Met and Lys. Dinn et al. (1998) noted that urinary N excretion and fecal N of cows fed lower protein diets (16.7 and 15.3%) supplemented with RP Met and Lys were reduced compared with a higher-protein (18.3%) diet that was not supplemented with RP Met and Lys. Dinn et al. (1998) also reported that although milk N was increased, N retention was not affected, indicating that supplementing RP Met and Lys improved N utilization in the form of milk N but not in form of N retention. Archibeque et al. (2002) found that RP Met supplementation tended to decrease urinary N and to increase N retention. The improvement of N utilization could be attributed to the reduction of NH₃ in the rumen and, in turn, reduced absorption of NH₃ into the portal vein and reduced ureagenesis in the liver, which was supported by the reduction of urea N concentration in plasma. In response to an increased NH₃ load from the digestive tract, ovine hepatic ureagenesis is stimulated with a requirement for additional N substrates. This may reduce a proportion of feed N available to the animal as an anabolic form, and further hepatic removal of NH₃ may require net utilization of ingested AA (Lobley et al., 1995). Another explanation could be the altered AA profile by the supplementation of Met and Lys, which matches more closely to that in muscle protein and has a better efficiency for protein anabolism (Shan and Tan, 2004).

The PUN concentration was reduced by supplementing PML and CML compared with control in this study. This observation agrees with the findings reported for Angora goats (Sahlu and Fernandez, 1992) and in cows (Dinn et al., 1998). Sun et al. (2007) reported that PUN was increased by decreasing the amounts of Met and Lys infused to the gut of growing goats. The decrease in PUN concentration is associated with an increase in protein or AA utilization (Ponnampalam et al., 2005).

Wiley et al. (1989) reported that a lower ruminal NH₃-N concentration was observed in cows supplemented with RP Met compared with cows supplemented with soybean, and the result is in agreement with results obtained in this study. The lower ruminal NH₃-N observed may be due to the ability of RP AA to bypass the rumen, escaping rumen microbial degradation and production of NH₃.

Duodenal AA Flows and Plasma AA Concentrations
This study shows that supplementation of Met-Zn and Lys-Mn chelates increased the duodenal flows of Met, Lys, and TEAA and that a provision of both AA in free form in diets made no significant alteration to the duodenal AA flows. Plasma concentrations of Met, Lys, and TEAA changed correspondingly. Furthermore, the lipid-coated chelate had relatively greater values compared with the uncoated counterpart. Related results were also reported by other researchers. Bernard et al. (2004) reported that supplementation of free Lys was not effective in increasing the flows of AA to the duodenum of lactating cows. Lynch et al. (1991) noted that the concentration of plasma Met was increased by supplementing RP Met and RP Lys in the diet of lactating ewes. Dinn et al. (1998) also reported that plasma Met concentration of lactating dairy cows was increased by adding RP Met and RP Lys in diets containing 16.7 and 15.3% CP (DM basis), respectively, compared with the control diet containing 18.3% CP (DM basis). Our data supports these findings.

The results showed that lipid-coated Met-Zn and Lys-Mn chelates are effective in increasing the duodenal AA flows of themselves, His, Val, and TEAA and also the flow of MP, indicating that lipid coating and chelating of AA provide protection to the AA from microbial degradation in the rumen and possibly has a potential effect on the rumen fermentation, which increases MP supply. An increased duodenal flow of Met, Lys, and the other AA alters the AA profile in the small intestine and in plasma, which leads to an improved N economy in the body. This technology could improve productive performance and be of potential benefit to ruminant production if cost-effective products are developed.

	Footnotes

 
¹ This study was financially supported by National Natural Science Foundation of China (30600436) and National Natural Science Foundation of China (30571352). We gratefully acknowledge the support of K. C. Wong Education Foundation, Hong Kong, and the Chinese Academy of Science/the Academy of Science for Developing World for providing postdoctoral fellowship funding for one of us.

² Corresponding author: zltan{at}isa.ac.cn

Received for publication November 2, 2006. Accepted for publication August 7, 2007.

	LITERATURE CITED

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 


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