INTRODUCTION. A. L. Donsbough, S. Powell, A. Waguespack, T. D. Bidner, and L. L. Southern 2

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1 Uric acid, urea, and ammonia concentrations in serum and uric acid concentration in excreta as indicators of amino acid utilization in diets for broilers 1 A. L. Donsbough, S. Powell, A. Waguespack, T. D. Bidner, and L. L. Southern 2 School of Animal Sciences, Louisiana State University Agricultural Center, Baton Rouge ABSTRACT Five experiments were conducted to determine if serum uric acid, serum urea N (SUN), serum ammonia, and the uric acid content of the excreta (UAE) could be used to determine the efficacy of amino acid (AA) utilization in diets for broilers. All experiments were conducted with Ross Ross 308 or 708 broilers from 0 to 14 or 0 to 18 d posthatching in brooder batteries. s had 6 or 7 replications with at least 6 broilers per replicate pen. All diets were corn and soybean meal-based and formulated to contain 1.0% Ca and 0.45% nonphytate P and to meet or exceed the requirements of all nutrient requirements except total Lys, Met, and Thr (experiment 1) or Met (experiments 2 to 5). Experiment 1 consisted of 2 dietary treatments. Diet 1 was formulated to be deficient in Lys, Thr, and Met and diet 2 was formulated to be adequate in all nutrients. Broilers fed the AA-adequate diet had increased (P < 0.01 to 0.03) ADG, ADFI, and G:F compared with broilers fed the AA-deficient diet. Serum uric acid, SUN, serum ammonia, and UAE were Key words: broiler, amino acid, urea, uric acid not affected (P = 0.34 to 0.70) by dietary treatment. In experiments 2 to 5, diets contained 1.35% total Lys, 2 levels of Met (0.50 or 0.76 TSAA:Lys), and without or with Gly supplementation up to 2.32% Gly + Ser. Broilers fed diets containing supplemental Met in experiments 2 to 5 had increased (P = 0.01 to 0.03) ADG, ADFI, and G:F. Gain:feed was increased (P = 0.01 to 0.07) in broilers fed supplemental Gly. Serum uric acid and SUN were decreased (P < 0.01) after a 2-h fast in broilers fed supplemental Met and Gly. Serum uric acid and SUN also were decreased at other times after fasting, but the 2-h fast gave the most consistent response. Uric acid content of the excreta was decreased (P < 0.01) in broilers fed supplemental Met. Serum ammonia was decreased (P < 0.01 to 0.02) in experiments 2, 3, and 4 at varying times postfeeding but was not affected by diet in experiment 5. The results of this research indicate that serum uric acid, SUN, and UAE concentrations can be used as an indicator of AA utilization in broilers fed AA-adequate and AA-deficient diets Poultry Science 89 : doi: /ps INTRODUCTION Corn and soybean meal prices are increasing for livestock production. It is therefore important to formulate diets to efficiently meet the needs of animals. Most importantly, diets must be formulated to contain the correct amount of amino acids (AA) for optimum performance. Previous research has used ADG, ADFI, and G:F to estimate AA requirements of broilers. In swine, the use of plasma urea N (PUN) has been shown to be an accurate variable for estimating AA requirements (Coma et al., 1995; Knowles et al., 1997; Guzik et al., 2010 Poultry Science Association Inc. Received August 13, Accepted October 24, Approved for publication by the director of the Louisiana Agricultural Experiment Station as manuscript number Corresponding author: lsouthern@agcenter.lsu.edu 2005). In broilers, uric acid (UA), and not urea, is produced as the main end product of N metabolism. Therefore, plasma UA (PUA) and excreta UA (UAE) should be viable response variables to determine AA requirements of broilers or the efficiency of AA utilization. Some research examining PUA and UAE has been conducted with broilers fed very low to excessive amounts of CP (Pudelkiewicz et al., 1968; Featherston, 1969; Okumura and Tasaki, 1969; Hevia and Clifford, 1977). This research indicates that both PUA and UAE increase as dietary N intake increases. However, inconsistent results have been reported when using PUA as a response variable to assess AA utilization. Xie et al. (2004) reported both increases and decreases in PUA concentrations when Met was increased in the diet. Miles and Featherston (1974) reported decreases in both PUA and UAE when dietary Lys was increased in the diet. However, Corzo et al. (2003, 2005) reported no changes in PUA concentrations when dietary Lys 287

2 288 or Trp were increased in the diet. Research also shows that fasting and refeeding schedules may need to be implemented to determine differences in PUA concentrations (Okumura and Tasaki, 1969; Wilson and Miles, 1988; Kolmstetter and Ramsay, 2000). The objective of this research was to determine if serum UA (SUA), serum urea N (SUN), and serum ammonia (SA) could be used to determine the AA adequacy of a diet for broilers, much like that of PUN for swine. General MATERIALS AND METHODS All experimental animal use was in compliance with the Louisiana State University Agricultural Center Animal Care and Use Committee. Five experiments were conducted using Ross Ross 308 or 708 male or female broiler chicks. For each experiment, broilers were weighed, wing-banded, and randomly placed onto dietary treatments on d 0 posthatching. Each experiment lasted 0 to 14 or 18 d. Broilers were housed in environmentally controlled brooder batteries (Petersime Incubator Co., Gettysburg, OH) with raised wire floors and continuous fluorescent lighting. Broilers had ad libitum access to feed and water. All diets were corn and soybean meal-based and formulated to contain 1.0% Ca and 0.45% nonphytate P and to meet or exceed the requirements of all other nutrients except total Lys, Met, and Thr (experiment 1) or Met (experiments 2 to 5), as recommended by the NRC (1994). A basal diet was prepared for each experiment that lacked only the supplemental AA and cornstarch. Individual treatment diets were made by adding the necessary AA or cornstarch (Table 1). At the termination of each experiment, broilers and feeders were weighed for the determination of growth performance. Excreta Collection and Analysis Excreta samples were collected from each pen of broilers daily for the last 4 d of each experiment to determine the amount of UA excreted. Samples from all 4 d were combined and mixed thoroughly, dried, and ground for analysis of UA content by methods described by Marquardt (1983). Blood Sampling At the termination of each experiment, broilers were bled via cardiac puncture for determination of SUA, SUN, and SA. Blood was placed into 10-mL serum tubes (BD Vacutainer, Franklin Lakes, NJ) and samples were held on ice until centrifugation at 1,734 g at 0 C for 20 min. Serum (0.5 ml) from each broiler was collected and pooled by replicate pen. The pooled serum was then analyzed for SUA (Fossati et al., 1980) and SUN Donsbough et al. (Mathies, 1960) using commercial reagent kits (Pointe Scientific, Canton, MI). The SA was analyzed by the same method as SUN but urease was not added to the reaction solution. Experiment 1 Eighty-four male Ross Ross 708 broilers were allotted to 2 dietary treatments (Table 1) to determine if SUA, SUN, or SA could be used to determine the AA adequacy of a diet. Diet 1 was formulated to be deficient in Lys, Thr, and Met, and diet 2 was formulated to be adequate in all AA. Cornstarch was added to replace supplemental AA in diet 1. s contained 7 replications with 6 broilers per replicate pen. At the termination of the experiment, individual broilers were bled. Before bleeding, all broilers had access to feed, and no fasting and refeeding schedule was implemented. Experiments 2 to 5 Experiments 2 to 5 were conducted to determine the SUA, SUN, and SA concentrations of broilers bled during various fasting and refeeding schedules. There were 6 replications of each treatment. Dietary treatments (Table 1) were arranged as a 2 2 factorial with or without supplemental Met (TSAA:Lys of 0.50 or 0.76) and with or without supplemental Gly (total Gly + Ser of or 2.32%; Dean et al., 2006). All diets contained 1.35% total Lys with 0.25% l-lys HCl. Cornstarch was added in place of Met and Gly in diets without supplemental Met or Gly. Experiment 2 was conducted for 18 d to determine the blood N concentrations in fasted and fed birds. One hundred forty-four Ross Ross 308 female broilers were used with 6 broilers per pen. At the termination of the experiment, broilers were fasted for 2 h and 2 broilers from each pen were bled. Feeders were reintroduced to each pen and 2 broilers were bled 30 and 60 min postfeeding. Experiment 3 was conducted for 14 d to determine the optimal time after fasting for blood collection to determine SUA, SUN, and SA concentrations. Dietary treatments were similar to experiment 2. Three hundred sixty male and female Ross Ross 308 broilers were used and the experiment was conducted for 14 d. There were 3 replications of males and 3 replications of females with 15 birds per pen. The fasting and refeeding schedule for this experiment consisted of an initial 2-h fast, replacement of feeders to each pen for 20 min to ensure that each broiler had the opportunity to consume feed, and feeders were then removed. Three broilers per pen were bled at 0 (when feeders were removed), 1, 2, and 3 h postfeeding. Experiment 4 was conducted for 14 d to extend the fasting period that was used in experiment 3. Four hundred thirty-two Ross Ross 708 female broilers were used with 18 broilers per pen. The fasting and refeeding

3 SERUM URIC ACID AS AN INDICATOR OF AMINO ACID UTILIZATION 289 Table 1. Percentage composition of corn-soybean meal negative control diets with and without supplemental amino acids (AA) in experiment 1 to 5, as-fed basis 1 Item Experiment 1 (AA deficient 2 ) Experiments 2 to 5 ( Met Gly 3 ) Ingredient Corn Soybean meal (47.5%) Soy oil Monocalcium phosphate Limestone Salt Mineral mix Vitamin mix Choline chloride dl-met l-lys HCl l-thr Gly l-val l-arg HCl l-ile Cornstarch Calculated composition ME (kcal/kg) 3,164 3,180 CP (%) Ca (%) Nonphytate P (%) Lys (%) (1.260) TSAA (%) (0.907) (1.026) 8 Thr (%) (0.882) Trp (%) Leu (%) Gly + Ser (%) (2.526) (2.32) Arg (%) Ile (%) Val (%) A basal diet was mixed to contain the minimum of all ingredients except supplemental Gly, dl-met, l-lys HCl, l-thr, and cornstarch, which were added as needed for each diet. 2 The AA-adequate diets had 0.274% dl-met, 0.250% l-lys HCl, 0.156% l-thr, 0.75% Gly, and 1.47% cornstarch (instead of 2.95% cornstarch). 3 The diets with added Met and Gly were supplemented with 0.356% dl-met and 0.415% Gly, respectively. 4 Provided the following per kilogram of diet: copper (copper sulfate), 7 mg; iodine (calcium iodate), 1 mg; iron (ferrous sulfate monohydrate), 50 mg; manganese (manganese sulfate), 100 mg; selenium (sodium selenite), 0.15 mg; zinc (zinc sulfate), 75 mg; with calcium carbonate as the carrier. 5 Provided the following per kilogram of diet: vitamin A, 8,003 IU; vitamin D 3, 3,004 IU; vitamin E, 25 IU; vitamin K, 1.5 IU; vitamin B 12, 0.02 μg; biotin, 0.1 μg; folic acid, 1.00 mg; niacin, 50 mg; pantothenic acid, mg; pyridoxine, 4.00 mg; riboflavin, mg; thiamin, 3.00 mg. 6 Contains 600,000 mg/kg of choline chloride. 7 Calculated composition of diets with supplemental Lys, Met, Thr, and Gly. 8 Calculated composition of diets with supplemental Met and Gly. schedule was similar to the previous experiment, except broilers were bled at 0, 2, 3, 4, and 5 h postfeeding. Experiment 5 was conducted for 14 d to confirm the results of the previous experiments. One hundred fortyfour Ross Ross 708 female broilers were used with 6 broilers per pen. Three broilers per pen were bled at 0 and 2 h postfeeding. Data were analyzed by ANOVA procedures (Steel and Torrie, 1980) as completely randomized designs using the GLM procedure in SAS (SAS Inst. Inc., Cary, NC). In experiment 3, the effect of sex was included in the model. The pen of broilers served as the experimental unit for all data. means for experiments 2 to 5 were separated by orthogonal contrasts appropriate for a 2 2 factorial arrangement of treatments. Actual P-values are provided for all effects, but we considered P < 0.10 as significant. RESULTS In experiment 1 (Table 2), broilers fed the AA-adequate diet had an increased ADG, ADFI, and G:F compared with broilers fed the AA-deficient diet (P < 0.01 to 0.03). Serum UA, SUN, SA, and UAE (Table 2) were not affected by the addition of supplemental AA (P > 0.10). In experiment 2 (Table 3), broilers fed diets containing supplemental Met had increased ADG, ADFI, and G:F compared with broilers fed diets without supplemental Met (P < 0.01 to 0.03). Feed efficiency was also increased in broilers fed diets containing supplemental Gly (P < 0.07). Serum UA and SUN were decreased after the 2-h fast in broilers fed diets containing supplemental Met and Gly or the combination (P < 0.01 to 0.08). Serum urea N was decreased after 60 min of ac-

4 290 Donsbough et al. Table 2. Growth performance, serum N data, and uric acid content of the excreta of broiler chicks fed amino acid (AA)-deficient and AA-adequate diets in experiment 1 1 Item ADG (g) ADFI (g) G:F (g/g) SUA 2 (mg/dl) SUN 3 (mg/dl) SA 4 (mmol/l) UAE 5 (mg/g) 1) AA deficient ) AA adequate SEM P-value Data are means of 7 replications of 6 broilers per replicate pen. 6 Overall treatment P-value. cess to feed in broilers fed diets containing supplemental Met (P < 0.02). Serum ammonia was decreased after a 2-h fast and after 30 min of access to feed in broilers fed diets containing supplemental Met (P < 0.01). Uric acid content of the excreta was decreased in broilers fed diets containing supplemental Met (P < 0.01) and increased in broilers fed Gly (P < 0.02). In experiment 3 (Table 4), broilers fed diets containing supplemental Met had increased ADG, ADFI, and G:F (P < 0.01). Gain was greatest when Gly and Met were added together (P < 0.03). Feed efficiency was increased in broilers fed diets containing supplemental Gly (P < 0.03). Males had a greater ADG (P < 0.06) and G:F (P < 0.03) than females (data not shown). Serum UA and SUN (Table 5) were decreased at all times in broilers fed diets containing supplemental Met (P < 0.02). Serum ammonia was decreased at 1 and 3 h postfeeding in broilers fed diets containing supplemental Met (P < 0.05) and was decreased most at 0 and 3 h postfeeding when Gly and Met were added together (P < 0.09). Sex and sex diet interactions were observed for some of the responses, but these data are not shown in Table 5. Serum UA concentrations were higher (P < 0.05) in males at 0 and 3 h postfeeding. Serum urea N concentrations were higher (P < 0.04) in females at 1 h postfeeding. At 0 h postfeeding, there was a sex diet interaction for SUN (P < 0.01). Female broilers fed no supplemental Met had greater SUN concentrations than males. Serum ammonia concentrations were higher in females at all times (P < 0.01). Uric acid content of the excreta was decreased (P < 0.01) by the addition of supplemental Met and was increased by supplemental Gly (P < 0.01). Females had higher UAE (P < 0.05) compared with male broilers ( mg/g, mg/g). In experiment 4 (Table 6), broilers fed diets containing supplemental Met had increased ADG, ADFI, and G:F (P < 0.01). Gain:feed was increased in broilers fed diets containing supplemental Gly (P < 0.05) and was highest in broilers fed Gly and Met added together (P < 0.04). Serum UA (Table 7) was decreased in broilers fed diets containing supplemental Met at 0 and 2 h Table 3. Growth performance, serum N data, and uric acid content of the excreta of broiler chicks fed supplemental dl-met and Gly in experiment 2 1 Item ADG (g) ADFI (g) G:F (g/g) SUA 2 (mg/dl) SUN 3 (mg/dl) SA 4 (mmol/l) 2 h 6 30 min 60 min 2 h 30 min 60 min 2 h 30 min 60 min UAE 5 (mg/g) 1) Met Gly ) +Met ) +Gly ) +Met +Gly SEM P-value Gly Met Gly Met Growth performance data are means of 6 replications of 6 broilers per pen and serum data are means of 6 replications of pooled serum of 2 broilers per replicate pen per time. 6 Broilers were fasted for 2 h, feeders were reintroduced to each pen, and broilers were bled 30 and 60 min postfeeding. 7 Overall treatment P-value.

5 SERUM URIC ACID AS AN INDICATOR OF AMINO ACID UTILIZATION 291 Table 4. Growth performance of broiler chicks fed supplemental dl-met and Gly in experiment 3 1 Item ADG (g) ADFI (g) G:F (g/g) 1) Met Gly ) +Met ) +Gly ) +Met +Gly SEM P-value Sex Gly Met Gly Met Data are means of 6 replications of 15 broilers per replicate pen. 2 Overall treatment P-value. 3 Sex diet interactions were not significant and were removed from the model. postfeeding (P < 0.04) and was increased at 5 h postfeeding (P < 0.05). Serum urea N was decreased in broilers fed diets containing supplemental Met at all times (P < 0.04) and was also decreased in broilers fed diets containing supplemental Gly at 2 h postfeeding (P < 0.01). Serum ammonia was decreased in broilers fed diets containing supplemental Met at 0 and 4 h postfeeding (P < 0.02). Uric acid content of the excreta was decreased (P < 0.01) by the addition of supplemental Met. In experiment 5 (Table 8), broilers fed diets containing supplemental Met had higher ADG, ADFI, and G:F (P < 0.01), and when Gly and Met were added together, the effect was greater (P < 0.08). Gain:feed was also increased in broilers fed diets containing supplemental Gly (P < 0.07). Serum UA and SUN concentrations were decreased in broilers fed diets containing supplemental Met and Gly at 2 h postfeeding (P < 0.04), and SUA was further decreased in broilers fed diets containing both Gly and Met (P < 0.08). Serum ammonia concentrations were not affected by diet (P > 0.10). Uric acid content of the excreta was decreased (P < 0.01) by the addition of supplemental Met. DISCUSSION The results of this research indicate that SUA, SUN, and UAE can be used as indicators of AA utilization in a diet. These response variables changed in a manner indicating an increased AA utilization, which was consistent with the positive changes in growth response. The most consistent response in SUA and SUN was after a period of time without feed, and the most consistent time was 2 h of fast after a time of concentrated feed consumption. The responses in SUA and SUN were more consistent than SA, but SA generally was reduced as AA utilization increased. In experiment 1, in which birds with full access to feed were bled without any fasting or refeeding period, there was no effect on any serum N variable. In experiments 3 to 5, in which birds were bled 20 min after exposure to feed at time 0 min fast, SUA and SUN were decreased in two of these experiments but not in one experiment. We have no explanation for this discrepancy in response within these 3 experiments. However, the discrepancy in SUA and SUN at time 0 min fast in experiment 1 vs. experiments 3 to 5 may be explained by the method in which the birds were allowed feed before bleeding. In experiments 3 to 5, the birds were fasted for 2 h then allowed to consume feed for 20 min, and it was after this 20 min of feed consumption that the birds were bled for the 0-min fast. In the latter experiments, we can be fairly certain that the birds had consumed feed within the previous 20 min before bleeding, whereas in experiment 1, we have no idea when those birds consumed feed before bleeding. This Table 5. Blood N data and uric acid content of the excreta of broiler chicks fed supplemental dl-met and Gly in experiment 3 1 Item SUA 2 (mg/dl) SUN 3 (mg/dl) SA 4 (mmol/l) 0 h 1 h 2 h 3 h 0 h 1 h 2 h 3 h 0 h 1 h 2 h 3 h UAE 5 (mg/g) 1) Met Gly ) +Met ) +Gly ) +Met +Gly SEM P-value Sex Sex diet Gly Met Gly Met Data are means of 6 replications with pooled serum from 3 broilers per replicate per time. 6 Overall treatment P-value. 7 Sex diet interactions were not significant and were removed from the model.

6 292 Table 6. Growth performance of broiler chicks fed supplemental dl-met and Gly in experiment 4 1 Item ADG (g) ADFI (g) G:F (g/g) 1) Met Gly ) +Met ) +Gly ) +Met +Gly SEM P-value Gly Met Gly Met Data are means of 6 replications with 18 broilers per replicate pen. 2 Overall treatment P-value. inconsistency in time of eating and access to feed may be the reason that SUA and SUN were not affected by AA adequacy or deficiency for experiment 1. The AA used in this research to evaluate serum N responses to AA adequacy or deficiency were Gly and Met in experiments 2 to 5 and Lys, Met, and Thr in experiment 1. The results we observed for SUA response to Met are similar to the results observed from Met on PUA in ducks (Xie et al., 2004). However, not all research indicates that SUA can be used as an indicator of AA utilization. Xie et al. (2004) reported both increases and decreases in PUA concentrations when Met was increased in the diet. Miles and Featherston (1974) reported decreases in both PUA and UAE when dietary Lys was increased in the diet. Corzo et al. (2003, 2005) reported no changes in PUA concentrations when dietary Lys or Trp was increased in the diet. These inconsistencies may be due to the actual AA that was deficient. A Met deficiency seems to result in an increase in SUA, but in our experiment in which Met, Lys, and Thr were all deficient, there was no response in SUA. Again, those birds did not undergo a fast refeeding schedule. Supplemental Gly was not as consistent as Met in affecting N responses in serum. Recent data in Donsbough et al. our laboratory indicate that SUA cannot be used to estimate the Lys requirement of broilers. Previously, Featherston (1969) and Okumura and Tasaki (1969) reported much more dramatic effects of AA and CP excesses on SUA concentrations compared with our research. The major difference in our experiments compared with previous experiments is that all our diets were similar in CP but deficient in AA. Previous researchers varied casein from 5 to 40% (Okumura and Tasaki, 1969) or isolated soy protein from 25 to 75% (Featherston, 1969). In contrast to these observations, Russell and Weber (1934) reported no differences in PUA in laying hens fed low (13%) or high (19%) CP diets. Effects of fasting and refeeding schedules on PUA and blood urea N (BUN) previously have been evaluated. Results indicate that PUA increased and peaked 2 h after feeding and decreased as the fasting period increased (Okumura and Tasaki, 1969; Wilson and Miles, 1988; Kolmstetter and Ramsay, 2000). These observations are similar to our research because SUA is most consistently affected by AA or CP amendment to the diet 2 h after eating. In our research, the birds were fed and then fasted for 2 h. In the research of Okumura and Tasaki (1969) and Wilson and Miles (1988), the birds were bled 2 h after being fed, but it is not clear if the birds had access to feed at the time they were bled. Kolmstetter and Ramsay (2000) reported that penguins given ad libitum access to their normal diets had significantly increased PUA 0 to 2 h postprandial compared with preprandial and 4 to 6 h postprandial PUA concentrations. It was noted that the penguin that ate constantly for the 2 h that feed was allowed had the highest postprandial PUA concentration. Our research shows changes in SUA if birds are bled while they have access to feed, but the response is much more consistent after a 2-h fast. The use of PUN as an indicator of AA utilization in swine does not require a fasting or refeeding schedule to observe differences in PUN due to Table 7. Blood N data and uric acid content of the excreta of broiler chicks fed supplemental dl-met and Gly in experiment 4 1 Item SUA 2 (mg/dl) SUN 3 (mg/dl) SA 4 (mmol/l) 0 h 2 h 3 h 4 h 5 h 0 h 2 h 3 h 4 h 5 h 0 h 2 h 3 h 4 h 5 h UAE 5 (mg/g) 1) Met Gly ) +Met ) +Gly ) +Met +Gly SEM P-value Gly Met Gly Met Data are means of 6 replications with pooled serum from 3 broilers per replicate per time. 6 Overall treatment P-value.

7 SERUM URIC ACID AS AN INDICATOR OF AMINO ACID UTILIZATION 293 Table 8. Blood N data and uric acid content of the excreta of broiler chicks fed supplemental dl-met and Gly in experiment 5 1 Item ADG (g) ADFI (g) G:F (g/g) SUA 2 (mg/dl) SUN 3 (mg/dl) SA 4 (mmol/l) 0 h 2 h 0 h 2 h 0 h 2 h UAE 5 (mg/g) 1) Met Gly ) +Met ) +Gly ) +Met +Gly SEM P-value Gly Met Gly Met Growth performance data are means of 6 replications of 6 broilers per pen and serum data are means of 6 replications of pooled serum from 3 broilers per replicate per time. 6 Overall treatment P-value. diet (Coma et al., 1995; Knowles et al., 1997; Guzik et al., 2005). We do not know why the most consistent effect of SUA as an indicator of AA utilization in broilers occurs after a fast. There is very little research on changes in SUN in broilers. In our experiments, changes in SUN concentrations were as consistent as changes in SUA concentrations. Serum urea N has been reported previously in avian serum (Stevens, 1996), but the origin of the SUA is not known. Kolmstetter and Ramsay (2000) observed differences in BUN concentrations in fed and fasted penguins. Two hour postprandial BUN concentrations were significantly lower than preprandial and 4 to 6 h postprandial BUN concentrations. No significant differences in BUN were observed by Russell and Weber (1934) in laying hens fed low (12.71%) or high (19.14%) protein diets. Uric acid content of the excreta was decreased by the addition of Met in experiments 2 to 5, which confirms that Met deficiency limits AA utilization in cornsoybean meal diets. The decrease in UAE is related to the decrease in SUA that is observed in all experiments when broilers are fed diets containing supplemental Met. In experiment 2 and 3, Gly increased UAE, which is thought to be due to readily available Gly clearing excess AA. This effect is similar to previous research by Pudelkiewicz et al. (1968) and Miles and Featherston (1974) in that changing the amount of N intake by increasing supplemental AA in the diet or by increasing N from 4 to 7% changed UAE. The UAE values observed in these experiments range from 28 to 49% of DM, which are agreement with the percentage of UAE reported by Pudelkiewicz et al. (1968), Marquardt (1983), and Marquardt et al. (1983). Therefore, UAE can be used as indication of AA utilization. The results of these 5 experiments indicate that SUA and SUN can be used as indicators of AA utilization in broilers. A fed, fasting, and refeeding schedule results in the most consistent response, but differences were observed in some experiments in fed birds. The most optimal time for blood collection is 2 h postfeeding. REFERENCES Coma, J., D. Carrion, and D. R. Zimmerman Use of plasma urea nitrogen as a rapid response criterion to determine the lysine requirement of pigs. J. Anim. Sci. 73: Corzo, A., E. T. Moran, and D. Hoehler Lysine needs of summer-reared male broilers from six to eight weeks of age. Poult. Sci. 82: Corzo, A., E. T. Moran Jr., D. Hoehler, and A. Lemme Dietary tryptophan needs of broiler males from forty-two to fifty-six days of age. Poult. Sci. 84: Dean, D. W., T. D. Bidner, and L. L. Southern Glycine supplementation to low protein, amino acid-supplemented diets supports optimal performance of broiler chicks. Poult. Sci. 85: Featherston, W. R Nitrogenous metabolites in the plasma of chicks adapted to high protein diets. Poult. Sci. 48: Fossati, P., L. Principe, and G. Berti Use of 3,5-dichloro- 2-hydroxybenzene sulfonic acid/4-aminophenazone chromogenic system in the direct enzymic assay of uric acid in serum and urine. Clin. Chem. 26: Guzik, A. C., J. L. Shelton, L. L. Southern, B. J. Kerr, and T. D. Bidner The tryptophan requirement of growing and finishing barrows. J. Anim. Sci. 83: Hevia, P., and A. J. Clifford Protein intake, uric acid metabolism, and protein efficiency ratio in growing chicks. J. Nutr. 107: Knowles, T. A., L. L. Southern, and K. R. Robbins The use of serum urea nitrogen concentrations for the determination of the lysine requirements of barrows. Prof. Anim. Sci. 13: Kolmstetter, C. M., and E. C. Ramsay Effects of feeding on plasma uric acid and urea concentrations in blackfooted penguins (Spheniscus demersus). J. Avian Med. Surg. 14: Marquardt, R. R A simple spectrophotometric method for the direct determination of uric acid in avian excreta. Poult. Sci. 62: Marquardt, R. R., A. T. Ward, and L. D. Campbell A rapid high-performance liquid chromatographic method for the quantitation of uric acid in excreta and tissue samples. Poult. Sci. 62: Mathies, C. J Adaptation of the Berthelot color reaction for the determination of urea nitrogen in serum and urine to an ultramicro system. Clin. Chem. 10:

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