Effects of adding arginine and histidine to dairy calf milk replacers

Effects of adding arginine and histidine to dairy calf milk replacers

The Professional Animal Scientist 27 (2011):565–570 ©2011 American Registry of Professional Animal Scientists Carginine S : Effects of adding and hi...

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The Professional Animal Scientist 27 (2011):565–570

©2011 American Registry of Professional Animal Scientists

Carginine S : Effects of adding and histidine to dairy ASE

TUdY

calf milk replacers

T. M. Hill,1 H. G. Bateman II, J. M. Aldrich, PAS, and R. L. Schlotterbeck Nurture Research Center, Provimi North America, Lewisburg, OH 45338

ABSTRACT Male Holstein calves, 2 to 3 d of age, were fed a 27% CP (whey based), 17% fat milk replacer (MR) at 0.66 kg of DM per calf daily to provide control or test treatments with either added l-Arg or l-His. Calves were also fed free choice corn, oat, and soybean meal–based starters and water. Each trial included an unsupplemented control and supplemented test MR. In trial 1, MR contained 0.28 and 0.44 Arg-to-Lys ratios. In trial 2, MR contained 0.27 and 1.0 Arg-to-Lys ratios. In trial 3, MR contained 0.2 and 0.42 His-to-Lys ratios. Measurements included ADG, feed efficiency, starter intake, hip width, and serum preweaning concentrations of albumin, alkaline phosphatase, amylase, creatinine, glucose, total protein, and urea N. Data from each trial were analyzed as repeated measures over time during preweaning (0 to 42 d in trials 1 and 2 and 0 to 28 d in trial 3), postweaning, and overall 56-d periods. Other than increases (P < 0.05) in test Arg and His intake, no measurements in any trial differed (P > 0.1). The lack of response to supplementing Arg and His to young dairy calves indicates that Arg and His in these whey protein–based MR were not limiting to normal growth and performance.

1 Corresponding author: mhill@provimi-na. com

Key words: amino acid, arginine, histidine, milk replacer

INTRODUCTION Little research has been conducted to establish the amino acid requirements of calves (Hill et al., 2008) less than 1 or 2 mo of age. Most of the research in calves has been conducted in older veal calves not fed dry feed. Hill et al. (2008) reported Lys and Met were limiting to ADG in the calf less than 1 mo of age and fed a milk-based milk replacer (MR) and starter, whereas Thr was marginally limiting in skim-based MR and not limiting in whey-based MR. Also, the amino ratios in MR fed by Hill et al. (2008) of His (0.21) and Arg (0.28) to Lys were lower than optimum ratios to Lys calculated by Williams and Hewitt (1979), van Weerden and Huisman (1985), Toullec (1989), and Gerrits et al. (1997) in older calves fed only MR. Fligger et al. (1997) reported that Arg supplemented to dairy calves increased ADG and altered some measurements of immunity. Wu and Knabe (1994) suggested that Arg in sow colostrum and milk was an amino acid limiting ADG of pigs. Wu et al. (1994) reported that Arg supplemented to MR enhanced plasma concentrations of insulin and growth hormone, as well as ADG, in pigs. The NRC

(1998) for swine outlines ideal proteins or ratios of Arg to Lys for pigs based on protein accretion (0.48) and body tissue (1.05). Those ratios and each are greater than the ratio of 0.28 reported to optimize ADG in calf MR by Hill et al. (2008). Little research has addressed His requirements in young calves. The swine ideal protein ratios to Lys for His were 0.32 for protein accretion and 0.45 for body tissue in the NRC (1998), and they are greater than the ratio of 0.21 reported in calf MR by Hill et al. (2008). Our objectives were to evaluate the effect of supplementing Arg and His to calf MR formulated with optimal amounts of Lys and Met. Our hypothesis was that calf ADG, feed efficiency, and serum urea N concentrations would be improved with supplemental Arg and His.

MATERIALS AND METHODS Male Holstein calves were cared for by acceptable practices as described in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 2010). Calves were initially 2 to 3 d of age and were sourced from one dairy farm and transported approximately 3.5 h to the Nurture Research Center in southwest Ohio, where the trials were conducted. Calves were housed

566 through d 56 in individual pens in a curtain-sided nursery with a ridge vent that used natural ventilation. The pens were 1.2 × 2.4 m with wire mesh sides and bedded with straw. Feed units are reported on a DM basis.

Trial 1 Calves (24 per treatment; initially 40.8 ± 1.22 kg of BW) were fed a 27% CP, 17% fat MR at 0.66 kg of DM per calf daily to provide 2 treatments (A: control, no added l-Arg,

Hill et al.

B: added l-Arg; Table 1). The l-Arg was added to achieve a theoretical 0.48 Arg-to-Lys ratio, which is approximately the ideal ratio for protein accretion in pigs (NRC, 1998). Whey protein concentrate (78% CP) was replaced with l-Arg to keep the CP, fat, and ME concentrations approximately equal. The MR powder was mixed as a 15% solution with water. On d 40, 41, and 42, calves were only fed the MR at the a.m. feeding to facilitate weaning. Calves were fed a coarse, textured, 20% CP starter consisting of 37% whole corn, 25% whole oats,

35% supplement pellets (providing 23.75% soybean meal to the complete starter plus other ingredients), and 3% molasses. Fecal scores were assigned daily based on a 1-to-5 system (1 being normal, thick in consistency; 2 being normal, but less thick; 3 being abnormally thin but not watery; 4 being watery; 5 being watery with abnormal coloring; modified from Kertz and Chester-Jones, 2004) through d 56. Medical treatments were recorded daily. Calves were weighed initially and every 7 d. Hip widths were measured with a caliper initially and

Table 1. Ingredient and analyzed nutrient composition of the milk replacers and analyzed composition of the starters1 in each trial Trial 1 Control

Item Ingredient, % as fed  Whey   Whey protein concentrate, 78% CP   Dry fat, 7% CP, 60% fat  Emulsifier   Flow agent   Deccox premix,2 0.5%   Ca carbonate   dl-Met   l-Lys  Flavor   Dicalcium phosphate   Vitamins, trace minerals   l-His   l-Arg Nutrients, % DM  DM  CP  Fat  Ash  Ca  P  Lys  Met  Thr  Arg   Arg:Lys ratio  His   His:Lys ratio   Metabolizable energy,3 Mcal/kg





46.5 23.4 23.4 3 0.96 0.68 0.68 0.37 0.35 0.2 0.2 0.26 — — 96.3 27.3 17.8 7.4 0.80 0.65 2.45 0.76 1.65 0.69 0.28 0.46 — 4.8

Trial 2 +Arg





46.5 23 23.4 3 0.96 0.68 0.68 0.37 0.35 0.2 0.2 0.26 — 0.4 96.2 27.4 17.9 7.5 0.83 0.62 2.43 0.75 1.63 1.08 0.44 0.48 — 4.8

Control

                                                             





46.5 23.4 23.4 3 0.96 0.68 0.68 0.37 0.35 0.2 0.2 0.26 — — 96.3 27.6 18.0 7.1 0.79 0.66 2.46 0.77 1.63 0.67 0.27 0.49 — 4.8

Trial 3 +Arg





46.5 21.7 23.4 3 0.96 0.68 0.68 0.37 0.35 0.2 0.2 0.26 — 1.7 96.3 27.4 17.8 7.4 0.82 0.64 2.46 0.78 1.62 2.46 1.00 0.47 — 4.8

Control

                                                             





46.5 23.4 23.4 3 0.96 0.68 0.68 0.37 0.35 0.2 0.2 0.26 — — 96.2 27.2 17.9 7.1 0.84 0.65 2.42 0.76 1.61 0.68 — 0.49 0.20 4.8

+His  



46.5 22.84 23.4 3 0.96 0.68 0.68 0.37 0.35 0.2 0.2 0.26 0.56 — 96.3 27.4 17.8 7.2 0.81 0.64 2.44 0.77 1.64 0.68 — 1.03 0.42 4.8

Trial 1: 88.1% DM, 20.7% CP, 3.9% fat, 0.79% Ca, 0.55% P, 0.92% Lys, 0.29% Met, 0.67% Thr, 1.13% Arg, 0.48% His. Trial 2: 88.2% DM, 20.8% CP, 3.8% fat, 0.81% Ca, 0.57% P, 0.93% Lys, 0.32% Met, 0.65% Thr, 1.16% Arg, 0.52% His. Trial 3: 88.2% DM, 20.5% CP, 3.9% fat, 0.79% Ca, 0.54% P, 0.91% Lys, 0.34% Met, 0.69% Thr, 1.19% Arg, 0.50% His. 2 Deccox (Alpharma Inc., Fort Lee, NJ). 3 Metabolizable energy calculated using NRC (2001) equations. 1

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Arginine and histidine for dairy calves

every 14 d. On d 7, 14, and 21, blood was sampled and serum harvested by centrifugation. Serum was analyed for albumin, alkaline phosphatase, amylase, creatinine, glucose, total protein, and urea N as described in Hill et al. (2007). The average nursery temperature during the trial was 22°C (ranged from 9 to 32°C) and average humidity was 63% (ranged from 52 to 99%) based on hourly measurements. Data were analyzed preweaning (0 to 42 d), postweaning (42 to 56 d), and overall (0 to 56 d) as a completely randomized design. Repeated-measures analyses over time were calculated using Proc Mixed in SAS (SAS Inst. Inc., Cary, NC). Calf was the experimental unit from 0 to 56 d.

B: added l-His; Table 1). The l-His was added to achieve a theoretical 0.45 His-to-Lys ratio, which is approximately the ideal ratio for body tissue accretion in pigs (NRC, 1998). Whey protein concentrate (78% CP) was replaced with l-His to keep the CP, fat, and ME concentrations approximately equal. Otherwise, feeding, management, and statistical analysis were the same as in trial 1. The average nursery temperature during the trial was 23°C (ranged from 13 to 34°C) and average humidity was 68% (ranged from 25 to 99%) based on hourly measurements.

General Management Calves were received at approximately 1100 h after a 3.5-h transit. The MR powder was fed at 0.330 kg per calf at the first p.m. and the following a.m. feeding. At 1100 h the day after arrival, the calves were weighed (initial BW), blood was sampled from the jugular vein to measure serum protein using an optical refractometer (ATAGO USA Inc., Bellevue, WA), and calves were randomly assigned to treatment (d 0). Equal samples of manufactured feeds were collected from every second bag (22.7 kg) of feed at the time of manufacturing and composited. Composites of feeds were analyzed (AOAC, 2000) for DM (oven method

Trial 2 Calves (24 per treatment; initially 39.4 ± 0.68 kg of BW) were fed a 27% CP, 17% fat MR at 0.66 kg of DM per calf daily to provide 2 treatments (A: control, no added l-Arg, B: added l-Arg; Table 1). The l-Arg was added to achieve a theoretical 1.05 Arg-to-Lys ratio, which is approximately the ideal ratio of body tissue in pigs (NRC, 1998). This greater concentration of Arg was selected because of the outcome in trial 1. Whey protein concentrate (78% CP) was replaced with l-Arg to keep the CP, fat, and ME concentrations approximately equal. The MR powder was mixed as a 15% solution with water. On d 26, 27, and 28, calves were only fed the MR at the a.m. feeding to facilitate weaning. Feeding, management, and statistical analysis were the same as in trial 1. The average nursery temperature during the trial was 22°C (ranged from 9 to 33°C) and average humidity was 63% (ranged from 52 to 99%) based on hourly measurements.

Trial 3 Calves (24 per treatment; initially 44.7 ± 0.77 kg of BW) were fed a 27% CP, 17% fat MR at 0.66 kg of DM per calf daily to provide 2 treatments (A: control, no added l-His,

Table 2. Effect of adding Arg (+Arg) to a calf milk replacer fed for 42 d in 56-d trial 1 Item

Control

+Arg

SEM

P-value

Initial serum protein, g/L Initial BW, kg Final BW, kg ADG, kg/d   0 to 42 d   42 to 56 d   0 to 56 d Starter intake, kg/d   0 to 42 d   42 to 56 d   0 to 56 d Arg intake, g/d   0 to 42 d   42 to 56 d   0 to 56 d Feed efficiency1   0 to 42 d   42 to 56 d   0 to 56 d Hip width change, cm   0 to 42 d   42 to 56 d   0 to 56 d Serum constituents   Urea nitrogen, mmol/L   Creatine, μmol/L   Alkaline phosphatase, U/L   Albumin, g/L   Total protein, g/L   Glucose, mmol/L

53 40.9 74.8   0.508 0.900 0.606   0.241 1.896 0.655   7.1 21.4 10.7   0.581 0.475 0.536   2.6 1.3 3.9   3.8 84.6 144 13.5 60 5.8

53 40.7 73.1   0.491 0.844 0.579   0.219 1.822 0.620   9.3 20.6 12.1   0.576 0.463 0.529   2.6 1.2 3.8   3.9 86.3 148 13.6 59 5.9

0.4 1.22 2.1   0.0194 0.0465 0.0224   0.0227 0.0664 0.0313   0.88 0.73 0.59   0.0153 0.0232 0.0114   0.11 0.19 0.18   0.15 3.93 8.9 0.35 0.5 0.32

0.99 0.86 0.54   0.54 0.39 0.41   0.49 0.43 0.44   0.01 0.23 0.04   0.96   0.74   0.66 0.43 0.59   0.34 0.51 0.36 0.46 0.84 0.53

Gain divided by starter plus milk replacer intake. Milk replacer intake averaged 0.633 kg of DM daily for d 0 to 42.

1

568 930.15), ash (oven method 942.05), CP (Kjeldahl method 988.05), fat (MR using alkaline treatment with Roese-Gottlieb method 932.06; starter using diethyl ether extraction method 2003.05), Ca and P (dry ashing, acid digestion, analysis by inductively coupled plasma method 985.01), and amino acids by HPLC. Amino acids were determined after acid hydrolysis (method 982.30 E[a]). Sulfur amino acids were determined after performic acid oxidation and then acid hydrolysis (method 982.30 E[b]). Calves received an intranasal tissue sensitive respiratory disease vaccine (TSV-2, Pfizer, Exton, PA) and subcutaneous injections of vitamins A, D, E (Vital E - A + D, Schering-Plough Animal Health, Union, NJ), and Se (MU-SE, Schering-Plough Animal Health) upon arrival. Calves received an intramuscular respiratory disease vaccine (Bovashield Gold 5, Pfizer, Exton, PA) at d 7 and again at d 28. At d 14 they received an intramuscular vaccine for types C and D clostridium (Ultra Choice 7, Pfizer). A pasturella vaccine (Presponse HM, Fort Dodge Animal Health, Fort Dodge, IA) was administered intramuscularly on d 28 and 42. Calves were castrated and dehorned at 36 d of age. Animals that required medication for sickness were treated per veterinary recommendation and treatments were recorded daily. Scouring was the only sickness observed, and it was diagnosed based on rectal temperatures greater than 39.5°C, lack of vitality, and fecal scores >2. Scouring was treated with subcutaneous ceftiofur sodium (Naxcel, Pharmacia & Upjohn, Kalamazoo, MI).

RESULTS AND DISCUSSION In trials 1 (Table 2) and 2 (Table 3), Arg intake was greater during d 0 to 42 and d 0 to 56 in calves fed the MR supplemented with Arg than in control calves. In trial 3 (Table 4), His intake was greater during d 0 to 28 and d 0 to 56 in calves fed the MR supplement with His than in control calves. Otherwise there were no differences in measurements made in trials

Hill et al.

1, 2, and 3. These trials were conducted during summer months with high average environmental temperatures (22 to 23°C). High temperatures are associated with increased maintenance requirements of calves (i.e., panting), reduced starter intakes, and reduced ADG compared with cooler temperatures (McKnight, 1978; NRC, 2001; Chester-Jones et al., 2008; Bateman et al., 2010; Hill et al., 2011). We are unaware of reasons why environmental temperature would affect the outcome of supplementing Arg and His. Kim and Wu (2004) reported a 65% increase in ADG and reduction in the concentration of plasma urea of pigs from 7 to 21 d of age when their MR was supplemented with Arg. Pigs were fed MR ad libitum,

and MR intake did not change. It has been reported that Arg in sow milk and colostrum is low relative to the requirement of a young pig (Wu and Knabe, 1994; Kim and Wu, 2004) and that the uptake of Glu and release of Arg and other amino acids in the small intestine of the young pig is the reason the young pig requires more Arg than older pigs (Wu et al., 1994). No trends for increased ADG or decreased serum urea N were observed in trials 1 and 2. Zhan et al. (2008) reported that microvascular development in the small intestine of the pig increased with low levels of Arg supplementation but decreased and had adverse effects with high levels of supplementation. We are unaware of this kind information for calves.

Table 3. Effect of adding Arg (+Arg) to a calf milk replacer fed for 42 d in 56-d trial 2 Item

Control

+Arg

SEM

P-value

Initial serum protein, g/L Initial BW, kg Final BW, kg ADG, kg/d   0 to 42 d   42 to 56 d   0 to 56 d Starter intake, kg/d   0 to 42 d   42 to 56 d   0 to 56 d Arg intake, g/d   0 to 42 d   42 to 56 d   0 to 56 d Feed efficiency1   0 to 42 d   42 to 56 d   0 to 56 d Hip width change, cm   0 to 42 d   42 to 56 d   0 to 56 d Serum constituents   Urea nitrogen, mmol/L   Creatine, μmol/L   Alkaline phosphatase, U/L   Albumin, g/L   Total protein, g/L   Glucose, mmol/L

53 39.7 70.3   0.466 0.787 0.546   0.304 1.888 0.700   7.8 21.9 11.3   0.497 0.417 0.465   2.7 1.7 4.4   3.9 83.9 139 13.8 59 5.7

53 39.1 68.1   0.458 0.695 0.517   0.297 1.786 0.670   19.0 20.7 19.4   0.492 0.389 0.452   2.6 1.5 4.1   3.7 85.2 142 13.8 59 5.8

0.9 0.68 2.14   0.0167 0.0435 0.0233   0.0226 0.0713 0.0337   1.09 0.83 1.05   0.0143 0.0221 0.0113   0.11 0.11 0.17   0.15 3.9 8.9 0.35 0.5 0.32

0.56 0.58 0.35   0.73 0.14 0.31   0.83 0.31 0.52   0.01 0.38 0.01   0.86 0.39 0.31   0.62 0.19 0.25   0.67 0.53 0.35 0.73 0.83 0.44

Gain divided by starter plus milk replacer intake. Milk replacer intake averaged 0.633 kg of DM daily for d 0 to 42.

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Arginine and histidine for dairy calves

Table 4. Effect of adding His (+His) to a calf milk replacer fed for 28 d in 56-d trial 3 Item

Control

+His

SEM

P-value

Initial serum protein, g/L Initial BW, kg Final BW, kg ADG, kg/d   0 to 28 d   28 to 56 d   0 to 56 d Starter intake, kg/d   0 to 28 d   28 to 56 d   0 to 56 d His intake, g/d   0 to 28 d   28 to 56 d   0 to 56 d Feed efficiency1   0 to 28 d   28 to 56 d   0 to 56 d Hip width change, cm   0 to 28 d   28 to 56 d   0 to 56 d Serum constituents   Urea nitrogen, mmol/L   Creatine, μmol/L   Alkaline phosphatase, U/L   Albumin, g/L   Total protein, g/L   Glucose, mmol/L

47 45.4 76.1   0.434 0.666 0.55   0.193 1.623 0.908   4.1 8.1 6.1   0.533 0.410 0.452   1.8 2.2 4.0   4.1 79.3 137 13.2 59 5.9

48 43.9 74.2   0.45 0.632 0.541   0.196 1.642 0.919   7.5 8.2 7.9   0.551 0.385 0.440   1.9 2.1 4.0   4.2 82.3 134 13.3 59 6.0

1.1 0.77 1.35   0.0195 0.0304 0.0201   0.0112 0.0564 0.0322   0.41 0.29 0.36   0.0205 0.0133 0.0107   0.09 0.13 0.16   0.19 3.45 8.3 0.39 0.4 0.28

0.75 0.93 0.41   0.89 0.89 0.30   0.52 0.68 0.63   0.01 0.49 0.01   0.34 0.81 0.67   0.49 0.91 0.65   0.55 0.46 0.73 0.42 0.43 0.65

Gain divided by starter plus milk replacer intake. Milk replacer intake averaged 0.621 kg of DM daily for d 0 to 42.

1

However, Fligger et al. (1997) reported that Arg supplemented in the MR of dairy calves increased ADG and altered some measurements of immunity. Little information could be found for His. The NRC (1998) for swine has a His requirement and cites only one study (Izquierdo et al., 1988). Izquierdo et al. (1988) suggested a His requirement for a corn and soybean meal diet fed to weaned pig to be approximately 0.4% His. The unsupplemented MR fed in each of our trials was 0.46 to 0.49% His (His-to-Lys ratio of 0.2).

IMPLICATIONS Supplementing dairy calf MR formulated with whey proteins with l-Arg and l-His did not change calf BW gains, feed efficiency, starter intake, and plasma concentrations of several constituents including urea N. The lack of calf responses in these 3 trials suggests that Arg and His are not limiting calf performance when the calves are fed whey-based MR and corn and soybean meal–based starters.

LITERATURE CITED AOAC. 2000. Official Methods of Analysis. Vol. I. 17th ed. AOAC, Arlington, VA.

569 Bateman, H. G., II, T. M. Hill, J. M. Aldrich, R. L. Schlotterbeck, and J. L. Firkins. 2010. Meta analysis for designing an empirical model to predict growth of neonatal Holstein calves through eight weeks of age. J. Dairy Sci. 93(Suppl. 1):398. Chester-Jones, H., D. M. Ziegler, R. Larson, C. Soderholm, S. Hayes, J. G. Linn, M. Raeth-Knight, G. Golombeski, and N. Broadwater. 2008. Applied calf research from birth to six months. Pages 106–112 in Proc. 4 State Dairy Nutr. Manage. Conf., Dubuque, IA. Iowa State Univ. Press, Ames, IA. FASS (Federation of Animal Science Societies). 2010. Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching. 3rd ed. FASS, Champaign, IL. Fligger, J. M., C. A. Gibson, L. M. Sordillo, and C. R. Baumrucker. 1997. Arginine supplementation increases weight gain, depresses antibody production, and alters circulating leukocyte profiles in preruminant calves without affecting plasma growth hormone concentrations. J. Anim. Sci. 75:3019. Gerrits, W. J. J., J. France, J. Dijkstra, M. W. Bosch, G. H. Tolman, and S. Tamminga. 1997. Evaluation of a model integrating protein and energy metabolism in preruminant calves. J. Nutr. 127:1243. Hill, T. M., J. M. Aldrich, R. L. Schlotterbeck, and H. G. Bateman II. 2007. Amino acids, fatty acids, and fat sources for calf milk replacers. Prof. Anim. Sci. 23:401. Hill, T. M., H. G. Bateman, J. M. Aldrich, and R. L. Schlotterbeck. 2011. Comparisons of housing, bedding, and cooling options for dairy calves. J. Dairy Sci. 94:2138. Hill, T. M., H. G. Bateman II, J. M. Aldrich, R. L. Schlotterbeck, and K. G. Tanan. 2008. Optimal concentrations of lysine, methionine, and threonine in milk replacers for calves less than five weeks of age. J. Dairy Sci. 91:2433. Izquierdo, O. A., K. J. Wedekind, and D. H. Baker. 1988. Histidine requirement of the young pig. J. Anim. Sci. 66:2886–2892. Kertz, A. F., and H. Chester-Jones. 2004. Invited review: Guidelines for measuring and reporting calf and heifer experimental data. J. Dairy Sci. 87:3577. doi:10.3168/jds.S00220302(04)73495-5. Kim, S. W., and G. Wu. 2004. Dietary arginine supplementation enhances the growth of milk-fed young pigs. J. Nutr. 134:625. McKnight, D. R. 1978. Performance of newborn dairy calves in hutch housing. Can. J. Anim. Sci. 58:517. NRC. 1998. Nutrient Requirements of Swine. 10th ed. Natl. Acad. Sci., Washington, DC. NRC. 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. Natl. Acad. Sci., Washington, DC. Toullec, R. 1989. Veal calves. Pages 109–119 in Ruminant Nutrition—Recommended Al-

570 lowances and Feed Tables. R. Jarrige, ed. INRA, London, UK. van Weerden, E. J., and J. Huisman. 1985. Amino acid requirement of the young veal calf. J. Anim. Physiol. Anim. Nutr. (Berl.) 53:232.

Hill et al. Williams, A. P., and D. Hewitt. 1979. The amino acid requirements of the preruminant calf. Br. J. Nutr. 41:311.

Wu, G., and D. A. Knabe. 1994. Free and protein-bound amino acids in sow’s colostrum and milk. J. Nutr. 124:415.

Wu, G., A. G. Borbolla, and D. A. Knabe. 1994. The uptake of glutamine and release of arginine, citrulline and proline by the small intestine of developing pigs. J. Nutr. 124:2437.

Zhan, Z., D. Ou, X. Piao, S. W. Kim, Y. Liu, and J. Wang. 2008. Dietary arginine supplementation affects microvascular development in the small intestine of early-weaned pigs. J. Nutr. 138:1304.