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J APPL POULT RES 2007. 16:574-582. doi:10.3382/japr.2007-00046
© 2007 Poultry Science Association
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Research Reports

Dietary Threonine Needs for Growth and Immunity of Broilers Raised Under Different Litter Conditions1

A. Corzo*,2, M. T. Kidd*, W. A. Dozier, III{dagger}, G. T. Pharr{ddagger} and E. A. Koutsos§

* Department of Poultry Science, Mississippi State University, Mississippi State, MS 39762; {dagger} USDA, Agricultural Research Service, Mississippi State, MS 39762; {ddagger} Department of Basic Sciences, Mississippi State University, Mississippi State, MS 39762; and § Department of Animal Science, California Polytechnic State University, San Luis Obispo, CA 93407

Correspondence: 2 Corresponding author: acorzo{at}poultry.msstate.edu


   SUMMARY
TOP
 SUMMARY
DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Two studies were conducted simultaneously and evaluated the Thr needs of male Ross x Ross 708 broilers. Broilers in the 2 studies were reared under 2 litter conditions: new (NL) vs. used built-up soft wood shavings (BL). Separated by a center aisle, all floor pens from 1 side of the close-sided house contained NL, whereas the opposite side contained BL. Broilers received common diets up to 21 d and then were fed 1 of 6 total dietary Thr levels that ranged from 0.51 to 0.86% total Thr until d 42. At 42 d, birds were processed. A subsample of birds from each experimental unit corresponding to either the 0.51 or 0.72% Thr treatments was taken, immune function was quantified, and lymphoid organs were weighed. Results for live performance and carcass traits are in close agreement with previously reported values in the literature. Quadratic responses were observed for BW gain, feed conversion, and carcass and breast meat absolute and relative weights. Depending on the variable, these responses were maximized from 0.71 to 0.74% Thr when broilers were raised on NL and from 0.73 to 0.78% Thr when broilers were raised on BL. Low Thr (0.51%) was without effect on most immune parameters. However, low Thr decreased relative thymus weight and increased monocyte NO production in built-up and new litter environments, respectively.

Key Words: breast meat yield • broiler • immunity • litter • threonine


   DESCRIPTION OF PROBLEM
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
The use of supplemental amino acids in poultry diets has provided nutritionists with considerable flexibility in formulation. The availability of Lys, Met, and Thr at competitive prices has enabled the vast majority of the poultry industry to include them as supplements in their diets and therefore allow for a reduction in pressure points caused by their degree of limitation in practical broiler diets.

Threonine is the amino acid that will become first-limiting in practical broiler diets once Lys and Met needs are met. It is also one of the most important amino acids at the intestinal level for maintenance purposes. It has been estimated that more than half of the dietary Thr consumed by a piglet [1] or a human [2] is retained at the intestinal level to fulfill these gut-maintenance functions and is primarily used in the synthesis of mucins. Mucin glycoproteins consist of peptides enriched in Thr and have been estimated to compose close to a third of the proteins utilized in mucin [3]. The type and amount of mucin produced in the gastrointestinal tract affects microbial communities (mucin serves as a substrate for bacterial fermentation and fixation), nutrient availability (via endogenous mucin losses as well as absorption of luminal nutrients), and immune function (via regulation of microbial communities and nutrient availability). In turn, the microbial communities in the gastrointestinal tract, as well as the exposure to nutrients and feed ingredients, will affect mucin dynamics [4]. Therefore, 2 simultaneous studies examined the effect of dietary Thr in broilers on performance and immune function. It was hypothesized that environmental conditions, particularly litter status, would influence Thr needs in growing broilers. Two simultaneous studies were conducted in similar experimental conditions but differing in the litter status [new soft wood shavings (NL) vs. used built-up litter (BL)] and fed diets differing in dietary Thr.


   MATERIALS AND METHODS
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Bird Husbandry
Ross x Ross 708 male 1-d-old chicks [5] were randomly distributed into floor pens of a closed-sided house that had thermostatically controlled heating (forced-air furnaces), curtains, and cross-ventilation. One side (48 pens) of the house had floor pens that had used built-up litter (BL) from 4 flocks (pens had been empty 10 d before this study), whereas the other side (48 pens) had unused new soft wood shavings (NL), each separated by a center aisle. Both sides were equipped with a nipple drinker line (3 nipples/pen) and a hanging feeder (22.5-kg capacity). The lighting program consisted of 23 h of light and 1 h of dark. Ventilation was accomplished by negative air pressure. Chicks were vaccinated for Marek’s disease (via in ovo administration at d 18), Newcastle disease, and infectious bronchitis (via coarse spray at hatch). Temperature was recorded daily and managed to provide thermoneutral conditions to the birds throughout the study. All birds received common corn-soybean meal feeds from placement to 21 d of age in crumbled form that met or exceeded NRC [6] nutrient recommendations.

Treatments
At 21 d of age, bird number was equalized among all pens (12 birds/pen; 0.1 m2/bird), and then treatments were assigned by blocking areas of the house at the start of both experiments (new vs. built-up litter). Treatments consisted of 6 dietary Thr levels (8 replications/treatment) that progressed from 0.51 to 0.86% total Thr at 0.07% Thr increments. To insure uniformity of mix, a common dose-response basal diet batch was made in a vertical mixer. Random aliquots of the dose-response diet were used to produce the dietary Thr treatments, accomplished by the addition of L-Thr at the expense of an inert filler. Composite samples of dietary treatments were obtained and analyzed for protein-bound and -supplemented amino acids [7], to assure that calculated and analyzed Thr values were in agreement. Formulation of the basal diet (Table 1Go) minimized Thr content while assuring the minimum levels of all other essential amino acids in a manner that would meet or exceed current NRC recommendations [6]. All experimental diets were fed in pellet form. Feed and water were offered ad libitum. The different levels of dietary Thr were simultaneously fed to all 48 floor pens in each study from 21 to 42 d of age.


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  		Table 1. Composition of experimental diet fed to male Ross x Ross broilers from 21 to 42 d of age
 
Measurements
The mean bird weight of all pens was recorded at the initiation and termination of the experimental phase for both studies. Feed consumption and mortality were recorded. Feed conversion was corrected for mortality and represents grams of feed consumed by all birds in a pen divided by grams of BW gain per pen. At 42 d of age, 5 birds per pen were randomly removed, weighed, and cooped for 12 h before processing them. Processing was manual, and hot carcass and abdominal fat weights were obtained and recorded. Carcasses were chilled for 4 h, and the breast muscles were manually de-boned and their weights recorded.

Several parameters of the immune system were examined in broilers fed either 0.51 or 0.72% Thr in both experiments, based on the hypothesis that broilers corresponding to these treatments would be theoretically deficient and adequate in dietary Thr, respectively. The primary antibody response was assessed following vaccination with sheep red blood cells (SRBC). Sheep red blood cells were obtained, washed twice with sterile saline, and a 10% solution was prepared for i.v. injection into a randomly selected bird per pen at 28 d of age. On d 35, each previously injected bird was bled via the brachial vein, and serum was obtained for evaluation of the primary antibody response by micro-hemagglutination. Briefly, serum was inactivated at 56°C for 30 min, and then an aliquot (25 µL) was added to the first well of a microtiter plate. Serial dilutions were performed, followed by an addition of 25 µL of 2.5% SRBC to each well. The plate was sealed and incubated for 60 min at 37°C. Primary antibody titers are expressed as base-2 logarithms of the highest serum dilution that agglutinated 0.05 mL of 2.5% suspension of SRBC in sterile saline.

Parameters of the innate immune system were assessed from whole blood from another randomly selected bird from each pen fed either 0.51 or 0.72% Thr at 35 d of age. Whole blood was immediately diluted in a 1:1 dilution of media (RPMI 1640, Sigma Aldrich number R8758) [8], and samples were shipped on ice to be processed within 24 h of collection. Each sample was assessed for leukocyte counts (determined after staining with Wrights-Geimsa stain and visualization with a hemacytometer under a light microscope). Additionally, adherent peripheral blood mononuclear cells (PBMC) were isolated using a density gradient [9, 10, 11].

After cooping the birds at d 42, 1 bird from each pen fed either 0.51 or 0.72% Thr and raised on NL and BL was randomly selected. Lymphoid organs (thymus, bursa, and spleen) were removed and weighed. Thymus weight represented the 3 proximal lobes on the left side of the bird. Organ weights are expressed relative to the live BW.

Statistical Analyses
Data obtained from each side of the house, NL or BL, were treated as individual experiments, experiments 1 and 2, respectively, and were analyzed and presented as such. Both experiments were evaluated by ANOVA in a randomized complete block design. Pen was used as the experimental unit for analysis. Percentage data for mortality were transformed to arcsine {surd}% for analysis. All data were analyzed by the GLM procedure of SAS Institute [12]. Only linear and quadratic effects are presented on the respective tables, because significance (P > 0.05) of higher-order polynomials was not observed. Regression analysis was used to estimate Thr optimization (95% of the maximum or minimum response) whenever a significant quadratic response (P < 0.05) was observed. Immune response means were tested and separated for Thr effects (P ≤ 0.05) using a t-test.


   RESULTS AND DISCUSSION
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Before experimentation (d 21), birds obtained satisfactory BW (mean: 816 g; SD: 35.8) based on comparison to the Ross x Ross 708 management guide, which displays a BW of 1.62 lb (735 g) at 21 d of age [5]. Amino acid analyses of dietary Thr in experimental diets were in close agreement with calculated values (Table 1Go).

This design encompassed clean litter or litter that had been used in 4 growouts. Also, it should be noted that the facility was newly constructed and had never been used for research.

Increasing levels of dietary Thr resulted in quadratic responses in both experiments (Table 2Go). Those broilers reared in NL maximized their BW gain at a dietary level of 0.74 Thr, compared with 0.77% Thr needed by those reared in BL. Feed conversion also exhibited quadratic responses for broilers reared under both litter conditions. Those birds reared in NL optimized their feed conversion at 0.72% Thr compared with 0.73% Thr needed by their BL counterparts. Feed intake exhibited a quadratic response for those birds reared on BL, but those reared on NL exhibited a decreasing linear effect with increasing dietary Thr. Mortality was minimal for the experimental period from 21 to 42 d of age (0.34% total) and was unaffected by dietary Thr. Dietary Thr needs observed for live performance measurements are in agreement with those previously published for birds during the same age period [13, 14, 15, 16, 17]. Kidd et al. [18] fed graduated Thr levels to Cobb x Cobb 500 fast-feathering broilers from 42 to 56 d of age that were exposed to 1 of 2 types of litter, either new soft wood shavings or used built-up litter. During live performance, Kidd et al. [18] observed quadratic responses for these birds for BW gain and feed conversion for birds reared on NL. However, Kidd et al. [18] were unable to calculate Thr requirements in broilers reared in BL, because only linear responses with dietary Thr for BW gain and feed conversion were observed.


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  		Table 2. Live performance of broilers fed incremental levels of dietary Thr and raised on new or built-up litter
 
After the birds were processed, the carcass weight responded in a similar fashion to BW gain (Table 3Go). For those birds in the study that used NL, the weight of the carcass was maximized at a Thr level of 0.72%, whereas those reared on BL optimized their carcass weight at 0.77% Thr. The value of carcass relative weight gave a response parallel to that of carcass absolute weight. Broilers raised on NL maximized their yield at a Thr level of 0.71%, whereas their BL counterparts maximized at a level of 0.78% total Thr. This last value is in close agreement with those previously reported by Kidd et al. [17] for 3 broiler strains reared on built-up litter.


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  		Table 3. Carcass and fat of broilers at 42 d of age after being fed incremental levels of dietary Thr and raised on new or built-up litter
 
The effect of dietary Thr on absolute and relative abdominal fat pad weight in broilers has typically been inconsistent [14, 17, 19, 20, 21]. In the present studies, quadratic and linear effects were observed for abdominal fat pad weight, for broilers reared on NL and BL, respectively (Table 3Go). In both instances, a tendency to increase these absolute weights with dietary Thr was perhaps as a result of increased feed intake, and such abdominal fat values are perhaps the reflection of increasing body mass. Abdominal fat pad values, when expressed as a percentage of the live BW, did not show a dietary Thr influence.

Breast meat development has been shown to necessitate both higher [20] and lower [17, 18, 19] Thr needs when compared with live performance measurements like BW gain. In the present studies, absolute weight of breast muscles was maximized at 0.73 and 0.77% Thr for broilers raised in NL and BL, respectively (Table 4Go). Their relative weight, also known as breast meat yield, displayed almost identical Thr needs. Broilers raised on NL required 0.73% Thr for maximization, whereas those reared on BL required 0.75% Thr. It would appear that breast meat needs for dietary Thr do not differ from those needs for live performance or carcass weight and yield for the Ross x Ross 708 male, based on the results observed in both of our studies.


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  		Table 4. Breast meat weight and yield of broilers at 42 d of age after being fed incremental levels of dietary Thr and raised on new or built-up litter
 
It must be pointed out that BL and NL pens were not randomly assigned adjacently due to knowledge of cross-contamination via litter movement due to bird dust-bathing, preening, and overall physical activity. Hence, litter types were maintained separately for validation of physiological effects rather than optimal experimental design. Therefore, even though litter type cannot be involved in the present statistical model as a factor, it does appear that Thr needs may be increased as litter ages. It is interesting to observe how all quadratic responses yielded higher Thr needs for those broilers reared on BL compared with those reared on NL. Similar results were observed by Kidd et al. [18], in which those broilers in clean environments exhibited total Thr needs ranging from 0.63 to 0.68% depending on the variable, but those raised in a dirty environment failed to display quadratic trends but did exhibit linear increases with increasing dietary Thr values. Kidd et al. [18] reported an apparent higher need for dietary Thr as broilers are exposed to less clean environments. Kidd et al. [18] also discussed the limitations of conducting a simultaneous study in which Thr needs could be evaluated depending on environmental conditions. The present studies were conducted simultaneously, using broilers from the same strain and the same flock of breeders, reared in a common broiler house, under a common lighting-temperature regimen, and similar ventilation conditions. However, due to the difficulty of blocking for environment, we believe this model is as close as one can compare nutrient needs in broilers under different litter types and under practical commercial conditions, unless an entire broiler house is used as an experimental unit or environmental chambers are used; in both of these cases, applicability and managerial reasons would be a disadvantage.

The basis for increased Thr needs for broilers exposed to unclean environments could be based upon the maintenance needs associated with the intestinal mucosa. Certain nutrients have direct involvement in the protection of the intestinal surface. In humans, arachidonic acid is used for prostaglandin synthesis, known to have cytoprotective properties [22]. It has also been shown that dietary phospholipids may improve the surfactant function of the stomach [23]. Glutathione has been associated with luminal surface protection of intestinal epithelial cells in dogs [24]. Mucin also serves as a surface protection agent, which in turn helps regulate the type and frequency of intestinal immune stimulation. Some amino acids, including Thr, have been linked to surface protection via intestinal mucin synthesis. The proteins in enteric mucin have been known to be composed with a minimum Thr content of 16% and as high as 30% [3, 25, 26]. Therefore, the maintenance needs for this amino acid have been shown to be remarkably high at the intestinal level [1, 2]. Furthermore, supplementation with Thr, along with L-Ser, L-Pro, and L-Cys, was shown to increase the number of Muc2-containing goblet cells around an ulcerated area and stimulated mucin synthesis in the colon of dextran sulfate sodium-treated rats [27]. In another case, Thr restriction reduced small and large intestine mucin synthesis [28]. The homology and composition of mucin in chickens has been shown to be of similar composition to other species, and even some specific mucin types have shown resemblance in their composition to those of humans [29]. Therefore, the observed higher needs for Thr in broilers reared in BL may reflect the different microbial exposure level to which these birds were exposed and the resulting changes in mucin production.

White blood cell counts (absolute and ratios) did not differ between broilers fed either the lowest Thr level in the study (0.51% Thr) or a dietary Thr value predicted to be nutritionally adequate (0.72% Thr; Table 5Go). Dietary Thr had no effect on these white blood cell profiles. The heterophil:lymphocyte ratio, known to be an indicator of the presence of physiological stress [30], was not affected by a dietary Thr deficiency. Similarly, no dietary Thr effect was observed for relative weights of bursa and spleen (Table 6Go). Results are in agreement with those presented by Kidd et al. [31] in that no dietary Thr effect was seen to influence relative organ weights of broilers raised in a clean environment. In broilers raised on the BL, there was an increase in relative thymus weight with Thr adequacy as compared with birds fed low Thr, which may reflect an increased need for dietary Thr under conditions of used litter, as seen in terms of BW gains as well. Thymus weight has been previously shown to be very sensitive to nutrient deficiency, specifically for vitamin D in chickens [32], Mg in rats [33], and Cu in mice [34]. However, low dietary Thr in the BL environment did not negatively affect any other immune system parameter.


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  		Table 5. White blood cell count (%) and ratios of broilers at 35 d of age after being fed incremental levels of dietary Thr and raised on new or built-up litter
 

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  		Table 6. Organ relative weights of broilers at 42 d of age after being fed incremental levels of dietary Thr and raised on new or built-up litter1
 
Regarding the broilers reared in the NL environment, low dietary Thr did not negatively affect any blood profile, organ relative weight, or mononuclear cell function. However, low dietary Thr increased the postlipopolysaccharide production of NO by adherent PBMC (Table 7Go), which may indicate a higher degree of responsiveness to innate immune challenge or a higher level of activation of the cells before isolation. The latter hypothesis may have resulted from impaired mucin production and consequent increases in bacterial stimulation of the gastrointestinal tract, and perhaps systemic immune system, although analysis of mucin production would need to be completed to address this hypothesis.


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  		Table 7. Peripheral blood monocyte NO, HD11 macrophage NO, and primary antibody response to sheep red blood cells of broilers at 35 d of age after being fed incremental levels of dietary Thr and raised on new or built-up litter
 
No Thr effect was observed for production of NO by HD11 macrophages (Table 7Go), indicating that any effect of low dietary Thr on PBMC NO production was likely related to the cells as opposed to the serum conditions under which they were incubated. Finally, there was no effect of dietary Thr level on the primary antibody response to SRBC (Table 7Go). In the future, a second antibody response to SRBC inoculation should be evaluated, but the borderline increase (P = 0.07) observed with birds fed 0.51% Thr may have been correlated to the increase in NO secretion and thus an activation of the immune system.

Results from these studies illustrate the importance of meeting critical amino acid needs for maximization of yields in a high-yielding, late-developing broiler like the Ross x Ross 708. There appears to be a possible rise in Thr needs for maintenance as broilers are reared in dirtier environments, but any potential environmental effects need to be statistically validated by being factored into a model before such assumption can evolve into a formal statement.


   CONCLUSIONS AND APPLICATIONS
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
REFERENCES AND NOTES
 

  1. Depending on the litter type used, BW gain, feed conversion, and carcass and breast meat absolute and relative weights were maximized at different total dietary Thr values.
  2. These variables were optimized with total Thr ranging from 0.71 to 0.74% (0.63 to 0.66% calculated digestible) for birds reared in new shavings; birds reared in used built-up litter optimized these parameters with total dietary Thr values that ranged from 0.73 to 0.78% (0.65 to 0.70% calculated digestible).
  3. Immune criteria used in this study did not provide a possible explanation to understand the mode of action behind the increase in dietary Thr with birds reared in built-up litter environments. However, data from growth and carcass criteria are suggestive that an increase in Thr needs in the gut due to microbial challenges may likely be a reason.


   ACKNOWLEDGMENTS
 
We wish to thank Degussa Corporation (Kennesaw, GA) for the amino acid analyses of the experimental diets.


   FOOTNOTES
 
1 This is journal article number J11074 from the Mississippi Agricultural and Forestry Experiment Station supported by MIS-322220. Use of trade names in this publication does not imply endorsement by the Mississippi Agricultural and Forestry Experiment Station and USDA-ARS of the products nor similar ones not mentioned. Back


   REFERENCES AND NOTES
TOP
 SUMMARY
 DESCRIPTION OF PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 

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