Growth performance, liver and kidney functions, blood hormonal profile, and economic efficiency of broilers fed different levels of threonine supplementation during feed restriction

Abstract

The objective of the existing investigation was to determine the effect of dietary inclusion of threonine amino acid at different levels during feed restriction on growth indices, liver and kidney function parameters, and some hormonal profiles along with economic indicators in broiler chickens. A total of 1,600 from 2 different breeds (800 Ross 308 and 800 Indian River) at 21-day-old age were incorporated. Chicks were randomly assigned into 2 main groups, control and feed-restricted (8 h/d), during the fourth week of age. Each main group was subdivided into 4 groups. The first group was fed a basal diet without adding extra threonine (100%), the second, third, and fourth groups were fed a basal diet with extra threonine levels of 110, 120, and 130%, respectively. Each subgroup consisted of 10 replicates of 10 birds. We noticed that the dietary inclusion of threonine at extra levels in the basal diets significantly enhanced final body weight, body weight gain, and better feed conversion ratio. This was mainly due to the enhanced levels of growth hormone (GH), insulin-like growth factor (IGF1), triiodothyronine (T3), and thyroxine (T4). Moreover, the lowest feed cost per kilogram body weight gain and improved return parameters were reported in control and feed-restricted birds fed higher levels of threonine than other groups. Also, a significant increase in alanine aminotransferase (ALT), aspartate aminotransferase (AST), and urea levels was observed in feed-restricted birds supplemented with 120 and 130% levels of threonine. Hence, we recommend supplementing threonine at levels of 120 and 130% in the diet of broilers to promote growth and profitability.

INTRODUCTION

Over the past few years, the demand for poultry products has steadily grown globally (FAO, 2013). As a result, various efforts are being made to look for new ways to enhance poultry productivity (Adedokun et al., 2014; Alagawany et al., 2014; Abbasi et al., 2018; Abou-Kassem et al., 2022). According to Kidd and Kerr (1996), threonine is poultry's third limiting amino acid. Threonine exhibits a vigorous role in cellular development. It improves intestinal mucosa and digestive enzyme activity (Dozier et al., 2001), immune system functions, barrier integrity, and antioxidant capacity (Chen et al., 2016; Bi et al., 2018). Nearly a third of the proteins used in mucin are estimated to be found in peptides rich in threonine (Corzo et al., 2007; Rehman et al., 2017; Soomro et al., 2017; Saeed et al., 2020; Chandra et al., 2021a,b). Threonine deficiency in meat-type poultry causes decreased growth performance, feed consumption, and carcass weight (Zhang et al., 2014). Financial losses can arise from slight dietary shortages of digestive threonine because of elevated feed efficiency and decreased breast meat production (Khan et al., 2008).

Feed restriction programs have become more popular due to rising feed costs and their ability to decrease abdominal fat deposition and enhance feed efficiency. Broiler production is characterized by a fast growth rate linked with impaired reproductive competence, ascites, lameness, and mortality. Hence feed restriction has been applied (Ebeid et al., 2022; Ogbuagu et al., 2023).

The effectiveness of several feed restriction strategies, such as controlling the amount of time that animals can access food each day (Bordin et al., 2021; Tůmová et al., 2022), removing food for up to 8 h at a time or skipping a day of feeding, permitting animals to eat only once every hour, and feeding only once every other day (Boostani et al., 2010; Saffar and Khajali, 2010) have been assessed, but the findings have been varied (Khetani et al., 2009; Ghazanfari et al., 2010). The present study hypothesized that supplementing threonine to broiler diet may have a positive impact on its growth and profitability. Hence, the existing treatise aimed to inspect the effect of threonine dietary inclusion with various levels in 2 breeds of broiler chickens, either feed-restricted birds or not, on growth performance, liver and kidney function parameters, hormonal profile, and economic efficiency.

MATERIALS AND METHODS

The care of the birds used in this experiment and the experimental methods followed the guidelines set forth by the Animal Care and Ethics Committee of the Faculty of Veterinary Medicine at Damanhur University in Egypt (DMU/VetMed-2021-/0013).

Experimental Birds, Design, and Management

A total of 1,061-day-old chicks from 2 different breeds (800 Ross 308 and 800 Indian River) were obtained from commercial chickens hatcheries, at 21-days old, birds for each breed. Birds were separated randomly into 2 main groups. The first group was fed ad libitum as a control (n = 400), while the second group was subjected to feed restriction as fed 8 h/d during the fourth week of age (n = 400). Within each of the last divided groups, birds were subdivided into 4 groups, the first group fed on the basal ration (Table 1) without any supplementation 100% (threonine level was 0.9%), the second group 110% fed basal diet threonine level, the third group fed 120% of basal diet threonine level. The fourth group fed 130% of basal diet threonine level (n =100). Each subgroup was divided into 10 replicates, each of 10 birds.

Table 1.

Basal diet composition and chemical analysis.

Ingredients	Starter diet %	Grower diet %
Soybean meal (44%)	32.6	29.5
Yellow corn	53.65	58.15
Vegetable oil1	2.0	2.0
Corn gluten (60%)	8.0	6.5
DCP2	1.7	1.5
Limestone3	1.3	1.6
Lysine4	0.05	0.05
DL-methionine5	0.15	0.15
Premix (vitamin)6	0.15	0.15
Mineral premix7	0.1	0.1
Salt	0.3	0.3
Chemical analysis
Crude protein	22.85	21.12
ME kcal/kg diet	3039.8	3058.7
Crude fiber	2.79	2.65
Moisture	12.14	12.98
Ash	6.79	6.55
Total phosphorus	0.73	0.68
Calcium	1.10	1.09
Methionine	0.67	0.56
Lysine	1.23	1.19
Ether extract	4.68	5.25

¹Vegetable oil (mixture of sunflower oil and cottonseed oil).

²DCP = dicalcium phosphate (18% P and 25% Ca).

³Limestone (34% calcium).

⁴Lysine = lysine hydrochloride (98.5% lysine).

⁵DL-methionine (produced by Evonic Co. and contain 99.5% methionine).

⁶The premix used was Heromix produced by Heropharm and composed of (per 1.5 kg) vitamin A 12,000,000 IU, vitamin D3 2,500,000 IU, vitamin E 10,000 mg, vitamin K3 2,000 mg, thiamin 1,000 mg, riboflavin 5,000 mg, pyridoxine 1,500 mg, cyanocobalamin 10 mg, niacin 30,000 mg, biotin 50 mg, folic acid 1,000 mg, pantothenic acid.

⁷Mineral premix: formulated and composed of (each 1 kg) 70,000 mg Mn, 60,000 mg Zn (using zinc oxide (ZnO) and replaced by zinc polysaccharide complex or nano zinc particles), 8,000 mg Cu, 1,000 mg I, 250 mg Se, and 150 mg Co.

Productive Performance

In this experiment, the growth indices, including weight gain (WG), body weight (BW), and feed conversion ratio (FCR), were measured. Birds were independently weighed over a weekly interval period from the start of the third week of age until the seventh week of age. In the early morning and before the starting, feed and birds were weighed weekly. The weekly WG of birds was considered by subtracting the bird's BW at a definite week from the BW of the same chicken the following week. For assessing the FCR, we were allocating the quantity of feed uptake (g) throughout the week by the WG (g) during a similar week.

Liver and Kidney Functions

The ALT kit of Bio-diagnostic determined alanine aminotransferase(ALT) and aspartate aminotransferase (AST) conferring to the technique previously reported by Reitman and Frankel (1957). Urea and creatinine were determined conferring to the procedures of Fawcett and Scott (1960) and Bartle (1972), respectively.

Hormonal Profile

Following the method described by Kelly and Alworth (2013), blood samples were picked up from the brachial wing vein (6 birds/group) and placed into sterilized tubes with anticoagulant at the termination of the experiment. Then, the collocated samples were preserved at room temperature for separating the plasma, centrifuged at 3,000 rpm/15 min, and then transferred into a sterilized Eppendorf tube (1.5 mL). Samples were preserved at −20°C until the exploration of hormonal assays.

The values of plasma hormones such as insulin-like growth factor-1(IGF-1) and growth hormone (GH) were assessed by radioimmunoassay utilizing the commercial kit (Diagnostic Products Corporation, Angeles, CA). The determinations of GH and IGF-1 followed the methods of Huybrechts et al. (1985) and Berghman et al. (1988), respectively. For assessing the plasma thyroid hormones such triiodothyronine (T3) and thyroxine (T4), we followed the RIA procedure stated by Akiba et al. (1982), utilizing the Gamma-Coat 125I RIA Kits.

Economic Evaluation

Expenses. The expenses and profits conferred by the dominant prices in the Egyptian market during the research. Feed cost (EGP/kg feed) was considered by multiplying the whole FI/chicken by the cost of 1 kg feed (11.30 EGP/kg feed). The cost of the threonine supplement (0 for the 100% group, 0.004 EGP/kg for 110% and 0.008 EGP/kg for the 120% group, and 0.012 EGP/kg for 130% group) was comprised in the feed costs of the experimental groups.

$$\begin{array}{c}\text{The}\text{average}\text{cost}\text{of}\frac{\text{feed}}{\text{kg}}\text{body}\text{weight}\left(\frac{\text{EGP}}{\text{kg}}\text{gain}\right)\hfill \\ =\frac{\text{feed}\text{cost}\left(\frac{\text{EGP}}{\text{kg}}\text{feed}\right)\times \text{FI}\text{per}\text{bird}\left(\text{kg}\right)}{\text{WG}\text{per}\text{bird}\left(\text{kg}\right)}\hfill \end{array}$$

where FI is the feed intake and WG is the weight gain.

Other costs, comprising the price of day-old chickens, disinfectant, veterinary supervision labor, housing, drugs, depreciation, and vaccines, were reflected in fixed expenses as the animals had identical management and equal (15.25 EGP)/bird. The sum of fixed and variable costs calculates the total costs.

Profits. The return was the income from selling birds, where the total return equals the bird live BW multiplied by the price of 1 kg of meat (31.98 EGP/kg live BW). The difference between the total return and total costs was calculated to obtain the net return. Net return per kg gain = net return/kg body BW. The economic efficiency was assessed according to Hassan and Awad (2017) as the following equation:

$$\text{Economic}\text{efficiency}=\text{net}\text{return}\text{per}\text{kg}\text{gain}/\text{feed}\text{cost}\text{per}\text{kg}\text{gain}$$

Statistical Analysis

The body weight records were normally distributed and subjected to analysis covariance for initial body weight data using the general linear model (GLM) of the SPSS software (ver. 20 for Windows, SPSS, Inc., Chicago, IL). The following statistical model was:

$${\mathrm{Y}}_{\text{ijkl}}=\mu +{\mathrm{W}}_{\mathrm{i}}+{\mathrm{B}}_{\mathrm{j}}+{\mathrm{F}}_{\mathrm{k}}+{\mathrm{T}}_{\mathrm{l}}+{\left(\text{FT}\right)}_{\text{kl}}+{\mathrm{e}}_{\text{ijkl}}$$

where Y_ijkl is the value of dependent variables; W_i is the effect due to covariance of initial body weight; B_j is the effect of the breed; F_k is the effect of feed restrictions; T_l is the effect of threonine treatment; (FT)_kl is the interaction between feed restriction and threonine treatment; e_ijkl is the error related to individual observation. While other records were normally scattered and subjected to statistical analysis using the GLM of the SPSS software (ver. 20 for Windows, SPSS, Inc., Chicago, IL). The test of Duncan's multiple ranges was utilized to determine the significance among the means and the significance was P ≤ 0.05.

RESULTS

Productive Performance

The effect of chicken breeds fed-restricted diets and various levels of threonine and their interactions on growth indices are listed in Table 2. It was clear that Ross and IR broiler breeds did not vary significantly in final BW, WG, FI, and FCR. Moreover, broilers fed control diets did not differ significantly from those fed the restricted diets in the final BW and FI. In addition, birds with control diets had a greater WG and FCR (P < 0.05) than those fed-restricted diets. Concerning the effect of threonine supplementation, broilers fed diets enriched with 130% threonine level had greater significant BW, WG and the greatest FCR compared to other treated groups. Regarding feed restriction*threonine interaction, both groups of broilers (control and feed restriction) received diets containing a threonine level of 130% had higher significant values for BW, WG, and best FCR with the lowest feed intake compared with other experimental groups.

Table 2.

The effect of broiler breeds fed-restricted diets and different levels of threonine and their interactions on growth performance criteria.

Items/breed		Final weight	Feed intake (g)	Total weight gain	Feed conversion ratio
Ross		2194.25 ± 13.79	2606.43 ± 9.74	1449.88 ± 13.42	1.80 ± 0.02
IR		2131.71 ± 18.07	2636.12 ± 9.22	1404.22 ± 18.28	1.88 ± 0.03
Feed restriction
Control		2153.26 ± 15.35	2627.76 ± 8.50	1477.58 ± 11.46a	1.78 ± 0.02b
Feed restriction (%)		2172.70 ± 17.56	2614.79 ± 10.60	1376.52 ± 18.21b	1.89 ± 0.03a
Threonine
100		2016.05 ± 21.02d	2652.59 ± 13.53	1432.13 ± 16.18b	1.85 ± 0.03b
110		2128.13 ± 14.67c	2615.04 ± 13.05	1356.94 ± 24.15c	1.93 ± 0.04a
120		2207.85 ± 17.13b	2632.55 ± 11.50	1395.68 ± 26.64bc	1.89 ± 0.04ab
130		2299.90 ± 13.13a	2584.92 ± 14.25	1523.46 ± 13.88a	1.70 ± 0.03c
Feed restriction*threonine (%)
Control	100	2025.40 ± 21.16d	2683.90 ± 24.52a	1429.21 ± 9.75c	1.88 ± 0.04b
	110	2097.85 ± 29.83c	2644.10 ± 19.30ab	1449.70 ± 16.91bc	1.82 ± 0.03bc
	120	2193.15 ± 25.08b	2620.15 ± 22.47bc	1513.05 ± 9.75ab	1.73 ± 0.03cd
	130	2296.65 ± 16.21a	2562.88 ± 19.73d	1518.35 ± 17.94ab	1.69 ± 0.02d
Feed restriction	100	2006.70 ± 36.85d	2621.27 ± 21.75bc	1435.05 ± 23.52c	1.83 ± 0.06bc
	110	2158.40 ± 19.84bc	2585.99 ± 33.41cd	1264.18 ± 18.00d	2.04 ± 0.04a
	120	2222.55 ± 23.52b	2644.95 ± 30.98ab	1278.31 ± 20.79d	2.07 ± 0.02a
	130	2303.15 ± 18.43a	2606.96 ± 19.99bcd	1528.56 ± 21.48a	1.71 ± 0.01d

The mean values in the same column within each division with different superscripts (a–d) differ significantly (P < 0.05).

Kidney and Liver Functions

Some kidney and liver functions of 2 broiler breeds fed-restricted diets and different threonine levels and their interactions are listed in Table 3. The results discovered that Ross and IR broilers did not vary (P > 0.05) in urea and ALT levels. In contrast, IR broilers exhibit higher creatinine (P < 0.05) than Ross broilers. On the other hand, Ross broilers had greater (P < 0.05) AST levels than IR broilers. Concerning the feed restriction effect, broilers fed-restricted diets showed a significant elevation in urea, creatinine, ALT, and AST levels compared to control birds fed full diets without restriction. The results regarding threonine supplementation revealed that broilers supplemented with threonine levels at 120 and 130% of basal diets contents increased (P < 0.05) urea and ALT levels, while threonine levels at 110% significantly increased creatinine levels compared to other groups.

Table 3.

The effect of broiler breeds fed-restricted diets and different levels of threonine and their interactions on kidney and liver functions.

Items/breed		Urea	Creatinine	ALT	AST
Ross		5.08 ± 0.04	0.48 ± 0.01b	21.35 ± 0.33	98.08 ± 1.08a
IR		5.10 ± 0.05	0.84 ± 0.19a	21.63 ± 0.28	94.88 ± 1.18b
Feed restriction
Control		4.89 ± 0.07b	0.47 ± 0.01b	20.48 ± 0.27b	91.43 ± 1.04b
Feed restriction		5.29 ± 0.02a	0.85 ± 0.19a	22.50 ± 0.25a	101.53 ± 0.57a
Threonine (%)
100		5.00 ± 0.06b	0.48 ± 0.01b	19.65 ± 0.36c	91.80 ± 1.58d
110		5.02 ± 0.09b	1.16 ± 0.37a	21.30 ± 0.34b	95.15 ± 1.55c
120		5.14 ± 0.04a	0.49 ± 0.01b	22.15 ± 0.31a	98.00 ± 1.43b
130		5.19 ± 0.05a	0.51 ± 0.01b	22.85 ± 0.36a	100.95 ± 1.32a
Feed restriction*threonine (%)
Control	100	4.83 ± 0.09ef	0.46 ± 0.02b	18.50 ± 0.34f	86.00 ± 1.16d
	110	4.73 ± 0.10f	0.45 ± 0.01b	20.30 ± 0.39e	89.40 ± 1.51cd
	120	4.98 ± 0.04de	0.48 ± 0.01b	21.30 ± 0.36cde	93.00 ± 1.73c
	130	5.01 ± 0.03cd	0.49 ± 0.01b	21.80 ± 0.41cd	97.30 ± 2.03b
Feed restriction	100	5.16 ± 0.04bc	0.49 ± 0.01b	20.80 ± 0.38de	97.60 ± 1.32b
	110	5.31 ± 0.05ab	1.86 ± 0.68a	22.30 ± 0.38bc	100.90 ± 0.75ab
	120	5.31 ± 0.03ab	0.51 ± 0.01b	23.00 ± 0.33ab	103.00 ± 0.33a
	130	5.37 ± 0.04a	0.53 ± 0.01b	23.90 ± 0.37a	104.60 ± 0.56a

The mean values in same column within each division with different superscripts (a–f) differ significantly (P < 0.05). ALT: alanine aminotransferase. AST: aspartate aminotransferase.

Moreover, threonine supplementation in broiler diets at 130% elevated (P < 0.05) AST level compared to other treated groups. Regarding the feed restriction * threonine interaction results, the results showed that broilers fed-restricted diets and supplemented with threonine levels at 110, 120, and 130% had increased levels of urea and AST. While broilers fed-restricted diets supplemented with 110% threonine level displayed an increase (P < 0.05) in creatinine levels compared to other experimental groups, restricted diets with 120 and 130% threonine levels also produced a significant increase in ALT levels related to other treated diets.

Hormonal Profile

The influences of breed feed restriction, threonine and feed restriction*threonine interactions on some hormonal parameters are presented in Table 4. Breed factor did not affect T3, T4, IGF1, and GH. T3 level of feed-restricted broilers didn't differ with the broilers fed ad libitum. However, feed-restricted birds disclosed a significant reduction in T4, IGF1, and GH related to control broilers. Threonine at level 120 and 130% of the basal diet increased T3 level compared to other levels. Moreover, threonine at 130% increased T4, IGF1, and GH levels more than broilers supplemented with other threonine levels. Regarding the feed restriction*threonine interaction effect, the T3 level increased significantly (P < 0.05) at 120 and 130% threonine levels in feed-restricted groups. But T4, IGF, and GH increased significantly (P < 0.05) in control birds supplemented with 130% threonine level compared to other supplemented groups.

Table 4.

The effect of broiler breeds fed-restricted diets and different levels of threonine and their interactions on some hormonal parameters.

Items/breed		T3	T4	IGF1	GH
Ross		1.04 ± 0.01	0.89 ± 0.01	27.60 ± 0.49	40.93 ± 0.71
IR		1.09 ± 0.02	0.89 ± 0.02	28.68 ± 0.73	39.33 ± 0.82
Feed restriction
Control		1.04 ± 0.01	0.93 ± 0.02a	29.18 ± 0.66a	42.23 ± 0.71a
Feed restriction		1.08 ± 0.02	0.85 ± 0.01b	27.10 ± 0.56b	38.03 ± 0.68b
Threonine (%)
100		1.01 ± 0.01b	0.79 ± 0.01d	24.10 ± 0.35d	34.50 ± 0.58d
110		1.03 ± 0.02b	0.83 ± 0.01c	26.05 ± 0.28c	38.80 ± 0.67c
120		1.11 ± 0.02a	0.95 ± 0.02b	29.15 ± 0.42b	41.95 ± 0.66b
130		1.11 ± 0.01a	0.99 ± 0.02a	33.25 ± 0.63a	45.25 ± 0.64a
Feed restriction*threonine (%)
Control	100	1.00 ± 0.01b	0.79 ± 0.01f	25.10 ± 0.46d	36.50 ± 0.58d
	110	1.02 ± 0.01b	0.85 ± 0.02e	26.60 ± 0.40cd	40.80 ± 0.77c
	120	1.08 ± 0.02ab	1.02 ± 0.01b	30.30 ± 0.47b	44.00 ± 0.42b
	130	1.08 ± 0.02ab	1.06 ± 0.01a	34.70 ± 0.92a	47.60 ± 0.60a
Feed restriction	100	1.01 ± 0.04b	0.79 ± 0.01f	23.10 ± 0.31e	32.50 ± 0.45e
	110	1.04 ± 0.04b	0.82 ± 0.01f	25.50 ± 0.34d	36.80 ± 0.64d
	120	1.14 ± 0.04a	0.88 ± 0.01d	28.00 ± 0.49c	39.90 ± 0.84c
	130	1.14 ± 0.01a	0.93 ± 0.01c	31.80 ± 0.61b	42.90 ± 0.43b

The mean values in same column within each division with different superscripts (a–f) differ significantly (P < 0.05). T3: triidothyronine. T4: thyroxine. IGF1: insulin-like growth factor. GH: growth hormone.

Economic Evaluation

The impacts of breed, feed restriction, threonine, and feed restriction * threonine interaction on some economic parameters are listed in Table 5. It was clear that Ross and IR broilers did not vary (P > 0.05) in feed cost/bird. Meanwhile, Ross broilers had the lowest (P < 0.05) feed cost per kg gain and the highest (P < 0.05) total return/bird, net return/bird, net return/kg gain, and economic efficacy compared to IR broilers. Concerning the feed restriction effect, the feed-restricted birds exhibit greater (P < 0.05) feed charge/kg gain and net return per/gain relative to control broiler groups. Regarding the threonine effect, diets containing 130% threonine resulted in inferior feed cost/kg gain (P < 0.05) and significantly increased total return/bird, net return/bird, net return /kg gain, and economic efficacy compared to broilers fed other threonine levels.

Table 5.

The effect of broiler breeds fed-restricted diets and different levels of threonine and their interactions on some hormonal parameters.

Items/breed		Total feed cost/bird (EGP)	Feed cost per kg gain (EGP)	Total cost (EGP)/bird	Total return/bird (EGP)	Net return/bird (EGP)	Net return per kg gain (EGP)	Economic efficiency
Ross		29.36 ± 0.19	20.34 ± 0.19b	44.61 ± 0.08	70.17 ± 0.38a	25.56 ± 0.38a	17.63 ± 0.01a	0.86 ± 0.01a
IR		29.59 ± 0.19	21.44 ± 0. 19a	44.84 ± 0.08	68.17 ± 0.57b	23.35 ± 0.57b	16.62 ± 0.02b	0.77 ± 0.01b
Feed restriction
Control		29.56 ± 0.19	20.11 ± 0.19b	44.81 ± 0.08	68.86 ± 0.57	24.05 ± 0.57	16.28 ± 0.19b	0.81 ± 0.01
Feed restriction		29.40 ± 0.19	21.67 ± 0.19a	44.65 ± 0.09	69.48 ± 0.57	24.83 ± 0.57	18.04 ± 0.57a	0.83 ± 0.01
Threonine (%)
100		29.57 ± 0.57	20.96 ± 0.01b	44.82 ± 0.15	64.47 ± 0.76d	19.65 ± 0.76d	13.72 ± 0.38c	0.65 ± 0.02d
110		29.37 ± 0.57	21.97 ± 0.02a	44.62 ± 0.15	68.05 ± 0.38c	23.44 ± 0.57c	17.27 ± 0.76b	0.79 ± 0.02c
120		29.62 ± 0.57	21.52 ± 0.01ab	44.87 ± 0.12	70.61 ± 0.57b	25.74 ± 0.57b	18.44 ± 0.38a	0.86 ± 0.02b
130		29.35 ± 0.57	19.10 ± 0.01c	44.60 ± 0.14	73.55 ± 0.19a	28.89 ± 0.57a	18.96 ± 0.19a	0.99 ± 0.02a
Feed restriction*threonine (%)
Control	100	29.93 ± 0.17	21.26 ± 0.36b	45.18 ± 0.19	64.77 ± 0.71d	19.59 ± 0.76d	13.70 ± 0.44d	0.64 ± 0.03d
	110	29.78 ± 0.17	20.59 ± 0.36bc	45.03 ± 0.19	67.09 ± 0.71c	22.17 ± 0.75c	15.29 ± 0.44c	0.74 ± 0.03c
	120	29.51 ± 0.17	19.56 ± 0.36cd	44.76 ± 0.17	70.14 ± 0.71b	25.38 ± 0.76b	16.77 ± 0.44b	0.86 ± 0.03b
	130	29.13 ± 0.17	18.99 ± 0.36d	44.38 ± 0.20	73.44 ± 0.71a	29.07 ± 0.57a	19.14 ± 0.44a	1.01 ± 0.03a
Feed restriction	100	29.21 ± 0.17	20.66 ± 0.36bc	44.46 ± 0.19	64.17 ± 00.71d	19.71 ± 0.75d	13.74 ± 0.44d	0.67 ± 0.03cd
	110	29.06 ± 0.17	23.34 ± 0.36a	44.31 ± 0.19	69.03 ± 0.71bc	24.72 ± 0.76b	19.55 ± 0.44a	0.84 ± 0.03b
	120	29.74 ± 0.17	23.48 ± 0.36a	44.99 ± 0.17	71.08 ± 0.71b	26.09 ± 0.76b	20.41 ± 0.44a	0.87 ± 0.03b
	130	29.58 ± 0.17	19.20 ± 0.36d	44.83 ± 0.19	73.65 ± 0.71a	28.82 ± 0.75a	18.86 ± 0.44a	0.98 ± 0.03a

The mean values in same column within each division with different superscripts (a–d) differ significantly (P < 0.05).

DISCUSSION

Generally, the breed differences between broiler strains in growth performance parameters are due to differences in the genetic makeup of different breeds (El-Tahawy et al., 2017; Nangsuay et al., 2017; Taha et al., 2019). The feed restriction program manipulates the broiler growth curve to improve production efficiency. Feed restriction aims to decrease metabolic and growth rates to about scope. It relieves some metabolic syndromes such as lameness, ascites, sudden death syndrome, and mortality, reduces feed cost and improves feed conversion (Sahraei, 2012).

In the existing study, feed restriction diminished the total weight gain and recorded the worst FCR. The improvement of broiler BW, WG, and FCR in feed-restricted and control broilers may be due to threonine's role in developing intestinal mucosa, intestinal villi, and digestive enzymes function (Dozier et al., 2001; Qaisrani et al., 2018). In an experiment, Najafi et al. (2017) fed broilers various levels of threonine (0.89, 0.93, and 0.97%) from 1 to 14 d of age alongside a control diet having 0.65% of threonine to examine the effects on growth performance, the authors reported that as compared to broilers on a control diet, broilers received diets containing 0.97% of threonine presented 5.1% greater FI, 1.4% better FCR, and 6.4% higher body WG. Moreover, Bassareh et al. (2023) informed that adding threonine (115% more than the basic requirements) positively affected BW and WG and improved the FCR in the broiler. Greater intestinal function (villus surface and crypt depth) was also detected in the birds fed with threonine (105% dietary, P < 0.05) (Bassareh et al., 2023; Hussein et al., 2023).

The serum biochemical profile details animals' immune systems and general health. The values of these biochemical parameters depend on factors like breed, species, sex, age, season, location, diet, and physiological conditions (Kokore, 2021). The increased levels of AST in birds and animals indicate degenerative liver changes and diseases (Beaufrère and Vergneau-Grosset, 2021).

Our outcomes agree with those acquired by Min et al. (2017), who supplemented broilers with various levels of threonine (85%, 100%, 125%, and 150% of recommended basal diet), and found that the excess dietary threonine levels significantly affected AST and ALT activities and these might be owing to the metabolism of extra amino acids. The increase in urea levels in higher levels of threonine-supplemented broilers, as reported by Wang et al. (2006), noticed that the consumption of true ileal digestible promotes augmented from 5.0 to 6.6 g/d, and the serum urea nitrogen levels also rose. Moreover, serum urea levels would raise once one or some amino acids are scarce or in a spare (Gong et al., 2005). The alterations in blood biochemical parameters owing to feed restriction were also informed by Karabayir and Mendes (2008) that increasing the period of feed restriction instigated a severe stage of stress on chickens, thus may lead to impairment in liver and kidney functions.

Feed restriction-induced stress may cause a significant alteration in plasma thyroid hormone levels. The most likely cause of the change in thyroid hormone levels in feed-restricted chickens is the change in the percentage of T4 deiodination by altering T3 degradation and hepatic D1 and D3 deiodinases (Farag and Alagawany, 2018).

During feed restriction, mono-deiodinase activity declines, which contributes to variations in the T3 level. Accordingly, it was hypothesized that metabolic frequency would decrease and hormonal activity related to growth would decline. In the current investigation, feed-restricted broilers had reduced T4 during the feed restriction phase. This may result from the birds conserving energy with a low basal metabolism (Zhan et al., 2007; Ogbuagu et al., 2023). Nevertheless, there was no statistically substantial difference between the 2 feed-restricted and ad libitum birds regarding the level of T3. According to the results, the T3 level, the active form of thyroid hormone in the restricted chicks, is normalized and maintains its value during compensatory growth as that of ad libitum broilers. A noteworthy observation is that early feed restriction statistically decreased (P < 0.05) levels of IGF-1 and GH compared to ad libitum birds.

Similarly, Giachetto et al. (2003) found that broiler chickens also had lower IGF-1 plasma levels during the feed restriction period. The results of GH levels are harmonious with Ghazanfari et al. (2010), who informed that growth hormone levels lowered in feed-restricted birds at the age of 22 to 32 d. Increasing dietary threonine was associated with greater levels of the T3 and T4 hormones in the chickens. This association was most likely caused by the occurrence of the receptors of threonine in the thyroid gland that are crucial for synthesizing the T3 hormone (Wu, 2013; Azzam and El-Gogary, 2015). The T4 is crucial for energy and protein metabolism (Decuypere et al., 2005; Jahanpour et al., 2020) and increases IGF-I hormone concentrations (Hemmati et al., 2019). A sufficient amount of T4 is required for improved GH activity, protein synthesis, and muscle mass conservation (Jahanpour et al., 2020). Our results agree with Al-Hayani's (2017) results, which showed that increasing threonine in the diet by 600 or 900 mg/kg raised GH concentration. Our findings strongly suggest that adding threonine at 120 and 130% in broilers' diets exposed to feed restriction programs enhances concentrations of GH and IGF-1.

Regarding feed restriction*threonine interaction, broilers fed 130% threonine level either in control and feed restriction groups exhibit lower (P < 0.05) feed cost per kg gain (P < 0.05) and significantly enhanced total return/bird, net return/bird, net return per kg gain, and economic efficacy compared to broilers of the other treated groups. A lowered feed cost per kilogram gain in feed-restricted groups supplemented with higher doses of threonine was obtained from higher body WG. Our findings agreed with Corzo et al. (2004), who concluded that amino acid supplements above recommended levels could increase flock homogeneity, which has a significant economic effect. Also, Yaqoob et al. (2018) found that higher levels of threonine do not affect total feed cost.

CONCLUSIONS

The dietary inclusion of threonine amino acid in feed-restricted broiler chickens enhanced growth performance parameters. This effect was linked to altered GH, IGF, T3, and T4 levels. Also, it was found higher levels of threonine in control or feed-restricted birds have a favorable effect on return parameters. In addition, the supplementation of threonine significantly increased urea and AST levels in feed-restricted birds. Ross breed exhibited a lower feed cost/kg gain and a higher profitability than IR breed.