Over the past 50 years, the world’s chicken production has grown significantly to meet the rising demand for poultry products. Typically, broiler chicks are fed diets high in protein or AA and grow quickly (ERS, USDA, 2001).  By using the digestible amino acids (DAA) principle, it may be possible to maintain optimal chicken performance while lowering the dietary protein concentration. The DAA concept has several benefits, including lowering environmental pollution, lowering feed costs,lowering the dietary requirements of the limiting amino acid (AA) and diverting a larger percentage of dietary amino acids (AA) for protein synthesis.  Corn and soybean meal (SBM) are used in the formulation of the majority of broiler chicken diets. In order for the fast-growing broilers of today to exhibit their full genetic potential, the AA contents of such diets are inadequate.  The amino acid requirements are thus met by supplementing the corn-SBM based meals with crystalline amino acids. High levels of well-balanced nutrients are necessary for the fast growth of broiler chickens, particularly amino acids for early marketing. These amino acids are crucial for the body’s defensive and structural building as well as for the activities of enzymes and tissues (NRC, 1994). Dietary Threonine is the third limiting amino acid in low crude protein (CP) poultry diets based on corn-soybean meal (Fernandez et al., 1994). This amino acid plays an important role in the maintenance of intestinal mucosal integrity, nutrient digestion and absorption (Dozier et al., 2001; Mao et al., 2011; Wang et al., 2009). Dietary threonine requirements for maximum productivity reduced as birds grew older (Jones et al., 2019). White leghorn layers performed well on low protein diets (i.e., 13.46% CP) supplemented with optimal doses of lysine (0.65%) and thronine (66% lysine) without affecting intestinal morphology (Nagaraja et al., 2021). In ovo supplementation with five amino acids (Lys, Met, Thr, Arg, Glu) with or without minerals, or a combination of four amino acids (Lys, Thr, Arg, and Glu) with Lys, lowered hatchability. The combination of four amino acids (Met, Thr, Arg, and Glu) without Lys at lower concentrations (1x) resulted in high hatchability (86.6%) (Saravanakumar et al., 2024). However, it is currently uncertain how much dietary protein can be replaced by digestible threonine supplementation without affecting broiler performance. The current biological investigation was aimed to carry out to assess the performance, slaughter characteristics and economics of commercial broiler diets supplemented varied concentration of digestible threonine in commercial broiler diets.

The study was carried out in commercial broilers using 250 day-old male broiler chicks (Ven Cobb-400) in a completely randomized design (CRD). After obtaining the chicks, they were all weighed and had their wings banded. They were then randomly assigned to five treatments, each with 10 replicates and five chicks in each. In electrically heated battery brooders, the chicks were raised in optimum brooding conditions throughout the experimental period six weeks. The biological experiment under report was conducted at the Poultry Experimental Station, Department of Poultry Science, College of Veterinary Science, PVNRTGVU, Rajendranagar, Hyderabad-500030,Telangana State, India.
       
The experiment was conducted from August-2021 to January-2022.
 
Experimental diets
 
According to Cobb breeder’s recommendations, a corn-soya based control diet was formulated and other experimental diets were formulated by reducing crude protein levels by 2, 1 and 1% during pre-starter, starter and finisher phases respectively, with graded concentrations of threonine 75, 70, 65 and 60% of digestible Lysine  during pre-starter (0-14 d); (0.885, 0.826, 0.767 and 0.708), starter (15-28 d); (0.787, 0.735, 0.682 and 0.630) and finisher period (29-42 d); (0.712, 0.665, 0.617 and 0.570) (Table 1).


 
Collection of data
 
The body weight was recorded on individual bird basis at weekly intervals up to six weeks of age and feed intake was calculated on replicate-wise at weekly intervals. The feed conversion ratio (FCR) was calculated as feed intake per unit bodyweight gain from 0-6 weeks of age at weekly intervals. At the end of 6^th week of each experiment, blood from one bird per replicate was collected for serum biochemical parameters by using diagnostic test kits (Arkray Healthcare Pvt. Limited, Sachin (Surat), India). The lipid peroxidation levels in the serum were estimated and expressed in nano mol Malondialdehyde (MDA)/mg protein (Ohkawa et al., 1979). The protein concentration was determined by the method of Lowry et al., (1951). At the end of experimental period (6 weeks), ten birds from each dietary group were sacrificed to evaluate the effect of supplementing synthetic amino acids to the corn soybean meal based diets on carcass parameters (dressing percentage, thigh weight and ready to cook meat) which were expressed as gram per kg live body weight. The mortality among the birds as per replicate / treatment wise is recorded throughout the experimental period and dead birds were subjected to necropsy examination to determine the cause of death. Body weight of dead bird was recorded and used to calculate the feed efficiency.
       
The feed cost was arrived for pre-starter, starter and finisher broiler diets separately on the ration formulated and the prevailing market price of the major and minor feed ingredients. The phase wise (pre-starter, starter and finisher) cost of production of broilers was calculated for all the treatment groups by multiplying feed intake with feed cost. The cost of production per kg live weight was calculated by multiplying cost of total feed intake and feed conversion ratio for all treatment groups. The total cost of production was subtracted from the sale amount of broilers for each treatment group was calculated to arrive the profitability. The 15^th version of the Statistical Package for Social Sciences (SPSS) one-way ANOVA was used to analyze the data. The difference between the treatment means was analyzed using Duncan’s multiple range test (Duncan, 1955) at P<0.05.

Performance
 
Supplementation of digestible threonine at decreasing concentration of (75, 70, 65 and 60% of Lysine) did not influence (P>0.05) body weight gain (BWG) at different weeks. Contrary to the present study, Mehri et al., (2012), Corzo et al., (2009), Kidd et al., (1999) who reported that, threonine (%) 0.78, 0.73 and 0.66 respectively increased the body weight gain significantly (P<0.05) in commercial broilers. The feed intake was significantly (P<0.05) increased in groups fed 65% of d Lysin concentration during 3rd week of age in commercial broilers. However, the feed intake values during the corresponding week in 75% threonine group are comparable to control group. The feed intake during third week in 70% threonine supplemented group showed a significant (P>0.05) reduction in feed intake compared to T4 and T5.Contrary to the findings of the present study, Star et al., (2010) who reported that a higher level of dietary Thr:Lys ratio (0.70) in treatment group significantly (P<0.05) increased feed intake compared to a threonine to lysin ratio of 0.65 in broilers. The inclusion level of digestible threonine 100% of the Cobb recommendation (0.77 %) in dietary group (T1) significantly (P<0.05) improved the weekly FCR during first and second weeks of age. However, poor FCR was significantly (P<0.05) recorded in dietary group T4 during 4th week of age. This is in agreement with the findings of the Corzo et al., (2009) who reported that threonine (replaced (0.76%) significantly affected the FCR of male broilers during 14-28 days age (Table 2).


       
Supplementation of varied concentrations of digestible threonine in broilers diets did not influence the slaughter parameters (dressing percentage, ready-to-cook yield and thigh weight), serum biochemical parameters (total protein) and oxidation activity (lipid peroxidation). These results are contrary with the findings of Min et al., (2017) who reported that concentrations of total protein (TP) increased quadratically with increasing threonine level and the highest concentrations of TP were obtained at the 125% threonine level (Table 3).

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Livability
 
Supplementation of crystalline threonine in graded concentrations at 75 to 60 % of the digestible lysine did not influence the livability of broilers during 0-42 days. The livability on the varied levels of threonine in the diets ranged from 94 to 98% during overall experimental period.  These results are in concurrence with the results of Awad et al., (2014) who reported that mortality rate was considerably low in reduced dietary crude protein (CP) level and dietary treatments fortified with crystalline amino acids to meet the ideal AA ratios concept in broiler diets.
 
Cost economics
 
The cumulative feed cost (Rs/bird) for 0-6 weeks age was decreased in gradient manner with decreasing concentration of dig threonine in the experimental diets. However no significant difference was recorded in weekly body weight gains. Hence the net revenue over the feed cost (Rs) increased with the reduction digestible threonine concentrations in the experimental diets. Among all groups the highest net revenue over the feed cost (Rs.71.09/bird) was generated in 0.708, 0.630 and 0.570 % dig. Threonine (60% of the digestible lysine) during pre-starter, starter and finisher phases respectively. These results are in agreement with those of Burnham (1992) who reported that replacing intact protein, soybean meal with crystalline amino acids, L-Threonine, L-Lysine and DL-Methionine to balance essential amino acid levels in practical broiler diets resulted in increased live weight performance and gave a financial advantage (Table 4).

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Based on findings of the present study, it is concluded that digestible threonine concentrations during the pre-starter, starter and finisher phases at 60% of the digestible lysine (0.708, 0.630 and 0.570%, respectively) are optimum for getting good economic returns without having negative impact on growth performance, slaughter parameters, serum biochemical parameters and livability in commercial broilers.

Authors are thankful to M/s Evonik (SEA) Pvt. Ltd. Singapore for providing financial assistance to conduct the present experiment on commercial broilers.

The authors declare that they have no conflict of interest.