Effects of dietary energy levels on performance and carcass yield of 2 meat-type broiler lines housed in hot and cool ambient temperatures.

Two meat-type broiler lines, line A and line B were fed experimental diets from 22-42 d with objectives to determine the effects of dietary metabolizable energy (ME) levels on feed intake (FI), performance, body composition, and processing yield as affected by environmental grow-out temperatures. Two thousand fifty male chicks from line A and 2,050 male chicks from line B were reared in 90-floor pens, 45 chicks per pen utilizing primary breeder nutrition and husbandry guidelines for starter (1-10 d) and grower (11-21 d) phases. Experimental finisher diets consisted of 5 increasing levels of apparent nitrogen corrected ME (2,800, 2,925, 3,050, 3,175, and 3,300 kcal/kg set at 19.5% crude protein and 1.0% dLys at each level) to represent 80, 90, 100, 110, and 120% ME of Evonik AminoChick energy level giving 2 × 5 factorial design and were fed from 22-42 d. All other amino acid levels in diets were formulated to a fixed ratio of dLys level. There were nine replicate pens for each diet and each line. The experiment was conducted twice-once in hot season (barn averages: 77.55 ˚F and 86.04% RH) and another in cool season (barn averages: 69.91 ˚F and 63.98% RH) of the year. Results showed that FI and feed conversion ratios (FCR) decreased (P < 0.05) linearly (R2 = 0.9) by 61.25 g and 0.073 units for every 10% increase in dietary ME for combined analysis of lines and seasons. The % fat mass of total body mass increased by 0.57%, whereas % protein mass decreased by 0.21% across ME levels (R2 > 0.9). However, there was no difference (P > 0.05) in % weights (of live weight) for wings, breast filet, tenders, or leg quarters across ME levels for both lines except % fat pad that increased (P < 0.05) by 0.20% for each 10% increment in dietary ME level. Line B had higher cumulative FI, BW gain, % lean, and protein mass of body mass than line A in hot season (P < 0.05). Feed intake was not different between lines in cool season (P > 0.05), whereas higher BW and improved FCR were observed for line A. Line A had higher % fat mass in both seasons. In summary, performance and yield results as affected by dietary ME levels were line specific and were affected by grow-out seasons. The optimal dietary ME level for the ME range studied (2,800-3,000 kcal/kg) at a constant recommended amino acid level lies in determining the best performance and profitability indices by taking into account the grow-out production inputs and processing yield outputs.

Introduction

The genetic progress of broiler's productive traits such as growth rate and breast yield has been exponential in the past 2 decades (Fancher, 2014). The selection for increased growth rate has sustained the improved efficiency seen with broilers (Carre, 2014). Feed intake (FI) has fueled this increased growth rate as broilers show the capacity to physiologically process an increasing amount of nutrients on a daily basis. The FI of broilers was thought to be regulated by the energy concentration of the diet (Fisher and Wilson, 1974; Leeson et al., 1996), a concept that is still strongly held by many nutritionists (Leeson, 2012). There is ongoing research to understand factors that govern FI of current broiler genetics such as the physiological ability of broilers to digest feed and dietary energy content (Marchini et al., 2016). Furthermore, Lemme (2005) showed that FI was regulated not only by dietary energy level but by the concentration of amino acids in the diet (balanced protein). Consideration in confounding factors such as amino acid levels (Delezie et al., 2010; Swennen et al., 2010; Classen, 2013; Maharjan et al., 2020a), energy levels (Dozier and Gehring, 2014; Maharjan et al., 2020b), and physical quality of the feed could further contribute to our understanding on what drives FI. More recently, Classen (2013) discussed the confounding factors that affect response of broilers to dietary energy such as broiler genotype and age, dietary ingredient and nutrient composition, feed form, environment, protein to energy ratio, etc. It was concluded that the energy level did not affect FI when confounding factors could be minimized and the energy level of diet needed to be determined based on the maximum protein accretion of the bird. In addition, no effect on FI was reported when birds were fed 80 and 90% of their amino acid requirement (Aviagen, 2007).

The broiler industry perceives that reducing the energy levels of the diets by 25–50 kcal/kg will not impact performance, and thus could reduce feed cost associated with high energy diet. Dozier III and Gehrin (2014) studied the effects of 6 dietary metabolizable energy (AMEn) levels ranging from 3,000 to 3,150 kcal/kg at 30 kcal difference between each diet on growth performance and reported no significant effects on body weight (BW), FI, and FCR. The study further showed the effect could be genotype specific. However, another study conducted in broilers housed in high ambient temperature showed the performance and yield benefits in treatment group fed 55 kcal/kg higher AMEn throughout the feeding phases (Zhai et al., 2014). Scant information is available exhibiting on how the differences in wider ranges (at 125 kcal/kg) in dietary energy levels affect on broiler FI and performance at different grow-out environmental conditions. In this present study, broilers were fed diets with increasing energy levels with same amino acid content and were evaluated for affecting performance and yield characteristics to understand what dietary energy density would be optimal for improved performance specific to broiler lines. Two modern meat-type broiler lines were used in the evaluation during their finishing feeding phase (22–42 d) in hot and cool season of the year. The standardized ileal feed amino acid digestibility and body composition changes (protein mass and fat mass) in these broiler lines were also evaluated due to the effects of dietary ME changes in both seasons.

Materials and methods

Birds and Husbandry

Two thousand twenty-five male chicks from line A and 2,025 male chicks from line B were placed in 90-floor pens (house 1 and house 2, 45 pens each house), 45 chicks per pen and reared under husbandry practices recommended by primary breeder guidelines. Both the lines utilized in the study were fast-growing current meat broiler lines. All procedures of bird care and handling followed University of Arkansas IACUC protocol #15048. This experiment consisted 2 study trials conducted once in the hot season (24-h grow-out environmental mean values during experimental feeding phase: ∼77.55 ˚F and 86.04% RH) and then repeated in cool season (24-hr grow-out environmental mean values during experimental feeding phase: ∼69.91 ˚F and 63.98% RH) of the year to represent higher and recommended grow-out ambient temperatures. Therefore, a total of 8,100 day-old chicks were used in the study. For hot season, line A and line B chicks were hatched from 33-week-old and 32-week-old hens, respectively. For cool season, line A and line B chicks were hatched from 36-week-old and 37-week-old hens, respectively. Ambient temperature and relative humidity recorded for both trials are reported in Figures 1A–1D.

Figure 1

Average temperature log for 2 grow-out houses in hot season (A, B) and in cool season (C, D). Mean daily average temperature values (21–42 d) recorded for house 1 and house 2 for hot season were 77.55°F and 78.07 °F, and for cool season were 69.65 °F and 70.17 °F, respectively. Relative humidity percentage recorded for house 1 and house 2 for hot season were 86.07 and 86.01; and for cool season were 63.98 and 62.55, respectively.

Experimental Diets and Design

Diets were formulated based on reported standardized ileal digestible (SID) amino acids and AMEn in accordance with Evonik AminoChick recommendation. Birds were fed a common starter feed from 0 to 10 d formulated to 3,030 kcal/kg and 1.27% digestible lysine (dLys) (Supplementary Tables 1 and 2). A common grower feed, fed from 11 to 21 d, was formulated to 3,080 kcal/kg and 1.09% dLys (Supplementary Tables 1 and 2). At 22–42 d, both broiler lines were placed on the 5 experimental diets (9 replicate pens for each diet and each line) with 5 increasing levels of AMEn (2,800, 2,925, 3,050, 3,175, and 3,300 kcal/kg set at 19.5% crude protein and 1.0% dLys at each level) to represent 80, 90, 100, 110, and 120% ME of typically recommended Evonik AminoChick nutrient guidelines for energy level giving 2 × 5 factorial design (Table 1 and Table 2). Amino acid levels in diets were kept constant in all treatment diets and were formulated to a fixed ratio of dLys level (Table 3). The analysis of AMEn involved analysis of gross energy, dry matter, and nitrogen in feed and excreta. Gross energy (GE) was determined with a bomb calorimeter (Parr 6200 bomb calorimeter, Parr Instruments Co., Moline, IL). Dry matter was analyzed by method 934.01 (AOAC, 1990) and nitrogen determined by the method 990.03 (AOAC, 1995). The titanium dioxide (TiO) in feed and excreta was measured utilizing the method described in the study by Myers et al. (2004). AMEn values of the treatment diets were evaluated while measuring amino acid digestibility as described below in the methods. The AMEn was calculated as follows: AMEn kcal/kg= (GEdiet−GEexcreata × (TiO2 diet/TiO2 excreta)−8.22 x (N diet−N excreta × TiO2 diet/TiO2 excreta). The SID AA calculations are discussed further below in the methods.

Table 1

Ingredient and nutrient composition of experimental finisher diets—Hot season.

Treatment	Finisher 1		Finisher 2		Finisher 3		Finisher 4		Finisher 5
	%		%		%		%		%
Corn	35.91		39.29		43.96		48.74		53.75
Soybean meal, 48%	20.65		20.08		20.78		25.29		29.37
Wheat middlings	20.00		20.00		16.12		11.42		5.99
Sunflower meal (1st)	9.01		9.01		9.01		4.06		0.00
Sunflower meal (2nd)	0.98		0.98		0.98		0.44		0.00
Dicalcium phosphate 19	1.66		1.65		1.68		1.75		1.82
Soybean oil	4.42		4.68		4.99		5.95		6.81
Limestone (CaCO3)	0.73		0.74		0.73		0.73		0.72
L-lysine-HCl	0.24		0.25		0.24		0.17		0.10
Sodium bicarbonate	0.39		0.40		0.39		0.34		0.29
MetAmino	0.25		0.25		0.24		0.25		0.25
Salt	0.21		0.20		0.21		0.24		0.27
Choline chloride 60%	0.07		0.06		0.06		0.08		0.09
ThreAmino	0.07		0.07		0.07		0.05		0.03
ValAmino	0.02		0.02		0.02		0.01		0.00
L-isoleucine	0.03		0.03		0.02		0.00		0.00
Vitamin premix1	0.10		0.10		0.10		0.10		0.10
Trace min premix2	0.10		0.10		0.10		0.10		0.10
Ethoxyquin3	0.02		0.02		0.02		0.02		0.02
mycoCurb4	0.05		0.05		0.05		0.05		0.05
Ameri-Bond 2x5	0.50		0.50		0.50		0.50		0.50
Filler solkafloc	4.57		1.50		0.00		0.00		0.00
Total	99.98		99.98		100.27		100.29		100.26

Vitamin premix: Vit A, 13,227 IU/kg; Vit D3, 3,968 IU/kg; Vit E, 66 IU/kg; Vit B12, .040 mg/kg; biotin, .254 mg/kg; menadione, 3.968 mg/kg; thiamine, 3.968 mg/kg; riboflavin, 13.228 mg/kg; Vit B6, 7.937 mg/kg; niacin, 110.229 mg/kg; folic acid, 2.205 mg/kg.

Trace mineral premix: Mn, 60 mg/kg (manganese sulfate); Zn, 60 mg/kg (zinc sulfate); Fe, 40 mg/kg (ferrous sulfate); Cu, 5 mg/kg (copper sulfate); I, 1.25 mg/kg (calcium iodide); Co, 0.5 mg/kg (cobalt sulfate).

Santoquin: Novus International, St. Charles, MO.

mycoCurb: Kemin Industries, Inc., Des Moines, IA.

Ameri-Bond 2X: Borregaard LignoTech, PO Box 162, 1701 Sarpsborg, Norway.

Analysis on as is basis.

Table 2

Ingredient and nutrient composition of experimental finisher diets—Cool season.

Treatment:	Finisher 1		Finisher 2		Finisher 3		Finisher 4		Finisher 5
Ingredient	%		%		%		%		%
Corn	37.79		41.52		51.51		58.23		61.09
Wheat middlings	20.00		20.00		7.44
Sunflower meal, 27% CP	10.00		10.00		10.00		4.73
Soybean meal, 48% CP	19.51		18.80		21.86		26.98		24.00
Corn gluten meal, 60% CP									3.93
Dicalcium phosphate 19	1.63		1.62		1.71		1.80		1.82
Soybean oil	4.53		4.77		5.11		6.05		6.72
Limestone	0.76		0.77		0.72		0.71		0.78
Sodium bicarbonate	0.39		0.39		0.35		0.28		0.32
Salt NaCl	0.21		0.20		0.23		0.28		0.25
L-lys-HCl	0.22		0.23		0.17		0.09		0.19
MetAmino®	0.22		0.21		0.20		0.21		0.19
Choline Chloride 60%	0.10		0.10		0.10		0.11		0.15
ThreAmino®	0.09		0.09		0.06		0.04		0.06
ValAmino	0.02		0.02
L-isoleucine	0.04		0.05		0.02				0.01
L-arginine
Vitamin premix1	0.10		0.10		0.10		0.10		0.10
Trace mineral premix2	0.10		0.10		0.10		0.10		0.10
Selenium premix 60%	0.02		0.02		0.02		0.02		0.02
Ethoxyquin3	0.02		0.02		0.02		0.02		0.02
moldCurb4	0.05		0.05		0.05		0.05		0.05
Filler (sand/solka-floc)	4.2		0.94		0.23		0.20		0.20
Total	100.00		100.00		100.00		100.00		100.00

Vitamin premix: Vit A, 13,227 IU/kg; Vit D3, 3,968 IU/kg; Vit E, 66 IU/kg; Vit B12, .040 mg/kg; biotin, .254 mg/kg; menadione, 3.968 mg/kg; thiamine, 3.968 mg/kg; riboflavin, 13.228 mg/kg; Vit B6, 7.937 mg/kg; niacin, 110.229 mg/kg; folic acid, 2.205 mg/kg.

Trace mineral premix: Mn, 60 mg/kg (manganese sulfate); Zn, 60 mg/kg (zinc sulfate); Fe, 40 mg/kg (ferrous sulfate); Cu, 5 mg/kg (copper sulfate); I, 1.25 mg/kg (calcium iodide); Co, 0.5 mg/kg (cobalt sulfate).

Santoquin: Novus International, St. Charles, MO.

mycoCurb: Kemin Industries, Inc., Des Moines, IA.

Analysis on as is basis.

Table 3

Dietary energy levels and amino acid levels in the study design.1

Treatment	Line	AMEn (kcal/kg)	dlys	dM + C:dLys	dThr: dLys	dVal: dLys	dIle: dLys	dTry: dLys	dArg: dLys
1	A/B	2,800	1.00∗	76	65	80	71	16	105
2		2,925
3		3,050
4		3,175
5		3,300

Crude protein (CP) of the experimental diets 19.50%; Ratio of amino acid is given in percent and is kept constant across dietary ME levels.

Broilers were also measured for protein turnover in pectoralis major in response to dietary ME levels in hot season and cool season were measured and are discussed in detail in another article (Maharjan et al., 2020b).

Performance Parameters and Yield Evaluation

For obtaining performance data, broilers and feed consumed were weighed at 0, 10, 21, and 42 d to determine BW, ADG, and mortality corrected FCR. Ten broilers per pen (n = 90 broilers/treatment; 900 broilers/season) were selected within one standard deviation of the average BW for each treatment for processing yield determination at 45 d and 42 d in hot season and cool season, respectively. Processing was performed after ∼8 h of feed withdrawal. Initial live weight (LW) and ready-to-cook parts—whole breast (boneless and skinless), tenders (boneless and skinless), wing, and thigh—were measured in terms of % LW after carcass chill (∼2 h). Abdominal fat pads were also evaluated.

Body Composition

Body composition was determined by utilizing dual-energy X-ray absorptiometry (GE, Madison, WI) equipped with Lunar Prodigy small animal software (version 12.2). Broilers were euthanized with CO2 gas and were scanned (feathers-on). Scanned broilers were used in previously determined equations (Caldas et al., 2018) to calculate the total lean, protein, fat mass, and energy content of scanned broilers. Two broilers per pen were selected at 42 d of age for scanning (18 broilers per treatment–180 broilers/season; 360 broilers in total). Broilers were selected to be within 1 SD of the average BW for the treatment.

Standardized Ileal Digestible Amino Acids

At 42 d, 6 broilers from line A and line B were selected from each dietary treatment to be within 1 SD of the mean BW for that treatment to determine the AA digestibility of the finisher diets. Broilers were acclimated to the digestibility cages for 2 d. The five experimental finisher diets with 0.2% TiO2 added were fed ad libitum. Feed was removed at 2,400 h for 8-h period on the evening of day 44. The broilers were fed ad libitum from 0,800 to 1,000 h (2 h period) on day 45 and were euthanized by CO2 inhalation. The digesta was removed from the terminal ileum and immediately frozen in liquid nitrogen. The digesta from each broiler was then freeze-dried and analyzed for AA content by reverse-phase HPLC utilizing AOAC 982.30 and AOAC 985.28 methods (AOAC, 1990) as discussed by Caldas et al. (2018). The experimental diets with 0.2% TiO2 were also analyzed for AA content. The titanium in feed and digesta was measured using the method described in the study by Myers et al. (2004). The apparent AA digestibility (AID) was calculated using the expression (Maharjan et al., 2019)where, Ci is the concentration of TiO2 present in diet, Co is the concentration of TiO2 present in digesta; Xo is the AA content in digesta; and Xi is the AA content present in diet. All values for Ci, Co, Xo, and Xi were expressed on % DM basis. Digestibility coefficient values for each AA were determined.

The AID values were converted to SID values using basal endogenous AA losses (BEL) for SID calculations (Blok and Dekker, 2017), and using the following expression

The SID coefficients were then used to calculate the % standardized ileal digestible AA in experimental diets for each AA using the expression;

% standardized ileal digestible AA = (SID coefficient ∗ % AA in diet ("as is")

Data Analysis

The data obtained for variables measured were analyzed by one-way ANOVA using JMPro 14 (SAS Institute, Inc., Cary, NC). Mean values were obtained for variables measured (BW, FCR, ADG, processing yields). One-way ANOVA was performed to understand the dietary treatment effects within each line. Significant means for response variable measured between dietary treatments within each line were measured using a Student t test or HSD test where appropriate. Means were considered significant with a P value ≤ 0.05. For understanding if there was any dietary ME level (factor A) and broiler line (factor B) interaction on the response variable, two-way ANOVA model was utilized as follows:where, μ = mean, Τi = effect of ith level of factor A, Βj = effect of jth level of factor B, (ΤΒ)ij = effect of interaction between the ith level of factor A and the jth level of factor B, eijk = random error associated with the kth replicate.

General linear regression analyses were applied for FCR, body composition, and yield data to determine the relationship of dietary ME levels on these response variables. The following equation was utilized: y = mx + c, where y = response variable x = ME levels, m = slope, and c = intercept.

Results

Feed and Nutrient Intakes

There was no line × diet interaction effect on cumulative 22–42 d FI, protein intake, or energy intake. Feed intake and protein intake decreased linearly as the dietary ME levels increased for both hot season and cool season (Table 4; Figure 2).

Table 4

Feed intake (FI) and nutrient intake per bird of 2 broiler lines fed 5 dietary ME levels (kcal/kg) in hot season and cool season.1

Line/ME level	Hot season						Cool season
	0–21 d			22–42 d			0–21 d			22–42 d
	Cumulative FI	Protein intake	Energy intake	Feed intake	Protein intake	Energy intake (ME)	Feed	Protein	ME	Feed	Protein	ME
	g	g	kcal/b	g	g	kcal/b	g/b	g/b	kcal/b	g/b	g/b	kcal/b
Line A	1,200b	248.5b	3,704.03b	3,510b	649b	10,693b	1,225	260	3,759	3,745	714	11,372
Line B	1,260a	260.8a	3,878.24a	3,602a	666a	11,679a	1,218	258	3,737	3,712	708	11,269
SEM	5	12.21	17.03	80	10	114	4.79	1.01	14.72	20.9	4.02	85
P-value	0.0004	0.0005	0.0051	<0.0001	<0.0001	<0.0001	0.2925	0.2517	0.3020	0.2706	0.2804	0.2609
A, 2,800				3,660a	677	10,248b				3,843a	754a	10,760
A, 2,925				3,620b	670	10,589b				3,789a,b	740a	11,083
A, 3,050				3,480c	644	10,614a,b				3,752a,b	708b	11,256
A, 3,175				3,410c	631	10,827a,b				3,644b	683c	11,570
A, 3,300				3,390c	627	11,187a				3,695b	683c	12,194
B, 2,800				3,790a	701	10,612b				3,850a	756a	10,780
B, 2,925				3,640a,b	673	10,647b				3,726a,b	727a	10,899
B, 3,050				3,590b,c	664	10,950a,b				3,723a,b	703b	11,169
B, 3,175				3,530c,d	653	11,208a,b				3,675b	689b,c	11,668
B, 3,300				3,460d	640	11,418a				3,584c	662c	11,827
SEM				80	20	255				46.58	8.96	190
P-value2				0.0057	0.0502	0.0064				0.0002	<0.0001	0.0625

Different letters in superscripts represent significantly different means between dietary ME levels within line in each column.

P-values presented are for combined lines for dietary ME levels. No diet by line interaction was observed.

Figure 2

∗Linear fit of lines for feed intake (g) for line A and line B for dietary ME levels in hot season (A) and cool season (B). ∗80 ME, 90 ME, 100 ME, 110 ME, and 120 ME is equivalent to 2,800, 2,925, 3,050, 3,175 and 3,300 kcal/kg of feed.

Hot season: There was a linear decrease in FI of 75 g and 77 g for line A (R2 = 0.93) and line B (R2 = 0.95) for every 10% increase in dietary ME for treatment diets (Figure 2). Cumulative FI for line B broilers was higher for 22–42 d than for the line A broilers (Table 4). This resulted in the line B broilers consuming higher protein and higher energy than the line A broilers from 22 to 42 d. Cumulative 22–42 d FI per bird was 3,510 g and 3,602 g for line A and line B broilers, respectively. Cumulative 22–42 d protein intake per bird was 649 g and 666 g for line A and line B broilers, respectively. Cumulative 22–42 d energy intake per bird was 10,693 kcal and 11,679 kcal for line A and line B broilers, respectively.

Cool season: There was a linear decrease in FI of 35 g and 58 g for line A (R2 = 0.80) and line B (R2 = 0.91) for every 10% increase in ME for treatment diets evaluated (Figure 2). Cumulative feed and nutrient intakes were not different between the 2 lines at any time during all feeding phases including day 22–42 (Table 4). For the 22–42 d period, the cumulative FI per bird was 3,745 and 3,712 kg for line A and line B broilers, respectively (P = 0.2706). For this period the line A and line B broilers consumed 714 g and 708 g of protein, respectively (P = 0.2804). The AMEn intake was 11,372 and 11,269 kcal per bird for line A and line B birds, respectively (P = 0.2609).

Body Weight, Average Daily Gain, and Feed Conversion Ratio

There was no line × diet interaction effect on FCR at any age period. The FCR dropped linearly as the dietary ME levels increased (Table 5; Figure 3).

Table 5

Body weight (BW) and FCR per bird of 2 broiler lines fed 5 dietary ME levels (kcal/kg) in hot season and cool season.1

BW	Hot season								Cool season
	BW					ADG	FCR		BW					ADG	FCR
	Day 0	Day 10	Day 21	Day 35	Day 42	22–42 d	22–42 d	0–42 d	Day 0	Day 10	Day 21	Day 35	Day 42	22–42 d	21–42 d	0–42 d
LINE/ME level	g	g	g	g	g	g	g:g	g:g	g	g	g	g	g	g	g:g	g:g
Line A	38.63b	270b	950b	2,240b	2,810b	88b	2.09	1.83	43b	302	988	2,350	3,029a	96a	1.94b	1.74b
Line B	41.04a	280a	1010a	2,350a	2,930a	91a	2.07	1.81	46a	302	986	2,303	2,922b	94b	2.01a	1.76a
SEM	0.0922	1.5	4.4	10.9	15.7	0.7	0.0215	0.0132	0.1	3.1	10.2	22.5	0.0283	0.6	0.0109	0.0058
P-value	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	0.0022	0.9785	0.2479	<0.0001	0.8644	0.9158	0.2686	0.0479	0.0233	<0.0001	0.0010
A,2,800	38.83	270	950	2,210b	2,760b	86b	2.21a	1.92a	44	306	984	2,306b	2,950b	94b	2.10a	1.84a
A,2,925	38.56	270	950	2,220b	2,810a,b	89a,b	2.13a,b	1.87a,b	43	304	985	2,313b	2,962b	93b	2.05a	1.80a
A,3,050	38.76	260	930	2,210b	2,780a,b	88a,b	2.1a,b	1.84b	43	300	990	2,376a	3,042a,b	97a	1.93b	1.73b
A,3,175	38.37	270	960	2,230b	2,800a,b	87a,b	1.04b,c	1.8b,c	43	302	997	2,389a	3,089a	98a	1.84c	1.67c
A,3,300	38.64	270	970	2,310a	2,900a	92a	1.96c	1.76c	43	301	986	2,365a	3,101a	98a	1.80c	1.64c
B,2,800	41.24	270	1,000	2,300b	2,860b	88b	2.18a	1.88a	46	301	972	2,244b	2,862b	91b	2.18a	1.87a
B,2,925	41.01	270	990	2,310b	2,870a,b	89a,b	2.17a,b	1.86a	46	306	993	2,307a	2,850b	94a,b	2.10a,b	1.82a,b
B,3,050	40.99	280	1,030	2,380a	2,970a,b	93a,b	2.06a,b	1.8a,b	46	297	986	2,306a	2,926a,b	93a,b	2.00bc	1.76b
B,3,175	40.92	280	1,020	2,390a	2,960a,b	93a,b	2.03b,c	1.78b,c	45	304	990	2,331a	3,001a	97a	1.90c,d	1.69c
B,3,300	41.05	280	1,010	2,390a	2,990a	94a	1.9c	1.71c	45	299	990	2,326a	2,970a,b	95a,b	1.87d	1.68c
SEM	0.2027	3.3	9.7	23.9	34.4	1.5	0.0419	0.023	0.2	4.2	13.9	30.8	38.7	1.2	0.0239	0.0129
P-value2	0.4256	0.1275	0.2041	0.0014	0.0027	0.0035	<0.0001	<0.0001	0.1666	0.1940	0.7037	0.0076	<0.0001	0.0002	<0.0001	<0.0001

Different letters in superscripts represent significantly different means between dietary ME levels within line in each column.

P-values presented are for combined lines for dietary ME levels. No diet by line interaction was observed.

Figure 3

∗Linear fit of lines for FCR (22–42 d) for Line A and Line B for dietary ME levels (kcal/kg) in hot season (A) and cool season (B). ∗80 ME, 90 ME, 100 ME, 110 ME, and 120 ME is equivalent to 2,800, 2,925, 3,050, 3,175 and 3,300 kcal/kg of feed.

Hot season: Line B had higher BW throughout the trial period (P < 0.05) than line A. ADG increased as the dietary ME level increased. The FCR decreased linearly by 0.059 and 0.07 units for line A (R2 = 0.98) and line B (R2 = 0.93) across ME levels (Figure 3). The cumulative FCR was lower for line B (P > 0.05) for 22–42 d or 0–42 d than for line A.

Cool season: There was no difference in BW between lines at day 21 and day 35 (P > 0.05), whereas the higher BW for line A was observed at day 42 (P < 0.05). Cumulative ADG was higher for line A (P < 0.05), whereas ADG increased across dietary ME levels for both lines. The FCR decreased linearly by 0.08 units for both lines (R2 > 0.97) across dietary ME levels (Figure 3). Similarly, line A had lower FCR values for 22–42 d and 0–42 d (Table 5).

Cumulative total body mass was higher for line B in hot season (P < 0.05), whereas it was not different between lines in cool season (P > 0.05). The % lean and % protein mass were higher for line B than for line A (P < 0.05) for both seasons (Table 6). Total body mass increased linearly for both lines across ME levels. The cumulative % fat mass at each ME level was higher for line A (P < 0.05) than for line B. There was a linear decrease in % protein (hot season: 0.14% for line A and 0.19% for line B; cool season: 0.28% for line A and 0.25% for line B), whereas the % fat linearly increased (hot season: 0.53% for line A and 0.38% for line B; cool season: 0.66% for line A and 0.74% for line B) across ME levels (Figure 4). The rate of increases for % fat mass across ME levels was higher for cool season as given by higher slope values (Figure 4) for both lines.

Table 6

Body composition in percentage of total body mass at 42 d of 2 lines of broilers reared from 22–42 d of age on 5 different dietary ME levels (kcal/kg).1

Line/ME level	Hot season					Cool season
	Total mass (g)	Lean%	Protein%	Fat%	Energy kcal/g	Total mass (g	Lean%	Protein%	Fat%	Energy kcal/g
Line A	2,957.24	88.4b	18b	8.6a	1.8a	3,140.2	86.1	17.6	9.9	1.86
Line B	3,061.06	89.7a	18.4a	7.9b	1.77b	3,117.2	87.1	17.8	9.4	1.84
P-value	<0.0001	0.0002	<0.0001	0.0003	0.0011	0.1760	0.0046	0.0068	0.0047	0.0037
A, 2,800	2,961a,b	89.6a	18.3a	8b	1.8b	3,094b	87.9a	18a	8.9c	1.81c
A, 2,925	2,871b	89.9a	18.3a	7.8b	1.8b	3,071b	87.9a	18a	8.9c	1.82c
A, 3,050	2,952a,b	88.9a	18.1a	8.3b	1.8b	3,127a,b	86.1a,b	17.6b	10b	1.86b
A, 3,175	2,954a,b	87.3b	17.8b	9.3a	1.8b	3,196a	85.4b	17.5b	10.3b	1.88b
A, 3,300	3,050a	86.1b	17.6b	9.9a	1.9a	3,213a	83.2c	17c	11.5a	1.94a
B, 2,800	3,043a,b	90.5a	18.5a	7.4b	1.7b	3,061b	89.4a	18.3a	8.1d	1.78d
B, 2,925	3,028b	91a	18.6a	7.1b	1.7b	3,079b	88.5a,b	18.1a,b	8.6c,d	1.80c,d
B, 3,050	3,062a,b	90a	18.4a	7.8b	1.8a	3,131a,b	87.4b,c	17.9bc	9.2b,c	1.83b,c
B, 3,175	3,088a	88.3b	18b	8.7a	1.8a	3,150a	86.7c	17.7c	9.6b	1.85b
B, 3,300	3,084a	88.7b	18.1b	8.5a	1.8a	3,165a	83.7d	17.1d	11.3a	1.93a
SEM	34.54	0.5	0.1	0.3	0	26.52	0.57	0.13	0.32	0.01
P-value2	0.0207	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

Different letters in superscripts represent significantly different means between dietary ME levels within line in each column.

P-values presented are for combined lines for dietary ME levels. No diet by line interaction was observed.

Figure 4

∗Linear fit of lines for total body mass (A, B), % fat of total body mass (C, D), and % protein of total body mass (E, F) for line A and line B at 42 d for dietary ME levels (kcal/kg) in hot season and cool season. ∗80 ME, 90 ME, 100 ME, 110 ME, and 120 ME is equivalent to 2,800, 2,925, 3,050, 3,175, and 3,300 kcal/kg of feed.

Processing Yield

There were differences in cumulative % weights (of LW) for wing, breast, tenders, and leg quarters for both lines and seasons (P < 0.05) (Table 7). However, the % weight of LW for wings, breast filet, tenders, and leg quarters were not different when compared between treatment diets in hot season (P > 0.05). The % breast weight and % tenders' weight of the LW between dietary ME levels (the lowest and highest ME) were different mainly in cool season (P < 0.05). The % fat pad of total LW increased linearly by 0.22 and 0.21% in hot season and 0.2 and 0.18% in cool season for line A and line B, respectively (R2 > 0.95), for every 10% increase across dietary ME levels. The cumulative % fat pad for combined ME levels was higher for line A (P < 0.05) for both seasons, with overall fat pad % higher for hot season than cool season.

Table 7

Ready-to-cook and fat pad yield in percentage of live weight (LW, g) of 2 broiler lines fed dietary ME levels (kcal/kg) in hot season and cool season.1

Line/ME level	45-d processing, hot season						42-d processing, cool season
	LW	Fat pad	Wings	Breast filet	Tenders	Leg quarters	LW	Fat pad	Wings	Breast filet	Tenders	Leg quarters
Line A	3,016b	2.5a	7.7a	20.5a	4.34a	23b	3,173	1.4a	7.6b	20.2a	4.3a	22.5
Line B	3,135a	2.3b	7.6b	20.1b	4.27b	23.2a	3,162	1.2b	7.7a	19.7b	4.2b	22.6
P-value	<0.0001	<0.0001	<0.0001	<0.0001	0.0010	0.0201	0.4124	<0.0001	0.0325	0.0019	0.0088	0.2840
A, 2,800	2,956	2.1c	7.8	20.6	4.4	23.2	3,088	1c	7.6	20.4a	4.4a	22.6
A, 2,925	3,001	2.2c	7.6	20.6	4.4	23	3,083	1.1c	7.7	20.4a	4.4a	22.2
A, 3,050	3,017	2.5b	7.7	20.6	4.3	22.9	3,187	1.5b	7.5	20.3a	4.3a,b	22.5
A, 3,175	3,014	2.8a	7.7	20.3	4.3	23.3	3,216	1.5b	7.5	19.5b	4.2b	22.7
A, 3,300	3,089	2.9a	7.8	20.7	4.3	22.8	3,291	1.8a	7.4	20.2a	4.2b	22.1
B, 2,800	3,050	1.9c	7.6	20.3	4.3	23.1	3,082	0.9d	7.6	20.1a	4.3a	22.5
B, 2,925	3,077	2c	7.7	20	4.3	23.2	3,153	1d	7.7	20a,b	4.2a	22.5
B, 3,050	3,160	2.2b	7.7	20.1	4.3	23.2	3,128	1.2c	7.6	19.7a,b	4.3a	22.7
B, 3,175	3,169	2.5a	7.6	19.9	4.3	23.3	3,228	1.4b	7.6	19.6b,c	4.3a	22.7
B, 3,300	3,217	2.7a	7.5	20.3	4.3	23.1	3,221	1.6a	7.6	19.2c	4.1b	22.6
SEM	19.41	0.08	0.04	0.16	0.04	0.12	20.84	0.01	0.01	0.05	0.01	0.04
P-value2	<0.0001	<0.0001	0.1871	0.1159	0.1053	0.0920	<0.0001	<0.0001	0.1262	0.0015	0.0018	0.3567

Different letters in superscripts represent significantly different means between dietary ME levels within line in each column.

P-values presented are for combined lines for dietary ME levels. No diet by line interaction was observed.

There was no difference between lines in standardized ileal digestibility (SID) coefficient values (P > 0.05) for combined analysis of treatment diets for hot season or cool season. The % standardized ileal digestible amino acids intake decreased (P < 0.05) for the most essential and nonessential amino acids for both lines with increasing dietary ME level in hot season whereas this trend was not observed for cool season (Table 8). Leucine was the only amino acid that the SID intake increased over the increase in dietary ME levels for both lines and seasons.

Table 8

Percentage of standardized ileal digestible amino acids intake (“as is” basis) of experimental diets in hot season and cool season.

Amino acid	Lys	Met	Thr	Trp	Arg	Val	Leu	Ile	His	Phe	Cys	Gly	Ser	Pro	Ala	Asp	Glu
Hot season
Line A	1.06	0.48	0.70	0.21	1.20	0.86	1.39	0.76	0.46	0.86	0.29	0.77	0.86	1.02	0.87	1.63	3.21
Line B	1.06	0.48	0.70	0.20	1.20	0.85	1.39	0.76	0.46	0.86	0.29	0.78	0.86	1.02	0.87	1.64	3.21
P-value	0.82	0.99	0.72	0.48	0.43	0.95	0.64	0.80	0.70	0.79	0.81	0.44	0.76	0.82	0.65	0.73	0.76
A,2,800	1.12	0.49	0.74	0.23	1.27	0.91	1.41	0.80	0.49	0.90	0.31	0.83	0.91	1.04	0.90	1.75	3.42
A,2,925	1.06	0.56	0.68	0.20	1.20	0.83	1.33	0.74	0.45	0.84	0.29	0.75	0.85	0.98	0.83	1.58	3.18
A,3,050	1.03	0.48	0.70	0.20	1.15	0.83	1.34	0.73	0.44	0.83	0.27	0.77	0.81	1.01	0.85	1.54	3.12
A,3,175	1.03	0.44	0.66	0.20	1.16	0.82	1.41	0.73	0.46	0.86	0.28	0.74	0.84	1.02	0.87	1.57	3.12
A,3,300	1.06	0.42	0.73	0.19	1.21	0.89	1.45	0.79	0.47	0.89	0.29	0.77	0.89	1.04	0.89	1.72	3.20
B,2,800	1.12	0.49	0.74	0.23	1.27	0.90	1.40	0.80	0.47	0.89	0.32	0.82	0.90	1.03	0.90	1.73	3.40
B,2,925	1.09	0.58	0.72	0.21	1.24	0.87	1.39	0.76	0.47	0.87	0.32	0.81	0.89	1.04	0.87	1.68	3.30
B,3,050	1.04	0.48	0.71	0.19	1.16	0.84	1.35	0.73	0.44	0.84	0.28	0.77	0.82	1.02	0.85	1.55	3.13
B,3,175	1.02	0.43	0.66	0.20	1.16	0.81	1.41	0.73	0.46	0.84	0.27	0.74	0.83	1.02	0.87	1.57	3.10
B,3,300	1.03	0.41	0.69	0.19	1.18	0.86	1.41	0.76	0.46	0.87	0.27	0.75	0.87	1.01	0.86	1.67	3.12
P-value1	<.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	0.33	0.00	<0.0001	<0.0001
Cool season
Line A	0.79	0.43	0.51	0.19	0.89	0.63	1.14	0.61	0.35	0.68	0.16	0.53	0.62	0.85	0.67	1.17	2.66
Line B	0.74	0.42	0.45	0.18	0.82	0.53	1.04	0.56	0.32	0.61	0.19	0.45	0.56	0.77	0.60	1.02	2.43
P-value	0.12	0.15	0.02	0.50	0.08	0.01	0.07	0.07	0.05	0.06	0.71	0.02	0.01	0.03	0.07	0.04	0.03
A,2,800	0.90	0.45	0.57	0.21	0.99	0.72	1.17	0.68	0.39	0.73	0.21	0.63	0.64	0.95	0.70	1.30	2.94
A,2,925	0.70	0.45	0.52	0.17	0.79	0.60	0.87	0.53	0.29	0.55	0.03	0.47	0.62	0.74	0.53	0.94	2.50
A,3,050	0.82	0.45	0.52	0.20	0.95	0.65	1.19	0.65	0.38	0.71	0.22	0.61	0.64	0.88	0.71	1.30	2.77
A,3,175	0.77	0.42	0.47	0.18	0.89	0.57	1.08	0.58	0.34	0.65	0.17	0.50	0.60	0.78	0.63	1.18	2.45
A,3,300	0.77	0.41	0.49	0.19	0.82	0.61	1.40	0.62	0.35	0.73	0.19	0.47	0.61	0.92	0.77	1.14	2.64
B,2,800	0.76	0.41	0.44	0.17	0.83	0.55	0.96	0.56	0.32	0.61	0.15	0.47	0.54	0.79	0.57	1.01	2.51
B,2,925	0.77	0.46	0.45	0.17	0.83	0.54	0.95	0.56	0.32	0.60	0.15	0.48	0.57	0.78	0.58	0.99	2.58
B,3,050	0.68	0.40	0.42	0.18	0.81	0.50	0.96	0.53	0.31	0.58	0.02	0.44	0.54	0.70	0.57	0.98	2.24
B,3,175	0.70	0.43	0.46	0.19	0.82	0.50	0.99	0.53	0.32	0.58	0.20	0.42	0.57	0.70	0.57	1.01	2.28
B,3,300	0.79	0.38	0.47	0.19	0.83	0.55	1.34	0.59	0.34	0.71	0.42	0.45	0.59	0.87	0.72	1.10	2.55
P-value2	0.42	0.02	0.91	0.22	0.52	0.62	<0.0001	0.44	0.29	0.05	0.31	0.39	0.97	0.04	0.01	0.36	0.37

Significantly different means existed between dietary ME levels within line in hot season (P < 0.0001) for all amino acid analyzed.

P-values presented are for combined lines for dietary ME levels. No diet by line interaction was observed.

Discussion

Dietary energy is the largest cost factor for poultry operation. The utilization of dietary energy by modern broiler lines needs further exploration. Finding the appropriate protein to energy ratio in the diet can be crucial for the optimal performance of broilers. The performance and processing yield benefits by increasing dietary amino acid density is becoming more evident (Temim et al., 1999; Viera and Angel, 2012; Maharjan et al., 2020c), whereas less is known regarding dietary energy density. The present study evaluated the effects of dietary ME levels ranged from 2,800 to 3,300 kcal/kg on performance parameters and processing yield characteristics.

Decreasing dietary ME levels resulted in a FI increment of 61.25 g/b for every 10% increase in energy level in 22–42 d for combined lines and seasons in this study. Even though there was a decreased FI during 22–42 d with higher ME diets (>100% ME levels), there were still increased energy intake per bird. The FCR values dropped linearly as the dietary ME levels increased for both lines and seasons (Table 5; Figure 3). Reduced FCR for broilers fed higher ME diets as reported in this study were also observed in other studies conducted with meat broilers fed high energy diets (3,250 kcal ME/kg vs. 3,050 kcal ME/kg) reared at grow-out temperatures from 21.1 C to 35 C (Cheng et al., 1997; Gopinger et al., 2017; Liu et al., 2019). The increased FI of broilers fed lower ME diets (<100% ME levels) resulted in higher protein intake, which resulted in an increase in % whole body protein (Table 6). Broilers fed high energy density diets did not produce different processing characteristics (% breast, tenders, wings or leg quarters of LW) in present study (at least for hot season) as compared with low energy density diets except for % fat pad of LW (Table 7). Similar results for yield were reported in other studies that tested various dietary energy levels (Gopinger et al., 2017; Liu et al., 2019; Maynard et al., 2019), which was attained by actually increasing the FI as observed in present study. The decreased FI of broilers with higher ME diets (>100% ME levels) and thus decreased protein intake, compared with lower ME diets potentially indicated improved use of available dietary protein with higher ME level diets. High energy diets could optimize the dietary amino acid utilization which increases the protein accretion potential of broilers. This is also justified by higher SID intake for leucine across dietary ME for both lines and seasons as leucine almost entirely mediate translation initiation processes in protein synthesis of skeletal muscle (Norton and Layman, 2006; Stipanuk, 2007). Jackson et al. (1982) studied the effects of alteration of nutrient densities of 6 levels of dietary protein (16–36%) and energy (2,600–3,600 kcal/kg) on performance and found significant interactions between dietary protein and energy suggesting the need for balanced protein-to-energy ratio.

All animals including avian species possess natural ability to control FI based on intent to normalize energy intake as per the body requirement. When energy intake is decreased, there is an increased intake of amino acid per Mcal that leads to decreased deposition of carcass fat or vice versa (Leeson et al., 1996). There was increased fat mass (P < 0.05) as dietary ME level increased in both seasons in current study. Whole-body % protein mass of total body mass measured at day 42 linearly dropped for both lines across all ME levels for both seasons (R2 = ∼0.9). Another study found a similar response in broilers with protein and fat body composition when fed lowered energy diets in the finishing phase as observed in this study (Leeson et al., 1996).

Protein deposition that occurs through protein turnover processes is energy-dependent process (Waterlow, 1984). Based on the calculated energy cost for protein synthesis, 4 mol ATP + GTP per mole of peptide bond would require a minimum of 0.86 kcal/g protein of average composition yet (Waterlow, 1984; Bergen, 2008). The protein synthesis is followed by the almost equivalent amount of breakdown which the exact stoichiometry for breakdown energy cost has not been completely worked out yet (Waterlow, 1984; Bergen, 2008); however, it is predicted that it is half the amount (0.40 kcal/g) compared with synthesis (Fan et al., 2008). The balance of protein: energy is essential for optimal protein turnover (Maharjan et al., 2020a). Optimal protein (digestible amino acids)-to-energy (calorie) ratio in diet potentiates for optimal PT gain for skeletal protein deposition, the concept discussed earlier in many other studies (Kyriazakis and Emmans, 1992; Maharjan et al., 2020b). Broilers had the highest fractional synthesis rate (FSR) in pectoralis major for finishing phase consuming ∼3.48 g digestible Lys/Mcal (Maharjan et al., 2020a) which is higher than the g dLys consumed/Mcal in present study. Increasing the g digestible Lys/Mcal ≥3.48 as reported by Maharjan et al. (2020a,b) during the cool season improved the FSR by ∼5% per day. This indicates that FSR is more responsive to increased dietary digestible amino acid than to dietary energy changes. Achieving the optimal g dLys/Mcal is important; however it is equally essential to have adequate energy level of the diet to attain optimal amino acid utilization for protein synthesis.

Processing data in the present study showed no difference in yield (P > 0.05) in % breast, tenders, leg quarters, and wing weights of live weight for ≤ 100% ME level diets compared with > 100% ME levels. Broilers at end of finishing phase (42 d) would require higher maintenance energy (Em) because of larger BW. Broilers that consumed treatment diets ≤100% ME level utilized major portion of dietary energy in the form of maintenance energy (Em), and thus energy was deficient for optimal protein synthesis. A study conducted in humans showed that FSR and associated synthetic intracellular signaling proteins (insulin-like growth factor 1, protein kinase B, and eukaryotic initiation factor 4E binding protein 1 phosphorylation) were downregulated in response to energy-deficient diet (80% of estimated energy requirements) fed for 10 d, and skeletal FSR was significantly reduced (Pasiakos et al., 2010; Lu et al., 2017). Energy-deficient diets can also affect the ubiquitin-proteasome pathway (of protein degradation), and an ATP-dependent pathway resulting in altered protein degradation.

The excess energy that a broiler consumes is deposited as fat. Heat stress can affect the energy partitioning into either fat deposition or protein deposition. Adipokines such as leptin and adiponectin and their receptors are upregulated during heat stress (Bernabucci et al., 2009; Morera et al., 2012). High energy diets fed to broilers has been shown to increase fat deposition in hyperthermic birds (as compared with high protein diets) (Le Belego et al., 2002) which resonated the findings in the present study with hot season producing increasing % fat pads of BW compared with cool season (Table 7; Figure 5). Line A had higher fat mass than line B in both seasons across all ME levels, indicating higher lipogenesis occurring in line A than line B.

Figure 5

∗Linear fit of lines for % fat pad of total live weight (A, B) at 42 d for line A and line B for dietary ME levels in hot season and cool season. ∗80 ME, 90 ME, 100 ME, 110 ME, and 120 ME is equivalent to 2,800, 2,925, 3,050, 3,175, and 3,300 kcal/kg of feed.

The findings of present study showed that dietary energy levels negatively impacted the FI of both broiler lines linearly (R2 of 0.94 and 0.86 in hot season and cool season, respectively).

Broilers fed lower ME diets consumed more feed to provide the physiological energy requirements resulting in lower feed efficiency. The % fat deposition and % fat pad were positively correlated across dietary ME in both seasons (R2 > 0.9). Line A deposited more % body fat and % fat pad than line B in both seasons (P < 0.05). The overall findings suggested that results for performance and yield as affected dietary ME levels were line specific and were affected by grow-out ambient environmental temperatures. The optimal dietary ME level for the ME range studied (2,800–3,000 kcal/kg) at a constant recommended amino acid level lies in determining the best performance and profitability indices by taking into account the grow-out production inputs and processing yield outputs. The industry needs a continuing research aimed at scientific understanding on broiler growth nature and type of gain (protein and fat) in relation to feed energy intake and energy metabolism. The impact dietary energy and amino acid levels have on endocrine function and intermediary metabolism in broilers could shed more light on the fate of these macronutrients and perhaps further understand the effects of these nutrients in feed intake regulation.