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Growth Performance, Meat Yield, and Economic Responses of Broilers Fed Diets Varying in Metabolizable Energy from Thirty to Fifty-Nine Days of Age¹

W. A. Dozier, III^*,2, C. J. Price, M. T. Kidd, A. Corzo, J. Anderson and S. L. Branton^*

^* USDA, Agriculture Research Service, Poultry Research Unit PO Box 5367, Mississippi State, MS 39762-5367; Sanderson Farms Inc., Laurel, MS 39441; and Department of Poultry Science and Department of Agricultural Economics, Mississippi State University, Mississippi State 39762

² Corresponding author: bdozier{at}msa-msstate.ars.usda.gov

	SUMMARY

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

 
Two studies examined responses of broilers to diets varying in AME from 30 to 59 d of age. A 59-d termination allows for evaluation of energy needs applicable to "big bird" programs, because research on nutritional needs for such programs is warranted. Two experiments were conducted: experiment (exp.) 1 having low temperatures whereas exp. 2 used moderate temperatures. The treatments in exp. 1 and 2 were AME concentrations ranging from 3,175 to 3,310 kcal/kg with CP, TSAA, and Lys being identical across all treatments. In exp. 2, an additional treatment consisted of a diet containing 3,310 kcal of AME/kg with CP, TSAA, and Lys being increased by 4% of those specifications used in the other treatments so as to minimize differences in energy:CP ratio.

In both experiments, feed consumption and conversion decreased linearly as dietary AME increased, but breast meat yield was reduced with the high AME diet and only increasing amino acids in exp. 2 ameliorated the negative effect. Live production costs and gross feeding margin (bird return over feed costs) were optimized in exp. 1 with 3,220 kcal of AME/kg of diet, but 3,310 kcal of AME/kg of diet and increased amino acids were needed in exp. 2 for improved monetary returns. In both experiments, broilers had similar caloric consumption indicating that these broilers can compensate to varying dietary AME concentrations within practical limits. These results demonstrated that the response to dietary AME was more pronounced under moderate ambient temperatures.

Key Words: broiler • dietary energy • feeding regimen • metabolizable energy • nutrient density • temperature

	DESCRIPTION OF PROBLEM

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

 
In the United States, a trend of marketing more birds at heavier weights has been increasing over the last several years. At the present time, approximately 16% of the total broilers in the United States are marketed at 3.2 kg or greater [1]. Because processing fixed costs per shackle remain the same regardless of bird size, processing larger broilers can reduce cost per unit of meat produced on a weight basis by increasing the amount of saleable meat processed within a given time without expanding the plant. Heavy broilers grown to 3.7 to 4.0 kg of BW consume about 7.5 kg of feed, with approximately 70% of this occurring between 5 and 9 wk of age. Dietary energy-contributing ingredients are a major cost to the broiler diet [2], especially as the bird ages and the relative energy need increases. Furthermore, the efficiency of energy use decreases as the bird advances with age [3]. Thus, a consideration of the effect of dietary energy level on the profitability of the production of heavy broilers is warranted.

Given the energy cost of the diet and lower efficiency of nutrient use in heavy broilers, the optimum dietary AME concentration late in development based on growth, meat yield, and economics is of utmost importance to broiler companies. Increasing dietary AME via fat supplementation has been shown to improve feed conversion by reducing feed consumption in broilers marketed at low or moderate BW [3, 4, 5, 6, 7, 8]. Providing increasing concentrations of dietary AME while maintaining a constant ratio to CP and amino acids of broiler diets improves feed conversion without increasing fat deposition [9, 10, 11, 12]. Most previous research did not evaluate the response of breast meat yield to dietary energy. However, Hidalgo et al. [3] reported no differences in carcass parts in 1.9-kg broilers fed diets differing in energy. Other research found no differences in carcass parts in response to dietary AME with broilers grown to 9 wk of age [9, 10]. These studies did not address economic considerations based on diet cost, chicken meat prices, and amounts of carcass parts.

Reece and McNaughton [13] reported that increasing dietary AME improved 49-d BW and 23- to 49-d feed conversion of broilers at 18.3°C, but BW was not affected by dietary AME at 26.7°C. Furthermore, the response of feed conversion to dietary AME was more pronounced at 18.3 than 26.7°C [13]. In agreement, Veldkamp et al. [14] found the response to dietary AME to be less pronounced in high vs. low ambient temperatures with male turkeys. Therefore, the response of heavy broilers to dietary AME may be different according to the season of the year.

Reports on dietary AME needs are limited with heavy broilers (>3.2 kg of BW) during the latter periods of production. Research has documented dietary AME responses of broilers grown to heavy BW, but results may have been influenced by carryover effects from previous feeding periods [9, 10]. Therefore, the present studies examined dietary AME needs of Ross x Ross 308 broilers from 30 to 59 d of age. Measurements were extended beyond growth performance and meat yield to include an economic sensitivity analysis of gross feed margin that encompassed varying chicken meat prices and diet costs.

	MATERIALS AND METHODS

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

 
Bird Husbandry
In 2 exp., 2,500 1-d-old Ross x Ross 308 [15] chicks were obtained from a commercial hatchery and placed in a common rearing environment from 1 to 29 d. Vaccinations for Marek’s disease, Newcastle disease, and infectious bronchitis were administered at the hatchery, and for (Gumboro) vaccine at 12 d of age. At 30 d, 1,920 broilers were randomly distributed to 32 floor pens (30 males and 30 females/pen; pen size = 5.57 m²) in exp. 1, and 1,800 broilers were randomly distributed to 30 floor pens (30 males and 30 females/pen; pen size = 5.57 m²) in exp. 2. Before initiation of the dietary treatments, initial BW was equalized across all treatments. The experimental facility was a solid-sided building with controls for light and ambient temperature. Both experiments were conducted in the same facility (exp. 1 = January to February 2005, low temperature; exp. 2 = May to June 2005, moderate temperature). Each pen was equipped with 2 pan feeders and a nipple drinker line having 15 nipples (flow rate = 70 mL/min) and fresh pine shavings. Birds had access to feed and water ad libitum. In exp. 1, temperature was set at 22°C at 30 d and then reduced as the birds progressed in age, with a final temperature of 18°C at 56 d [16]. Ambient temperature was held constant at 24°C in exp. 2 [17]. The lighting schedule used in both exp. was continuous with a light intensity of 3 lx.

Dietary Treatments
In both exp., birds were fed common diets from 1 to 29 d of age. Finisher (30 to 47 d; Table 1) and withdrawal (48 to 59 d; Table 2) diets were fed during the experimental periods. Dietary treatments were formulated to contain 3,175, 3,220, 3,265, and 3,310 kcal of AME/ kg in exp. 1. Supplemental fat was added in the mixer before pelleting. The amounts of CP, TSAA, and Lys in the experimental diets were identical. In exp. 2, dietary treatments were the same as in exp. 1 with an additional treatment being a diet formulated to contain 3,310 kcal of AME/kg with CP, TSAA, and Lys specifications increased by 4% compared with the other treatments. This additional treatment was added in exp. 2 to minimize any adverse affects on breast meat yield due to a reduction in feed consumption of the high AME diet. It had been observed that feed consumption was reduced 4% with feeding the high AME diet in exp. 1. All feed was steam pelleted. In exp. 2, percentage pellets and pellet durability index (PDI) were determined based upon a standard procedure [18].

Economics
A sensitivity analysis was conducted by varying diet cost, breast meat, and carcass prices to calculate gross feed margin per bird. Diet cost ranged from 80 to 120% of the base price (100%) and breast fillet, breast tender, and whole carcass prices varied from 70 to 130% of the base (100%) price. The range of values for diet cost and meat prices were intended to represent the array of price fluctuations occurring during a 12-mo period experienced by an integrated broiler operation. Diet cost was formulation cost rather than supplemental fat cost. Formulation cost was used because amounts of corn, soybean meal, and supplemental fat varied across all diets to accommodate differences in AME while maintaining similar CP, Lys, and TSAA concentrations. The base diet cost was determined from local ingredient prices as of July 2005. The price of fat used in all diets was $0.373/kg. Base meat prices were $1.32, $3.32, and $3.97/kg to represent carcass, breast fillet, and breast tender prices, respectively. The diet cost and meat prices used allowed the evaluation of 24 different economic scenarios as influenced by the dietary treatments. Gross feeding margin represents the per-bird return over feed costs, calculated as follows: output (quantity of meat produced multiplied by meat price) minus input (quantity of feed consumed multiplied by diet cost) [19].

Statistics
Both exp. were conducted as a randomized complete block design. Experiment 1 had 8 replicate pens per treatment, whereas exp. 2 had 6 replications per treatment. Three analyses were conducted: 1) ANOVA followed by least significant difference test comparing dietary AME means; 2) ANOVA using a linear trend to explain potential dietary AME effects; and 3) a quadratic trend to explain potential dietary AME effects. From the linear trend analysis, a slope estimate and a standard error of the slope were determined. In general, a quadratic trend was not significant (P > 0.05) for the variables measured in this study with the exception of abdominal fat percentage in exp. 2. Analyses were performed by PROC MIXED [20]. Statistical significance was established at P 0.05.

	RESULTS

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

 
Exp. 1
Graded increments of dietary AME decreased (P 0.0005) feed consumption and feed conversion from 30 to 47 and from 30 to 59 d of age (Table 3). The feed conversion response to dietary AME was more pronounced from 30 to 59 d than from 30 to 47 d. The linear trend analysis indicated the slope was –0.043 from 30 to 59 d. This implies that, as dietary AME was increased by each unit of 45 kcal/kg, feed conversion would be decreased by 4 points. An 8-point reduction in feed conversion occurred as dietary AME was increased from 3,220 and 3,265 to 3,310 kcal/kg from 30 to 59 d of age. Body weight, BW gain, and the incidence of mortality (P 0.06) were not affected by the dietary treatments from 30 to 47 or 30 to 59 d of age.

	DISCUSSION

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

Environmental temperature influences the energy need for maintenance [24]. When broilers are exposed to temperatures above their thermoneutral zone, metabolic heat is removed by a process referred to as latent heat loss (heat of evaporation) as broilers pant to remove the heat. Panting expends additional energy to remove excess metabolic heat, thus reducing the amount of energy available for growth. May and Lott [25] reported that the optimum temperature for BW gain and feed conversion of Ross x Ross 308 male broilers from 2.5 to 3.0 kg was 12 and 16°C, respectively. In exp. 2 of the current research, the temperature was maintained at 24°C. Thus, the temperature was above the birds’ thermoneutral zone resulting in birds removing heat via latent heat loss.

The addition of dietary fat has been shown to improve feed conversion of broilers exposed to high temperatures [8, 22, 23] and this improved efficiency may be related to a lower heat increment following absorption and during metabolism [26]. In the current research, dietary fat supplementation ranged from 0.82 to 4.60% as AME increased from 3,175 to 3,310 kcal/kg. Dietary fat was used to increase AME because it is the method of choice in the United States broiler industry.

Providing broilers feeds with poor pellet quality adversely affects growth performance [27, 28] and also increases energy and time for prehension [29]. Thus, less energy is available for growth. Fat supplementation reduces pellet quality [30, 31, 32, 33] by impeding the penetration of steam into grain particles to form a high quality pellet. In the current research, caloric use was not reduced to a great extent as dietary AME increased from 3,265 to 3,310 kcal/kg (without increased CP and amino acids) in exp. 1 and 2, which may be due to poor pellet quality in the high AME diets. Plavnik et al. [30] reported that slopes for growth rate and feed efficiency were less pronounced with fat supplementation than carbohydrate addition with increasing dietary energy. These authors concluded that the reduction in pellet quality with fat supplementation might have decreased the response to dietary energy compared with the carbohydrate supplementation.

Increasing concentrations of dietary AME will not alter abdominal fat percentage if the ratio of calories to CP remains constant [3, 9, 10, 12]. In the present study, gradient increments of dietary AME did not significantly lead to a linear trend in abdominal fat percentage. Dietary calorie:CP ratio increased by 7 units (from 185 to 192) and 8 units (from 196 to 204) with diets formulated to contain 3,175 to 3,310 kcal of AME/kg provided from 30 to 47 d and 48 to 59 d, respectively. Conversely, Bartov et al. [12] reported an increase in skin fat concentration of broilers provided diets having a 19-unit increase in calorie:CP ratio (158 vs. 176). Therefore, it is likely that the calorie:CP ratio of the diets in the present study may not have differed enough to cause a significant difference in abdominal fat percentage.

In the current research, increasing dietary AME decreased feed consumption without adversely affecting BW gain, but reductions in breast fillet and total breast meat yields were observed when the high AME diets (3,310 kcal/ kg) were fed. This decrease in meat yield may have occurred due to a reduction in CP, TSAA, and Lys intake. In exp. 1, increasing dietary AME from 3,220 to 3,310 kcal/kg decreased Lys intake/bird from 48.40 to 47.75 g from 30 to 59 d. Decreasing amino acid density has been shown to negatively affect breast meat yield of broiler chickens [34, 35, 36, 37]. In exp. 2, increasing CP, TSAA, and Lys in the high-energy diet (3,310 kcal/kg) by 4% of those values used in the other dietary treatments resulted in Lys intake of 40.87 g, whereas the diets formulated to contain 3,220 and 3,310 kcal/kg had Lys intakes of 39.18 and 37.36 g, respectively. Increasing Lys intake/bird to 40.87 g led to similar breast meat yield of broilers fed the diet formulated to contain 3,220 kcal of AME/ kg (Lys intake/bird = 39.18 g). Therefore, increasing CP and amino acid concentrations in high-energy diets can alleviate negative effects on meat yield. In agreement with the meat yield data, the addition of CP, TSAA, and Lys concentrations to the high-energy diet was advantageous for gross feeding margin. Increasing the CP and amino acid concentrations of the high-energy diet while compensating for decreased amino acid intake was economically beneficial during moderate temperatures. Integrated broiler companies may need to evaluate nutritional feeding strategies differently for the summer vs. winter grow-outs by increasing AME concentration in the diet in the finisher and withdrawal diets by 35 to 40 kcal/kg during summer production. When feeding diets high in AME, CP/amino acids should be increased to compensate for decreased nutrient intake so that meat yield is not limited.

Sensitivity analysis was conducted to evaluate the effect of changing diet costs and meat prices on gross feeding margin under each of the diet formulations. This analysis consisted of varying diet cost from 80 to 120% of the base diet cost and varying meat price from 70 to 130% of the base meat price. Ranges of diet costs and meat prices investigated in the sensitivity analysis are similar to the typical variability in these economic variables observed over the course of a year for a broiler company. This analysis reveals that several combinations of input and output values will affect the economic outcome of any of the diet formulations being considered here.

In exp. 1, gross feed margin per bird was higher with the diet formulated to 3,220 kcal/ kg than with any of the other formulations investigated in that experiment. No economic benefit was realized by increasing dietary AME beyond 3,220 kcal/kg with changing diet and meat prices. In exp. 2, increasing dietary AME from 3,220 to 3,310 kcal/kg decreased gross feeding margin. However, increasing CP, Lys, and TSAA by 4% in the 3,330 kcal/kg formulation led to a higher gross feeding margin across the range of diet and meat prices considered here. At base diet and meat prices, feeding additional CP, Lys, and TSAA increased gross feeding margin by $0.083 per bird compared with birds fed the diet containing 3,220 kcal/kg. This translates to an $83,000 increase in revenue for a plant processing 1 million broilers/wk. Increasing dietary AME beyond 3,220 kcal/kg adversely affected breast meat yield; gross feeding margin thus being negatively affected. In exp. 2, the diet containing 3,310 kcal/kg supplemented with additional CP, Lys, and TSAA rectified the reduction in breast meat yield. As a result, the increase in gross margin was probably due to the reduction in feed consumption.

	CONCLUSIONS AND APPLICATIONS

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

 

1. Gradient increases of dietary AME concentration decreased feed consumption and improved feed conversion from 30 to 59 d.
   
2. Heavy broilers have the ability to adjust to similar AME intake when provided diets varying in ME within concentrations used in commercial practice.
   
3. The reduced amino acid intake associated with the diets formulated to high dietary AME decreased breast meat yield in exp. 1, but increasing CP and amino acids by 4% rectified the reduction in breast meat yield with the high AME diets in exp. 2.
   
4. In general, supplementing a high AME diet (3,310 kcal/kg) with additional CP and amino acids resulted in greater gross feeding margins with broilers exposed to moderate temperatures. Conversely, providing broilers with a diet formulated to 3,220 kcal/kg optimized gross feeding margin with low ambient temperatures.
   

	FOOTNOTES

 
¹ Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA.

	REFERENCES AND NOTES

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

 

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16. Temperature set points were 22.0°C at 30 d, 21.0°C at 36 d, 19.0°C at 46 d, 18.5°C at 50 d, and 18.0°C at 56 d. Actual temperatures were: 21.9 ± 0.9°C from 30 to 35 d, 20.8 ± 1.2°C from 46 to 49 d, 20.0 ± 0.9°C from 50 to 55 d, and 18.7 ± 2.2°C from 56 to 59 d.
17. Temperature was set as 24°C from 30 to 59 d of age. Actual temperatures were: 25.0 ± 0.8°C from 30 to 59 d.
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20. SAS Institute. 2004. SAS User’s Guide. Statistics. Version 9.1 Edition. SAS Institute, Inc., Cary, NC.
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