J. Appl. Poult. Res.
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J APPL POULT RES 2009. 18:54-65. doi:10.3382/japr.2008-00053
© 2009 Poultry Science Association
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Research Reports

The effect of genotype, choice feeding, and season on organically reared broilers fed diets devoid of synthetic methionine

A. L. Rack, K. G. S. Lilly, K. R. Beaman, C. K. Gehring and J. S Moritz1

Division of Animal and Nutritional Sciences, West Virginia University, Morgantown 26506-2008

1 Corresponding author: Joe.Moritz{at}mail.wvu.edu


   SUMMARY
TOP
 SUMMARY
DESCRIPTION OF THE PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
In response to the impending ban on synthetic methionine in organic poultry diets, researchers have focused on finding alternative strategies to supply this amino acid. The objectives of this study were to assess performance and carcass characteristics of broilers fed diets devoid of synthetic methionine by using 1) a slow-growing and a fast-growing genotype, 2) choice-feeding management (supplying grain and a complementary premix in 2 separate feeders), and 3) pasture access and seasonal variation. Inclusion of fish meal and high percentages of soybean meal enabled the specific genotype methionine requirement to be met. All diets were certified organic. The experiment was conducted during the grower and finisher phases in 4 different seasons: late fall, spring, summer, and early fall. Pasture access was assessed either by housing broilers on the West Virginia University Organic farm and giving them outdoor access, or by housing broilers on the West Virginia University Animal Sciences farm and giving them no outdoor access. Fast-growing genotypes were superior in performance and carcass characteristics compared with slow-growing genotypes, and choice-feeding management did not improve performance or carcass characteristics. Pasture access tended to have no effect on slow-growing broilers and decreased the performance of fast-growing broilers. Performance was decreased in late fall, likely because of cold ambient temperatures.

Key Words: methionine • broiler • organic


   DESCRIPTION OF THE PROBLEM
TOP
 SUMMARY
 DESCRIPTION OF THE PROBLEM
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Methionine is an essential amino acid, used as a methyl donor to facilitate methyl transfer reactions in the body, and is also the major precursor of keratin [1]. When poultry are consuming a corn- and soy-based diet, Met is the first limiting amino acid [2]. In conventional poultry production, least-cost diet formulation incorporates synthetic DL-Met to obtain optimal bird growth. The supplementation of this ingredient has shown a consistent effect in increasing bird performance and meat yield compared with no supplementation. In broilers especially, an increase in weight gain and decrease in FCR have been reported with synthetic DL-Met use in diets [3, 4].

Despite standard use in conventional poultry production, a growing movement exists to have synthetic DL-Met prohibited from diets fed to organic poultry. Currently, the National Organic Standards Board plans to ban synthetic DL-Met from use in organic poultry diets by October 2010. The National Organic Standards Board made this recommendation because evidence exists that DL-Met is necessary only for obtaining maximal growth, rather than being needed for bird health or well-being [5]. In addition, the methods used to produce this ingredient have come into question. Methionine is commercially synthesized by condensing acrolein and methyl mercapton. The resulting compound is reacted with ammonia and hydrogen cyanide to form a racemic mixture of the D and L isomers of Met that is effectively 100% pure [6]. All the above chemicals are toxic, flammable, and explosive to varying degrees, and several have been classified as ecological hazards as well. Thus, the use of this synthetic ingredient does not fit with the principles espoused by organic or sustainable production.

Past research has shown that there are several alternative methods of replacing synthetic DL-Met in organic diets without detrimentally affecting broiler production. One way is simply to remove it from the grower and finisher diets and instead supply the birds with adequate pasture [7]. However, an "adequate" pasture area of 27 ft2/bird (8.23 m2/bird) has been argued as being impractical for the commercial organic industry. Furthermore, pasture quality and digestibility changes from season to season [7–9], so the forages that supplement the diet are not consistent throughout the year.

Recent research has focused on using slower growing poultry genotypes in organic production, because preliminary data showed that this may result in reduced Met requirements [10]. Slow-growing broilers are classified as reaching market weight in 12 wk compared with the typical 6- to 7-wk production cycle for conventional broilers. Phenotypically, these birds have elongated bodies, smaller breast sizes, and larger legs than a conventional commercial broiler [11]. The longer grow-out cycle of the slow-growing birds reduces susceptibility to typical broiler ailments such as tibial dyschondroplasia and ascites [12]. In addition, their inclination to forage makes them well suited to the principles of organic production.

Another possible alternative to synthetic DL-Met is to use alternative feedstuffs. Supplementing corn gluten meal or fish meal in the diet, both naturally high in available Met, can raise the Met in feed to amounts comparable to a diet that includes synthetic DL-Met. However, availability of these ingredients, especially in the organic form, remains problematic. In addition, a concern with organic fish meal is lipid oxidation. Organically certified fish meal cannot contain the antioxidant ethyoxyquin, and must rely on the addition of natural preservative ingredients [13]. As a result, feeds using organic fish meal may spoil more rapidly. In addition, using fish meal at high inclusion amounts may cause palatability issues, both in the feed presented to the birds and in the meat harvested from those birds [14].

A fourth possibility to alleviate the need for synthetic DL-Met is to use choice-feeding management. Choice feeding, or self-selection, is a strategy that provides the birds with 2 or more feed rations and allows choice of intake. Unlike providing 1 complete diet, the use of choice feeding allows the bird to consume only those nutrients required at any point in time, and may decrease the production cost if birds improve their FCR [15, 16]. Allowing birds the choice between corn and a complementary concentrate may enable them to balance changing Met requirements. Past research has shown that broilers are able to choose between a Met-adequate and a Met-deficient diet and eat a combination better suited to their requirement [17, 18]. Utilizing choice feeding may not only remove the need for synthetic DL-Met, but may also help reduce feed costs. Choice-fed birds tend to have lower feed consumption than birds on a complete diet [15], which can be especially beneficial to organic producers because organic feed may cost 3 times as much as conventional feeds [19].

Further research addressing problems associated with removal of synthetic DL-Met from broiler diets is needed. Therefore, the objectives of this study were to assess the performance and carcass characteristics of broilers fed diets devoid of synthetic Met by using 1) a slow-growing and a fast-growing genotype, 2) choice-feeding management, and 3) pasture access and seasonal variation.


   MATERIALS AND METHODS
TOP
 SUMMARY
 DESCRIPTION OF THE PROBLEM
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
One-day-old slow-growing (Gourmet Black) broilers (n = 275) [20] and 1-d-old fast-growing (Cobb 500 x Cobb 500) broilers (n = 275) [21] were obtained from commercial hatcheries. In accordance with previous research, the date that birds were acquired was staggered so that all broilers reached market weight on the same date based on the anticipated growth rates [22]. Starter diets were formulated to be genotype specific (Table 1Go). Broilers were brooded in pens in a negative-pressure house for 28 d (Gourmet birds) or 21 d (Cobb birds). Each pen contained pine shavings, nipple drinkers [23], and a feed pan and hopper [24]. Brooding temperature was initially set to 35°C (95°F) and was gradually reduced over 3 wk to acclimate chicks to outdoor temperatures. Continuous lighting was gradually reduced to match daylight hours for the appropriate seasonal study period. Broilers were fed a certified organic mash starter diet that contained no synthetic Met from hatch onward (Table 1Go). Feed and water were provided ad libitum.


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  		Table 1. Diet formulations and descriptions of nutrient content
 
At the end of the starter period (21 d for Cobb birds or 28 d for Gourmet birds), broilers (n = 150) of each genotype were blocked by weight and transported to the West Virginia University Organic farm or kept where chicks were brooded at the conventional farm for further experimentation. Birds on the Organic farm were placed in one of five 3.05 x 3.05 m (10 x 10 ft) houses. Each house was divided into four 1.52 x 1.52 m (5 x 5 ft) pens. Broilers were allocated 15 per pen, for an indoor stocking density of 0.46 m2 (1.5 ft2) per bird. Each pen was equipped with nipple drinkers [23], 2 feed pans and hoppers [25], and 2 doors for outside access. Each door permitted access to a fenced pasture of 6.40 x 7.62 m (21 x 25 ft). Pastures were equipped with a 5-gal water fount [24] and a tarp to provide partial shading of the pasture. Broilers were rotated weekly to an adjacent, identical pasture to ensure fresh and clean forage access. Broilers were given outdoor access during daylight hours and were free to move in and out of the houses. Predation was addressed by locking birds inside houses at night, and by surrounding the 5-house production system with a solar-powered electric fence [26]. All treatments were replicated indoors at the conventional farm, in 5 blocks of 4 pens. These birds were held at an identical indoor stocking density to assess the effect of pasture access on broiler performance.

The experimental design was a genotype x feeding management x season factorial arrangement of treatments within a split plot (access x no access) and was implemented by using a randomized complete block design. Genotype and choice feeding each provided 2 levels, resulting in 4 treatments: Gourmet choice, Gourmet no choice, Cobb choice, and Cobb no choice. All treatments were allocated in each of the 4 study periods: late fall (October 5 to November 29, 2006), spring (April 12 to June 7, 2007), summer (June 14 to August 9, 2007), and early fall (August 23 to October 18, 2007). The blocking criterion was organic house location or adjacent pen location, thus resulting in 5 treatment replications per treatment indoors and outdoors. All grower and finisher diets were certified organic, contained no synthetic Met, and were fed as mash (Table 1Go). The no-choice treatments consisted of complete diets placed in both feeders. The choice treatments used the same diet formulation; however, to establish choice feeding, ground corn was placed in 1 feeder and the remaining ingredients were mixed and placed in the other feeder. Feeders were equidistant from drinkers and randomly rotated. Gourmet birds were fed a grower diet from 28 to 56 d and a finisher diet from 56 to 83 d. Cobb birds were fed a grower diet from 21 to 38 d and a finisher diet from 38 to 54 d. Diets were formulated to be genotype specific (Table 1Go).

Pens of broilers were weighed at the end of the starter, grower, and finisher phases. Average daily gain (ADG) was calculated for each period, and FCR was determined by dividing total feed consumed by the sum of pen live weight gain plus pen mortality weight. The Met conversion ratio (MCR) was determined by dividing the grams of analyzed Met consumed per pen by the sum of kilograms of live weight gain per pen and kilograms of mortality weight. At the end of the finisher period, a cockerel and a pullet from each pen that were representative of the average pen weight for each sex (within ±0.1 kg for Cobb birds and ±0.2 kg for Gourmet birds) were selected and processed at the West Virginia University pilot processing plant. A wider range was needed for Gourmet birds because they had a greater variation in individual weights within a pen than did Cobb birds.

Boneless, skinless hot breast (HBP), gizzard (GP), and abdominal fat (FPP) were collected from these representative broilers. The breasts were deboned by hand, weighed, and chilled for 24 h on ice. The weight after chilling was divided by the hot weight and was considered the chill loss. All right breasts were then separated and cooked on raised wire racks to an internal temperature of 74°C. The weight after cooking was divided by the weight after chilling and was considered the cook loss.

Statistical Analysis
A genotype x feeding management x season factorial split plot design was used to explore the main effects and interactions of all treatments on performance and carcass characteristics. Pasture access was considered the main plot. Treatments were blocked by organic house and by adjacent pen location for broilers with and without pasture access, respectively. A male:female ratio was used as a covariate for performance and processing data. Single degree-of-freedom contrasts were conducted and used to explain significant interactions (data not shown). All statistical analyses were performed by using the GLM procedure of SAS Institute [27], and {alpha} was designated as 0.05. Data are presented in histograms with SE bars to represent the error associated with the mean of each treatment.


   RESULTS AND DISCUSSION
TOP
 SUMMARY
 DESCRIPTION OF THE PROBLEM
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS AND APPLICATIONS
 REFERENCES AND NOTES
 
Environmental conditions in the spring and early fall (2007) study periods were similar (Table 2Go), especially mean and maximum temperatures. The summer (2007) period did not contribute to excessive heat stress. However, the late fall (2006) study period coincided with much cooler temperatures, which likely influenced broiler performance.


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  		Table 2. Outside environmental conditions of the various seasons in which the 4 studies were conducted1
 
The final BW (data not shown) of the majority of birds was 2.0 kg or greater. Mortality (data not shown) was low, below 5%. Birds were not fed a coccidiostat or administered a coccidiosis vaccine. The incidence of tibial dischondroplasia was low and was confined to the Cobb birds.

Numerous main effect interactions were observed among performance variables. Average daily gain and MCR were affected by an access x breed x season interaction (P = 0.02 and 0.01, respectively). Cobb birds had a greater ADG than did Gourmet birds, and pasture access significantly decreased Cobb ADG (Figure 1Go). Gourmet ADG was not affected by pasture access. Late fall (2006) ADG was less for Gourmet birds but greater for Cobb birds compared with the 2007 seasons. Cobb birds had a significantly smaller MCR than did Gourmet birds (Figure 2Go), which agrees with prior research [7]. Cobb MCR was improved without pasture, whereas Gourmet MCR was not affected by pasture access. The summer study period also showed a significant decrease in MCR for Cobb birds that was not displayed by Gourmet birds compared with other study periods. These results were expected for Gourmet birds. Gourmet birds have been shown to take an additional 4 wk to reach the same market weight as Cobb birds [20]; therefore, they tend to have less ADG and greater MCR. However, the lack of a negative response when given access to pasture suggests that the Gourmet birds may be better suited to outdoor production.


Figure 1
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  		Figure 1. Total average daily gain of broilers for combined grower and finisher periods.

 

Figure 2
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  		Figure 2. Methionine conversion ratio for combined grower and finisher phases. Methionine conversion ratio was calculated by dividing the grams of analyzed Met consumed per pen by the sum of kilograms of live weight gain per pen and kilograms of mortality weight.

 
An access x feeding x season interaction was also observed for ADG (P ≤ 0.05). Broilers performed better without pasture than with pasture access. No-choice management improved performance over choice management. Environmental variations among study periods may have affected forage quality [8, 9], and thus feeding behavior, and may have contributed to the interaction. The differences between choice and no-choice management and between pasture and no pasture were significant in all study periods, but were most pronounced in fall study periods. Pasture likely differed in quality in early and late fall compared with spring and summer. This factor may have contributed to increased ADG in the latter periods. Choice management resulted in intakes that varied from the formulated ratio (data not shown) and contributed to decreased performance.

A breed x feeding x season interaction was observed for FCR (P = 0.03; Figure 3Go). Cobb birds demonstrated improved FCR compared with Gourmet birds, and no-choice management was better than choice management; both effects have been observed in previous research [28–30]. The 2007 study periods displayed better conversions than the 2006 study period, likely because of ambient temperature differences.


Figure 3
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  		Figure 3. Feed conversion ratio for combined grower and finisher phases. Feed conversion ratio was calculated by dividing total feed consumed by the sum of pen live weight gain plus pen mortality weight.

 
Compared with performance variables, similar patterns were observed for processing and carcass characteristics. Breed x feeding and access x breed interactions were observed for HBP (P = 0.02 and 0.03, respectively; Figure 4Go). No-choice diets resulted in greater breast percentages for both breeds, although the difference was greater in Cobb birds. Cobb birds consistently had a greater breast percentage than did Gourmet birds. This was expected, because one of the phenotypic characteristics of slow-growing broilers is a smaller, narrower breast [11]. Cobb HBP was greatest when birds were allowed no pasture access, whereas Gourmet birds had greater HBP with access.


Figure 4
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  		Figure 4. Boneless, skinless hot breast weights expressed as a percentage of hot carcass weight.

 
Both FPP (Figure 5Go) and GP (data not shown) demonstrated an access x feeding x season interaction (P = 0.04 and P = 0.001, respectively). Allowing pasture access decreased FPP, most likely because of increased activity of the birds. Greater FPP occurred in the warm study periods, suggesting that birds in the cold season were expending more maintenance energy than their warm season counterparts, and that they may have been subjected to cold stress. This difference was more pronounced for birds with pasture access than those without. Implementing choice management increased the FPP above that of no-choice management, except for spring no-choice management with pasture access. Further analysis demonstrated that birds in the choice management treatment tended to consume above the formulated corn:premix ratio (data not shown). Increased intake of corn would have increased energy intake above the requirement and may be associated with increased fat deposition. This is similar to results found in prior choice-feeding research [28].


Figure 5
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  		Figure 5. Abdominal fat pad weights expressed as a percentage of hot carcass weight.

 
Gizzard percentage (data not shown) also had an access x breed x season interaction (P = 0.03). Choice management resulted in greater gizzard weight than no-choice management, with the exception of no choice with pasture access. Gourmet birds also showed larger GP, likely because of a genotypic influence on foraging behavior; slow-growing broilers tend to forage and show more exploratory behavior than fast-growing industry broilers [12, 28–30]. Birds reared with pasture access also had larger gizzards, most likely because of increased opportunity for foraging. Finally, the summer and spring study periods showed reduced gizzard weights, with a sharp increase in early fall. This probably is a reflection of variations in forage quality. Late-season forages tend to be less digestible [8] and would have triggered greater gizzard activity.

There were no significant main effects or interactions for breast chill loss (data not shown). Cook loss, however, showed an access x breed x feeding interaction (P = 0.03; data not shown). Gourmet birds had similar cook loss despite feeding management when given pasture access, but no-choice management was greater and choice management less with no access. Cobb birds generally had less cook loss than did Gourmet birds, with the exception of choice-managed Cobb birds on pasture. A similar cook loss was exhibited by Cobb birds with no pasture access, whereas on pasture, the cook loss of choice-managed Cobb birds increased and that of no-choice managed Cobb birds decreased. This may possibly be a genotypic difference arising from the rate of intramuscular fat deposition and from differences in energy intake between choice and no-choice birds.

The lack of synthetic Met did not detrimentally affect the performance of either Gourmet or Cobb broilers in terms of unacceptable percentages of morbidity and mortality or decreased growth compared with industry-reared broilers. Natural sources of Met provided in adequate amounts were shown to meet broiler requirements even without pasture access.

The idea that different breeds of broilers have different requirements for Met [31–33] seems supported by the data. This may be due either to a difference in the rate of feathering [34], or perhaps to a difference in the rate of Met conversion to Cys [31].

As expected, a pronounced effect of genotype was observed. Performance was markedly decreased in the Gourmet birds compared with Cobb birds. Part of the difference in performance may be attributed to the different behavior of Gourmet birds. As stated previously, Gourmet birds tend to show more foraging behavior, more exploratory behavior, and a greater activity level than did Cobb birds [28–30]. Thus, they spend more energy on these behaviors and perhaps less is available for growth. In addition, the slow-growing genotypes have not been under the same rigorous selection pressures as industry broilers. The speculation has been made that selection for increased growth rate and breast yield may have unintentionally caused a decrease in fitness and survivability [33].

Choice-feeding management appeared to have little or no positive impact on performance. As in prior research, birds overconsumed corn, leading to increased fat pads and decreased muscle accretion [28]. One theory on why this may occur is that broilers are not trying to maximize growth but to maximize their long-term survival [28]. Whether birds show this level of self-awareness and nutritional wisdom is debatable. Alternatively, the overconsumption of corn could simply be due to the high palatability of that ingredient. Growth is then limited by amino acid imbalances.

One note of interest is that in the finisher period of the late fall study period, corn was actually underconsumed by all birds (data not shown). This period was when the birds were subjected to the coldest temperatures. Increasing consumption of the protein fraction of the diet may have caused an increase in the total heat increment metabolized [35]. This would help the birds compensate for possible cold stress, and past research has demonstrated that birds change their feed selection patterns to compensate for heat stress [36].


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

  1. Synthetic Met is not needed for rearing organic broiler chickens to achieve market weight or to prevent unacceptable percentages of morbidity and mortality.
  2. Fast-growing genotypes outperformed slow-growing genotypes.
  3. Choice-feeding management was not a viable feeding alternative, even in a small-farm setting.
  4. Pasture access tended to have no effect on slow-growing broilers and decreased the performance of fast-growing broilers.
  5. Slow-growing and fast-growing broilers performed better in warmer seasons.


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

  1. Nelson, D. L., and M. M. Cox. 2005. Lehninger Principles of Biochemistry. 4th ed. W. H. Freeman and Company, New York, NY.
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  3. Waldroup, P. W., C. J. Mabray, J. R. Blackman, P. J. Slagter, R. J. Short, Z. B. Johnson, and G. M. Pesti. 1981. Effectiveness of the free acid of methionine hydroxy analogue as a methionine supplement in broiler diets. Poult. Sci. 60:438–443.[Web of Science][Medline]
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