ARTICLES

 

The impact of dietary protein content and lighting programme on breast meat yield in broiler chickens

 

 

P. Sodella; Z. Rani^#; R.M. Gous

School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Pietermaritzburg, South Africa

 

 

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ABSTRACT

Effects of the interaction between lighting programme and dietary protein content on broiler performance and meat yield were investigated. The hypothesis was that low breast meat yield from birds managed under short day lengths could be improved by increasing dietary protein content. The treatments consisted of four lighting programmes and four dietary protein levels. They were applied to sexed broilers from one day old to 35 days old. Eight light-tight rooms each contained eight pens with 50 birds, sexes separate, in each pen. Each feed x sex treatment was replicated twice in each room, with the four lighting treatments also being replicated twice. Multiple regression analysis was used to measure responses to the three factors. There was no significant interaction between dietary protein content and lighting programme in feed intake, feed conversion efficiency, bodyweight gain, carcass chemical composition and breast meat yield. Breast meat yield was linearly related to the number of hours of light, the highest yield occurring on the longest day length. Food intake was the same on the shortest day length and the longest, yet breast meat yield was greater on the 23-hour light programme. The decreased breast meat yield in broilers given short day lengths was therefore not the consequence of a shortage of dietary protein, and this hypothesis therefore had to be rejected.

Keywords: breast meat, day length, dietary protein, feed conversion efficiency, feed intake

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Introduction

Lighting programmes and dietary protein content influence broiler performance and meat yield. Lewis et al. (2009) conducted an experiment in which a range of constant photoperiods was applied to broiler chickens up to 35 days old. All performance characteristics except breast meat yield (BMY) benefited from a 12L:12D (L: light; D: dark) programme. This was unfortunate because BMY is of paramount importance in broiler production owing to its high value and demand. This may not be an issue where live birds are sold, but in most Western markets breast meat is the primary source of income from poultry carcasses (Mehaffey et al., 2006). To ascertain whether BMY could be enhanced on a 12L:12D schedule by increasing the protein content of the feed supplied to the broilers on this lighting treatment, a follow-up trial was conducted by Mlaba et al. (2015). The rationale was that feed intake (FI) by broilers might be lower under short day length regimes than with longer day length, and thus the protein that was ingested not be sufficient ensure maximum BMY. However, the higher protein feeds did not enhance BMY in that trial. The question posed in that investigation was whether short day lengths per se resulted in reduced BMY. To evaluate this possibility, day length was reduced even further in the present investigation using intermittent lighting, which is known to influence the performance of broilers.

Broilers respond to light (Lewis & Morris, 2006), although reports on the effects of various treatments are often contradictory. Lighting treatments during the growing period may be continuous, of different length, increasing or decreasing, and intermittent (repeated short dark cycles over 24 hours) (Schwean-Lardner et al., 2012). Traditionally it was believed that continuous or near-continuous lighting schedules resulted in increased FI and growth rate. Rodrigues and Choct (2019) found that broilers reared under continuous lighting had increased performance (growth rate and FI) compared with broilers on 6-, 12-, and 18-hour day lengths. In another study, bodyweights (BW) of broilers exposed to 23 hours light were heavier than those on 12 hours when they were raised to 42 days old (Ingram et al., 2000). Brickett et al. (2007) reported that birds on 12 hours light were more feed efficient than those on longer day lengths, whereas Lewis and Gous (2007) found that broilers on eight hours light were more feed efficient than those on 16 hours. In an experiment by Classen (2004), short day lengths resulted in lower BW. The assumption was that darkness reduced the available feeding time. However, Lewis and Gous (2007) showed that BW was unaffected by day lengths greater than six hours. Feed intake was reduced when broilers were maintained on photoperiods less than12 hours, but at 35 days their BW was superior to those reared on day lengths greater than 16 hours. Broilers consumed feed during dark periods as long as these were longer than 8 hours and less than 18 hours (Lewis et al., 2009). The proportion of feed consumed during the dark period increased with time of exposure to darkness (Classen, 2004; Lewis & Gous, 2007; Schwean-Lardner, 2011). These studies showed that BW and FI depended on photoperiod.

The current study was designed to determine whether BMY in broilers reared from one day to 35 days old was related to photoperiod and whether a higher protein feed would increase it to a greater extent on short photoperiods than on those more than 12 hours.

 

Materials and Methods

Use of animals and approval for all experimental protocols was granted by the University of KwaZulu-Natal Animal Care Committee (reference AREC/061/018M). The trial was conducted at Ukulinga Research Farm, Pietermaritzburg. A total of 3200 one-day-old Ross broiler chicks were reared in 8 light-tight rooms, each room being populated with a total of 400 broilers, 50 chicks being placed in each of the 8 pens (measuring 1.8 x 3.07 m) per room. males and females were penned separately. The floor in all rooms was covered with 10 cm wood shavings. Initially, the room temperature was set at 32 °C. This was decreased gradually to 22 °C by 21 days and kept at this temperature until 35 days, when the trial was ended. In each room, two gas-fired spot-brooders were used to provide heat, each mounted 750 cm from the floor. Minimum ventilation was initially set at 10% and was increased at fixed intervals to reach 100% at 21 days.

Treatments consisted of four levels of dietary protein, four lighting programmes, and two sexes. Two basal starter and finisher feeds (Table 1) were formulated to contain 0.85 and 1.30 of broiler nutrient requirements recommended by Aviagen (2009). The chemical composition of these diets is presented in Table 2. These basal feeds were mixed by a commercial feed company and blended on the farm to produce two intermediate feeds forming a dilution series, with levels of 0.85, 1.00, 1.15, and 1.30 times these recommendations. Starter feeds were fed from one old to 21 days, then finisher feeds from 22 days to 35. The feeds were not pelleted. Chicks were initially fed with open trays (two per pen). However, after seven days two self-feeders were used. Water was supplied in chick fonts for the first seven days, then bell drinkers were used. These four lighting treatments were applied in the trial, namely 23L:1D, 18L:6D, 12L:12D, and 1L:3D repeated 6 times daily. These treatments were introduced after the first 24 hours, with continuous lighting being used initially.

Initial bodyweights (BWs) were measured on the day of arrival by weighing a sample of six boxes (600) of chicks of each sex, subtracting the weight of the boxes, and calculating the mean BW per sex. At 21 and 35 days, birds were bulk weighed per pen, from which their bodyweight gains (BWG) were calculated. Feed intake (FI) was calculated as the difference between the feed allocated and that remaining at the end of each phase, divided by the average number of birds in the pen during the period. Feed conversion efficiency (FCE) (g gain/kg feed) was calculated from average daily gain and FI.

At 35 days, two birds were randomly selected from each pen to measure their chemical and physical properties. Individual BW was measured before electrical stunning and exsanguination. When bleeding had stopped, birds were weighed to measure blood loss and were weighed again after plucking with a drum plucker. Breast meat (without bone), thigh, wing, and drum (with skin and bone) were dissected and weighed separately. The remainder of the body was also weighed, after which all portions were placed in a plastic bag, sealed, labeled and placed in a freezer. Frozen samples were passed twice through a mincing machine, after which approximately 80 g of the mince was sampled for further analysis. Samples were freeze-dried for seven days using Edwards freeze drier, then milled with Retch ultra-centrifugal mill. Freeze-dried samples were then analysed to determine crude protein (CP) with the LECO FP200 nitrogen analyser. For CP analysis, 0.2 g was sampled from the dried sample. Lipid content was determined from the water content of the carcass using the unpublished equation that was generated from broiler carcass data accumulated over many years:

The responses to lighting programme and dietary protein content and their interaction for both sexes were predicted using multiple linear regression with groups in Genstat (VSN International, 2016). This analysis applied both linear and quadratic terms, with their interactions, with only those showing significance (P <0.05) being used to describe the combined response. In addition, constant terms and regression coefficients were fitted for each sex to determine whether these differed significantly (P <0.05).

 

Results and Discussion

Mean BW, FI and FCE of sexed broilers subjected to four dietary protein levels and four lighting programmes are given in Table 3. There was no significant interaction between light and protein level for any of the three variables. The interaction term and non-significant second-order terms (P >0.05) were removed from the regression analysis and are not shown in the tables. Multiple regression coefficients relating these variables to the two treatments are given in Table 4. Bodyweight rose linearly as day length increased and curvilinearly to dietary protein content, with the lowest BW being on the lowest protein content. These terms were common to males and females. However, the constant term differed between the sexes, with males being heavier at 35 days than females. Feed intake responded curvilinearly to light, but dietary protein had no effect on this variable (t = 0.80). Over all treatments, males consumed more feed than females, but the responses to light and protein were the same. The goodness of fit (R²) was low (16.1%). Feed conversion efficiency declined curvilinearly as day length increased, and rose linearly with dietary protein content, with males and females exhibiting the same responses to light and protein, but with a higher overall FCE for males (higher constant term).

It was not expected that feed intakes would differ markedly between treatments, because broilers learn to feed in the dark on day lengths shorter than 18 hours (Lewis et al., 2009) However, intake increased with day length up to 18 hours and then decreased on 23 hours light, which was reported by Lewis et al. (2009), but differed from other reports (Downs et al., 2006; Lien et al., 2007), where intake was highest on 23 hours light. Buyse and Decuypere (1988) found that birds on 6 hours intermittent lighting consumed about 80 % of the daily feed intake consumed by the controls on 23.5 hours light. In the present trial, weight gain increased linearly with the number of hours of light, with the highest gain on 23 hours light. In previous trials the gains were the same on all day lengths (Lewis et al., 2009) or higher on 23 hours (Ingram et al., 2000; Rodrigues & Choct, 2019). As a consequence of the higher gain and lower intake, broilers on 23 hours light had the highest FCE of all lighting treatments. These levels of performance were contrary to the results of Lewis & Gous (2007) and Lewis et al. (2009). However, Aviagen (2009) suggested that performance was optimized at day lengths between 17 and 20 hours of light.

An increase in dietary protein content resulted in rises in FI, bodyweight gain and FCE, with the performance on the highest protein content (1.3 times requirements) being the same as on the next lower (1.15) level. This asymptotic response was expected and had been demonstrated (Mlaba et al., 2015). But as in Mlaba et al. (2015) there was no interaction between lighting and dietary protein with the response in FCE being the same under all lighting regimes.

The weights of physical parts of the broilers, anmely deboned breast, thigh, drum and wing are shown in Table 5. The multiple regression coefficients relating these weights to the two treatments are given in Table 6. No interaction between light and protein content was evident in any of these variables. Second-order terms that were not significant (P >0.05) were dropped from the analysis, but all linear terms are given in the table, together with the effect of sex on the constant term, which in all cases was significant, with male weights being heavier than those from females. As with body weight, FI and feed conversion, the physical parts of the broiler responded to light and to dietary protein. Day length was related linearly to both BMY and drum weight, with these weights being numerically lower on the 12 hours light treatment. Thigh and wing weights were unaffected by the length of the photoperiod. The range of day lengths used in this trial was greater than that used by Mlaba et al. (2015) (18 hours as opposed to 11), yet the responses were similar, indicating that broilers respond in the same way to intermittent lighting and to constant day lengths. This refuted the suggestion that reduced energy expenditure and stress under intermittent lighting (Onbasilar et al., 2007) might improve BMY in broilers (Skrbic et al., 2011; Yang et al., 2015). Breast meat yield increased linearly with hours of light and curvilinearly with dietary protein content, whereas thigh weight increased linearly with both light and protein content. Drum weight increased linearly with dietary protein content, but was unaffected by an increase in the number of hours of light. Similarly, dietary protein content had a greater influence on wing weight than day length, although the linear response was not quite significant (t = 0.055).

Broiler producers continually seek methods of improving profitability by increasing performance or by reducing the cost of production. The cost of electricity is such that considerable savings could be made by reducing the number of hours of light used in a broiler house, provided that performance was not affected adversely. Conventionally, a 23 L:1D programme was used in broiler houses on the assumption that birds required light to find the feeders and drinkers (Lewis & Morris, 2006). This changed with the European Union Council (2007) directive, enforceable from 30 June 2010, which stipulated that after seven days old broilers must be given a lighting regime that followed a 24-hour rhythm and had at least 6 hours total darkness, of which at least 4 hours must be uninterrupted. This led to a number of studies in which various day lengths were applied to ascertain their effect on the performance of commercial broilers. Among many trials, Lewis et al. (2009) demonstrated advantages in growth rate, feed efficiency, bone strength, and liveability when using 12-hour photoperiods, but this treatment reduced BMY significantly compared with a 23-hour lighting regime.

Breast meat yield is of paramount importance in broiler production. So it was deemed worthwhile to ascertain whether this problem could be overcome with higher dietary protein feed, since FI was lowered on a short day length, thus reducing the supply of dietary protein for body protein synthesis. Mlaba et al. (2015) confirmed the benefits of a 12-hour day length on performance, but did not improve BMY by feeding additional protein to broilers on 12 hours light. The current study was designed to determine whether BMY was related to photoperiod per se by introducing an intermittent lighting programme that provided just 6 hours light/day, and whether a higher protein feed would increase BMY to a greater extent on short photoperiods than on those longer than 12 hours. Because this was a response trial, in which the levels of the two factors were not independent, the trends or responses to the imposition of dietary protein level and day length were important, and not whether there were differences between treatment means (Morris, 1983; 1999).

Body water, protein and lipid contents (g/kg) of the sampled broilers at 35 days are given in Table 7. These three chemical components all responded linearly to dietary protein content, but not to the number of hours of light. There was no interaction between light and dietary protein content. Since the lipid contents were calculated from body water content, the lipid contents were affected in the same way as the water, except that the correlation was negative.

The following equations describe the response in body water (BW, g/kg), protein (BP, g/kg) and lipid (BL, g/kg) to the relative protein content of the diet (DPC) and sex (female = 0, male = 1):

Day length did not have any influence on the chemical composition of the broilers in this trial. This was contrary to reports by Apeldoorn et al. (1999), who used a 6-hour programme, and Yang et al. (2015), who used two intermittent lighting regimes (2L:2D and 4L:4D). In both cases fat content was low compared with the continuous lighting. Conversely, an intermittent programme of 1L:2D used by Ohtani & Leeson (2000) resulted in higher abdominal fat content compared with continuous lighting.

Dietary protein content had a greater effect on the growth of the physical parts than day length. Breast meat yield rose curvilinearly with an increase in protein content, whereas thigh, drum and wing weights all rose linearly, corresponding to increases in body protein and water contents. Danisman & Gous (2011; 2013) demonstrated that there were allometric relationships between the physical parts of the body and body protein content, with the weights of the physical parts increasing with body protein content. Because more lipid is deposited in the body on low protein feeds as opposed to high (Gous et al., 1990) the weights of the physical parts would be expected to increase on low protein feeds at a given body protein content, because of this increase in lipid content, thus increasing the constant term in the allometric regression. This was corroborated by Mlaba et al. (2009) because the body lipid content increased as dietary protein content decreased. This would account for the observation by Brickett et al. (2007) that birds fed low protein feeds had heavier wing and drum weights at 35 days as a result of the intramuscular fat that had been deposited.

 

Conclusion

The responses in feed intake, weight gain, carcass composition and in the weights of the physical parts of the broiler were greater over the range of dietary protein level fed than from the range of constant day lengths imposed on the birds. There were no interactions between dietary protein and lighting programme in any of the variables. Although the birds consumed less food with 6 hours of light compared to those having light for 12 and 18 hours, birds on the regime providing 23 hours of light consumed the same amount of food but grew faster and had heavier breasts than broilers on the 6-hour intermittent lighting programme. The decreased breast meat yield in broilers given short day lengths was not the consequence of a shortage of dietary protein, and thus this hypothesis was rejected.

 

Acknowledgements

The authors would like to express appreciation to the Moses Kotane Institute for financial support for the senior author, and to Ukulinga Research Farm senior technician Masefo Mokoma for ensuring that the trial was conducted successfully.

 

Authors' Contributions

PS was responsible for conducting the experiment, data collection, analysis of results, and write up. ZR was responsible for supervising and editing the written paper, ensuring that the paper followed the guidelines. RG oversaw project planning and statistical analysis of the results and assisted in editing the paper.

 

Conflicts of Interest Declaration

The authors state under confidence that there are no conflicts of interest with regard to the publication of the manuscript.

 

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Submitted 21 March 2021
Accepted 4 November 2021
Published 10 February 2022

 

 

# Corresponding author, email: Raniz@ukzn.ac.za