https://orcid.org/0000-0003-0386-5694
ABSTRACT
1. The study evaluated the effect of dietary metabolisable energy (ME) content and crude protein (CP) level on the growth performance and behaviour of ducks.
2. A total of 720, Cherry Valley ducks were allocated to 36 pens in groups of 20 birds. For the initial period, from 1 to 21 d age, six diets, containing a standard (SME), low (LME) and high (HME) ME of 12.14, 11.93 and 12.35 MJ/kg, and standard (SCP) or high (HCP) CP contents of 210 or 220 g/kg diet, respectively, were mixed. For the period from 22 to 42 d age, the diets contained ME of 12.98 (SME), 12.77 (LME), 13.19 (HME) MJ/kg and the levels of CP were 170 (SCP) or 180 (HCP) g/kg, respectively.
3. An ME by CP interaction was seen from 1 to 21 d age in ducks fed HME + HCP diet, which had greater weight gain than those fed LME + SCP (P < 0.05). Compared to LME + SCP, dietary HME decrease feeding but increased walking behaviour compared to LME + SCP and SME + SCP (P < 0.05). High CP in LME and SME diets increased drinking behaviour (P < 0.05), but there was no change in HME diet. Compared to LME, feeding HME reduced ground pecking (P < 0.05). Feeding HME reduced feeding behaviour (P < 0.05) from 22 to 42 d age. During the same period, standing behaviour was reduced in HCP + LME (P < 0.05). Drinking was reduced in LME + SCP compared to SME + HCP and HME + HCP (P < 0.05).
4. A diet formulated with HME and HCP is effective for enhancing growth performance of ducks aged 1–21 d and saving time for feeding or ground pecking, which may induce spending more time on other activities.
Introduction
Ducks are known to engage in more social contact, although in recent years, ducks have been reared under intensive housing systems. Therefore, growth performance and meeting behavioural needs is important in ensuring success in duck production. Growth performance is greatly influenced by dietary metabolisable energy (ME) and crude protein (CP) which plays a major role in feed costs (Zhao et al. Citation2007). While a higher dietary level of ME and CP is costly, lower levels may affect growth performance of ducks (Xia et al. Citation2019). However, high dietary ME content in broiler diets can cause deposition of excess abdominal and carcass fat, and low dietary CP has decreased meat yields and increased fat deposition (Tumova and Teimouri Citation2010). Nutritional factors affect poultry behaviour, which is key for understanding welfare. When chickens are deficient in nutritional requirements, they spend time less resting and more feed-searching, which may cause an increase of stress hormones (Aranibar et al. Citation2020; El-Sabrout et al. Citation2022). This can assist farmers in managing poultry by determining nutrient concentration and how it influences behaviour. Therefore, there is a need to identify which level of ME and CP would enhance growth performance and minimise duck unsuitable behaviours. Most researchers have focused on the performance (Dozier et al. Citation2006; Yang et al. Citation2022; Zeng et al. Citation2015), whereas little is known about duck behaviour with level of ME and CP. In recent years, it has been found that broiler chicks excrete more water due to higher protein intake. Increasing the level of dietary ME could improve the feed conversion ratio of broilers by reducing feed intake (Dozier et al. Citation2006). Lower dietary ME increases feeding time, resulting in a similar ME intake of laying hens (Wu et al. Citation2005). Broilers fed diets with lower ME and CP spent more time feeding and appeared calmer than those fed higher ME and CP (de Los Mozos Citation2017). However, the behaviour of ducks is different from chickens, because ducks are less aggressive and engage in more social contact, and these factors contribute to synchronised feed intake and increase comforting behaviours, activity and resting phases among flock mates (Dong et al. Citation2021). Most researchers to date have not evaluated the level of ME and CP among the different growth stages of ducks. The optimal dietary ME level for minimising behavioural issues while maintaining growth performance remains unknown. Therefore, the present experiment investigated the effect of dietary concentrations of ME and CP on the performance and behaviour of ducks.
Materials and methods
Ethical statement
All managements of ducks and experimental procedures were conducted in accordance with the Institutional Animal Care and Use Committee of National Institute of Animal Science in Republic of Korea.
Dietary treatments, birds and management
A total of 720, one-d-old mixed sex Cherry Valley ducks (56.54 ± 0.09 g) were allocated to one of three levels of ME × two levels of CP factorial treatments give six pen replicates (20 birds/replicate). The experimental period was divided into two phases; starter (1–21 d) and finisher (22–42 d). The ME contents were 11.93, 12.14, and 12.35MJ/kg for 1–21 d and 12.77, 12.98, and 13.19 MJ/kg for 22–42 d periods, respectively. The two CP contents were 210 and 220 g/kg for 1–21 d and 170 and 180 g/kg for 22–42 d periods, respectively ().
Table 1. Dietary composition of experimental diets of ducks aged 1–21 d.
Table 2. Dietary composition of experimental diets of ducks aged 22–42 d.
The ME in a diet formulated with the guidelines set forth in the Korean Feeding Standard for Poultry (Citation2022) recommendation was designated as standard ME (SME; 12.14 MJ/kg for 1–21 d and 12.98 MJ/kg for 22–42 d periods). The other dietary ME concentrations were formulated to provide low ME (LME; SME-0.21 MJ/kg) and high ME (HME; SME +0.21 MJ/kg). The CP was formulated according to the guidelines set forth in the Korean Feeding Standard for Poultry (Citation2022) recommendations, designated as standard CP (SCP; 210 g/kg for 1–21 d and 170 g/kg for 22–42 d). The other dietary CP concentration was formulated as high crude protein (HCP; SCP +10 g/kg). The chemical composition of the two experimental diets was determined as described by Abdulla et al. (Citation2021) and Watts et al. (Citation2021).
Ducks were bedded on rice husk, and stocking density (6 birds/m2) was equal in each pen. Supplementary heat was provided by hot water pipes (85 cm) under the floor. The room temperature was maintained at 32°C for the first week and then reduced by 2–3°C per week until it reached 20°C on 28 d, which was maintained until the end of the experiment. The mean relative humidity was 60–65% and was kept constant within this range throughout the experiment. Birds had ad libitum access to commercial pellets and water. When the temperature and humidity were exceeded, a fan was operated automatically and circulated fresh air inside the pen. To prevent any preferences for familiar odours interfering with the dietary treatments, each pen was cleaned daily. A light emitting diode (LED) were located 150 cm above from floor and held on luminous intensity (30 lux), luminous flux (600 ± 15 lm) and light colour (dayglow colour; 5,700 k). The ducks in each group received the same amount of light.
Growth performance
Body weight and feed intake were determined at weekly intervals. Weekly body weight gain was calculated by subtracting the weight recorded at the end of the previous week from the current weight of the birds. Feed conversion ratio (FCR) was calculated by dividing the feed intake with weight gain.
Behavioural observations
Video cameras (HCO-7010RA, Hanwha Techwin, Korea) were set to give a full view of each pen whereby the central arena of each pen was visible. Behaviour was recorded for the following activities: consumption (feeding and drinking), active (walking), inactive (standing and sitting), pecking (preening, ground pecking and peck objects) and comfort (wing flapping, wing stretching, tail wagging, body shaking and head shaking). A software program (Behaviour Observer Program, Noldus, The Netherlands), specifically designed for such use, estimated ducks’ behaviour based on movement as assessed through mean pixel count changes per frame of video. A duck was considered to be walking only when it was locomoting and not engaged in any other activity; short periods of immobility were recorded as standing or standing whilst not being engaged in any other activity. Eating and drinking were considered to have stopped as soon as the bird stood inactive, even if it was in front of a feeder or a drinker. A bird was considered to be ‘drinking’ when they stood in front of the drinker with a raised head and the beak was touching the drinker nipple. The observer noted ‘feeding’ when a duck ate from the feeder. It is difficult to distinguish among sitting, lying, resting, and sleeping behaviours; thus, these were jointly categorised as sitting. Other than at a feeder or drinker, attention to other objects was considered pecking. Preening was when plumage was manipulated with the beak. Ground pecking was recorded only when away from a feeder. Extending both wings out from the body simultaneously was considered flapping behaviour. Stretching included the head, wings and legs. Wagging was recorded when the tail moved rapidly from side to side.
These behaviours were recorded on video twice daily (1 h each in morning and afternoon) for 5 d/week. During the 42 d trial, video observations (both morning and afternoon) regarding feeding, drinking, sitting, walking, standing, preening, wing flapping, wing stretching, tail wagging, head shaking, body shaking, ground pecking, and the peck object and social interaction were recorded as time counts and expressed as counts per hour. The video was analysed at 5 min intervals (Sultana et al. Citation2013), and the duration of behaviour was recorded.
Data from 10 ducks/pen were averaged; therefore, each pen produced one data point for each behaviour both in the morning and afternoon. Because the data of each observation in a pen were linked, 10 birds per pen, 5 d/week for 6 weeks with two replicate pens were used to measure each variable. During the observation time of the day, 100 data units (10 ducks per pen × five observation days in a week × two replicate pens) were considered for each behaviour.
Statistical analysis
The experiment was conducted using a completely randomised design with a factorial structure. Data were analysed as 3 × 2 (three levels of ME and two levels of CP) arrangement of treatments by two-way analysis of variance using the general linear model (GLM) procedure in SAS (v. 9.1, Cary, NC, 2002). Results were considered significantly different if their P-values were <0.05.
Results
Performance
During the first 21 d (), interactions (P < 0.05) between ME and CP were observed for weight gain in birds fed HME+HCP which contained 12.35 MJ/kg ME and 220 g/kg CP. There were no interactions between ME and CP on either feed intake or feed conversion ratio of ducks.
Table 3. Effect of dietary concentrations of ME and CP on the performance of ducks.
When the growth period was extended further, from 22 to 42 d, the interactive effects between ME and CP on the performance of ducks were not shown. In addition, there were no significant differences on the performance of ducks in main effect both ME and CP.
Behaviour
The behaviour results are shown in . Ducks spent the majority of their time sitting, drinking and feeding, whereas tail wagging, stretching, and shaking were observed less frequently. Feeding, drinking, walking, preening and social interacting were performed in an age-dependent manner. Feed intake and activity of ducks fed dietary concentrations of ME and CP are shown in . There were interactive effects (P < 0.05) on feeding and drinking in ducks aged 1–21 d, whereby feeding behaviour was higher in birds fed LME+SCP, and drinking was higher in Group 2 (LME+HCP). When the rearing period was extended from 22 to 42 d of age, ducks spent significantly (P < 0.05) more time-consuming feed and water as seen by the interaction of ME and CP in the diets, which increased feeding behaviour in birds fed LME+SCP and LME+HCP and drinking behaviour in those fed SME+HCP and HME+SCP. More (P < 0.05) walking in young ducks (1–21 d age) was seen in those fed HME+SCP and HME+HCP. In addition, walking was ME dependent, as higher (P < 0.05) ME diets resulted in longer walking duration. However, there was no effect of diet on standing and sitting. While walking and sitting duration for 22–42 d old ducks showed no differences among the groups, standing behaviour was longer (P < 0.05) in those fed LME+SCP than in other groups.
Table 4. Effect of dietary concentrations of ME and CP on the consummatory and active behaviour (min/h) of ducks.
Table 5. Effect of dietary concentrations of ME and CP on the pecking behaviour (min/h) of ducks.
Table 6. Effect of dietary concentrations of ME and CP on the comfort behaviour (min/h) of ducks.
Preening behaviour and pecking at objects were not affected by any dietary interaction or affected by ME or CP during the experimental period (). Ground pecking increased significantly (P < 0.05) in birds fed LME+SCP and LME+HCP, with an interaction between ME and CP from 1 to 21 d old ducks, although the groups spent similar times pecking at 22–42 d of age. There was no difference for the peck object focus of ducks fed various levels of both ME and CP during the experimental period.
The comfort behaviour (wing flapping, wing stretching, tail wagging, body shaking, and head shaking) was not affected by different levels of ME and CP, and data is shown in .
Discussion
During the first 21 d, it was confirmed that weight gain was significantly increased in ducks due to the effect of feeding diets containing 12.35 MJ/kg ME and 220 g/kg CP. Similarly, Zeng et al. (Citation2015) reported interactive effects of dietary levels between ME (11.8, 12.8, 13.8 MJ/kg) and CP (150, 170, 190 g/kg) on weight gain and diet digestibility (dry matter, energy, and nitrogen) in ducks. It is well known that productive performance of ducks is influenced by ME:CP ratio, which has been demonstrated when surplus CP is fed, as the ME content should be increased concurrently (Liu et al. Citation2019; Zhao et al. Citation2007). To realise optimal performance for poultry, it is important to balance ME and CP, which demonstrates an interaction between ME and CP on the weight gain of ducks. In the current results, higher weight gain was seen for ducks aged 1–21 d fed a diet contained 12.35 MJ/kg ME compared to those receiving 11.93 and 12.14 MJ/kg ME. With respect to the weight gain of ducks fed at different ME levels, the findings were similar to those of Liu et al. (Citation2019), who reported higher growth rate, less feed intake and improved FCR when ducks were fed diets containing 13.19 MJ/kg ME compared to 11.93 MJ/kg ME during the starter period (15–40 d age). In another study, Xie et al. (Citation2010) used various ME levels (10.26, 10.88, 11.50, 12.14, 12.75 MJ/kg) in diets for ducks aged 1–21 d. The highest growth rate was achieved with 12.75 MJ/kg ME. Similarly, Mirza et al. (Citation2016) reported that high nutrient density was positively correlated to growth performance in turkeys. This was in line with the performance results from the current experiment, which revealed dietary ME concentrations had a significant effect on weight gain.
Duck behaviour is a factor influencing animal welfare levels (Cartoni Mancinelli et al. Citation2020; Makagon and Riber Citation2022), which can help farmers judge whether they are comfortable adapting to the farm environment and rearing conditions (Abdel-Hamid et al. Citation2020; Mi et al. Citation2020; Mohammed et al. Citation2019). Duck behaviour is influenced by both nutrients and concentrations in the diet (Abdel-Hamid and Abdelfattah Citation2020; Mahmoud et al. Citation2021). Previous studies have shown that ducks under restricted feeding management spent less time being active and drinking and more time eating and engaging in aggressive behaviours (Ahmed Citation2022). Lower nutritional status can lead to higher corticosterone concentrations by inducing gut stress mechanisms, which can make ducks more aggressive or hypersensitive (Bozakova et al. Citation2012; Fraley et al. Citation2013; Kraimi et al. Citation2019).
The current results showed that intake behaviour (feeding and drinking) was influenced by the ME and CP level in the diet. The ducks fed lower ME and CP (11.93 MJ/kg × 210 g/kg for 1–21 d and 12.77 MJ/kg × 170 g/kg for 22–42 d) spent more time feeding, leaving less time for activities and resulting in reduced walking and drinking activity. There appears to be few studies which investigated the behaviour of ducks regarding ME and CP level. Nevertheless, according to one relevant report (Leterrier et al. Citation2008), time spent eating was linearly affected by high ME and low CP in a diet and that sufficient ME and CP concentration increased gait score in broilers (Leterrier et al. Citation2008). This may have been due to feed intake decreasing in line with dietary ME and CP, which was in agreement with results from broiler chicks (De Los Mozos Citation2017; Prayitno et al. Citation1997).
Higher body weight requires more nutrient intake, which increases feeding motivation. The increase in drinking was caused by higher dietary CP level in the current trial. This was in agreement with research in broilers by El Sayed et al. (Citation2017), who reported that drinking was increased when feeding a diet with surplus CP level (210 g/kg) compared to normal CP levels (200 g/kg). This may have been related to protein digestion, which requires greater water intake to excrete excess nitrogen (Alleman and Leclercq Citation1997; Van Emous et al. Citation2019; Van Harn et al. Citation2019).
In the current trial, there were no main effects of interactions for dietary ME and CP contents on comfort behaviour (preening, wing flapping, wing stretching, tail wagging, head shaking, and body shaking). Similarly, Abdel-Hamid and Abdelfattah (Citation2020) showed that ducks fed varying dietary CP levels (140, 180, 220 g/kg) spent similar time on preening, leg stretching and head shaking. In another study, De Los Mozos et al. (Citation2017) reported that nutritional density (ME, CP, Ca, P, and amino acids) in a diet for broiler breeders did not influence preening, wing stretching and sand bathing.
In conclusion, there was an effect of dietary ME and CP levels on the weight gain of ducks at starter period, which was higher in group fed a diet with 12.35 MJ/kg ME and 220 g/kg CP. However, weight gain was not affected by ME and CP levels at finisher period. Ducks fed diets with a higher ME × CP (12.35 MJ/kg × 220 g/kg in the starter phase and 13.19 MJ/kg × 180 g/kg in the finisher periods) spent less time feeding or ground pecking and more time walking during starter and drinking during finishing periods. The results showed that a diet with sufficient ME and CP is effective is enhancing growth of ducks at an early age and may provide the opportunity for ducks time to spend other activities, which are beneficial to their welfare, rather than just focusing on eating.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Additional information
Funding
References
- Abdel-Hamid, S. E., and E. M. Abdelfattah. 2020. “Effect of Different Dietary Protein Levels on Some Behavioral Patterns and Productive Performance of Muscovy Duck.” Advance in Animal and Veterinary Science 8 (6): 661–667. https://doi.org/10.17582/journal.aavs/2020/8.6.661.667.
- Abdel-Hamid, S. E., A. S. Y. Saleem, M. I. Youssef, H. H. Mohammed, and A. I. Abdelaty. 2020. “Influence of Housing Systems on Duck Behavior and Welfare.” Journal of Advanced Veterinary and Animal Research 7 (3): 407–413. https://doi.org/10.5455/javar.2020.g435.
- Abdulla, J. M., S. P. Rose, A. M. Mackenzie, and V. R. Pirgozliev. 2021. “Variability of Amino Acid Digestibility in Different Field Bean Cultivars for Broilers.” British Poultry Science 62 (4): 596–600. https://doi.org/10.1080/00071668.2021.1891525.
- Ahmed, F. A. 2022. “Early Feed Restriction Can Affect the Behavior and Welfare of Mule Ducks.” PSM Veterinary Research 7 (1): 43–54.
- Alleman, F., and B. Leclercq. 1997. “Effect of dietary protein and environmental temperature on growth performance and water consumption of male broiler chickens.” British Poultry Science 38 (5): 607–610. https://doi.org/10.1080/00071669708418044.
- Aranibar, C. D., C. Chen, A. J. Davis, W. I. Daley, C. Dunkley, W. K. Kim, and J. L. Wilson. 2020. “Impact of an Alternate Feeding Program on Broiler Breeder Pullet Behavior, Performance, and Plasma Corticosterone.” Poultry Science 99 (2): 829–838. https://doi.org/10.1016/j.psj.2019.12.025.
- Bozakova, N. A., K. T. Stoyanchev, S. Popova-Ralcheva, N. V. Georgieva, V. T. Gerzilov, and E. B. Valkova. 2012. “Behavioral Study of Mule Ducks with Subclinical Muscular Dystrophy Under Ecological Comfort and Stress Conditions.” Bulgarian Journal of Agricultural Science 18 (4): 511–518.
- Cartoni Mancinelli, A., S. Mattioli, A. Dal Bosco, A. Aliberti, M. Guarino Amato, and C. Castellini. 2020. “Performance, Behavior, and Welfare Status of Six Different Organically Reared Poultry Genotypes.” Animals 10 (4): 550. https://doi.org/10.3390/ani10040550.
- de Los Mozos, J., A. I. García-Ruiz, L. A. den Hartog, and M. J. Villamide. 2017. “Growth Curve and Diet Density Affect Eating Motivation, Behavior, and Body Composition of Broiler Breeders During Rearing.” Poultry Science 96 (8): 2708–2717. https://doi.org/10.3382/ps/pex045.
- Dong, Y., D. M. Karcher, and M. A. Erasmus. 2021. “Self-And Conspecific-Directed Pecking Behavior of Commercial Pekin Ducks.” Applied Animal Behaviour Science 235:105223. https://doi.org/10.1016/j.applanim.2021.105223.
- Dozier, W. A., C. J. Price, M. T. Kidd, A. Corzo, J. Anderson, and S. L. Branton. 2006. “Growth Performance, Meat Yield, and Economic Responses of Broilers Fed Diets Varying in Metabolizable Energy from 30 to 59 Days of Age.” Journal of Applied Poultry Research 15 (3): 367–382. https://doi.org/10.1093/japr/15.3.367.
- el Sayed, R., D. Ibrahim, and E. N. Said. 2017. “Effect of Dietary Calorie and Protein Content on Performance, Behaviour, Expression of Some Growth-Related Genes and Economic of Broiler Chickens.” Zagazig Veterinary Journal 45 (4): 326–339. https://doi.org/10.21608/zvjz.2017.7863.
- el-Sabrout, K., A. El-Deek, S. Ahmad, M. Usman, M. R. T. Dantas, and J. B. F. Souza-Junior. 2022. “Lighting, Density, and Dietary Strategies to Improve Poultry Behavior, Health, and Production.” Journal of Animal Behaviour and Biometeorology 10 (1): 1–17. https://doi.org/10.31893/jabb.22012.
- Fraley, G. S., E. Coombs, E. Gerometta, S. Colton, P. J. Sharp, Q. Li, and I. J. Clarke. 2013. “Distribution and Sequence of Gonadotropin-Inhibitory Hormone and Its Potential Role as a Molecular Link Between Feeding and Reproductive Systems in the Pekin Duck (Anas platyrhynchos Domestica).” General & Comparative Endocrinology 184:103–110. https://doi.org/10.1016/j.ygcen.2012.11.026.
- Korean Feeding Standard for Poultry. 2022. Nutrient Requirement of Poultry. Wanju, Republic of Korea: National Institute of Animal Science, RDA.
- Kraimi, N., M. Dawkins, S. G. Gebhardt-Henrich, P. Velge, I. Rychlik, J. Volf, and C. Leterrier. 2019. “Influence of the Microbiota-Gut-Brain Axis on Behavior and Welfare in Farm Animals: A Review.” Physiology & Behavior 210:112658. https://doi.org/10.1016/j.physbeh.2019.112658.
- Leterrier, C., C. Vallée, P. Constantin, A. M. Chagneau, M. Lessire, P. Lescoat, and I. Bouvarel. 2008. “Sequential Feeding with Variations in Energy and Protein Levels Improves Gait Score in Meat-Type Chickens.” Animal 2 (11): 1658–1665. https://doi.org/10.1017/S1751731108002875.
- Liu, J. B., H. L. Yan, Y. Zhang, Y. D. Hu, and H. F. Zhang. 2019. “Effects of Dietary Energy and Protein Content and Lipid Source on Growth Performance and Carcass Traits in Pekin Ducks.” Poultry Science 98 (10): 4829–4837. https://doi.org/10.3382/ps/pez217.
- Mahmoud, U. T., M. A. Mahmoud, M. Abd-Elkareem, F. A. Ahmed, and N. S. Abou Khalil. 2021. “Prebiotics Reduce Feather Pecking Behavior, and Improve Trace Element Profile and Redox Balance in Mule Ducks.” Journal of Veterinary Behavior 43:28–38. https://doi.org/10.1016/j.jveb.2021.03.001.
- Makagon, M. M., and A. B. Riber. 2022. “Setting Research Driven Duck-Welfare Standards: A Systematic Review of Pekin Duck Welfare Research.” Poultry Science 101 (3): 101614. https://doi.org/10.1016/j.psj.2021.101614.
- Mi, J., H. Wang, X. Chen, K. Hartcher, Y. Wang, Y. Wu, and X. Liao. 2020. “Lack of Access to an Open Water Source for Bathing Inhibited the Development of the Preen Gland and Preening Behavior in Sanshui White Ducks.” Poultry Science 99 (11): 5214–5221. https://doi.org/10.1016/j.psj.2020.08.018.
- Mirza, M. W., V. Pirgozliev, S. P. Rose, and N. H. C. Sparks. 2016. “Dietary Modelling of Nutrient Densities: Effect and Response in Different Growing Phases on Growth Performance, Nutrient Digestibility, Litter Quality and Leg Health in Turkey Production.” Journal of World’s Poultry Research 6 (3): 161–190.
- Mohammed, H. H., A. I. Abdelaty, A. S. Y. Saleem, M. I. Youssef, and S. E. Abdel-Hamid. 2019. “Effect of Bedding Materials on Duck’s Welfare and Growth Performance.” Slovenian Veterinary Research 56 (22–Suppl): 149–156. https://doi.org/10.26873/SVR-752-2019.
- Prayitno, D. S., C. J. Phillips, and D. K. Stokes. 1997. “The Effects of Color and Intensity of Light on Behavior and Leg Disorders in Broiler Chickens.” Poultry Science 76 (12): 1674–1681. https://doi.org/10.1093/ps/76.12.1674.
- Sultana, S., M. R. Hassan, H. S. Choe, and K. S. Ryu. 2013. “Impact of Different Monochromatic LED Light Colours and Bird Age on the Behavioural Output and Fear Response in Ducks.” Italian Journal of Animal Science 12 (4): e94. https://doi.org/10.4081/ijas.2013.e94.
- Tumova, E., and A. Teimouri. 2010. “Fat Deposition in the Broiler Chicken: A Review.” Scientia Agriculturae Bohemia 41 (2): 121–128. http://www.sab.czu.cz.
- van Emous, R. A., A. Winkel, and A. J. A. Aarnink. 2019. “Effects of Dietary Crude Protein Levels on Ammonia Emission, Litter and Manure Composition, N Losses, and Water Intake in Broiler Breeders.” Poultry Science 98 (12): 6618–6625. https://doi.org/10.3382/ps/pez508.
- van Harn, J., M. A. Dijkslag, and M. M. van Krimpen. 2019. “Effect of Low Protein Diets Supplemented with Free Amino Acids on Growth Performance, Slaughter Yield, Litter Quality, and Footpad Lesions of Male Broilers.” Poultry Science 98 (10): 4868–4877. https://doi.org/10.3382/ps/pez229.
- Watts, E. S., S. P. Rose, A. M. Mackenzie, and V. R. Pirgozliev. 2021. “Investigations into the Chemical Composition and Nutritional Value of Single-Cultivar Rapeseed Meals for Broiler Chickens.” Archives of Animal Nutrition 75 (3): 209–221. https://doi.org/10.1080/1745039X.2021.1930455.
- Wu, G., M. M. Bryant, R. A. Voitle, and D. A. Roland Sr. 2005. “Effect of Dietary Energy on Performance and Egg Composition of Bovans White and Dekalb White Hens During Phase I.” Poultry Science 84 (10): 1610–1615. https://doi.org/10.1093/ps/84.10.1610.
- Xia, W. G., K. F. M. Abouelezz, A. M. Fouad, W. Chen, D. Ruan, S. Wang, and C. T. Zheng. 2019. “Productivity, Reproductive Performance, and Fat Deposition of Laying Duck Breeders in Response to Concentrations of Dietary Energy and Protein.” Poultry Science 98 (9): 3729–3738. https://doi.org/10.3382/ps/pez061.
- Xie, M., J. N. Zhao, S. S. Hou, and W. Huang. 2010. “The Apparent Metabolizable Energy Requirement of White Pekin Ducklings from Hatch to 3 Weeks of Age.” Animal Feed Science and Technology 157 (1–2): 95–98. https://doi.org/10.1016/j.anifeedsci.2010.01.011.
- Yang, Z., C. Xu, W. Wang, X. Xu, H. M. Yang, Z. Y. Wang, S. P. Rose, and V. Pirgozliev. 2022. “Dietary Amylose and Amylopectin Ratio Changes Starch Digestion and Intestinal Microbiota Diversity in Goslings.” British Poultry Science 63 (5): 691–700. https://doi.org/10.1080/00071668.2022.2079398.
- Zeng, Q. F., P. Cherry, A. Doster, R. Murdoch, O. Adeola, and T. J. Applegate. 2015. “Effect of Dietary Energy and Protein Content on Growth and Carcass Traits of Pekin Ducks.” Poultry Science 94 (3): 384–394. https://doi.org/10.3382/ps/peu069.
- Zhao, F., S. S. Hou, H. F. Zhang, and Z. Y. Zhang. 2007. “Effects of Dietary Metabolizable Energy and Crude Protein Content on the Activities of Digestive Enzymes in Jejunal Fluid of Peking Ducks.” Poultry Science 86 (8): 1690–1695. https://doi.org/10.1093/ps/86.8.1690.