Dietary crude protein is known to play a major role in maximizing growth performance. However, it is one of the main factors affecting the cost of Pekin duck diets [1,2]. High protein poultry diets are not only expensive but also cause environmental problems owing to ammonia emissions and nitrogen excretion . However, adverse environmental impacts can be alleviated simply by decreasing the level of protein in poultry diets [4–8].
Despite the importance of dietary crude protein level on the growth performance of birds, only a couple of studies have demonstrated that feeding birds with a higher dietary crude protein improved their average daily gain and feed efficiency [9–11]. However, it is often reported that excessive intake of dietary crude protein (i.e., over 22.5%) deteriorates growth performance in birds [12,13]. Conversely, birds fed insufficient dietary crude protein (i.e., under 20.0%), were shown to have compromised growth performance due to amino acid imbalance .
The National Research Council (NRC) recommended a 16% crude protein level in Pekin duck diets for 14–49 days . However, this study was conducted many years ago and is, therefore, no longer applicable to the formulation of modern diets. It has been reported that feeding 19% crude protein level provided for 15–35 days is suitable for maximizing growth performance and carcass traits without wasting crude protein . However, limited data are available for recommendations for optimal crude protein levels in White Pekin duck diets 21 days after hatching. Therefore, the objective of this study was to examine crude protein level requirements in White Pekin duck diets 21 days after hatching by evaluating growth performance and carcass traits.
MATERIALS AND METHODS
An experiment was conducted using 432 males White Pekin ducklings in a completely randomized design with six levels of crude protein (n = 6 replicate pens per treatment and 12 ducklings per pen). Ducklings were fed their respective experimental diets from the d one to twenty-one.
One-d-old male White Pekin ducklings were obtained from a local hatchery (Jang Sung duck farm, Jangseong, Jeonnam, Korea). On the same d, ducklings were weighed individually and randomly allocated to one of six dietary treatments with varying levels of crude protein content (15%, 17%, 19%, 21%, 23%, and 25% respectively). Twelve birds were housed in each pen (1.7 × 1.3 m2), with a mean BW of 55.8 ± 0.31 g (mean ± standard error of mean [SEM]). These ducklings were reared on floor pens littered with rice husk and each pen was equipped with 3 nipple drinkers and a feeder. Birds were offered the experimental diet on an ad libitum basis for the period of the study; the freshwater was available at all times, and lighting was continuous for 24 hours. The ambient temperature was maintained at 32°C from days one to three, and then gradually decreased to 25°C until the ducklings were 21 days of age. Six dietary treatments were formulated to contain a level of crude protein content from 15% to 25% in a 2.0% scale (Table 1). Diets were iso-caloric and formulated to meet or exceed NRC  specifications for ducklings 21 days of age. Crystalline amino acids (lysine, methionine, isoleucine, valine, arginine, leucine) also were added to the diet to meet or exceed dietary amino acid requirements in diets. All experimental diets were prepared as mash form.
1) Supplied per kilogram of total diets: Fe (FeSO4 · H2O), 80 mg; Zn (ZnSO4 · H2O), 80 mg; Mn (MnSO4 · H2O) 80 mg; Co (CoSO4 · H2O) 0.5 mg; Cu (CuSO4 · H2O) 10 mg; Se (Na2SeO3) 0.2 mg; I, (Ca(IO3) · 2H2O) 0.9 mg; vitamin A, 24,000 IU; vitamin D3, 6,000 IU; vitamin E, 30 IU; vitamin K, 4 mg; thiamin, 4 mg; riboflavin, 12 mg; pyridoxine, 4 mg; folacine, 2 mg; biotin, 0.03 mg; vitamin B8, 0.06 mg; niacin, 90 mg; pantothenic acid, 30 mg.
2) The values are calculated according to the values of feedstuffs in NRC .
Body weight (BW) was recorded at the beginning and on d 7, 14, and 21 of the experimental periods. Pen-based feed consumption was recorded together with BW to calculate average daily gain (ADG), average daily feed intake (ADFI), and feed conversion ratio (FCR).
On d 21, two ducklings (closest to the median BW) were selected from each pen. Individual live BW of the selected birds was recorded and euthanized via cervical dislocation for sample collection after bleeding. Blood samples were collected from the brachial vein into vacutainer tubes coated with lithium heparin (BD Vacutainer, BD SSTTM, Franklin Lakes, NJ, USA). Collected blood samples were quickly transferred to a laboratory for plasma separation. After evisceration, empty bodies were weighed. Drumsticks (skinless) and breast meat were removed from carcasses and weighed. The empty BW, drumstick, and breast meat weight were expressed as proportions relative to slaughter live BW.
Collected blood samples were centrifuged (1248R, GYROZEN, Gimpo, Korea) at 3,000×g for 10 min at 4°C. Plasma samples were separated and stored at −80°C before analysis that the concentrations of total protein (TP), blood urea nitrogen (BUN) were determined using commercial kits (Asan Pharmaceutical, Seoul, Korea) for an automatic biochemical blood analyzer (Model HITACHI 7180 chemistry analyzer, HITACHI, Tokyo, Japan).
Data were analyzed as a completely randomized design, using the general linear model procedure of ANOVA of SPSS software version 24 (IBM Corp, 2016), with a pen used as the experimental unit for growth performance data. Data from selected individual birds for carcass traits and blood metabolites were pooled to get an average value per pen before statistical analysis. When dietary treatment was significant (p < 0.05), means were separated using Tukey’s multiple range test procedures of SPSS software version 24. To determine the optimum crude protein level, linear-plateau and quadratic-plateau regression analysis were conducted using a Nutritional Response Model (Version1.1; ) as described previously by .
Health and growth performance were good for all birds over the 21 days of the experiment. BW, ADG, and FCR were higher in White Pekin ducks fed the higher crude protein diet (p < 0.05) on days 7 and 14 than those in White Pekin ducks fed the lower crude protein diet. However, compared to the White Pekin ducks fed higher crude protein diets from days 1 to 21, the ducks fed the diet containing under 21% crude protein exhibited improved BW, ADG, and FCR on d 21 (p < 0.05, Table 2). On days 7 and 14, compared to the ducks fed the lower crude protein diets, White Pekin ducks fed diets with less than 19% crude protein exhibited an increase in ADFI.
The level of crude protein required to improve growth performance from hatch to 21 d was determined using both linear-plateau and quadratic-plateau models (Figs. 1 and 2). The requirements for attaining maximum ADG were crude protein levels of 19.71% (linear-plateau model) and 21.55% (quadratic-plateau model). Synthetically, there suggested crude protein levels of 20.62% on ADG from hatch to d 21. The maximum proportion of FCR was attained in birds fed diets containing crude protein levels of 21.30% (linear-plateau model) and 25.19% (quadratic-plateau model). Based on both the linear-plateau model and quadratic-plateau model, the suggested crude protein requirement for improving the proportion of FCR was 23.25% fed from hatch to d 21.
The effect of dietary protein level on the voluntary protein intake of White Pekin ducks is shown in Table 3. Increased protein intake for 21 days after hatching was observed (p < 0.05) with increasing dietary protein levels in White Pekin ducks exposed to a higher protein feed.
No significant differences (p > 0.05) were observed in empty body weight, drumstick, and breast meat of ducks exposed on diverse levels of crude protein, on d 21 after hatching (Table 4).
|Item||Dietary protein content (%)||SEM||p-value|
|Empty body weight (%)||70.51||71.14||71.24||70.85||71.00||71.32||0.171||0.791|
|Breast meat (%)||6.33||6.50||6.86||7.34||6.36||7.22||0.154||0.235|
White Pekin ducks exposed to a higher protein diet, showed (p < 0.05) increased blood urea nitrogen and total protein levels on d 21 after hatching (Table 5).
|Item||Dietary protein content (%)||SEM||p-value|
|Blood urea nitrogen (mg/dL)||0.49c||0.51c||0.59bc||0.59bc||0.73ab||0.81a||0.026||0.001|
|Total protein (g/dL)||3.44c||3.45c||3.47c||3.51bc||3.71ab||3.86a||0.038||0.001|
In this study, the crude protein requirements for improved growth performance of White Pekin ducks, from hatch to d 21 were evaluated. Although a previous study on the crude protein requirements of White Pekin duck diet exists, it may not be completely applicable to the formulation of new diets due to the evolved genotype of White Pekin ducks. Baéza et al.  reported the crude protein requirements for improved FCR in White Pekin duck diet from d 1 to 21% to be 23.2%, which may not apply to diets with varying energy levels (i.e., up to 3,000 kcal/kg metabolizable energy) in feed formulation. Moreover, Jiang et al.  recommended crude protein requirements based on only two crude protein levels (17% and 21%). In addition, due to differences in protein digestibility between White Pekin ducks and broiler chickens, the crude protein requirement for broiler chickens cannot be adequately applied to White Pekin ducks , as White Pekin ducks have a higher basal endogenous amino acid loss than do broiler chickens. Therefore, it is important to evaluate the crude protein requirements for White Pekin ducks under various conditions.
Protein sources are the most important components of poultry diets after energy sources. Protein is essential in poultry diets because, it supplies amino acids for the growth of muscle and synthesis of egg protein. The synthesis of muscle requires, physiologically, 20 amino acids in poultry diets. Ten of these twenty amino acids cannot be synthesized, while the other ten can be synthesized slowly in the interior of a poultry body. Therefore, sufficient amino acids must be supplied in poultry diets . However, there are limitations in terms of the expense associated with the proportion of the protein source and the environmental contamination resulting from excessive protein diets.
Studies evaluating crude protein requirements have been conducted using a variety of methods (e.g., a statistical model for analysis) to ensure the accuracy of the estimated nutrient requirements [23,24]. Although the statistical model for analysis shows appropriate fit, its output cannot be regarded as the optimal level of nutrient requirement because it does not consider the physiological differences among the individuals in a population . In addition, a broken-line analysis is generally applied by underestimating nutrient requirements . To supplement the analysis of the statistical model method, both the linear-plateau model and the quadratic-plateau model methods have been used in recent studies on the estimation of nutrient requirements [26–28]. Consequently, the aforementioned methods (i.e., an average of the linear-plateau and quadratic-plateau models) were used to estimate the crude protein requirements of the White Pekin duck diet for 21 days after hatching, by evaluating the growth performance and carcass traits of the ducks.
In this study, when the level of protein was increased, BW, ADG, and FCR improved over the first 14 days, whereas data acquired on days 14–21 showed different results. The diet containing 21% crude protein elicited a greater improvement in BW, ADG, and FCR of the ducks from d 14–21 than a diet containing 25% crude protein. This result is probably due to higher concentrations of the ratio of amino acids as compared to the ratio before the diet experiments. Based on an average of both the linear-plateau and quadratic-plateau model analysis, the optimum requirements of crude protein for were 20.63% and 23.25%, for maximal ADG and FCR, respectively, for 21 days after hatching. Similarly, Baéza et al.  reported that a level of 21% crude protein in the diet could improve feed efficiency; however, crude protein levels higher than 21% may result in an increase in the nitrogen content of White Pekin duck feces. Jiang et al.  reported improved ADG in White Pekin ducks in response to the crude protein level of 20.89% in the diet, from d 1 to 21. Moreover, Xie et al.  observed that a diet containing 19.58% crude protein improved the BW, ADG, and feed efficiency of White Pekin ducks from d 1 to 19.
According to recent reports, significant differences were observed in the carcass traits exhibited by White pecking ducks in response to different crude protein levels. According to Baéza et al. , compared to that with a lower crude protein diet, a higher crude protein diet was observed to increase the breast weight of Pekin ducks, as measured on d 21. However, our results for the carcass trait were not affected by the crude protein requirements probably due to sex, metabolizable energy, and amino acid balance.