The Growth Curves with Non-Linear Models of in Mixed-Sex of Caucasian Pheasants (Phasianus Colchicus Colchicus)

ABSTRACT

The Caucasian pheasant (Phasianus colchicus colchicus) is a Phasianidae family poultry species that is an important animal-based protein source. This study aimed to estimate the growth traits of body weight (BW) in pheasants using Logistic (L), Gompertz (G) and Von Bertalanffy (VB) models. A total of 1,765 data records of BW from hatching to 18 weeks of age were collected from the pheasant population in the Alternative Poultry Unit at Prof. Dr. Hümeyra Ozgen Research and Application Farm, Selҫuk University, Türkiye. The results showed that the coefficient of determination (R²) value in each model was 0.94. However, the lowest values for Akaike’s information criterion (AIC) and Bayesian information criterion (BIC) among models were found in the G model. In general, the pheasants had the weight of inflection (W_i) and time of inflection (t_i) of 535.11-592.00 g and 9.00-10.01 weeks, respectively. Moreover, about 80-90 g/week of the average weight gain was reached at the inflection point of the pheasants. In conclusion, the parameters of the goodness-of-fit criteria (AIC and BIC) indicated that the G model was the best formula to predict the growth curve of body weight in Caucasian pheasants.

Keywords:
Body weight; estimate; Gompertz; inflection point; poultry

INTRODUCTION

The Caucasian pheasant (Phasianus colchicus colchicus) is one of the bird species of Phasianidae with high economic value (Kirikci, 2012). In general, pheasants are distributed from Siberia to China, Iran, the Caucasus and the southeastern Balkans (Iftikhar & Yaqoob, 2024). The Phasianus colchicus in Asia can be classified into 10 subspecies, i.e. P.c. shawii, P.c. vlangalii, P.c. strauchi, P.c. kiangsuensis, P.c. karpowi, P.c. torquatus, P.c. elegans, P.c. satscheuensis, P.c. suehschanensis and P.c. edzinensis (Zhang et al., 2014; Liu et al., 2019) .A study of the mitochondrial Cyt-b diversity in five subspecies of pheasant populations (P.c. strauchi, P.c. satscheuensis, P.c. kiangsuensis, P.c. suehschanensis, P.c. edzinensis) revealed 35 haplotypes and two clades of pheasants (Zhang et al., 2014).

The Caucasian pheasant or common pheasant is classified as a species of Least Concern for conservation, according to the IUCN Red List (BirdLife International, 2016). Therefore, this species can be domesticated to produce meat with high economic value (Kirikci, 2012). Cetin et al. (1997) state that the body weights of pheasants at the fourth, eighth, fourteenth and eighteenth weeks were 153.73, 462.87, 835.61 and 955.17, respectively. On the other hand, Tepeli et al. (1999) reported the average body weights of P. colchicus at the fourteenth, sixteenth and eighteenth weeks as 877.00, 918.00 and 1058.00 g, respectively. Kuźniacka & Adamski (2010) reported that the body weight of common pheasants at 24 weeks of age was 1,400 g for males and 978 g for females. At 17 weeks of age, the pheasants can produce about 825.00±91.70 g (males) and 688.00±41.70 g (females) of carcass weight (Kokoszyński et al., 2014). Moreover, pheasants at 17 weeks of age have been able to reach the body weight of 1156.10 g (males) and 867.30 g (females) under an intensive system, as reported by Yamak et al. (2020). In addition, pheasants at 16 weeks of age can reach the slaughter weight and carcass weight of about 1,090 g and 797 g (Kokoszyński et al., 2012). Despite this, the egg weight of pheasants has been reported to be about 30.70 g at 13 weeks of age (Kuzniacka et al., 2005) and 31.03±2.26 g at 44 weeks of age (Putra et al., 2023).

Evaluating the growth characteristics of pheasants is important to obtain the optimal slaughter age, and prepare appropriate feeding processes and selection programs (Sariyel et al., 2017). The growth of animals consists in an increase of body size per unit of time. Thus, growth is a continuous function during the animal’s life from embryonic stages to adult age and is mathematically explained by growth curve models (Lawrence & Fowler, 2002). The growth curve is defined as the regular change generated by the live weight or some part of the animal with increasing age and illustrated as a S-shaped (sigmoid) curve (Narinc et al., 2017). The growth curve in poultry is characterized by an accelerating growth phase from hatching, followed by an inflection point where the growth rate is maximum, until a phase where growth rate is decelerating to reach the mature weight (Fitzhugh Jr, 1976).

Studying the growth curve of animals can provide a set of parameters that are used to describe growth patterns over time. Despite this, growth curves can be used by breeders to predict the weight of animals at a specific age and detect the stage of growth rate reduction (Yakupoglu & Atil, 2001). In general, three non-linear regression models - namely Logistic (L), Gompertz (G) and Von Bertalanffy (VB) - have been used to estimate the growth curve of poultry (Topal & Bolukbasi, 2008). Therefore, these models can predict the inflection weight at about 50% (L) or 30% (G & VB) from the asymptote of the animal (Ricklefs, 1968). In a study on Rock partridges (Alectoris graeca), the L and G models could predict the body weight of birds overtime with the coefficient of determination (R²) value of 0.98 (Putra & Kirikci, 2021).

Unfortunately, studies on the growth curve of pheasants are very limited. Milby & Henderson (1937) estimated the growth curve of the body weight in pheasants with the Brody model, but without an inflection point of weight (W_i) and time (t_i) information. Subsequently, Labisky et al. (1969) also studied the growth curve of pheasants with different regression models, without W_i and t_i information. The aim of this study was to analyze growth curves of mixed-sex of Caucasian pheasants (Phasianus colchicus colchicus) using non-linear models. The results from this study are important initial information for the development of common pheasant breeding.

MATERIALS AND METHODS

Ethical Approval

This study was approved by the Ethics Committee of Selcuk University, Faculty of Veterinary Medicine, Experimental Animal Production and Research Centre (Decision number: 2024/168, Meeting date: 31.10.2024).

Research Site

This research was performed in the rearing facility of pheasants at the Alternative Poultry Unit at Prof. Dr. Hümeyra Ozgen Research and Application Farm, Selҫuk University, Konya, Türkiye. This area is located at an altitude of 1,027 m, a latitude of 38° 03’ 7273” N, and a longitude of 32° 50’ 5539” E. The climate is characterized by an air temperature between 7.99-16.77°C, 58.3% of relative humidity, and 360 mm of precipitation per year.

Bird Management

Hatching chicks were housed for the first five weeks in a 5-storey heated brooder cages, with each floor measuring 37x100x20 cm. 70, 35 and 20 pheasant chicks were housed in each floor of the heated brooder cages in respectively the first, 2-3^rd and 4-5^th weeks. The temperature of the heated brooder cages was set at 33-35°C during the first week, and was decreased by 3ºC each week. After the first 3 weeks, no heat was applied to the pheasants. Animals were fed ad libitum between 0-5 weeks of age, with a ration containing 28% HP and 2900 ME kcal/kg. Water was provided through automatic nipples. Lighting was applied for 24 hours during the first five weeks of age.

Pheasant chicks were moved to semi-open poultry pens (4 x 5 x 2.5 m indoor and 4 x 5 x 2.5 m outdoor areas) with 35 pheasants per pen between 6-10 weeks of age. During this period, pheasants were fed ad libitum with a ration containing 22% HP and 3000 ME kcal/kg. Between 11-18 weeks of age, birds were fed a ration containing 21% HP and 3100 ME kcal/kg; water was provided by automatic drinkers and no lighting other than daylight was applied.

Live weights of the chicks were determined weekly from the day of hatching. For the first 6 weeks, chicks were weighed using a digital balance (KER PFB 100) with a precision of 0.01 g without sex discrimination; and between 6-18 weeks of age, chicks were weighed using an electronic hand scale (Portable Electronic Scale) with sex discrimination.

Data Analysis

A total of 1,765 data records of body weight (BW) in mixed-sex pheasants were used for estimating growth curve models, and were collected from 137 birds. Three non-linear regression models - Logistic (L), Gompertz (G) and Von Bertalanffy (VB) - were used to estimate the growth curve of BW in pheasants using the SPSS 16.0 software. The growth parameters of birds were calculated using the mathematical formulas from Yang et al. (2006), as shown in Table 1.

Table 1
Mathematical formulas of growth parameters with the Logistic, Gompertz and Von Bertalanffy methods*.

Model	Yt	Wi	ti	GR
Logistic	A(1+Be-kt)-1	A/2	(Ln.B)/k	k(Wi) [1-(Wi/A)]
Gompertz	A exp(-Be-kt)	A/e	(Ln.B)/k	k(Wi) [Ln.((Wi/A)-1)]
Von Bertalanffy	A(1-Be-kt)3	A(8/27)	(Ln.3B)/k	3k(Wi) [(Wi/A)-1/3-1]

• Y_t: body weight (g) of birds at t week of age; W_i: weight of inflection (g); t_i: time of inflection (week); exp: exponent; GR: growth rate (g/week); A: the asymptotic weight (g), when times goes to infinity; B: the scaling parameters (constant of integration); k: maturing rate (g/week); e: constant (2.72); t: time (week) *Yang et al. (2006)

Therefore, the best model to describe the growth traits was selected according to values of coefficient of determination (R²), root of mean squared error (RMSE), Akaike’s information criterion (AIC) and Bayesian information criterion (BIC) values, using the mathematical formulas from Hojjati & Ghavi Hossein-Zadeh (2018) as follows:

$R^2=1-\left(SSE/SST\right)$

$RMSE=\surd \left(SSE/\left(N-p-1\right)\right)$

$AIC=N*Ln\left(SSE/N\right)+2p$

$BIC=N*Ln\left(SSE/N\right)+p*Ln\left(N\right)$

Where, R² is the coefficient of determination; RMSE is the root of mean squared error; AIC is the Akaike’s information criterion, BIC is the Bayesian information criterion; SSE is the sum of squared error; SST is the total sum of squares; p is the number of parameters; and N is the number of observations (datasets).

RESULTS

The body weight (BW) of Caucasian pheasants from hatching to 18 weeks of age is presented in Table 2. Generally, the BW of males were higher than those of females. In the mixed-sex pheasants, the A-value (asymptotic weight) was highest in the VB model and lowest is the L model (Table 3). The A-value in the mixed-sex pheasants was between 1184.00 and 1806.00 g and was reached at >15 weeks of age (Table 2). The weight of inflection (W_i) in mixed-sex pheasants was reached at about 500 to 600 g, while the time of inflection (t_i) was reached between 9.00 and 10.00 weeks of age. The three non-linear regression models in this study had similar coefficient of determination (R²) values (0.94), as shown in Table 4. However, the G model had the lowest root of mean squared error (RMSE), Akaike’s information criterion (AIC), and Bayesian information criterion (BIC). Therefore, the G model was confirmed as the best model to describe the growth characteristics of the mixed-sex pheasants under study. In this study, a sigmoid growth curve was obtained based on all studied models, as shown in Figure 1. The growth curve of the VB model had the highest growth pattern among the growth curve models.

Table 2
The average body weight of Caucasian pheasants (P.c. colchicus).

Age (weeks)	Male		Female		Total
	N	BW (g)	N	BW (g)	N	BW (g)
Hatching	-	-	-	-	137	21.88±2.11
1	-	-	-	-	117	30.75±7.77
2	-	-	-	-	115	59.03±13.09
3	-	-	-	-	113	119.63±24.58
4	-	-	-	-	112	167.16±25.07
5	-	-	-	-	109	211.77±45.95
6	-	-	-	-	106	277.79±46.70
7	42	392.44±38.67	52	290.65±39.17	94	336.13±63.95
8	42	475.71±41.35	42	366.62±41.93	84	421.17±68.73
9	42	553.33±44.04	42	434.88±46.37	84	494.11±74.64
10	41	672.44±58.59	42	534.52±53.72	83	602.65±89.05
11	41	782.07±61.72	41	622.80±64.19	82	702.44±101.67
12	41	819.15±71.33	41	635.85±70.73	82	727.50±116.13
13	41	887.00±81.72	41	672.68±55.31	82	763.60±114.79
14	41	1037.07±99.57	41	817.07±55.93	82	927.53±135.94
15	35	1069.57±96.02	41	826.83±59.36	76	938.62±144.54
16	32	1201.56±91.52	38	906.45±71.22	70	1041.36±168.55
17	28	1247.86±89.47	39	949.74±64.35	67	1074.33±166.15
18	30	1283.97±77.15	39	964.50±60.87	69	1098.77±171.21

• N: number of birds; BW: body weight

Table 3
Growth curve parameters for the body weight of Caucasian pheasants (P.c. colchicus).

Model	A	B	k	Wi	ti	GR	Iteration
Logistic	1184.00±13.67	22.28±0.84	0.31±0.01	592.00	10.01	91.76	7
Gompertz	1473.00±32.34	4.11±0.07	0.15±0.00	541.54	9.42	81.28	6
Von Bertalanffy	1806.00±61.87	0.82±0.01	0.10±0.00	535.11	9.00	80.26	5

• W_i: weight of inflection (g); t_i: time of inflection (week); A: the asymptotic weight (g), when times goes to infinity; B: the scaling parameters (constant of integration); k: maturing rate (g/week); GR: growth rate (g/weeks).

Table 4
The goodness-of-fit criteria in the growth curve models for body weight of Caucasian pheasants (P.c. colchicus).

Model	R2	RMSE	AIC	BIC
Logistic	0.94	94.22	16048.86	16065.28
Gompertz	0.94	92.51	15984.50	16000.93
Von Bertalanffy	0.94	92.73	15992.67	16009.10

• R²: coefficient of determination; RMSE: root of mean squared error; AIC: Akaike’s information criterion; BIC: Bayesian information criterion.

Figure 1
The growth curve of the body weight of Caucasian pheasants (P.c. colchicus) estimated with Logistic (green line), Gompertz (red line) and Von Bertalanffy (blue line) models. Dots (•) represent actual weight data points.

The relationship between t_i, and GR is illustrated in Figure 2. The time of inflection and weight of inflection in the L growth model was higher than in the other growth curve models. According to Figure 2, the maximum GR in the mixed-sex pheasants ranged from 80.00 to 90.00 g/week and was reached at 9-10 weeks of age.

Figure 2
The growth rate of the body weight of Caucasian pheasants (P.c. colchicus) estimated with Logistic (green line), Gompertz (red line) and Von Bertalanffy (blue line) models.

The relationship between ti, and GR (growth rate) is clearly illustrated in Figure 2.

According to Figure 2, the maximum GR in the mixed-sex pheasants ranged from 80.00 to 90.00 g/week and was reached at 9-10 weeks of age.

DISCUSSION

The sexual dimorphism of pheasants manifests in plumage color and the difference of body weight (size). In birds, sexual dimorphism is associated with social and sexual behavior (Owens & Hartley, 1998). Generally, the BW of males is higher than that of females, which applies to many avian species such as Guinea fowl (Numida meleagris), ostrich (Struthio camelus), Pekin duck (Anas platyrhynchos), Turkey (Meleagris gallopavo), goose (Anser anser), partridge (Alectoris chukar), pigeon (Columba livia), chicken (Gallus domesticus) and Cairina moschata or Muscovy duck (Nahashon et al., 2006; Ramos et al., 2013; Faridi et al., 2014; Sogut et al., 2016; Ibtisham et al., 2017; Mohammed & Al-Barzinji 2022; Zannah et al., 2022; Tian et al., 2024; Yuan et al., 2024). In contrast, the BW of female quails (Coturnix japonica) is higher than those in males, as reported by Camargo Júnior et al. (2023). Sexual dimorphism in avian species may be caused by steroidogenesis, which determines the ratio of androgens/estrogens produced by the gonads (Bruggeman et al., 2002).

The BW of male and female pheasants in this study were higher at the same age than those in the study by Yamak et al. (2020), which may be caused by differences in the management system, feed composition and climate. In this study, the A value of pheasants under study with the VB model was close to male Italian Padovana Argentata chicken (1859.00 g) as measured by the L model (Rizzi et al., 2013). Interestingly, many studies have obtained similar growth parameters between chickens and pheasants. Osei-Amponsah et al. (2014) obtained A and W_i values in female Ghanaian chickens with the L model of about 1169.70 g and 585.00 g, respectively. In addition, N’dri et al. (2018) obtained A and W_i values in mixed-sex Cote d’Ivoire chickens with the G model of about 1506.90 g and 554.01 g, respectively. In both previous studies, the growth parameters of Ghanaian chickens and Cote d’Ivoire chickens were close to those of the pheasants studied according to the same growth curve model. Furthermore, the A-value of guinea fowls with the L model was 1,474.91-1,548.52 g for males and 1,457.22-1,540.77 g for females (Nahashon et al., 2006), close to the studied pheasants as predicted by the G model.

The t_i value in the pheasants under study was close to Italian crossbred chickens (Padovana Cosmosciata × Berlanda) according to the G model, i.e. 10.29 weeks for males and 9.43 weeks for females (Rizzi et al., 2013). Male Muscovy ducks had a t_i value of 10.95 weeks based on the VB model (Tian et al., 2024), close to the studied pheasants as measured by the L model (10.01 weeks). In this study, the G model (R²=0.94) was confirmed as the best model to describe the growth characteristics of pheasants due to the lowest RMSE value. Many studies reported the G model to be the best model to predict the growth curve of body weight in chickens (Putra & Fajrina, 2021, Mancinelli et al., 2023), quails (Guler et al., 2022), geese (Li et al., 2022) and partridges (Wen et al., 2019).

The growth characteristics of chickens and pheasants can be similar, as both species are grouped in the Galliformes clade. Consequently, modified chicken management systems can be used to keep pheasants effectively. In general, the pheasants under study can be slaughtered for meat at 9-10 weeks of age with about 500-600 g of BW. The genetic potential of pheasants can be increased by selection programs. Rizzi et al. (1994) reported that the heritability (h²) value of BW at 120 days of age in common pheasants was 0.30±0.14 and indicated that it is possible to improve this trait by selection. Unfortunately, most of Phasianidae species in Southeast Asia have been classified as threatened species (Brickle et al., 2008). Therefore, developing pheasant farming in Southeast Asia is important to promote their product in this region.

CONCLUSION

Three growth curve models - Logistic, Gompertz and Von Bertalanffy - can be used to describe the growth characteristics of mixed-sex Caucasian pheasants (P.c. colchicus), with the R² of 0.94 for each model. However, the Gompertz model was the most accurate to explain pheasant’s growth, obtaining the lowest RMSE value. In general, the pheasants under study can reach the inflection time (t_i) at 9-10 weeks of age and the inflection weight (W_i) of 500-600g.

ACKNOWLEDGEMENTS

We would like to express our sincere gratitude to Selçuk University Scientific Research Projects for supporting this project.

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• FUNDING
  This study was supported by Selcuk University Scientific Research Projects. (Project no:24401217).
• DATA AVAILABILITY STATEMENT
  Data will be available upon request.
• DISCLAIMER/PUBLISHER’S NOTE
  The published papers’ statements, opinions, and data are those of the individual author(s) and contributor(s). The editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions, or products referred to in the content.

Edited by

• Section Editor:
  Irenilza de Alencar Nääs

Data availability

Data will be available upon request.

Publication Dates

• Publication in this collection
  27 Oct 2025
• Date of issue
  2025

History

• Received
  07 Apr 2024
• Accepted
  03 June 2025