




































 

 

 
22 

† Corresponding author 
© 2016 Conscientia Beam. All Rights Reserved. 

 

APPARENT METABOLISABILITY OF DIETS CONTAINING CONCENTRATE, ZOSTERA 
NOLTII OR TARAXACUM OFFICINALIS IN ANAS PENELOPE 

 

Carla Fabro1† --- Matteo Del Fabbro2 --- Barbara Piani3 --- Stefano Filacorda4 --- Piero A. Susmel5 
1,3,4,5Department of Agricultural and Environmental Science – University of Udine 
2Green Solutions S.R.l. via Piave, Martignacco (UD) - Italy 

 

ABSTRACT 

Four metabolisability trials on captive Wigeons were conducted comparing a complete pelleted diet with others where 

Taraxacum officinalis and Zostera noltii were added. The daily dry matter (DM) intake of Wigeons varied from 54.4 to 65.5 

g/day and the amount of nutrients received from the four diets was similar. The nitrogen (N) content of droppings statistically 

diminished when Wigeons were fed diets containing Zostera noltii. Correlation among cell wall components (CWC) of 

droppings and that of intakes was always highly significant and positive. The DM metabolisability of the four diets was 42-51 

%. The crude protein (CP) metabolisability varied significantly from 21 % for the diet with Taraxacum to 39 % for that with 

the Zostera collected in June. The metabolizabilities of CWC also differed significantly among diets. The apparent 

metabolizabilities of ash with the Zostera diets were significantly higher when compared to those of two other diets. 

Keywords: Wigeons, Bird captivity, Metabolisability, Zostera noltii, Taraxacum officinalis. 

 

Received: 31 July 2015/ Revised: 17 February 2016/ Accepted: 22 February 2016/ Published: 26 February 2016 

 

Contribution/ Originality 
This study is one of very few studies, which have investigated the metabolic responses of captive Wigeons 

(Anas penelope) to four different diets and compared the results of nutritional aspects to those of their wild 

counterparts.  

 

1. INTRODUCTION 

The Eurasian Wigeon, Anas penelope, the smallest grazing Anseriform, is a migratory herbivorous bird, flying 

from the northern regions in early autumn to winter to warmer southern coastal areas and wetlands until early 

spring in order to feed on nutrient rich plants high in protein and low in fibre [1, 2]. Ring recoveries suggest that 

the reproductive area of Wigeons wintering in northern Italy are found across different Palearctic regions. 

Migrations towards wintering sites begins during the months of September and October while spring back-

migration occurs during March and April. If the reproductive sites do not ensure ideal conditions for moulting, 

Wigeons may make an earlier juvenile or post-reproductive and pre-moult migration to safer and more adequate 

southern feeding grounds [3]. Many environmental investigations deal with the role of this widespread dabbling 

duck, migratory and herbivore, on the ecology of wet grassland or marshes. 

Early research carried out on Wigeon dietary habits dates back to the turn of the century [4]. It was reported 

that this dabbling duck fed primarily on phanerogame species on coastal mudflats [5] Its diet consists mainly of 

leaves, shoots, and seeds of Enteromorpha spp. and Zostera spp., and of a varying, but a significant amount of molluscs, 

chironomid larvae, and arthropods [6, 7]. The worldwide classified Zostera spp. are 16 [8]. Zostera noltii 

Animal Review 
2016 Vol. 3, No. 1, pp. 22-25 
ISSN(e): 2409-6490 
ISSN(p): 2412-3382 
DOI: 10.18488/journal.ar/2016.3.1/101.1.22.35 
© 2016 Conscientia Beam. All Rights Reserved 

. 

 

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Animal Review, 2016, 3(1): 22-35 

 

 
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© 2016 Conscientia Beam. All Rights Reserved. 

(Hornemann) and Z. Marina (L.) are widespread along the tidal zones of the European Atlantic [9] and 

Mediterranean coasts [10, 11] where it flourishes in perennial meadows [12, 13]. Rhizomes secure it to the sandy 

or muddy substrates forming beds. Zostera take root in April and maximum development stems and leaf 

development is reached over a short period of time. In autumn the leaves reach a length of 30-40 cm, when the 

incidence of generative shoots is higher [14]. Short stems grow out from extensive, white branching rhizomes. 

More or less mature inflorescences and infructescences can be found from July to October; mature seeds are 

released in autumn and the natural decay of the plant is triggered with the onset of winter [15].  

The ribbon-like leaves and shoots of this plant are eaten in large quantities (about 80 %), while rhizomes and 

stems make up about 5 % of the diet, and seeds about 10 % [16]. The submerged or floating vegetation is 

intensively grazed most of the day and night, preferably during ebbtide. Other authors also reported regular inland 

feeding, mainly on saltmarshes (Puccinellia and Salicornia spp.) but also on flooded inland pastures (Glyceria, Festuca, 

Poa pratensis, Agrostis stolonifera, Lolium perenne, Dactylis glomerata [17] Ranunculus repens, and Trifolium repens [18-

20]. These plants differ in nutrient content and vegetal structure and consequently in palatability and digestibility. 

The Taraxacum spp. is consumed by the Wigeons together with other inland plants to integrate the Zostera diet 

when necessary, as during reproduction, moulting, and before migration.  

A Wigeon, weighing about 700 g, is subject to the digestive constraints of small vertebrate herbivore [21]. 

Like other Anatidae it has a simple gut and its caeca are not as well developed as other avian grazers, so it relies on 

ingesting very large quantities of food, which passes through the gut rapidly and is only in part digested [22]. The 

two caeca, positioned backwards along the terminal portion of the ileum, are blind sacs consisting of a narrowly 

constricted open end and a dilated thinner-walled blind portion. The caeca retain the liquid and soluble components 

of the intestinal contents for long periods. Caecal length and activity in herbivorous birds are influenced by 

captivity [23, 24] fibre content of the diet [16, 25, 26] and ingestion rates [27].  

Studies on apparent digestibility or metabolisability of feedstuffs by Wigeon are few. E.g., according to 

Mayhew [5] the average digestive efficiency of dry matter (DM) by Wigeons is 28.8 %; [2, 28, 29] considering 

that some grouse and waterfowl digest 15-35 % of the cell wall components (CWC).  

The present study was conducted on Anas penelope kept in captivity to measure the apparent metabolisability of 

four diets containing similar amounts of concentrate pellet (the habitual feed, given alone), Zostera noltii and 

Taraxacum officinalis. Two successive trials were performed to compare Zostera collected in June, which should 

represent the feed consumed during reproductive and moulting period, to that vegetating in late September, at the 

time when Wigeons arrive for wintering. Taraxacum, a perennial wild herb native to the northern hemisphere, was 

also given as it is a common species along the lagoon banks of our region and represents a widespread source of 

complementary forage for Wigeons. The results obtained on captive animals at an experimental facility were 

compared with those previously published obtained on wild animals. 

 

2. MATERIALS AND METHODS 

Four trials were carried out to test the apparent digestibility. This consisted of administering either a pelleted 

complete feed (P), feed alone as a control, or with Zostera noltii collected in June(PZJ) and in late September (PZS), 

in the Grado and Marano lagoons (Friuli Venezia Giulia – Italy), or Taraxacum officinalis (PT) leaves, harvested just 

before flowering (April) at the University farm, where the experiments were conducted. 

The same 4 three-year-old European Wigeons (2 males and 2 females) born and reared on the farm were used. 

The animals were habitually housed outdoors, in an aviary and given ad libitum pellets of a complete feed (Table 1) 

and on occasion some seasonal fresh forage available on the farm. 

To establish the amount of daily feed to administer during digestibility trials, preliminary feeding tests were 

carried out offering forage (Taraxacum officinalis and Zostera noltii) ad libitum to accustoming Wigeons to the feed 

and to appraise their palatability. It appeared that Taraxacum was more palatable than concentrate, while the daily 



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© 2016 Conscientia Beam. All Rights Reserved. 

intake of Zostera appeared to be less regular and residues were always recovered in the feeders. To maintain a 

comparable daily intake and digestibility condition, even with less palatable Zostera, as well as prevent weight loss 

during the experimental periods, the original idea was that of administering a maintenance diet consisting of at 

least an equal amount of pellet and forage on a wet basis. Given the chemical differences among forages, the 

experimental diets were formulated to obtain the chemical characteristics and nutritive proprieties as close as 

possible, with particular attention to crude protein (CP) and energy content (Table 2). The nutrient and energy 

contents of concentrate pellets, Taraxacum officinalis, and Zostera noltii are reported in Table 1. Thus, the scheme of 

the amount of daily food offered to Wigeons was outlined (Table 2).  

 

Table-1. DM and nutrient content of single feed used in experimental diets (% on DM) 

Diet 
Energy DM 

% 
 CP CF EE Ash NDF ADF NFE WSC  HEMI CELL 

kJ/kg DM 

P 18581 91.9 18.9 15.3 3.36 7.7 36.7 18.6 54.8 33.4 18.1 12.2 
PT 16890 12.8 16.7 14.7 3.8 16.4 27.1 22.0 48.3 35.9 5.1 14.1 

PZJ 11968 19.7 10.0 11.5 0.4 39.3 42.3 32.0 33.9 7.9 10.4 25.4 
PZS 12982 20.7 10.2 9.5 0.4 33.3 38.9 25.0 46.5 17.1 13.9 21.2 

CF: crude fibre; CP: crude protein; EE: ether extract; NDF: neutral detergent fibre; ADF: acid detergent fibre; OM: organic matter; NFE: nitrogen free extracts; 

WSC: water - soluble carbohydrate; HEMI: hemicellulose; CELL: cellulose. 

 

The metabolisability trials were performed in November (P), April (PT), June (PZJ), and September (PZS). 

During the digestibility trials, each bird was kept and individually fed with pelleted concentrate and forages daily 

harvested, in a single cage (60 x 50 x 50 cm), fitted with a removable tray for excreta collection.  

After a preliminary adjustment period of 5-7 days, all Wigeons were individually weighed. The lengths of the 

experimental trials were 5 days for P, 7 days for PT, 7 days for PZJ, and 12 days for PZS. As the amount of the DM 

daily intake resulted more variable when forages were added to the diet, the option of extending the length of the 

collection period was chosen when mixed diets were administered. Wigeon live weights were recorded at the 

beginning and end of each experimental period to check that all animals maintained their live weight. After each 

trial, birds were brought back to their habitual housing.  

 

Table-2. Ingredients of the experimental diets 

  Feeds 

  Pellet Forage Total Pellet Forage Total 

Diet As sampled  As sampled  As sampled  DM (g) DM (g) DM (g) 

  (g) (g) (g)       
P 66   66 61.4 

 
61.4 

PT 60 100 160 55.8 12.8 68.6 
PZJ 60 85 145 55.8 16.7 72.5 
PZS 60 85 145 55.94 17.6 73.5 

Pellet: complete feed; Forage: Taraxacum officinalis or Zostera noltii. 

 

Wigeons were fed twice daily, at 8:00 a.m. and 5:00 p.m., before and during the experimental periods. Fresh 

water was always available, and animals were given grit (sand) to support the feed breakdown in the gizzard. Feed 

consumption during the experimental period was recorded daily. Spilled food was separately weighed to obtain the 

effective intake. At the same time, droppings were collected, weighed and then dried, and ground.  

The water content of feeds and dropping samples was determined by oven-drying (55° C). Proximate Analysis; 

Neutral Detergent Fibre (NDF), Acid Detergent Fibre (ADF), and Acid Detergent Lignin (ADL) were determined 

on samples to calculate by difference Hemicellulose and Cellulose [30, 31] Water-soluble carbohydrate contents 

(WSC) on DM was calculated according to the following: WSC = 100 % - % Ash - % Ether Extract (EE) - % NDF - 



Animal Review, 2016, 3(1): 22-35 

 

 
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© 2016 Conscientia Beam. All Rights Reserved. 

% CP; Nitrogen-free extract (NFE) was calculated according to the following: 100 % - % Ash - % EE - % Crude 

Fibre (CF) - % CP. An adiabatic calorimeter (IKA Werke C7000) was used to measure the gross energy content of 

feeds and dropping samples.  

The apparent metabolisability -uncorrected for endogenous losses- of different nutrients was calculated using 

the following formula: metabolisability (%) = [(1-Intake (g)/Excreta (g)] x 100. Data were statistically analysed by 

ANOVA (SPSS Statistics 17.0), setting the significance levels at P < 0.05; the Pearson test to detect correlation 

among variables was also applied. 

 

3. RESULTS 

We sampled and analysed Zostera noltii in our coastal lagoon at different times, obtaining a set of different data 

on DM, nutrients, and energy contents. The vegetative state primarily determines the proportion and the chemical 

composition of rhizomes, shoots and leaves. A fraction of the leaves prevailed in the Zostera collected in June, while 

in September Wigeons received a higher quota of rhizomes and shouts. It seems that Wigeons prefer the green 

shoots and young leaves. The higher incidence of young shoots in autumn could support the differences of ADF, 

and consequently of hemicelluloses, cellulose, and of soluble carbohydrates (NFE and WSC) contents observed 

between Zostera handpicked in June and in September (Table 1). The comparison of the chemical composition of 

Zostera noltii with Taraxacum illustrates that the former contains less protein, fat, carbohydrates, and gross energy, 

but more ash, hemicelluloses, and celluloses (Tables 1 and 2). Table 3 shows the effective feed intake measured 

across the four trials. The DM intake of concentrate and of forages examined separately was not significantly 

different among diets. When forages were added to the ration, the total daily DM intake significantly increased 

with respect to P diet and varied from 61.0 to 65.5 g/d. When the birds were given Taraxacum and Zostera collected 

in September DM intake was significantly greater when compared to those measured with P diet (Table 3). The 

intake of PZJ was intermediate and not statistically different from the other three diets. The same trend was 

observed for organic matter (OM) intake. 

 

Table-3. Concentrate, forage, and nutrients intake (DM, g/day) 

 Contents P PT PZJ PZS F 

Energy KJ 1009 1168 1089 1167 3.37 
DM 54.4b 64.0a 61.0ab 65.5a 4.52 
CF 7.4a 9.9b 9.2b 9.9b 13.28 
CP 11.6 11.1 10.5 11.2 1.25 

EE 1.8a 2.4b 1.8b 1.8b 24.36 
ASH 4.6c 5.9b 6.5b 7.3a 13.26 
NDF 19.6b 19.9b 21.4ab 26.7a 16.03 
ADF 9.3c 11.9b 13.3a 13.1a 11.71 
OM 49.8a 58.1b 53.4b 58.2b 4.64 
NFE 28.1c 34.8ab 31.8bc 36.1a 8.69 
WSC 16.8c 24.8a 20.8b 18.4bc 10.83 
HEMI 10.4b 9.0c 8.6c 11.5a 13.64 
CELL 5.7c 8.3b 10.2a 8.1b 43.07 

a-bMeans within a row with different superscripts differ significantly for P < 0.05 

CF: crude fibre; CP: crude protein; EE: ether extract; NDF: neutral detergent fibre; ADF: acid detergent fibre; OM: organic matter; NFE: nitrogen free extracts; 

WSC: water - soluble carbohydrate; HEMI: hemicellulose; CELL: cellulose. 

 

The ingestion of CP does not vary among diets, while a statistically higher intake of soluble carbohydrates was 

observed when Wigeons were fed with forages. With Zostera collected in September, the ash intake of Wigeons 

significantly increased. The daily individual average amount of excretions is reported in Table 4. The amount of 

daily excretion was also variable within the same feeding treatment, as the number of droppings collected per diem 



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© 2016 Conscientia Beam. All Rights Reserved. 

ranged between 50 and 85. The NDF constitutes the major component of DM Wigeon dropping in all diets. The 

chemical composition of droppings sampled differed among treatments in CP and in some CWC components (Table 

4). A higher amount of DM was excreted when the Wigeons were fed with Zostera, corresponding to a significantly 

higher content of NDF, which indicates a poorer nutritional quality of the fibre found in this forage. 

The CP content of DM was always higher in droppings than in the rations consumed by Wigeons, as well as, 

rather appropriately occurred for ash and for the components of the cell wall. The nitrogen (N) content of 

droppings of diet containing Zostera noltii, poorer in CP than other feeds (Tables 1 and 4), compared to those from P 

diet statistically diminished. The addition of Taraxacum officinalis also caused a reduction of N content of dropping, 

lower but still significant. It is highly probable that these results do not solely depend on the N content of the diets, 

but also on the different chemical characteristics and on the fermentability of forages CWC. In fact, correlation 

between CWC, NDF, and ADF of droppings and intake is always significantly and positively correlated (r = 0.97, r 

= 0.92, and r = 0.88, respectively). The correlations between intake and droppings calculated for other constituents, 

as CP, ash, and EE are also statistically correlated, but to a lesser degree.  

 

Table-4. Average daily excretion and chemical composition of droppings 

 Contents P PT PZJ PZS F 

g/bird 235.6 ± 48.8 311.4 ± 39.1 240.5 ± 49.0 301.8 ± 21.7 
 g DM/bird 26.7c 32.0b 29.5bc 37.9a 10.22 

Energy kJ 468.1d 555.7b 510.6b 643.0a 9.72 
Chemical composition         

DM (%) 11.5ab 10.4b 12.4a 12.6a 4.01 
CF % on DM 23.3b 25.4a 25.0a 22.7b 6.08 
CP % on DM 29.2a 27.2b 21.8c 21.3c 176.58 
EE % on DM 0.7b 1.2a 0.8b 0.6bc 29.89 
ASH % on DM 14.1 14.7 14.1 13.2 2.6 
NDF % on DM 49.4bc 45.5c 55.4b 66.8a 10.65 
ADF % on DM 24.6c 37.4a 35.3a 28.5b 55.5 
OM % on DM 85.9ab 85.3b 85.9ab 86.8a 2.6 
NFE % on DM 32.7c 31.6c 38.3b 42.3a 51.36 
WSC % on DM 6.7b 11.6a 7.9ab 10.9a 3.2 
HEMI % on DM 24.8ab 14.2c 20.1b 25.7a 8.4 

CELL % on DM 19.2b 22.5b 30.8a 18.1b 13.41 
Energy kJ/kg DM 17981 16132 15266 17115 2.34 

a-bMeans within a row with different superscripts differ significantly for P < 0.05 

CF: crude fibre; CP: crude protein; EE: ether extract; NDF: neutral detergent fibre; ADF: acid detergent fibre; OM: organic matter; NFE: nitrogen free extracts; 

WSC: water - soluble carbohydrate; HEMI: hemicellulose; CELL: cellulose. 

 

The overall effect following the addition of different forages to the ration on the amount and composition of 

dropping is better defined by energy excretion, which was significantly greater in PZS (643 kJ/d), even if the 

energy concentration of excreta did not statistically differ among treatments. 

The average DM metabolisabilities measured with four diets varies from 42.2 % for PZS to 51.4 % PZJ, the 

former value being significantly lower than the others (Table 5). Apart from the least result, from the outcome of 

DM metabolisabilities some distinct associative effects between forages and pellet can be excluded. 

A similar trend of assimilation efficiencies were found for OM, even if the digestive and metabolic utilization of 

ash statistically differed among diets. 

The apparent metabolisability of CP ranged from 21.4 % for diets containing Taraxacum to 38.8 % with PZJ. 

The protein of the diet containing Zostera harvested in June appears to have been more efficiently metabolised, more 

so than that of PZS, while the presence of Taraxacum statistically reduced the protein metabolisability of the diet. 



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As expected, the metabolisability of NDF differed significantly across diets, being the highest in the control 

diet P. The average value of ADF metabolisability was significantly lower in PT than in P and PZJ. The 

metabolisability of hemicelluloses was statistically higher in PT and lower in PZS and PZJ. The cellulose content of 

diets containing forages was less digestible than that of P diet. The values of WSC metabolisability was lower for 

the PZS diet. 

 

Table-5. Apparent metabolisability of diet contents (%) 

Contents P PT PZJ PZS F 

Energy 53.6a 52.4a 52.9a 43.7b 6.45 
DM 50.9a 50.1a 51.4a 42.2b 4.87 
CF 16.2 17.6 19.9 13.6 0.55 

CP 32.6ab 21.4bc 38.8a 28.3b 7.31 
EE 90.0a 84.3c 86.8b 87.7b 5.7 
ASH 18.0b 20.0b 36.0a 31.7ab 3.54 
NDF 33.2a 27.1ab 23.3b 21.0b 5.44 
ADF 29.1a 11.5b 21.2ab 18.0b 4.07 
OM 53.9a 53.1a 53.2a 43.5b 7.86 
NFE 69.9a 71.1a 65.4b 54.5c 29.13 
WSC 89.0 85.1 88.5 78.3 2.81 
HEMI 37.0ab 50.0a 30.8b 15.5c 9.43 
CELL 10.7 13.2 12.1 15.6 0.29 

a-bMeans within a row with different superscripts differ significantly for P < 0.05 

CF: crude fibre; CP: crude protein; EE: ether extract; NDF: neutral detergent fibre; ADF: acid detergent fibre; OM: organic matter; NFE: nitrogen free extracts; 

WSC: water - soluble carbohydrate; HEMI: hemicellulose; CELL: cellulose. 

 

The EE metabolisability was significantly lower only when Wigeons were given Taraxacum. Both Zostera diets 

showed the highest ash metabolisability, so that the OM metabolisability resulted only partially limited by the high 

ash content of Zostera. 

 

4. DISCUSSION 

Very little information is available on the chemical composition of Zostera noltii, however the composition of 

Zostera collected and offered Wigeons in June and September are consistent with those reported by Mathers and 

Montgomery [32]. The vegetal part of foodstuff collected always contained leaves, shoots, and rhizomes, but in 

different proportions. Shoot and rhizome fractions prevailed in September. According to [32, 33] Wigeon prefer to 

dabble for green shoots or short leaves and [34] observed that the above ground parts predominated in the diet. 

The contents of DM, NDF, and CP were similar while June Zostera contained more ADF and cellulose, but less 

WSC, and hemicellulose. Fox [35] found that the rhizomes of autumnal Zostera contain significantly less DM, CP, 

fibre or ADF, and ash, but more WSC than shoots. Mathers, et al. [36] chemically differentiated shoots and 

rhizomes, observing that rhizomes comprise 56 % of DM of the whole plant and are lower in fibre and higher in 

WSC than shoots. The high content of the ash of Zostera noltii, as that of other saltmarsh species, was not indicated 

or commented in the literature, but does not seem to affect food selection and intake [32]. 

The gross energy content of September Zostera given to Wigeons was 13 kJ/g DM, higher than June plants 

(12 kJ/g DM), values comparable with those were reported by Mathers, et al. [36]. Inger, et al. [37] observed that 

the energetic value of Zostera shoots varied widely between sites: at Strangford it was 13 kJ/g [32] compared with 

20 kJ/g for southern England [38] 19 kJ/g for the Wadden Sea [34] and 18.6 kJ/g for Lindisfarne in north-

eastern England [39]. 

In our experiments, when forages were available, the DM intake significantly increased from 13 % (PZJ) to 20 

% (PT) (Table3). 



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The observed levels of DM intakes we observed are comparable to those (65.2 g/d ash free dry weight) 

measured by Madsen [34] in Wigeons eating Zostera, as the sole feed source. A higher intake of Zostera (120 g/day 

DM) [40] of grass (91.6 g/day DM) [5] and of Salicornia (110.7 g/day DM) [41] was found. Whereas in another 

experiment the same forage determined a lower intake of 53.1 g/d DM [41]. All these investigations were 

conducted on wintering Wigeons. The gross energy intake measured during the trials was 1009 kJ/d with P and 

increased to an average of 1141 kJ/d (1470 kJ/kg0.75), when forages were also administered. Woollhead [6] 

estimated that the energy consumption of a wild Wigeon was 2055 kJ/d, that is 2630 kJ/kg0.75/d, which is 

consistent, but almost about double our values, measured at about maintenance level. 

There is evidence that, even if captive, the rationed Wigeons have maintained an aptitude to consume forages 

to attain a higher intake of utilizable DM. Miller [25] found that not only did gut length increase in mallards on a 

high fibre diet, but that the food consumption of these birds also increased. The diet quality regulates the intake: if 

digestive efficiency decreases in terms of bulk, the animal may try to eat more resulting in gut volume enlargement, 

and retention time is increased to cope with extra food to process each day [24]. The modification in gut size and 

feed consumption elicited by diet quality is often ephemeral, having its allometric downsides resulting in weight 

modifications and limitations to flight capability with a higher risk of predation [42]. In fact, the energetic cost of 

flight increases proportionally to Body Mass1.56 [43]. Captivity can have a major effect on gut size, morphology, 

and digestive physiology of Anatidae [23, 44]. In a select number of dead captive animals the length of the paired 

caeca measured 12.0 and 12.5 cm while that of wild Wigeons shot in the lagoons (n = 71) was 19.1±2.8 and 

19.3±2.4 cm (Fabro and Susmel, unpublished data). The dimensions of the caeca of wild Wigeons we measured are 

consistent with those reported by [5, 45]. 

The chemical composition of dropping differed statistically among diets and was related to the intake and 

discloses the complexity of the digestive process. DM excretions are not directly comparable with other results of 

other experiences, as it depends on DM intake, on the quality of forages, or on the NDF digestive outcomes. Similar 

to our figures, Madsen [34] along with an intake of 64.8 g DM/d of Zostera,  measured a defecation of 35 g DM/d, 

but [18] collected 65.2 g DM/d droppings from Wigeon ingesting only 91.6 g DM/d of grass. The high CP 

content of droppings not only represents the digestibility of protein source, but also endogenous faecal excretion 

composed of different components, including the remains of intestinal bacteria and the metabolic excretion of uric 

and other N chemical compounds (Table 3). Average CP excretion was statistically lower in the PZJ diet than in 

other diets. Considering the PZJ and PZS excretions, it seems clear that CP emissions go hand in hand with the 

DM and NDF ones. If less protein were enzymatically digested and assimilated, less N would also be excreted in 

urine. The droppings of Wigeons given the Zostera diets contain a higher percentage of NDF and hemicelluloses. 

For all diets the correlation between NDF and hemicellulose excretions are statistically positive (r = 0.81), 

indicating that the hemicellulose digestion is progressively limited when the intake increases (r = 0.78) while for 

cellulose there is no significantly correlation (r = 0.30). When intake and excretions of these two components of the 

cell wall are correlated, cellulose fermentation is found to be very limited and higher intake linearly results in 

higher excretion (r = 0.91), whilst the excretion of hemicellulose increases to a lower extent (r = 0.74). 

Methodologically, it may be argued that the results obtained with the criteria of partial substitution also have 

some limits, as the level of substitution of a test ingredient in a diet may per se change the metabolisability values. 

Farrell [46] authoritatively asserts that there is little experimental evidence to support this, concluding that the 

effects of substitution, even at high levels, appear to be additive rather than associative. Our results suggest that the 

DM metabolisability of Taraxacum and Zostera collected in June appears to be similar to that of P diet, close to 50 %, 

and significantly greater than that of PZS. A comparable value was found by Madsen [34] who on Wigeons 

grazing on Zostera meadows measured a DM digestive efficiency of 46 %. These values are higher than the average 

value of 28.8 % measured by Mayhew [18] on wild Wigeons eating grass. 



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The metabolisabilities of NDF and ADF diminished at various extents when pellet was associated to forage 

(Table 5), displaying in this case a dissimilarly negative associative effect. A number of papers settle that Anatidae 

can digest fibre to a variable extent [33]. [2, 28, 29] indicate that some grouse and waterfowl species digest 15-35 

% of CWC, which is in the range of the metabolisability of NDF, measured in our experiments in Wigeons. The 

rare findings on Anas penelope, report values of 30.8 % and 5.0 % of NDF and ADF digestibility of autumnal grass, 

respectively [33]. These values are similar to those measured on PT diet. Jamroz, et al. [47] measured the 

metabolisability of NDF, ADF and hemicellulose of cereals obtaining values of 18.1±8.6 %, 2.8±10.0 % and 

27.8±12.4 % respectively, which are quite different from the metabolisability of P diet. The metabolisability of the 

hemicellulose of P diet is similar to that of seeds and tubers (35.0 %) measured by Bruinzeel, et al. [48]. 

Waterfowls can partly solubilise and digest hemicellulose through acid hydrolysis in the proventriculus and in 

the gizzard followed by rapid fermentation in the lower small intestine and caeca [49, 50]. The role of caecal 

microorganisms in fermenting CWC has been studied in different bird species. What is lacking is specific research 

on Wigeons with less developed caeca. Grazing geese [51-53] are considered to poorly digest [18] or not digest 

cellulose at all, as cellulolytic bacteria failed to show up in the intestinal tract [54]. Instead, Durant [33] reported 

that some ADF digestibility was measured in Wigeons. During the experimental trials, our captive Wigeons were 

able to metabolise from 15.5 % to 50.0 % of hemicellulose and from 10.7 % to 15.6 % of cellulose. The question 

raised by the data is whether cellulose can be prevented from entering the caeca. 

Little information is available on protein metabolisability on wild and, even less, on captive Anseriformes. 

Buchsbaum, et al. [2] depicts the CP apparent digestibility in different herbivore waterfowls in a range of values 

between 61 % and 80 %, but CP digestibility is always higher than metabolisability. The addition of less than 20 % 

of forages to the concentrate quota was sufficient to significantly modify the CP metabolisability among diets. Our 

results, varying from 21.4 % for a Taraxacum diet to 38.8 % for a PZJ one, are more comparable to the value of 40 % 

for rye grass CP reported by Van Eerden [55]. This author suggests that the digestibility of foliage CP is not 

affected by body mass, as that of CWC, but could also indicate that Wigeons are less efficient than other herbivores 

at digesting protein. In our study the CP metabolisability coefficients are not correlated with protein intake (r = - 

0.19), but the coefficient is negative. The correlation coefficient between CP intake and excretion is fairly positive (r 

= 0.60) and consequently the figure between CP metabolisability and content of droppings is higher and negative (r 

= - 0.90), or highly dependant on CP excretion. In brief, high CP excretion might be attributable to intake, which 

over exceded the maintenance requirement while the imputable fraction coming from the weight loss (see below), 

can be estimated to be no more than 10 % of total CP excretion (0.20 g N/kg0.75). 

In fact, the Wigeons retained on average 0.67g N/kg0.75/d - more than enough to cover the maintenance 

requirements or at least to prevent the depletion body protein in the short term –, that [56] equalled to 0.49 g 

N/kg0.75/d. The smallest daily N retention was 0.46 g N/kg0.75 (PT) and the highest 0.84 g N/kg0.75(PZJ). 

The metabolisability of lipids by Wigeons is high, suggesting they may efficiently absorb the soluble 

compounds of a large molecular mass.  

The ash content of Zostera is elevated and its apparent metabolisability resulted higher when compared to those 

of the other two diets. This result should not be considered positive because Wigeons, since their salt glands are not 

fully developed to reduce levels of salt in the blood, respond mostly by drinking fresh water [57]. Secondly, the 

excretion of salt is likely to have a high energy cost due to the process of the active transport of Na+ and K+ ions 

[58]. 

The net benefit of a feed can be measured in terms of the changes in the body weight over time [59]. During 

the experimental period, an average weight loss of 7 g/d was measured. As the results of the trials demonstrate, the 

diets cover the N and energy requirements, and weight loss is most likely attributable to the disturbance due to the 

change of rearing conditions during experimental periods. A higher weight loss of 18.2 and 36.6 g/day were 

measured during digestibility trials [41] due to inadequate energy intake. Wigeons are considered quite sensitive 



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to human presence and activities [18, 60] and their behaviour was constantly observed as being timid or fearful, 

even if born in captivity. The inevitable tending, handling and feeding Wigeons in metabolisability cages generate a 

state of anxiety in the animals. Mayhew [5] noted that Wigeons might be under a great deal of stress when 

handled or when in contact with researchers, even if they have been imprinted by them. 

Independently from the cause, weight loss contributes to the energy excretion of endogenous origin and could 

affect energy or protein metabolisability. If we accept the indication given for Wigeons by Durant, et al. [41] for an 

equivalent 22.6 kJ/g of weight loss-quoted as half from fat and the other half from muscular tissues, our animals 

would rate a daily energy loss of 170 kJ/d. Otherwise [61] proposed to subtract 34.4 kJ from metabolisable energy 

(ME) for each gram of N lost to account for the energy required in the excretion of urinary nitrogen. For a loss of 

3.5 g/d of muscular mass, which is about 170 mg/d of N while the amount of ME is negligible at approximately 6 

kJ. Then, we should conclude that the energy from weight loss is almost entirely converted into heat and a 

correction to N equilibrium for the scope of this type of research appears not to be necessary [62]. 

We measured the energy assimilated by the difference between the energy contents of feeds and droppings, just 

to estimate the level of nutrition achieved over the course of the trials. In fact, we were not able to find in the 

literature data on intake of metabolisable DM or OM of Wigeons receiving comparable mixed diets, but only 

occasional information on energy intake from forages. 

ME is the conventional measure of the energy available to birds from their diet. In avian energetics, ME is used 

to convert daily energy budgets into the weight of food required to supply energy needed by individuals or 

populations [62, 63]. The energy values measured of feeds and droppings collected from test birds, fed at 

maintenance, using a bomb calorimeter is recognised as a proper measure of energy metabolisability. ME can be 

expressed as either apparent or true metabolisable energy. The true ME value, in most cases measured on starved 

animals, is adjusted by the energy of non-food origin lost through faeces and urine. This component of excretion is 

rather independent of energy consumption, in fact as energy intake increases, the true ME value progressively 

approaches that of the apparent ME. According to Miller and Reinecke [61] in the nearness of maintenance, the 

energy requirement weighs up at 2.5 times the basal metabolic rate (BMR) in Anas plutyrhynchos (1 kg of body 

weight), the difference between the two values is small, about 3 %. 

In our experiments, the differences between gross energy in foods on the one hand and droppings on the other 

amount to 540 kJ/d with pellets and an average of 571 kJ/d in birds fed mixed diets. No other experiences have 

been found where the metabolisability was measured on captive animals kept in metabolic cages.  

In feeding ecology researches, few balanced measures of food energy available to birds were instead obtained in 

semi-natural conditions on wild birds, using tracers, mobile aviaries, roost, and grazing patch enclosures. Wintering 

on grass pastures Wigeons grazed for 17.5 h/d, Mayhew [18] measured a ME intake of 630 kJ/d. Madsen [34] 

observed that wintering Wigeons spent 12.7 h/d feeding on Zostera, and evaluated a ME intake of 592 kJ/d, which 

he assumed to be the average daily energy expenditure (DEE) value for Wigeons (700 g of body weight). Durant, et 

al. [41] using mobile aviaries to keep Anas penelope grazing on Salicornia marsh, quantified a daily ME intake of 182 

and 345 kJ/d, in two successive experiments. 

In ecological and behavioural studies, the energy intake is quoted as DEE (otherwise indicated as Field 

Metabolic Rate), an all - inclusive field maintenance requirement, which conceptually excludes productive and 

growth exigencies [64]. DEE is measured, or more often estimated as a multiple of one category of the metabolic 

parameters. 

The minimum maintenance metabolic rate (resting, post - absorptive, non growing, non-reproductive at 

thermo neutral environmental temperature), or BMR, measured under experimental conditions, represents a well-

defined baseline energetic (heat) parameter concerning the animal [65-67]. Allometric equations have been widely 

used to predict avian BMR from metabolic weight – BMW = kg0.75 - [68-72]. The BMR was calculated using the 

average body weight of our Wigeons (722 g) with the non-passerines equation of Aschoff and Pohl [73] correspond 



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to 242.3 kJ/d. Kendeigh, et al. [63] for a Wigeon weighing 723 g gives a BMR of 241.8 kJ/d (2.799 W). Instead, 

McNab [74] provides a non-passerines equation, containing six variables matching factors which accounts for 97.7 

% of the variation in avian BMR measures to adjust the estimate of BMR to specific environmental conditions. 

When applied to our situation, using the suggested corrections, the BMR of Wigeons averaged 259.9 kJ/d. 

Standard Metabolic Rate (SMR; basal, not thermo neutrality) results slightly higher. For example, with a non-

passerines equation the value adapted to our data is 258.3 kJ/d [18, 75]. Resting Metabolic Rate (RMR; minimal 

activity, not post-absorptive), which accounts for the conditions under which data are obtained from test animals, is 

more variable and higher. An example could be that of using a general equation valid for dabbling ducks, the 

requirement corresponding to the weight of our experimental Wigeons should be 354.9 kJ/d [76]. The choice of 

the metabolic parameter among the numerous possible options for an assessment of DEE depends upon the 

conditions of the study and influence the multiplication factor. The factor value used to derive the DEE is 

differently designated, between 2.5 and 3 [54, 77]. 

With reference to Anas penelope, in two of the above reported studies, DEE was also calculated. Madsen [34] 

evaluated the DEE, as suggested by [73, 78] which resulted in 617 kJ/day, close to the measured value. Mayhew 

[5] calculated a DEE of 631 kJ/d [75, 78]. Notwithstanding the difference of experimental conditions, these values 

are very similar. In fact, although Anas penelope has different and varying patterns of activity during the day and 

night, energy expenditures for behaviour and for diurnal and nocturnal habits are almost the same [79]. In a 

wintering area of Anas penelope in the Wadden Sea, to maintain the balance of vegetal biomass grazed by waterfowls, 

Jacobs, et al. [40] quantified the number of Wigeons feeding on seagrass bed and the consumption of Zostera noltii. 

They estimate the ME content of Zostera and the average DEE of Wigeons through their SMR [75] choosing a 

multiplication factor of 3. The calculated SMR was 226 kJ/d for Wigeons (700 g of body weight), which is a DM 

intake of 54.1 g/d.  

Theoretically, in adult animals at maintenance, daily ME intake should correspond to DEE. The ME intake we 

have measured shows a close correspondence to DEE values determined in the studies mentioned above. The fact 

that Wigeons were kept in metabolic cages should have reduced the energy demand for maintenance. Robbins [56] 

and Kirkwood [80] devised that the maintenance requirement of adult captive birds in energy balance and kept in a 

comfortable thermal environment, come close to twice the BMR. If we consider that the average BMR of our 

Wigeons ought to be 260 kJ/d, as calculated by the McNab [74] equation, the ratio with a ME intake of 564 kJ/d 

equals 2.2, but the factor approaches 1.6 if compared with the BMR value proposed by Miller and McaEadie [76]. 

In any case, the level of nutrition which were applied in our trials were satisfactory, but unlike protein, did not 

appreciably exceed the maintenance requirements. 

 

5. CONCLUSION  

This set of experiments dealt with two related objectives. The immediate one was to evaluate the metabolisable 

energy and nutrients concentration using the standard procedure of total collection on captive Wigeons. The 

second purpose, essentially speculative, was to compare the results of the nutritional response of captive birds to 

those of their wild counterparts. Methodological, ecological and behavioural aspects, which characterize the 

experimental settings, represent unavoidable differential elements. So scientific knowledge has to help in defining 

which part of nutritional explanation represents unmodifiable specific physiological characteristic and which parts 

of the feeding response depend upon the flexible and adaptable process, which is given in the biochemical and 

functional digestive and metabolic configuration. In other words, what is functional and what is behavioural? 

Captive Wigeons have less developed caeca compared to other herbivorous ducks, but we could assume that the 

basic functionality is maintained. Wild grazing Wigeons rely on ingesting highly variable and relatively large 

quantities of fresh food, higher than captive birds, but in both situations ingesta pass through the gut rapidly and is 

inefficiently digested. The digestive system of herbivorous Anas penelope can be regarded as an adaptation to 



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differences in energy requirements and environmental conditions. The same way of thinking could apply to other 

aspects, like faecal and urinary excretion or BMR. These aspects are different from feeding behaviour, preferences, 

or real energy expenditure of waterfowl in natural conditions.  The differences among diets were determined by the 

addition of two types of forages, which represented less than the 20 % of total DM intake. The differences of DM 

intake among the four diets denote a preference or palatability for forages rather than a response to a need for a 

greater intake of energy or DM. Statistically significative changes in droppings excretion and in dietary nutrients 

metabolisability were observed. Most of the differences in chemical composition between Zostera noltii leaves 

harvested in June and September significantly affected metabolisability of CP, OM, NFE, and WSC. Not all the 

differences in metabolisability observed between Taraxacum and Zostera are attributable to chemical contents, but 

rather to the quality of the fibre. The in vivo trial seem to have screened the differences in the quality of forages 

more effectively than the chemical analysis. From the results it appears that, cellulose was almost partly digested 

leading to the consideration that caeca of captive Wigeons have and keep the ability to ferment cellulose. The CP 

metabolisability coefficients were low and failed to correlate with protein intake, while this occurred significantly 

with the correlation coefficient between CP metabolisability and CP in droppings. More likely, the protein 

positively interacts in the digestion of other nutrients, particularly CWC. In fact, CP, ash, hemicellulose and 

cellulose intake was statistically correlated with crude protein metabolisability. The ash content of Zostera also 

requires further investigation. In principle, the inorganic matter may be considered detrimental but this does not 

match the fact that this phanerogam represents a common feed for Wigeons. This experiment suggests that with 

captive birds it is possible to obtain some useful information on the nutrition of Wigeons, which can be extended to 

feeding in the wild. Although the consequences arising from fear and stress that Wigeons still exhibit in relation to 

man remain difficult to elude. 

 

Funding: This study received no specific financial support. 
 

Competing Interests: The authors declare that they have no competing interests. 
 

Contributors/Acknowledgement: All authors contributed equally to the conception and design of the study.  

 

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