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6 March 2026

Digestibility, Energy Value, and Performance of Lemna minor as a Novel Protein Source in Broiler Chicken Diets

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1
Faculty of Agricultural Sciences and Landscape Architecture, University of Applied Sciences Osnabrück, 49090 Osnabrück, Germany
2
Section of Environmentally Sustainable Animal Nutrition, University of Kassel, 37213 Witzenhausen, Germany
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Faculty of Agricultural and Environmental Sciences, University of Rostock, 18059 Rostock, Germany
4
Life Science and Engineering, University of Applied Sciences Bingen, 55411 Bingen am Rhein, Germany
Poultry2026, 5(2), 24;https://doi.org/10.3390/poultry5020024
This article belongs to the Collection Poultry Nutrition
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Abstract

Global population growth is expected to increase poultry meat demand, intensifying the need for sustainable protein sources. Soybean meal, the primary protein feed for poultry, has negative associations with deforestation and long transport distances. Duckweed has emerged as a possible, more sustainable alternative due to its high growth rate and protein yield. The nutrient digestibility and performance effects of the duckweed species Lemna minor (L. minor) in broiler diets were investigated in two experiments. Experiment 1 determined the ileal digestibility of crude protein, amino acids, phosphorus, and metabolizable energy in L. minor. The digestibility of most amino acids in L. minor ranged from 70% to 96%, with lysine and methionine at 87% and 86%, respectively. At 48%, the digestibility of cysteine was markedly lower than that of the other amino acids. However, the digestibility of P exceeded 90%. The energy values of dry matter were 7.05 MJ AME and 6.13 MJ. Experiment 2 tested the inclusion of L. minor (up to 10%) in isoenergetic and isonitrogenous diets. No significant effects on nutrient digestibility, weight gain, feed intake, or feed conversion ratio were observed. Both experiments demonstrate that L. minor cultivated under controlled conditions is a highly digestible, reliable feed source. Its inclusion in broiler diets is feasible, as it does not impair performance, yet provides amino acid balance whilst ensuring biomass quality. These findings support L. minor as a novel protein alternative and warrant further research on higher inclusion rates.

1. Introduction

The EU relies heavily on the importation of protein for animal nutrition [1]. Soybean meal imported from North and South America accounts for 48% of the imported protein-rich feed [2]. Soybean production is associated with environmental issues such as deforestation and the carbon footprint caused by long transportation distances [1]. Moreover, the importation of protein-rich feed contributes to excessive nutrient accumulation in regions with intense livestock production [3]. Furthermore, the production of conventional agricultural protein sources is becoming increasingly limited in the EU as the area available for agriculture decreases [1]. Therefore, novel protein sources have to be considered to alleviate these problems.
In this context, the duckweed plant family represents an alternative protein crop. The term duckweed describes a group of plants of the family Lemnaceae [4] with five genera and 36 species [5]. These floating freshwater plants are protein-rich and have a high growth potential [6]. The biomass of duckweed can contain up to 45% crude protein (CP), with a favorable amino acid composition [7]. Several experiments have investigated the nutritional value of dried duckweed biomass for broiler chickens. Ahammad et al. [8] reported that a diet including up to 6% of L. minor (LM) increased weight gain but decreased feed intake. Studies by Kabir et al. [9] revealed reduced feed efficiency when L. minor was added to the diet. In the case of the duckweed species Wolffia globosa, a decrease in broiler chickens’ feed intake and growth performance was observed [10]. The reasons for the contrasting results in these studies still need to be understood. In all the trials, feed was formulated without considering amino acid or phosphorus digestibility, and the metabolizable energy was calculated using estimation formulae. In a previous experiment, the digestibility of CP from different duckweed batches ranged from 40 to 84%, emphasizing the importance of using digestibility values in feed formulation to avoid nutrient imbalances [11].
Other possible explanations for the incompatible results of the three aforementioned experiments are the varying biomass compositions and the use of different duckweed species. Abiotic factors, such as light, temperature, and, particularly, the nutrient composition of the cultivation medium, can influence biomass composition [12,13]. Soilless plant cultivation methods (e.g., hydroponics) operated in a controlled environment allow regulation and adjustment of all abiotic parameters according to the crop’s demand for year-round production, independent of the climatic conditions. An indoor vertical farm (IVF) consists of vertically stacked cultivation layers with artificial lighting and is a novel and space-efficient strategy for crop production [14,15,16]. Re-circulating the nutrient solution within the IVF can minimize the nutrient and water requirements compared to batch cultivation systems [15]. Coughlan et al. [16] and Roman and Brennan [17] describe an IVF for duckweed production. Petersen et al. [18] developed and practically operated a small-scale re-circulating IVF. These models were used in the experiments performed in the current study.
In the present study investigating the digestibility, energy value, and performance of L. minor, the nutritional value of the L. minor biomass derived from standardized IVF production and outdoor cultivation was evaluated in a broiler feeding experiment. This investigation represents the first systematic approach to assessing L. minor from different cultivation systems as a potential protein source for broiler chickens. Specifically, we aimed to examine the effects of cultivation methods on the digestibility of crude protein, amino acids, and phosphorus, as well as on the metabolizable energy content of L. minor. The amino acid digestibility is particularly critical when evaluating novel protein sources.
We aimed to assess whether the digestibility of amino acids is transferable to other batches produced in the indoor vertical farm. Therefore, a three-phase feeding trial was subsequently conducted to integrate another batch of indoor-cultivated L. minor into broiler diets according to its digestible amino acid profile. In this second experiment, various levels of L. minor were used to assess their suitability, and the contents of digestible amino acids were recorded.

2. Materials and Methods

2.1. Biomass Production and Composition

The L. minor batches investigated in this experiment were produced under different growing conditions.
The first batch (LM1) was cultivated in an outdoor pond of 9.7 × 14.0 m with a water depth of approximately 1.0 m. The starting medium was a mixture of rainwater, biological effluent, and the liquid fraction of pig manure. These waste streams were sampled at the manure treatment facility of IVACO, Eernegem, Belgium. The liquid fraction of pig manure was obtained by separating the solid fraction from raw pig manure by centrifugation. This process reduces the P content in the liquid fraction of pig manure. Subsequently, the ammonia of the liquid fraction of pig manure was nitrified and denitrified in separate tanks during the biological treatment, resulting in the biological effluent stream with a reduced N content. The nutrient medium was produced using the rainwater discharged into the pond during winter. Then, 20 L of liquid fraction and 200 L of biological effluent were added to the pond. The composition of the nutrient medium was analyzed as described by Devlamynck et al. [19]. The average electrical conductivity was 513 µS cm−1 and pH 7.31. Weekly, 10 L of liquid fraction was added to the pond to keep nutrient concentrations sufficient for duckweed cultivation. L. minor was cultivated from 18 June 2021 until 6 August 2021 at an average temperature of 12 °C. The biomass was collected each week, rinsed with rainwater, and dried in small perforated plastic bags in an oven set at 60 °C.
The biomass of batch LM2 was produced in a re-circulating IVF with artificial lighting, as described by Petersen et al. [15]. It consisted of nine vertically stacked basins with a total production area of 25.5 m2. For fast growth and a high protein content of the L. minor biomass, a nutrient medium with a nitrate-N to ammonium-N ratio of 75% to 25% was applied [13]. The electrical conductivity averaged 699 µS cm−1, and the average pH level was 6.06. Local tap water was used as the source of water. The dosage of nutrients was determined based on the electrical conductivity. The light intensity of the LEDs was 105 µmol m−2 s−1 [18], and the photoperiod was set to twelve hours of light and twelve hours of darkness per day. The temperature of the nutrient medium was set to 23 °C and the flow rate to 10 L min−1. This production standard was applied to yield plant biomass with a high CP content. The biomass was harvested weekly using an integrated harvesting system. The harvested biomass of LM2 was then oven-dried in paper bags at 65 °C. The nutrient medium composition of both batches is shown in Table S1.
The third batch (LM3) was likewise cultivated in the re-circulating IVF system; however, several cultivation parameters were adjusted based on recent research findings to improve the biomass yield and quality. Specifically, the target electrical conductivity was increased to 1.3 mS cm−1, and the pH level was adjusted to 7.1 (comparable to that in experiment 1). The ratio between the different nutrients in the media remained equal compared to batch LM2. The photoperiod was extended to sixteen hours per day [20,21,22]. Harvest and drying were conducted using the same method as for LM2.
The three batches of L. minor were milled with a cutting mill (3 mm matrix; Fritsch Pulverisette 25, Fritsch GmbH, Idar-Oberstein, Germany) for a comparable particle size distribution. Batches LM1 and LM2 were used in the first experiment, and batch LM3 was used in the second experiment. Their chemical composition is given in Table 1, and the amino acid composition of both batches is in Table S2.
Table 1. Chemical composition (g kg−1 as fed, except where stated) of two batches of Lemna minor (LM) biomass.

2.2. Birds and Management

Two bird and management experiments were conducted at the University of Applied Sciences Bingen (Life Science and Engineering, 55411 Bingen am Rhein, Germany). These experiments trialed the effects of different diets. Animal welfare and trial protocol were controlled and approved by ‘Landesuntersuchungsamt Rheinland-Pfalz’, 56068 Koblenz, Germany (approval number: 23 177-07, 4 November 2020). In both experiments, temperature, humidity, and ventilation were controlled automatically. With a starting temperature of 30 °C, it was gradually reduced to 25 °C during the second week. Artificial light was provided for eighteen hours per day. Birds and housing conditions were checked twice a day.
For the first experiment, 48 one-day-old male Ross 308 broiler chicks were obtained from Probroed & Sloot (Vreden, Germany) and raised with commercial broiler diets. On day 16, the birds were transferred to 48 individual cages (0.440 m length × 0.355 m width × 0.335 m height, floor area 0.156 m2) and adapted to the pelleted control feed (without an indigestible marker) until day 18. On day 18, the birds were randomly assigned to one of the three feed treatments for the adaptation phase, resulting in sixteen replications (birds in individual cages) for each treatment. From days 21 to 26, feces were collected from six animals per treatment. On day 26, all chicks were slaughtered by concussion stunning and cervical dislocation. Subsequently, digesta was sampled from the terminal half of the ileum. The ileum was defined as the portion between Meckel’s diverticulum up to approximately 4 cm anterior to the ileo-caecal junction. All samples were subsequently stored at −20 °C.
In the second experiment, sixty-four one-day-old male Ross 308 broiler chicks were obtained from Probroed & Sloot (Vreden, Germany). They were randomly assigned to one of the four treatments. The feed differed in its L. minor content. From day 1 to day 21, the birds were raised in groups of sixteen and fed with the specific feed for their treatment. On day 21, nine birds per treatment were transferred to the same individual cages. Feces were collected from days 23 to 28. On day 28, all chicks were slaughtered by concussion stunning and cervical dislocation to receive the ileum samples as described in experiment 1.

2.3. Experiment Feed and Treatments

In the first experiment, the basal diet had a calculated content of N-corrected apparent metabolizable energy (AMEn) of 12.8 MJ kg−1 as fed. The ingredient composition is shown in Table 2.
Table 2. Botanical composition (g kg−1 as fed) of the basal diet in experiment 1.
The available biomass contained approx. 7.5% of L. minor CP. Thus, the diet of T1 consisted of 40% LM1 and 60% basal diet, and the diet of treatment T2 consisted of 25% LM2 and 75% basal diet. The pure basal diet was used for a control treatment and to calculate the digestibility and the metabolizable energy contents of the individual L. minor batches (see 2.5). All individual treatment diets contained 0.5% of a premix and TiO2. The TiO2 was used as an indigestible marker to calculate digestibility coefficients. The chemical composition of the treatment diets and the feed additives used in the premix are given in Table 3. The diets were pelleted with a 3 mm matrix. The diets and L. minor biomass were sampled during production, and the samples were stored at −20 °C.
Table 3. Analyzed chemical composition (g kg−1 as fed except where stated) of the treatment diets for broiler chickens from days 18 to 26 post-hatch.
In the second experiment, the poultry received a three-phase feeding program. Feed mixtures were offered to the animals ad libitum during the following three periods: starter: days 1–10, grower: days 11–21, and finisher: days 22–28.
Whilst the same ingredients were used for all diets, the ratio of ingredients was varied. For the integration of duckweed, the levels of soybean meal and wheat were progressively reduced, while the proportions of feed oil and supplemental amino acids were increased. The four diets were designed to have the same protein and energy levels, with amino acids balanced according to standardized ileal digestibility (SID) values. The amino acid digestibility values of duckweed were derived from the results for batch LM2 of experiment 1. The calculated composition of the experimental diets is presented in Table 4. All diets complied with the German government’s requirements for reduced nitrogen and phosphorus feeding. In the final diet, 1% titanium dioxide (TiO2) was included as an indigestible marker to enable digestibility calculations.
Table 4. Ingredients and calculated nutrient composition of the diets in experiment 2.
To record performance parameters, the chickens were weighed individually on days 1, 10, 20, 22, and 28. From day 22 onwards, additional feed intake and FCR were measured individually for the 36 animals in the cages. FCR was obtained as the ratio of total feed intake to body weight gain.

2.4. Sample Preparation and Chemical Analyses

In both feed and treatment experiments, samples were prepared and analyzed in the same way. Individual digesta and feces samples were freeze-dried (P8K-E-54-4, Dieter Piatkowski—Forschungsgeräte, Munich, Germany). Digesta samples were randomly pooled to obtain four mixed samples per treatment for analysis, with four samples of individual animals in each. All samples (including L. minor samples, treatment diets, and digesta and feces samples) were individually milled with a centrifugal mill (UZM 200, Retsch GmbH, Haan, Germany) with a 0.5 mm sieve. Bomb calorimetry was executed following DIN EN ISO 18125:2017-08 [24], and CP was analyzed following ISO 16634-1:2008-11 [25]. Some compounds were determined following Commission Regulation (EC) No 152/2009 [26]: dry matter and moisture content, method A; crude ash, method M; crude fat, method H procedure B; crude fiber, method I; starch, method J; total sugars, method L; tryptophan, method G; and all other amino acids, method F.
The total amino acids were determined by acid hydrolysis. For the total amino acid analysis, 1 g of sample material was hydrolyzed with concentrated hydrochloric acid. To stabilize sulfur-containing amino acids, oxidation was performed prior to hydrolysis, followed by decomposition of excess oxidizing agents. Hydrolysis was carried out over 23 h either under reflux (open system) or in closed vessels at an elevated temperature. After cooling, the hydrolysates were neutralized and adjusted to a pH value of 2.2. Depending on the requirements of the analyzer used, the volume was reduced before pH adjustment, where necessary. Internal standards were added to improve quantification. The prepared sample solutions were supplemented to a defined volume with citrate buffer, and the solutions were mixed thoroughly and filtered through 0.22 µm membrane filters prior to analysis. The quantitative determination of amino acids was performed by ion exchange chromatography (Biochrom 30+, Laborservice Onken GmbH, Gründau, Germany) with post-column ninhydrin derivatization. Calibration was performed using standard amino acid solutions, ensuring a linear detector response and sufficient chromatographic separation, especially of critical amino acid pairs.
DIN EN 15621:2017-10 [27] was applied to analyze calcium and phosphorus. An acid detergent fiber and an acid detergent lignin were analyzed according to the DIN EN ISO 13906:2008-11 [28], and a neutral detergent fiber (NDF) was analyzed according to ISO 16472:2006-04 [29]. The trypsin inhibitor activity was determined by DIN EN ISO 14902:2002-02 [30], and the tannin content was analyzed following the European Pharmacopeia method 2.8.18 (PY) [31]. Inositol phosphate esters were determined according to Zeller et al. [32]: To analyze the InsP isomers in duckweed, a 1.0 g sample was extracted for 30 min with 10 mL of a solution of 0.2 m EDTA and 0.1 m sodium fluoride (pH = 10) as a phytase inhibitor on a rotary shaker. The samples were centrifuged at 12,000× g for fifteen minutes, and the supernatant was removed and stored on ice. The residue was resuspended in 5 mL solution of EDTA–sodium fluoride, and then extracted for another 30 min. The supernatants from both extraction steps were then combined. A volume of 1 mL of the pooled supernatant was centrifuged for 15 min at 14,000× g, and 0.5 mL of the resulting supernatant was filtered through a 0.2 µm cellulose acetate filter (VWR, Radnor, PA; USA) into a Microcon filter system (cut-off 30 kDa; Millipore, Burlinton, MA, USA). The sample was then centrifuged for a further 30 min at 14,000× g. Throughout the extraction process, the samples were kept at a temperature below 6 °C. The filtrates were analyzed by high-performance ion chromatography with UV (Dionex ICS-3000 LCMS, Dionex Corporation, Sunnyvale, CA, USA) detection at 290 nm after post-column derivatization using an ICS-3000 system.

2.5. Calculation and Statistics

In vitro digestibility (IVD) of CP was calculated as the quotient of pepsin-soluble crude protein and CP. The energy contents and the digestibility coefficients of the L. minor batches were calculated in two steps. In the first step, the respective parameters were calculated for the treatment diets, and in the second step, they were calculated for the L. minor batches following the alternate method [33]. All parameters had the unit g kg−1 except where stated otherwise. The contents of the apparent metabolizable energy (AME) and AMEn of the treatment diets were calculated by Formulas (1) and (2) as described by Yang et al. [34]:
AMED (MJ kg−1) = GED − GEE × TiD/TiE
AMEnD (MJ kg−1) = AMED − [0.03654 × (ND − NE × TiD/TiE)]
AMED and AMEnD represent the apparent metabolizable energy and N-corrected apparent metabolizable energy of the treatment diet, respectively. GED and GEE are the gross energy contents in the diet and excreta. TiD and TiE are the titanium contents in the diet and excreta. ND and NE are the nitrogen contents in the diet and excreta. A correction factor of 0.03654 MJ per gram of nitrogen was used to account for retained nitrogen [35].
Digestibility coefficients of all treatment diets were calculated based on Formulas (3) and (4) in accordance with Ravindran et al. [36]:
AIDD (%) = 1 − [(XI/TiI)/(XD/TiD)]
AIDD represents the apparent ileal digestibility of amino acids, CP, or P, XI and XD are the amounts of amino acids in the ileal digesta and diet, respectively, and TiI and TiD are the titanium concentrations in the ileal digesta and diet. The AID was compensated for basal endogenous losses by the following formula to obtain the standardized ileal digestibility (SID) for amino acids and N:
SIDD (%) = AIDD + XB/XDMI
SIDD is the standardized ileal digestibility of an amino acid or CP; XB and XDMI are the basal endogenous losses and the content per kg of dry matter intake of the respective amino acid or CP. The individual values for basal endogenous losses are from a nitrogen-free diet, according to Adeola et al. [37]. The following two formulae, (6) and (7), were used in accordance with Nalle et al. [33] to calculate the digestibility coefficients and the energy content of the L. minor batches:
DLM (%) = [(DD × XD) − (DBD × SBD × XBD)]/(SLM × XLM)
DLM, DD, and DBD represent the digestibility of the L. minor biomass, the treatment diet, and the basal diet (Control). XLM, XD, and XBD are the nutrient contents of the L. minor biomass, the treatment diet, and the basal diet, respectively. SBD and SLM represent the proportions of the basal diet and the L. minor biomass in each treatment diet. The following formula was used to calculate the energy content of the L. minor biomass:
ELM (MJ kg−1) = [ED − (EBD × SBD)]/SLM
ELM, ED, and EBD represent the energy content (AME and AMEn) of the L. minor biomass, the treatment diet, and the basal diet, respectively.
A statistical analysis was performed with SPSS (Version 26.0.0.0, IBM Corp., Armonk, NY, USA) using the one-way ANOVA. The threshold of significance was set to p ≤ 0.05. Multiple comparison tests were performed using the Bonferroni correction with significance set at p ≤ 0.05.

3. Results

3.1. Experiment 1

The AID and the SID of some amino acids differed between the two duckweed batches. While the SID of methionine, threonine, tryptophan, valine, and aspartic acid were higher in LM2, the digestibility of histidine was lower (Table 5). No difference in AME and AMEn content was discovered. Table 6 gives an overview of the energy contents of the duckweed batches LM1 and LM2.
Table 5. Ileal digestibility (%) of crude protein, amino acids, and phosphorus in two batches of Lemna minor (LM) for broiler chickens on day 26 post-hatch.
Table 6. Apparent metabolizable energy (AME) and nitrogen-corrected apparent metabolizable energy (AMEn) in two batches of Lemna minor (LM) for broiler chickens from days 21 to 26 post-hatch.

3.2. Experiment 2

In experiment 2, the contents of ileal digestible amino acids in the four treatment diets were analyzed. Therefore, only amino acids optimized during feed formulation were recorded. No significant differences were detected. Table 7 gives an overview of the detected values.
Table 7. Contents of standardized ileal digestible amino acids in the four treatment diets on day 28 post-hatch (g kg−1).
The treatments had no significant influence on performance parameters at any time (Table 8). During all phases, live weights were close together. Feed intake and feed conversion could only be evaluated in the last feeding phase; during this phase, the four treatments did not differ. Overall, the integration of duckweed had no influence on the performance of the birds.
Table 8. Zootechnical parameters of broiler chickens fed with increasing amounts of L. minor.

4. Discussion

4.1. Biomass Composition

The CP content of the investigated L. minor batches is within the range of 12% to 40% CP, and these values of L. minor were also reported by Zhao et al. [38] and Khanum et al. [39]. The CP contents are comparable to those of other studies testing L. minor as a protein source for broiler chickens [8,9]. Common protein feedstuffs such as soybean meal or canola meal contain CP at 45% and 34%, respectively [40]. Strategies to increase the CP, as discussed by Petersen et al. [15], are therefore necessary to increase the nutritional value of the L. minor biomass.
The biochemical composition of the batches (particularly regarding the CP content) in experiment 1 is related to the respective cultivation conditions, especially the nutrient medium composition [41]. Iatrou et al. [42] reported that the CP content of L. minor increased from 21.9% to 39.4% with increased ammonium-N concentrations from 0.3 to 31.9 mg L−1. A nutrient medium with an ammonium-N concentration of 27.6 mg L−1 resulted in a CP content of 40.2% [39]. Increasing nitrate-N quantities in the nutrient media from 0.13 to 50.00 mg L−1 increased the CP contents from 15 to 29% [19]. Devlamynck et al. [19] also concluded that an increased nitrate-N concentration could affect the CP content in a L. minor biomass. However, concentrations above 50 m L−1 nitrate-N are proven to be ineffective [19]. All studies agree that nitrate and ammonium can influence the CP contents. In this context, Petersen et al. [13] investigated different ammonium-to-nitrate ratios in the nutrient media for L. minor. Partial substitution of nitrate-N with ammonium-N with a constant total-N concentration increased the CP contents of L. minor, indicating that ammonium-N could be more effective concerning the CP content in L. minor biomass. The adjustment of cultivation conditions in the IVF led to an increased crude protein content. It can be assumed that the increased EC value, as the main parameter for the nutrient medium, caused an increased ammonium and nitrate content in the media. This resulted in an increased CP content in batch LM3, which achieved values similar to those of rapeseed meal [40].
For the species L. minor, starch contents of up to 50% were investigated [43]. However, L. minor batches contained approximately 1% starch. The values are related to the higher N and P concentrations in both nutrient media, as investigated by Zhao et al. [44]. Under N limitation, Landoltia punctata could accumulate starch up to approximately 50% [45]. The crude fiber contents of L. minor ranged between 5% and 20% in previous investigations [46,47]. These values are consistent with the crude fiber contents in the present study. The varying crude fiber contents of the investigated batches could be associated with the different nutrient media. In the experiment of Culley et al. [48], low fiber contents were related to higher nutrient concentrations in the media. Nitrogen levels below 5 mg L−1 resulted in fiber contents of 17% for Lemna gibba, while the fiber content averaged 10% with an N level above 30 mg L−1 [48]. In agreement with the protein content, the different fiber contents could be related to the nutrient content in the media. In particular, the nitrogen form and concentration are relevant factors for L. minor biomass composition and nutritional value.
Compared to outdoor systems, indoor cultivation enables full control of abiotic factors. Light intensity, photoperiod, and temperature—parameters that cannot be regulated outdoors—have a significant influence on duckweed growth and biomass quality. Light intensity, a key determinant of plant growth, cannot be regulated under outdoor conditions, yet it has been shown to significantly influence both growth performance and biomass quality of duckweed [18]. In the present experiment, the day length was also kept constant, whereas under outdoor conditions, this parameter would vary seasonally, thereby affecting growth and hindering continuous production. The same limitations apply to temperature control as reported by Pasos-Panqueva et al. [49].

4.2. Digestibility and Energy Content

The amino acid digestibility of LM2 appears comparable to conventional protein sources such as soybean meal or canola meal (e.g., methionine: LM2 85.6%, soybean meal 91%, and rapeseed meal 87%) [40]. Demann et al. [11] investigated comparable amino acid digestibilities for Lemna obscura (methionine: 90.4%). Cysteine digestibility was below the digestibility of other amino acids in both batches, which is supported by the results of Demann et al. [11]. Considering the importance of cysteine in poultry for the development of feathering, this endorses the relevance of digestibility for feed formulation. Significant variations in the levels of digestibility have been investigated for some amino acids, showing the influence of cultivation conditions on digestibility. In a previous study, the digestibility of all individual amino acids differed significantly between batches of different duckweed species, and the differences were more substantial [11]. Given the minor differences in only a few amino acids in the current study, the question arises as to what extent amino acid digestibility can be affected by cultivation conditions and whether the amino acid digestibility is more characteristic of a particular species.
In order to validate the SID amino acids in batch LM2, the contents of SID amino acids in the complete diets in experiment 2 using batch LM3 were determined. The contents of ileal digestible amino acids did not differ between the treatment diets in experiment 2. Thus, the SID values detected in experiment 1 can be used for the L. minor biomass produced under the described cultivation conditions in the IVF. The increased target EC with constant nutrient ratios has likely not affected the amino acid digestibility.
Demann et al. [11] reported reduced digestibility of amino acids and CP for batches with tannin concentrations of 2943 and 2890 mg kg−1. More considerable differences between SID and in vitro digestibility have been detected in these batches [11]. IVD represents real digestibility in vivo and is unlikely to be influenced by anti-nutritional factors (ANF) [50]. Given the slight difference between IVD and SID in batch LM1 and the little difference between SID in LM1 and that in LM2, it is questionable whether the detected tannins could affect the SID of amino acids and crude protein in LM1. Considering that histidine had an even higher digestibility in the LM1, the effect of tannins would likely affect all amino acids. A substantial difference between IVD and SID was found in LM2. The IVD reflects the real digestibility in vivo but also considers solubility [51]. Thus, IVD may overestimate the real digestibility because soluble nitrogen might not be completely digestible [51,52]. As only tannins and trypsin inhibitors were analyzed and tannins were detected only in batch LM1, other ANFs could be present in LM2, reducing the SID.
In agreement with the results of the present study and those of Demann et al. [11], higher tannin levels occur with lower protein levels of approximately 20%. In other studies, tannin levels ranged from 9 to 16 g kg−1 of dry matter, with CP contents ranging from 18.6 to 28% [53,54,55,56]. One possible explanation for a higher tannin content in LM1 could be that L. minor cultivated outdoors is more likely exposed to biotic and abiotic stress. Controlled cultivation conditions, such as in an IVF, may prevent such stressful situations. Predominantly mechanical damage, e.g., insect wounding, can cause an accumulation of tannins in the cell wall to make plant tissues more stable [57,58].
To this date, no studies have given a deeper insight into the ANF of duckweed and its influence on digestion. Therefore, further research is necessary to identify ANF and their mechanisms, as well as the cultivation parameters that promote the formation of ANF in duckweed. Future studies could focus on factors affecting tannin content in duckweed and related anti-nutritional properties.
The apparent ileal digestibility of phosphorus differed significantly between duckweed batches. However, it is unlikely that the low phytate contents affected P digestibility. Demann et al. [11] reported an AID of P ranging from 50.8% to 78.9%. The phytate P contents were comparable and ranged from 0.17 to 0.43 g kg−1 [11]. Other protein sources can cause decreased P digestibility due to higher phytate contents. In canola meal and soybean meal, phytate P accounts for 60% of the total P [40]. Thus, LM2 can be considered a low phytate and highly digestible P source.
The energy content did not differ between L. minor batches and is comparable to the AMEn levels of sunflower and canola meal at 6.2 and 5.9 MJ kg−1 as fed [40]. Soybean meal has AMEn levels ranging from 9.2 to 9.7 MJ kg−1 [40]. Digestible fractions of protein, carbohydrates, and ether extracts contribute to AME and AMEn [59].

4.3. Zootechnical Parameters

The high quality of the L. minor biomass resulted in comparable animal growth across all treatments, consistent with breeder performance data. While Kabir et al. [9] reported negative growth effects after 14 days of duckweed feeding, other studies observed no differences until day 21 or 28 [8,10,60]. These studies indicate unaffected growth during the first two weeks, but reduced performance at inclusion levels as low as 6–8%. In the final feeding phase, only the 3% duckweed inclusion improved growth, which aligns with the lowest level tested by Ahammad et al. [8]. Similar reductions at higher inclusion rates were also reported by Haustein et al. [61] for L. gibba.
In the present experiment, feed intake could only be statistically evaluated for the final seven days, as the animals were housed individually in cages. There were no significant differences between the groups. These results are also consistent with previous studies. Feed intake was not negatively affected in any broiler trial. This suggests a high palatability regardless of species [8,9,10,60]. Demann et al. [11] used between 25% and 75% duckweed in diets to determine digestibility. Examining a duckweed content of up to 50%, no negative effects on feed intake were observed. Diets comprising 75% duckweed resulted in a sharp reduction in feed intake, which, however, can also be attributed to the significantly increased protein content in the diets [62].
Over the final seven days, feed conversion remained unaffected by the inclusion of duckweed in all experimental groups. In contrast, Kabir et al. [9] reported a significant deterioration in the feed conversion ratio as duckweed proportions increased, with adverse effects observed even at inclusion levels as low as 4%. Conversely, Ahammad et al. [8] observed an improvement in FCR at 3% and 6% duckweed inclusion rates, whereas a further increase to 9%—comparable to group T4 in the present experiment—resulted in a marked decline in feed efficiency. Only Zaffer et al. [60] were able to prevent this deterioration in FCR by supplementing the diet with an unspecified enzyme complex. Assuming these were non-starch polysaccharide (NSP)-degrading enzymes, their inclusion may have supported nutrient utilization in fiber-rich diets by reducing digesta viscosity and improving the availability of otherwise indigestible carbohydrates such as xylose [63].
L. minor biomass was integrated into the treatment diets based on equal crude protein and energy contents. Amongst others, digestible AA, P, MJ, and AMEn were used to optimize individual diets (see Table 4). In contrast to the present study, Zaffer et al. [60] and Kabir et al. [9] do not provide any information about the diet ingredients nor their AA profile. In addition, Chantiratikul et al. [10] significantly increased the content of full-fat soybeans in the experimental diets to compensate for the lower energy content. As a result, these diets contain higher levels of digestive inhibitors, such as trypsin inhibitors, which have well-documented negative effects on the performance of broilers [64].
Previous studies insufficiently addressed amino acids and the possible effects of their digestibility. Ahammad et al. [8] and Chantiratikul et al. [10] provide information on the gross amino acid content in the diets for methionine, lysine, and tryptophan. No information is provided on other AAs. This may explain the poorer performance in the test groups. On one hand, the different digestibilities were not taken into account, and on the other hand, imbalances may have existed in AAs that were not investigated [65]. To avoid this problem, the AA profile in experiment 2 was optimized on the basis of digestible AAs as determined in the first experiment. Due to the high phosphorus content of duckweed, it was possible in treatment 4 to dispense with monocalcium phosphate in all feeds and to supply the animals with only the native phosphorus from the feed, without negatively impacting the zootechnical parameters.

5. Conclusions

The L. minor batch from the IVF can be considered a highly digestible amino acid and P source, and a suitable protein feed for broiler chickens. However, cultivation conditions significantly affected the nutritional value of L. minor, mainly due to the chemical composition of the batches. Cultivation also played a role in the digestibility of some amino acids and phosphorus to a certain level. Therefore, the control of all abiotic parameters in the duckweed cultivation process ensures a product quality suitable for use as a protein feed for broiler chickens. An IVF particularly enables this level of control and standardization. Current vertical indoor farms for duckweed are operated on a reduced scale adequate for fundamental research. Although direct upscaling remains challenging, controlled production conditions and high area productivity suggest considerable potential for future large-scale, resource-efficient protein production.
Our findings show that the consideration of digestibility is a prerequisite for the large-scale use of duckweed in poultry nutrition and might elevate previously observed limitations in the use of duckweed in broiler diets.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/poultry5020024/s1, Table S1. Nutrient concentrations in the cultivation media for Lemna minor (LM); Table S2. Amino acid contents (g kg−1 as fed) in two batches of Lemna minor (LM) biomass.

Author Contributions

Conceptualization, J.N., J.D. and G.D.; methodology, J.N., J.D., F.P., G.D., R.D., A.U. and H.W.; validation, J.N., J.D. and G.D.; formal analysis, J.N. and J.D.; investigation, J.N., J.D. and F.P.; resources, R.D., A.U., H.-W.O. and H.W.; data curation, J.N. and J.D.; writing—original draft preparation, J.N., J.D., F.P. (materials and methods) and R.D. (materials and methods); writing—review and editing, J.N., J.D., F.P., G.D. and H.-W.O.; visualization, J.N. and J.D.; supervision, H.W.; project administration, H.W.; funding acquisition, A.U., H.-W.O. and H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Deutsche Bundesstiftung Umwelt (DBU), grant numbers 34223/01 and 38182/01.

Institutional Review Board Statement

This experiment was conducted at the University of Applied Sciences Bingen (Life Science and Engineering, 55411 Bingen am Rhein, Germany). Animal welfare and trial protocol were controlled and approved by ‘Landesuntersuchungsamt Rheinland-Pfalz’, 56068 Koblenz, Germany (approval number: 23 177-07, 4 November 2020).

Data Availability Statement

The original contributions presented in this study are included in the article or Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank the Institute of Animal Science, University of Hohenheim, Stuttgart, Germany, for phytate analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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