A moderate protein diet does not cover the requirements of growing rabbits with high growth rate

Targeted and untargeted metabolomic profiles in wild rabbit does (Oryctolagus cuniculus) of different breeding states (pregnant and lactating)

Ecological nutrition aims to unravel the extensive web of nutritional links that drives animals in their interactions with their ecological environments. Nutrition plays a key role in the success of European wild rabbit (Oryctolagus cuniculus) and could be affected by the breeding status of the animals and reflected in the metabolome of this species. As nutritional needs are considerably increased during pregnancy and lactation, the main objective of this work was to determine how the breeding status (pregnant and lactating) of European wild rabbit does affects nutritional requirements and their metabolome (using targeted and untargeted metabolomics), aiming to find a useful biomarker of breeding status and for monitoring nutritional requirements. To address this gap, 60 wild European rabbits were studied. Animals were divided according to their breeding status and only pregnant (n = 18) and lactating (n = 11) rabbit does were used (n = 29 in total). The body weight and length of each animal were analyzed. The relative and absolute chemical composition of the gastric content and whole blood sample were taken, and targeted and untargeted metabolomics were analyzed. As a main result, there were no differences in biometric measurements, gastric content, and targeted metabolomics, except for live weight and nonesterified fatty acids (NEFA), as pregnant animals showed higher live weight (+12%; p = 0.0234) and lower NEFA acid levels (-46%; p = 0.0262) than lactating females. Regarding untargeted metabolomics, a good differentiation of the metabolome of the two breeding groups was confirmed, and it was proven that pregnant animals showed higher plasmatic levels of succinic anhydride (3.48 more times; p = 0.0236), succinic acid (succinate) (3.1 more times; p = 0.0068) and propionic acid (3.98 more times; p = 0.0121) than lactating animals. However, lactating animals showed higher levels of N-[(3a,5b,7b)-7-hydroxy-24-oxo-3-(sulfoxide) cholan-24-yl]-Glycine (cholestadien) (2.4 more times; p < 0.0420), 4-maleyl-acetoacetate (MAA) (3.2 more times; p < 0.0364) and irilone (2.2 more times; p = 0.0451) than pregnant animals, any of these metabolites could be used as a potential biomarker. From these results, it can be concluded that the most notable changes were observed in the metabolome of individuals, with most of the changes observed being due to energy and protein mobilisation.

Biomarkers for ideal protein: rabbit diet metabolomics varying key amino acids

With the main aim of identifying biomarkers that contribute to defining the concept of ideal protein in growing rabbits under the most diverse conditions possible this work describes two different experiments. Experiment 1: 24 growing rabbits are included at 56 days of age. The rabbits are fed ad libitum one of the two experimental diets only differing in lysine levels. Experiment 2: 53 growing rabbits are included at 46 days of age, under a fasting and eating one of the five experimental diets, with identical chemical composition except for the three typically limiting amino acids (being fed commercial diets ad libitum in both experiments). Blood samples are taken for targeted and untargeted metabolomics analysis. Here we show that the metabolic phenotype undergoes alterations when animals experience a rapid dietary shift in the amino acid levels. While some of the differential metabolites can be attributed directly to changes in specific amino acids, creatinine, urea, hydroxypropionic acid and hydroxyoctadecadienoic acid are suggested as a biomarker of amino acid imbalances in growing rabbits' diets, since its changes are not attributable to a single amino acid. The fluctuations in their levels suggest intricate amino acid interactions. Consequently, we propose these metabolites as promising biomarkers for further research into the concept of the ideal protein using rabbit as a model.

Nutritional ecology of European rabbit (Oryctolagus cuniculus): Factors affecting chemical composition of gastric content

Nutritional ecology seeks to unravel the extensive web of nutritional links that directs animals in their interactions with their ecological and social environments. European rabbit (Oryctolagus cuniculus) populations its endemic locations are declining and it is considered a keystone species of the Mediterranean ecosystem prompteing the interest in its conservation. The main aim of this study was to determine the nutritional composition of the diet of European rabbits through the relative and absolute chemical composition of the gastric content. To address this objective, gastric content was collected from 80 European rabbits in a Mediterranean area for the analysis of its chemical composition. To this end, gastric content was analyzed for dry matter (DM), organic matter (OM), ash, crude protein (CP), highly digestible nonnitrogenous nutrients (HDNN), neutral detergent fibre (NDF), acid detergent fibre (ADF) and lignin. The rabbits were divided into two groups: EMPTY and FULL, depending on the level of stomach filling, directly related to food intake. Our results revealed a positive correlation between the rabbits weight with DM in the gastric content, total gastric content with DM in the gastric content, and DM in gastric content with all chemical parameters analysed. The mean relative values obtained were 8.8%, 25.5%, 40.4% and 25.4%, for ash, CP, NDF and HDNN, respectively. Moreover, EMPTY rabbits had both a proportional (+19%, p = 0.002 and -40%; p = 0.004, on NDF and HDNN, respectively) and absolute (-38%, p = 0.014, -52%; p = 0.012, -52%; p = 0.011 and +83%; p = 0.008 for OM, ash, HDNN, and lignin, respectively) different proportion of nutrients in gastric contents than FULL animals. Since there is a connection between this availability and the fitness of this species, understanding the chemical composition of the rabbit's diet can be utilised to delve into its biology. Our study provides information that will help elucidate the factors affecting the chemical composition of the gastric content of European rabbits to assist land use planners and conservationists in identifying sites for conservation in Mediterranean ecosystems.

Effect of Increasing the Methionine Level and Reducing the Threonine Level in the Diet of Fast-Growing Rabbits

The main aim of this work was to evaluate a novel combination of the first limiting amino acids (lysine, methionine, and threonine) in fast-growing rabbits (combination MHL, shown to minimise levels of plasmatic urea nitrogen in previous research, medium level of lysine, high level of methionine, and low level of threonine) and compare it with current recommendations (combination MMM, medium level of all amino acids). A total of 165 weaned rabbits (28 d old) from a paternal line selected for growth rate were used in the growth trial. The effect of the diet on apparent fecal digestibility, as well as the apparent and true ileal digestibility, were studied. Nutrient retention was also determined. Although no differences in digestibility between diets were observed, animals fed with Diet MHL improved the global average daily gain (+2.3 g/d; p = 0.0482) and feed conversion ratio (-0.10; p = 0.0229). Animals fed with Diet MMM reduced the protein-to-energy ratio retained (p = 0.0086). In conclusion, Diet MHL promoted an improvement in growth traits in a paternal line. Consequently, we propose its levels of 6.4, 5.4, and 5.0 g/kg DM of true ileal digestible lysine, methionine, and threonine, respectively.

Unravelling the Role of Metabolites for Detecting Physiological State of Wild Animals: European Rabbit's (Oryctolagus cuniculus) Case

European wild rabbit (Oryctolagus cuniculus) has been defined as a keystone species in the Mediterranean ecosystem. Rabbits have been classed as "endangered" by the IUCN within their native range. In this sense, animal nutrition may play a fundamental and limiting role in the conservation of wild species. The overarching goal of ecological nutrition is to unravel the extensive web of nutritional links that direct animals in their interactions with their ecological environments. The main aim of this work was to evaluate the effect of different feed intake, geographic location, animal sex, and reproductive stage on glucose, non-esterified fatty acids (NEFA), and plasmatic urea nitrogen (PUN), albumin, glutamate, and total protein metabolites. Additionally, we examined the potential of these metabolites as biomarkers. Full stomach contents and blood samples were collected from European wild rabbits (n = 89) for the analysis of the metabolites described above. Our work shows that the levels of these metabolites are affected by the sex of the animals, as well as by their reproductive stage (glucose, NEFA and albumin). There were signs of better optimisation of resources by females than by other groups of animals. These data may be interesting in the study of nutritional components that could be affecting physiological state of this species.

Nutritional Metabolites as Biomarkers of Previous Feed Intake in European Rabbit (Oryctolagus cuniculus): Applications on Conservation

European wild rabbit (Oryctolagus cuniculus) populations have drastically reduced, and recently, rabbits have been classed as “endangered” by the IUCN. This animal plays an important ecological role in Mediterranean ecosystems and its introduction could significantly contribute to ecological restoration. Rabbits have high nutrient requirements that apparently cannot be covered in all ecosystems, and there are clues that nutrition can limit their abundance and density. On the other hand, some studies reflect the effects of food availability on the metabolomic status of other animal species, but there are no specific studies on this keystone species. The main aim of this work is to find biomarkers to assess the previous levels of ingestion of European rabbits (Oryctolagus cuniculus). To address this gap, gastric content and blood samples were collected from European rabbits (n = 99) in a Mediterranean area for the analysis of glucose, non-esterified fatty acids (NEFA), plasmatic urea nitrogen (PUN), albumin, glutamate and total protein metabolites. Depending on their previous feed intake (gastric content and the ratio between the gastric content and the weight of the animal), the animals were divided into two groups (lower and normal previous feed intake). Our work shows that the metabolomic profiles of the animals were affected. Levels of glucose (+82%; p = 0.0003), NEFA (−61%; p = 0.0040) and PUN (+139%; p < 0.001) were different in the animals with lower previous feed intake than the animals with normal previous feed intake. This work summarises that metabolic phenotype can be interesting when seeking to discover the limiting nutrients and food availability in diets that could affect the ecological fitness and conservation of European wild rabbits. It is important to mention that in this work, only the effects on six different metabolites have been analysed and more studies are necessary to complement the knowledge of possible metabolites that indicate the level of ingestion in this species and others. These (and new) biomarkers could be used as a tool to provide information about individual or population characteristics that other physiological parameters cannot detect, improving the conservation physiology field.

Growth Performance, Digestive Efficiency, and Meat Quality of Two Commercial Crossbred Rabbits Fed Diets Differing in Energy and Protein Levels

The present study evaluated the effect of digestible energy (DE) and crude protein (CP) levels on growth, digestive efficiency, slaughter yield and meat quality in growing rabbits of two genotypes (Grimaud, G vs. Hyla, H). A total of 384 crossbred rabbits (192 G and 192 H), half males and half females, were divided into eight experimental groups according to a bi-factorial arrangement (2 genotypes × 4 diets; 48 animals/group) and reared in individual cages. From 33 to 64 days of age, rabbits were fed four diets obtained by combining two DE levels (high, HE—10.9 MJ/kg vs. low, LE—9.2 MJ/kg, on average) and two CP levels (high, HP—159 g/kg vs. low, LP—144 g/kg, on average). Then, all rabbits received the same fattening diet (10.7 MJ DE/kg; 156 g CP/kg) until slaughtering (77 days of age). Growth performance did not differ between genotypes for the entire trial (on average final live weight 3010 g; daily weight gain 50.5 g/d), but H rabbits showed a faster growth during the post-weaning period (p ≤ 0.01) compared to G rabbits, while exhibiting lower slaughter yield and lower carcass fatness (p ≤ 0.01). As DE level increased, feed intake decreased and feed conversion improved (p < 0.001), while caecal volatile fatty acid concentration increased and caecal pH and N-ammonia decreased (p ≤ 0.01). The reduction in the CP level did not affect performance and carcass traits. No significant interaction was detected between genotype and diet. In conclusion, G rabbits may have an advantage over H rabbits because of the higher slaughter yield. As for the nutritional characteristics of diets for high-producing crossbred rabbits, high energy levels are confirmed to enhance feed efficiency, whereas dietary CP can be reduced to 144 g/kg without negative effects on performance and carcass traits.

Urea Nitrogen Metabolite Can Contribute to Implementing the Ideal Protein Concept in Monogastric Animals

Simple Summary

Can urea nitrogen metabolite contribute to implementing the ideal protein concept in monogastric animals? This work aims to critically analyse how this metabolite can contribute to accurately implementing the ideal protein concept in monogastric animals, particularly in pig, poultry, and rabbit nutrition. This information will contribute to evaluating its potential and limitations as biomarker, as well as to standardizing the use of this metabolite in precise amino acidic monogastric nutrition.

Abstract

The ideal protein concept refers to dietary protein with an amino acid profile that exactly meets an animal’s requirement. Low-quality protein levels in the diet have negative implications for productive and reproductive traits, and a protein oversupply is energetically costly and leads to an excessive N excretion, with potentially negative environmental impact. Urea Nitrogen (UN), which corresponds to the amount of nitrogen in the form of urea circulating in the bloodstream, is a metabolite that has been widely used to detect amino acid imbalances and deficiencies and protein requirements. This review aims to critically analyse how UN can contribute to accurately implementing the ideal protein concept in monogastric animals, particularly in pig, poultry, and rabbit nutrition (14,000 animals from 76 published trials). About 59, 37, and 4% of trials have been conducted in pigs, poultry, and rabbits, respectively. UN level was negatively correlated to main performance traits (Pearson Correlation Coefficient [PCC] of −0.98 and −0.76, for average daily gain and feed conversion ratio, respectively), and lower UN level was related to higher milk yield and concentration. High level of UN was positively correlated to N excretion (PCC = 0.99) and negatively correlated to protein retention (PCC = −0.99). Therefore, UN in blood seems to be a proper indicator of amino acid imbalance in monogastric animals. Great variability in the use of UN was observed in the literature, including uses as determination medium (blood, plasma, or serum), units, and feeding system used (ad libitum or restricted), among others. A standardization of the methods in each of the species, with the aim to harmonize comparison among works, is suggested. After review, UN measurement in plasma and, whenever possible, the utilization of the same nutritional methodology (ad libitum conditions or restriction with blood sampling after refeeding at standardised time) are recommended. More studies are necessary to know the potential of UN and other bioindicators for amino acid deficiencies evaluation to get closer to the ideal protein concept.

Hematological and biochemical parameters and resistance of the organism of mother rabbits receiving sulfur compounds

Nanotechnologies are developing continuously, and their use is becoming more diverse and concrete, with high potential to improve the products of animal husbandry and animals in general. Yet, there have been few studies of nanotechnologies in those spheres. However, to confirm the efficiency and, mainly, safety of nanotechnologies, to avoid any harm to the environment or especially humans and animals, substantiated studies are still needed. A promising, but barely studied, direction of research is dosage of nanocompounds of biogenic elements in diets of animals, both modern supplements and alternatives for correcting mineral nutrition. Therefore, the objectives of the study presented here were changes in hematologic, biochemical and immune-biological parameters of the organism of mother rabbits under the influence of sulfur citrate and sodium sulfate for 14 days until insemination and until day 20 of lactation. The research was conducted on mother rabbits of the Hyla breed which were giving birth for the second time in LLC Horlytsia in Dobriany village of Horodok district of Lviv Oblast. Mother rabbits of the control group were fed ad libitum with complete granulated mixed feed with free access to water. Animals of experimental group I were fed feeds of the diet of the control group and during the day were watered sulfate citrate in dosage of 8 µg of S/kg of body weight. Females of experimental group II were fed the diet feeds of the control group and given sodium sulfate (Na2SO4) in the amount of 40 mg of S/kg of body weight. The mother rabbits received the supplements for 14 days both until insemination and for up to the 20th day of lactation. In the preparation period, on day 10 after the beginning of the study and on day 20 of lactation in the experimental period (65th day of watering the supplements), blood samples were taken from the marginal ear vein of the mother rabbits for hematological and biochemical studies. The study revealed that watering sulfate citrate in the dosage of 8 µg of S/kg of the body weight in the diet of mother rabbits for 14 days until insemination and up to day 20 of lactation increased the amount of erythrocytes by 19.5%, leukocytes by 37.5%, granulocytes by 38.3%, concentration of hemoglobin by 21.0%, mean corpuscular hemoglobin by 15.6%, and red cell distribution width by 14.7%, and activated protein metabolism, which manifested in 8.5% higher content of protein and activity of aspartate aminotransferase, alanine aminotransferase and alkaline phosphatase respectively by 12.9%, 29.6%, 19.8% and 51.8% lower content of triacylglycerols in the blood on day 20 of lactation compared with the control group. Intake of sulfur citrate heightened the parameters of immunobiological reactivity of the organisms of mother rabbits on day 65 of the experiment with significantly higher level of phagocytic number, phagocytic activity, lysozymic and bactericidal activities of blood serum, protein-associated hexoses, sialic acids and ceruloplasmin and immune globulins, compared with the control. Giving mother rabbits sodium sulfate in the amount of 40 mg/kg of body weight for 65 days led to less notable changes in the blood, expressed in 20.0% increase in mean corpuscular hemoglobin, higher phagocytic value, phagocytic activity, lysozymic and bactericidal activities of blood serum, protein-associated hexoses and ceruloplasmin. The results of the studies indicate possibility of additional use of supplement sulfur citrate in the amount of 8 µg of S/kg in diet of mother rabbits to improve metabolism, reproductive ability and immunological resistance in the period of increased physiological load.

What Are the Keys to the Adaptive Success of European Wild Rabbit (Oryctolagus cuniculus) in the Iberian Peninsula?

The European wild rabbit (Oryctolagus cuniculus) plays an important ecological role in the ecosystems of the Iberian Peninsula. Recently, rabbit populations have drastically reduced, so the species is now considered endangered. However, in some places, this animal is considered a pest. This is the conservation paradox of the 21st century: the wild rabbit is both an invasive alien and an endangered native species. The authors of this review aimed to understand the keys to the adaptive success of European rabbits, addressing all aspects of their biology in order to provide the keys to the ecological management of this species. Aspects including nutrition, genetics, immunity interactions with the environment, behaviour, and conflict with human activities were reviewed. Ultimately, rabbits are resilient and adaptable. The main adaptations that explain the rabbit's adaptive success are its nutrition (wide adaptation to food and good nutritional use of caecotrophy), immune system (powerful and developed), and other aspects related to genetics and behaviour. Rabbits' relationship with humans has led them to colonise other places where they have become pests. Despite these adaptations, populations in native places have been drastically reduced in recent years. Since it serves as a bastion of the Mediterranean ecosystem, a specific conservation program for this species must be carried out. Therefore, a study of the rabbit's response to diseases and nutrition (especially protein), as well as the interaction between them, is of special interest.

Do Growing Rabbits with a High Growth Rate Require Diets with High Levels of Essential Amino Acids? A Choice-Feeding Trial

As a consequence of the genetic selection process in growing rabbits, there are currently fast-growing animals exhibiting an average daily gain that may exceed 45 g/d. The protein requirements of these animals, namely amino acid requirements, may differ from animals with low growth rates. The objective of this work was to evaluate growth performance, the coefficient of total tract apparent digestibility (CTTAD), the apparent ileal digestibility (AID) of amino acids and nutrient retention of fast-growing rabbits when they had access to a diet with high levels of amino acids and/or a diet formulated with current nutritional recommendations in a choice-feeding trial. To this end, two diets were formulated: the M diet following current nutritional recommendations for growing rabbits (including 8.1, 5.8 and 6.9 g/kg dry matter (DM) of total lysine, sulphur amino acids and threonine, respectively) and the H diet with higher levels of total lysine, sulphur amino acids and threonine (9.4, 6.6 and 7.8 g/kg DM, respectively). A total of 220 weaned rabbits, from a paternal line selected for the growth rate, had free access to the M diet, the H diet or both (MH) diets from 28 to 63 days of age. The CTTAD of DM, crude protein and gross energy from 49 to 53 days of age as well as the AID of amino acids of the diets at 63 days of age were determined. Protein, amino acids and energy retained in the empty body from 28 to 63 days of age were also registered. No significant differences in growth performance, CTTAD, AID and nutrient retention between dietary treatments were observed. However, animals fed the H diet showed a higher AID of cysteine (p < 0.05) and higher threonine retention (p < 0.05) compared to the M diet. As regards the choice-feeding trial, MH animals showed a higher feed intake of the M diet compared to the H diet (+8.61%; p < 0.001), and furthermore, more than 50% of the animals preferred the M diet throughout the experimental period (p < 0.05). Our results suggest that animals with high growth rates do not show significantly higher productive traits when fed the H diet compared to the M diet. As regards choice feeding, MH animals were capable of choosing their preferred diet, showing high intra-individual repeatability in preference for the M diet. It would be interesting to continue studying this behaviour of choice based on amino acid levels.

Plasma urea nitrogen as an indicator of amino acid imbalance in rabbit diets

<p>In recent decades, recommendations on dietary protein content have been considerably reduced, while fibre content has been increased. Under these conditions, an adequate dietary amino acid balance could be crucial to optimise feed efficiency. Plasma urea nitrogen (PUN) level could be a good indicator of an amino acid imbalance and its potential has already been studied in other species, but not yet in rabbits. The main objective of the present work was to detect the possible interest of PUN in pinpointing amino acid deficiencies in rabbits. Two experimental diets were formulated from the same basal mixture, following all the recommendations for growing rabbits, except lysine, whose content was variable, following current guidelines in diet P8.1 or lower from those in P4.4 (with 8.1 and 4.4 g/kg dry matter of lysine and with 757 and 411 mg of lysine per MJ of digestible energy). Three different trials were designed: one where the animals were fed &lt;em&gt;ad libitum&lt;/em&gt; (AL) and two others in which fasting periods of 10 h were included; one where feeding was restored at 08:00 h (Fast8h) and the other at 18:00 h (Fast18h). A total of 72 three-way crossbred growing rabbits (24 animals for each trial in a split-plot trial) up to a total of 12 recordings were used. Blood samples were taken every 4 h in AL trial and every hour after refeeding up to a total of six controls, in trials Fast8h and<br />Fast18h. The differences between balanced and unbalanced diets in lysine were highest (&lt;em&gt;P&lt;/em&gt;&lt;0.001) between 04:00 h and 12:00 h in animals fed ad libitum, and at 3 h after refeeding (21:00 h) in Fast18h. These results suggest that PUN could be an adequate indicator to detect deficiencies in amino acids in growing rabbit<br />diets.</p>

Plasmatic Urea Nitrogen in Growing Rabbits with Different Combinations of Dietary Levels of Lysine, Sulphur Amino Acids and Threonine

Simple Summary

Formulating diets to maximize nutrient harnessing has positive effects on performance and environment. In the case of growing rabbits, clues exist indicating that animals with high growth rate when consuming current diets show lower protein retention than expected, and it could be related to amino acid supply. The aim of this work is to find the amino acid combination (27 experimental diets: 3 levels of the 3 main limiting amino acids: lysine, sulphur amino acids, and threonine) that would minimize the nitrogen excretion in the bloodstream, a marker of the efficiency in the amino acid use This combination is a good candidate to be tested in order to improve performance and reduce pollution.

Abstract

A total of 27 experimental diets were formulated starting from the same basal mixture, with a moderate content of crude protein and digestible energy (155 g and 9.86 MJ/kg of digestible matter (DM), respectively, both estimated). The contents of lysine, sulphur amino acids and threonine were variable. The first one, close to the current recommendations (Medium, M; 8.1, 5.8 and 6.9 g/kg DM for lysine, sulphur amino acids and threonine, respectively), and two other levels were on average 15% higher (High, H; 9.4, 6.6 and 7.8 g/kg DM for lysine, sulphur amino acids and threonine, respectively) or lower (Low, L; 6.7, 4.9 and 5.7 g/kg DM for lysine, sulphur amino acids and threonine, respectively). Diets were named with three letters, indicating lysine, sulphur amino acids and threonine levels, respectively. In total, 918 weaned rabbits (28 days old) were used (34 per diet). At weaning, animals were fed ad libitum with a commercial diet until day 46, day 47 each collective cage was randomly switched to one experimental diet. At day 48, blood samples were collected at 08:00h then the animals were subjected to 10 h of fasting and a second blood sample was extracted at 21.00h. At 08:00h, Pasmatic urea nitrogen (PUN) was higher with the L level of lysine (p < 0.001), unaffected by the level of sulphur amino acids and increased with the level of threonine (p < 0.001). At 21:00h, minimum PUN was observed with the MHL diet (14.72 ± 0.661 mg/dL). Taken into account the usual recommendations (established for a diet containing 11.3 MJ DE/kg DM, and then being 0.72, 0.51 and 0.61 g/MJ DE for lysine, sulphur amino acids and threonine, respectively), these results suggest that a diet containing more lysine and sulphur amino acids per energy unit (around 0.82 and 0.67 g/MJ DE) could better fit the growing rabbit requirements, although studies on the effects of such a diet on performance and protein retention are necessary.