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Simple Summary: There is evidence that the genetic progress in paternal rabbit lines is lower than expected due to animal- and environmental-related factors, as well as founding factors intrinsically linked to the effectiveness of the selection process. In response to this question, we conducted a trial to evaluate the response after 18 generations of selection for increased growth rate within a paternal line on rabbit performance. Our results indicate that there were no differences in the key indicators in selection for growth rate (average daily gain and feed conversion ratio) between both populations differing in the generation of selection. These findings indicate a lack of effectiveness in the genetic progress of paternal rabbit lines based on different works carried out with these same genetic lines and generations. A total of 338 weaned rabbits (from the R line, selected for post-weaning growth rate) were used to evaluate the response to 18 generations of selection for increased growth rate on rabbit performance. Animals were obtained from two vitrified populations of the R line: R19V, belonging to the 18th generation (n = 165), and R37V, belonging to the 36th generation (n = 173), were allocated in individual and collective pens (178 and 160, respectively). A fattening trial was conducted from weaning (28 d of age until 63 d of age). During the trial, the body weight (BW), daily feed intake (DFI), average daily gain (ADG) and feed conversion ratio (FCR) were weekly monitored. Additionally, mortality and morbidity were daily registered. On days 49 to 53, an apparent faecal digestibility trial was also performed (12 animals per generation). Our results indicate that the generation of selection for growth rate did not affect mortality and morbidity. There were no differences in the diet digestibility according to the generation of selection. Regarding performance traits, R37V animals showed higher global BW (+6.7%; p = 0.0011) than R19V animals. R37V animals showed the same BW at weaning; however, R37V animals showed higher BW values in the last three weeks compared with R19V animals. Animals from the R37V generation also showed a higher DFI from 56 to 63 d of age (+12%; p = 0.0152) than R19V animals. However, there were no differences in global ADG and FCR between generations. These results indicate that the selection for growth rate in growing rabbits has slowed down, suggesting a lack of effectiveness in the genetic progress. [ABSTRACT FROM AUTHOR]

Simple Summary: The use of diets with optimal levels of amino acids in animal production will cause the animal's productive traits to be better, to have less contamination, etc. This work compares two diets: Diet MMM, with an amino acid level according to the current recommendations (M = medium levels of lysine, methionine, and threonine, respectively), and Diet MHL, with higher levels of lysine (H), medium levels of methionine, and lower levels of threonine, which improved protein utilization in a previous experiment. This work analyzes the effects of this novel combination on performance, digestibility, and retention. Finally, it is concluded that this new combination, which reduced the excretion of urea, improves the growth rate, the feed conversion ratio, and the retention of the main nutrients in 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. [ABSTRACT FROM AUTHOR]

Simple Summary: This study aimed to evaluate rabbits' responses to the administration of six organic acids (OAs) through drinking water at three concentrations (pH levels). Animal drinking and feeding behaviour, pH evolution along the animal's gastrointestinal tract, and gastric pepsin activity were monitored in post-weaning rabbits for seven days. The screened combinations were used to identify the optimal OA candidates and pH values, while eliminating those that may exhibit early adverse effects in young rabbits. During our short-term assessment period, the OAs with the greatest potential for use in rabbit farming were formic acid, acetic acid, and citric acid at pH 4. However, further validation with a larger population of animals, and extending the duration of OA supplementation during the growing period could improve performance outcomes and enhance the modulation effect of the gastrointestinal environment. Organic acids (OAs) are employed in animal feed to regulate gastrointestinal disorders and diarrhoea thanks to their ability to modulate the gastrointestinal environment and their antimicrobial capacity. However, there is not enough evidence regarding the most adequate OA and its effectiveness in rabbit farming. Therefore, the aim of this study was to screen and evaluate the response of young rabbits to six OAs, administered via drinking water, at three different concentrations (pH levels). Organic acids (acetic, ACET; formic, FOR; propionic, PROP; lactic, LAC; citric, CIT; and butyric, BUT) were tested at three concentrations (pH 3, 4, and 5). A negative control (CON; non-acidified water) was also included. We used 240 weaned rabbits (28 days old) divided into 2 batches. In each batch, animals were randomly allocated to 1 of the 19 experimental treatments and were housed in group cages of 6 animals per cage, treatment, and batch. Among the 240 rabbits, an additional cage with 6 animals was included to determine the initial physiological state of the animals. All animals were fed with commercial pelleted feed throughout the whole experiment. The duration of the study was 7 days, until 35 days of age. At 31 and 35 days of age, in each batch, three animals per day and treatment were slaughtered. The pH of the digestive contents in the fundus, antrum, duodenum, jejunum, ileum, and cecum, as well as the gastric pepsin enzyme activity, was measured. Water and feed consumption per cage and individual body weight (BW) were recorded daily. The type and dosage of OAs affected water intake. ACET 3, PROP 3, and BUT 3 reduced water intake compared to CON, negatively impacting feed intake and weight gain. FOR and CIT acids led to the highest BW and weight gain at 35 days, compared to PROP, LAC, and BUT (p < 0.05); showing ACET intermediate values. While OAs had limited effects on gastric and small intestine pH, acidified water at pH 4 and 5 lowered ileum and caecum pH (p < 0.05) compared to pH 3. Acidified water at pH 4 showed the highest (p < 0.05) pepsin activity compared to pH 3 and pH 5. Considering the limited sample size and short-term assessment period of our screening test, the OAs with the highest potential for use in post-weaning rabbits were FOR, ACET, and CIT at pH 4. The selected combinations did not exhibit any early adverse effects in young rabbits. These results should be further confirmed in a broader population of animals. It would also be advisable to extend the application of OAs over longer periods to evaluate their effects throughout the entire growing period of rabbits. [ABSTRACT FROM AUTHOR]

Simple Summary: Beet pulp is a raw material widely used in feed for growing rabbits, because it is a good source of soluble fiber, and its inclusion is frequently associated with a reduction in the incidence of digestive disorders. Beet pulp may or may not be accompanied by the molasses that is also obtained in the beet sugar extraction process, but no information is available on the effect of molasses presence on the response of the rabbits. This work evaluates the effect of the inclusion of beet pulp, with or without molasses, on the growth performance, carcass, digestive tract, and fermentative profile of the cecum in growing rabbits. The results of the present work have shown that beet pulp linearly reduced the growth performance and carcass yield of growing rabbits and, although the inclusion of beet pulp could contribute to reducing the risk of digestive disorders, when beet pulp included molasses, even higher incidence was observed. To evaluate the effect of dietary level of beet pulp, with or without molasses, on growth performance, a total of 470 28-day-old rabbits were used (614 ± 6 g). Animals were randomly allocated into five dietary treatment groups: Control, without beet pulp; BP20, and BP40 with 20 and 40% of beet pulp without molasses, respectively; and BPM20 and BPM40, with 20 and 40% of beet pulp with molasses, respectively. Daily feed intake (DFI) and average daily gain (ADG) were controlled at 28, 49, and 59 days of age. Carcass and digestive tract traits were also determined at 59 days of age. Mortality and morbidity were controlled daily. Mortality during the growing period was higher in BPM than in BP groups (+9.2%; p < 0.05). The higher the inclusion of beet pulp, the lower the DFI and ADG of animals (5.5 and 4.6% for every 20% inclusion, respectively; p < 0.001), as well as the dressing out percentage, the liver proportion, and the dissectible fat percentage of their carcasses. However, the best feed efficiency during the last 10 days was obtained with the BPM40 group. The higher the inclusion of beet pulp, the higher the weight of the empty gastrointestinal tract and cecum (+2.4 and +3.0 percentage points for every 20% inclusion, respectively; p < 0.001). In fact, a higher inclusion of beet pulp decreased the pH and dry matter and decreased the total volatile fatty acids content of cecum richer in acetic acid but poorer in propionic, isobutiric, isovaleric, and valeric acids. Stomach weight was lower, and the capric acid content in the cecum was higher in the BPM than in the BP group. The inclusion of beet pulp in the feed reduced the growth performance and carcass yield of growing rabbits, and an even higher incidence of digestive disorders was observed when beet pulp included molasses. [ABSTRACT FROM AUTHOR]

The effect of dietary inclusion of distillers dried grains with solubles (DDGS) on carcass and meat quality of longissimus muscle was studied in 100 growing rabbits from 28 to 59 days old. Diets with no DDGS (C), barley (Db20), wheat (Dw20) and corn (Dc20) DDGS at 20% and corn (Dc40) DDGS at 40% were formulated. No effects on most of the carcass traits, texture and water holding capacity were found. Barley and corn DDGS led to a higher dissectible fat percentage. Meat redness was higher with Dw20 and pH was higher with Dw20 and Db20 than with Dc20. Protein and saturated fatty acids concentration declined as corn DDGS level increased. Dc40 led to the lowest saturated/unsaturated fatty acid ratio, atherogenic index and thrombogenic index. In conclusion, dietary inclusion of these DDGS at 20% did not affect most of the carcass and meat quality traits in rabbits. [ABSTRACT FROM AUTHOR]

Simple Summary: Two diets were designed to investigate the effect of the growth rate on amino acid requirements in growing rabbits: M diet (with medium levels of amino acids, following current nutritional recommendations for growing rabbits) and H diet (with high levels of lysine, sulphur amino acids and threonine compared with current nutritional recommendations). Performance, nutrient retention and digestibility (faecal and ileal) trials, as well as a choice-feeding test were conducted. We found no differences in animal performance, nutrient retention and digestibility parameters between diets, but the animals showed a preference for the M diet, expressed by a high intra-individual repeatability in preference. Our results provide useful information and nutrition tools to move towards precision protein nutrition in growing rabbits. 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. [ABSTRACT FROM AUTHOR]

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. 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. [ABSTRACT FROM AUTHOR]