A total of 190 rabbit females were used to evaluate five feeding programmes from 9 weeks of age to the first parturition: CAL, fed ad libitum with a control diet (C: 11.0 MJ digestible energy (DE) and 114 g digestible protein (DP)/kg dry matter (DM)) until first parturition; CR, fed ad libitum with C diet until 12 weeks of age and then C diet restricted (140 g/day) until first parturition; F, fed ad libitum with a low-energy, high-fibre diet (F: 8.7 MJ DE and 88 g DP/kg DM) until first parturition; FC, fed with F diet ad libitum until 16 weeks of age, and C diet ad libitum until first parturition; FCF, fed with F diet ad libitum until 16 weeks of age, then C diet ad libitum until 20 weeks and then F diet ad libitum until first parturition. The rabbits were artificially inseminated at 18 weeks of age. CAL group had a higher mortality rate compared with the other groups between 9 and 12 weeks of age (34% v. 3%; P < 0.05) and during the last 3 weeks of first pregnancy (14% v. 3%; P < 0.05). The CAL and FC females presented higher BW and perirenal fat thickness (PFT) than CR females at 11 days of pregnancy (+0.41 kg and +0.6 mm; P < 0.05), with F females showing medium values. The type of feeding procedure did not affect the fertility rate of young females at first artificial insemination. Differences in BW disappeared at parturition, when only CAL females presented a greater PFT than CR and FC females (+0.3 mm; P < 0.05). In comparison with FCF, CAL females had smaller and thinner live born litters (−2.5 kits and −139 g, respectively; P < 0.05), with CR, F and FC females showing medium values. The low number of kits born alive for CAL females was because of their lesser total number of kits born (−1.7 kits; P < 0.05) and the greater mortality of their litters at birth (+13.9%; P < 0.05) compared with FCF females. Non-esterified fatty acid was higher in the blood of females fed C diet (CAL and CR) than in others at partum day (on average +0.15 mmol/l; P < 0.05). In conclusion, the ad libitum use of diets for lactating rabbit does throughout the rearing period could lead young rabbit females to present a higher risk of early death and smaller litter size at first parturition. Feed restriction or earlier use of suitably fibrous diets led females to achieve the critical BW and fat mass at first mating to ensure reproduction.

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 ad libitum (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 andFast18h. The differences between balanced and unbalanced diets in lysine were highest (P<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 rabbitdiets.

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.

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.