Lysine, sulfur amino acids and threonine requirements of growing rabbits from a line selected by growth rate
Studies on protein nutrition in rabbits are scarce compared with other species, the estimation of their requirements is relatively old and no data are reported at ileal level. Moreover, their peculiar feeding behaviour, including caecotrophy, complicates more this study. In addition, in recent years, a relationship has been established between dietary crude protein and Epizootic Rabbit Enteropathy (one of the most important disease that affects this species) As a consequence, the protein content of diets has been reduced. On the other hand, there has been a loss of effectiveness of the selection for growth rate in the paternal lines, which could be related with a possible nutritional deficit in high growth rate rabbits when using the current commercial diets. Plasma urea nitrogen (PUN) could be a good indicator of an amino acid imbalance, but its use has not yet been tested in rabbits. Thus, the main hypothesis of this thesis concerns that growing rabbits selected by growth rate could have amino acid requirements different from current recommendations, and that another amino acid combination would maximize productive traits. To evaluate this hypothesis, a total of 1464 growing rabbits were used (mainly from the R line, a paternal line selected by growth rate, and, to a much lesser extent, from some other maternal lines with lower growth rates) 32 diets were formulated differing in the content of the three typically limiting amino acids (lysine, sulfur amino acids and threonine) Faecal and ileal digestibility trials were performed with 69 and 71 animals, respectively. Nutrient retention during their growing period was evaluated in 126 animals. Finally, PUN was analyzed in more than 2700 samples. From the first trial,we obtained indication about the possible existence of some limiting amino acid when current moderate protein diets are used in growing rabbits with high growth rates, as retained protein to retained energy ratio was clearly lower for animals of the paternal lines (this suggested a deviation of a certain proportion of protein as a source of energy, which is inefficient) Before considering what combination of amino acids would improve their utilization, it was necessary to evaluate the possible interest of PUN to detect deficiencies in amino acids and establish the appropriate methodology to optimize its use for this purpose in growing rabbits. In the second study we verified the applicability of PUN in rabbits and checked the highest differences between diets: from 04:00h to12:00h in animals fed ad libitum and at three hours after refeeding (21:00h) when a fasting period of 10 h was applied. At this point, knowing that amino acid deficiency could exist and that PUN could be an indicator of it, we formulated a matrix of 27 diets (in a 3x3x3 factorial design, with three levels of the three amino acids mentioned above, using the current recommendations and increasing or decreasing them by 15% searching for the combination minimizing PUN values and, hypothetically, improving the protein assimilation. From this trial, the diet MHL (8.1, 6.6 and 5.7 g/kg dry matter of lysine, sulphur amino acids and threonine, respectively) emerged as a candidate to replace the current recommendations (diet MMM; 8.1, 5.8 and 6.9 g/kg dry mater of lysine, sulphur amino acids and threonine, respectively) However a question arises: could this new combination improve productive traits? Finally, in the last study we verified the usefulness of this combination since it significantly improved both the growth rate and the feed conversion ratio. After digestibility assessment, we propose levels of 5.3, 4.3 and 2.9 g/kg dry matter of apparent ileal digestible lysine, sulphur amino acids and threonine, respectively. These data are intended to improve current knowledge on protein nutrition and move towards precision protein nutrition.