[Threonine requirements in high productivity does and fattening kits]

This study investigated the threonine requirements of rabbits. Five diets, containing from 0.54 to 0.72 per cent crude threonine, were formulated by supplementing a basal diet with L-threonine. The apparent fecal threonine digestibility (percentage) was 63.8 plus minus 1.3 in the basal diet and 93.0 plus minus 6.0 for L-threonine, as estimated by the difference method. Feeding trials were carried out using 370 rabbit does to measure the reproductive performance during three reproductive cycles and milk production in 80 lactations. One thousand, one hundred and fifty weaning rabbits (30 days of age) were used to determine growth performance. At the end of the growing period, 115 rabbits were slaughtered with a body weight of 1.8 to 2.1 kg to measure carcass dressing percentage and carcass quality. A digestability trial was conducted with 18 rabbits and plasma urea nitrogen concentration was determined in 40 animals. The results indicated that a minimal dietary concentration of 0.64 and 0.44 per cent of crude and digestible threonine, respectively, should be included to maximize performance of does and the optimal range of 0.60 per cent of crude threonine or 0.40 per cent of digestible threonine for rapidly growing in rabbits. Requirements were lowest for rabbits grown individually in commercial cages during the first two weeks of the fattening period (0.58 and 0.38 per cent of crude and digestible threonine respectively). An excess of dietary threonine tended to impair intake and performance and to reduce fat retention in carcass. The results of this study also showed the need for using digestible rather than crude units to express the threonine requirements of rabbits. However, in this study, it may have been necessary to use diets to provide a wider range of threonine concentrations to detect a significant effect of threonine concentration on plasma urea nitrogen.