Ninety-six young rabbits (48 males, 48 females) each weighing 08 kg were offered one of six diets to appetite (n = 8 per treatment) or at one of four restricted feeding levels (n = 2 per treatment). All animals were culled at ca. 2 kg body weight and carcass analysis was carried out. Nutrient retention was estimated by the comparative slaughter technique.
Five of the experimental diets (A–E) were formulated to contain increasing concentrations of a poor-quality grassmeal (0, 100, 300, 500 and 700 g/kg, respectively), the remainder of the diet consisting predominantly of ground oats and soya-bean meal. A sixth diet (F) was based on a common laboratory rabbit diet (SGI). All diets contained similar digestible crude protein (DCP): digestible energy (DE) ratios of ca. 13·1 g DCP/MJ DE. Acid detergent fibre concentrations in diets A–F were 146, 158, 213, 264, 321 and 180 g/kg dry matter (D.M.), respectively.
When feeding to appetite, growth rates were depressed at the highest inclusions of grassmeal (diets D and E). Growth rates (GR) and voluntary food intakes (VFI) for diets A–F were: GR (g/day): 40, 41, 43, 36, 30 and 40; VFI (g D.M./day): 93, 103, 120, 137, 159 and 111, respectively. The depression in growth rate on diets D and E was associated with a reduction in total daily DCP and DE intake. The data suggested that, when the DE content of the diet falls below 10·5 MJ/kg D.M., the young rabbit is unable to compensate further by increasing daily VFI. Males grew significantly faster than females (40 cf. 37 g/day) and females had fatter carcasses at slaughter (301 cf. 274 g fat/kg carcass D.M.).
Relationships between daily N and energy intake and their retention in the body of the growing rabbit were calculated using data from both restricted and ad libitum-fed animals. Daily energy retention (ER, kJ) was related to daily DE intake (DEI, kJ) by the equation: ER = –227 + 6 x DEI (b ranged from 0·407 to 0·502, according to diet).
The estimated maintenance energy requirement of the growing rabbit ranged from 351 to 434 kJDE/kg0·75 per day and the average value across all diets was 377 kJ DE/kg0·75 per day.
Daily N retention (NR, g) was related to daily digestible nitrogen intake (DNI, g) by the equation: NR = –0·398 + 6 x DNI (b ranged from 0·589 to 0·667, according to diet).
The estimated N requirement for maintenance ranged between 0·464 and 0·525 g DN/kg0·75 per day and the average value across all diets was 0·497 g DN/kg0·75 per day.