Dietary replacement of soybean meal by different varieties of narrow-leaved lupine seed in fattening rabbits: Effects on growth, nutrient digestion, hydrolytic activities of pancreatic enzymes, nitrogen balance, and nitrogen excretion

The efficacy of narrow-leaved lupine seeds (NLLs) as a dietary crude protein (CP) source compared to soybean meal (SBM) was evaluated in terms of growth performance, nutrient digestion, digestive enzyme activity, nitrogen (N) balance, and N retention in fattening rabbits. The SBM diet contained 60 g/kg SBM, whereas the other diets contained 110 g/kg of different NLL varieties (Boregine, NLLB diet; Jowicz, NLLJ diet; and Rumba, NLLR diet). The digestible protein (DP)/digestible energy (DE) ratio (10.9 g DP/MJ DE) was greater in the NLLJ diet than in the other diets (10.3 g DP/MJ DE on average). A total of 160 rabbits (40 animals per group, weaned at 32 days, sex ratio 1:1) were randomly assigned to one of the four experimental diets for a period of 42 days. An additional 40 rabbits (10 rabbits per group, weaned at 32 days, sex ratio 1:1) were included to investigate the coefficients of total tract apparent digestibility (CTTAD) of the experimental diets, N balance, N excretion, and pancreatic enzyme activities in small intestinal contents. Proteolytic activity tended to be lower in rabbits fed the SBM diet than in those fed the other diets (by an average of 193 mg azocasein/g dry matter digesta/h; P = 0.104). Compared with the other rabbits, the rabbits fed the NLLJ diet had a lower CTTAD of starch (by an average of 0.008; P = 0.048), higher urinary N loss (by an average of 0.36 g/day; P = 0.001), and higher total N excretion (by an average of 0.36 g/day; P = 0.013), which led to a lower N retention coefficient (by an average of 0.081; P = 0.008). Neither the final live weight nor the average daily weight gain of the rabbits was affected by the dietary treatments. Compared with the other rabbits, the rabbits fed the NLLJ diet had higher average daily feed intake between 53 and 74 days of age (by an average of 19 g; P = 0.001) and between 32 and 74 days of age (by an average of 10 g; P = 0.035), and consequently, they had worse feed conversion ratios between 53 and 74 days of age (by an average of 0.76; P = 0.001) and between 32 and 74 days of age (by an average of 0.34; P = 0.001). The results suggest that narrow-leaved lupine seeds may fully replace soybean meal only under appropriate dietary conditions, since not all lupine varieties showed comparable performance. To reduce ammonia volatilization and urinary nitrogen excretion while maintaining satisfactory health status, diets for fattening rabbits should be formulated to meet the recommended DP/DE ratio and current amino acid recommendations.