Comparison of different methods Comparison of different methods for nitrogen and amino acid evaluation in rabbit diets

Twenty New Zealand White x Californian doe rabbits weighing 3841±117 g were each surgically fitted with a glass T-cannula at ileum level. Animals were given ad libitum access to food and water throughout the experiment. Four diets were formulated to contain a maximal nitrogen proportion from lucerne hay (LH), barley grain (BG), wheat bran (WB) and sunflower meal (SM) according to diet. True ileal digestibility (TID) of crude protein (CP) and amino acids of LH, BG, WB and SM was determined and compared with digestibility coefficients obtained by using simplified balances (apparent ileal digestibility (AID) and faecal digestibility). The endogenous losses were determined by feeding rabbits with diets containing casein which was assumed completely digestible. On average, a positive balance of protein (0·568 g/day, on average, representing 0·18 of ileal flow) was observed, comparing the ileal flow of protein (3·17 g/day, on average) and the total protein excreted in caecotrophes and hard faeces (3·74 g/day, on average). All foodstuffs showed a higher CP digestibility at faecal than ileal level, as caecotrophes recycled on average a 0·36 of the total protein excreted which was mainly of bacterial origin (0·67, on average). An enrichment in lysine (0·63 of the ileal flow), methionine (0·95 of the ileal flow) and threonine (0·40 of the ileal flow) was also observed when comparing total excretion in hard faeces and caecotrophes with ileal apparent flow. The enrichment in these essential amino acids tended to be higher in diets based on LH, BG and SM than in those based in WB. This result was parallel to the lower proportion of microbial nitrogen in total nitrogen excreted in hard faeces observed in diet based on WB compared with the other diets. Although around one third of the total excretion of lysine and methionine was recycled with caecotrophes, ileal digestibilities of lysine and methionine were higher than at faecal level, which is the opposite trend to that found for CP. A different pattern was observed for threonine digestibility, which tended to be lower at ileal than at faecal level, although its concentration in the microbial protein is also relevant. Due to the large endogenous losses, higher TID than AID of CP and amino acids were observed in all the foodstuffs studied. Differences between TID and AID were of small magnitude in the high-protein foodstuff (SM) but much more pronounced in the low-protein foodstuffs (BG and WB). The relative value established among foodstuffs for the CP and the main essential amino acids changed depending on the digestibility method used and the amino acid concerned. In conclusion, the use of AID and apparent faecal digestibility lead to an underestimation of the ileal utilization of nitrogen and amino acid content of the foodstuffs compared with TID. The use of TID is recommended for a more precise evaluation of nitrogen in foodstuffs for rabbits.