The contribution of microbial amino acids through caecotrophy to tissue protein metabolism was investigated in lactating does. Attempts were made to vary microbial supply through a dietary antibiotic, Zn bacitracin, and to vary tissue demand through manipulation of litter size. Three groups of eight New Zealand does were fed different experimental diets from day 28 of pregnancy to day 26 of lactation. The control group received the basal diet formulated to meet requirements with grass hay, wheat, soyabean meal and barley grain. The second (no antibiotic) group and the third (bacitracin; BAC) group ingested the basal diet supplemented with ammonium sulfate (5 g/kg), initially unlabelled (day 1 to day 8) then labelled with 15N (day 9 to day 30), while the BAC diet was also supplemented throughout with antibiotic (Zn bacitracin; 100 mg/kg). From just after birth each group of does was subdivided into two groups, each of four females, with the litter size either five (LS5) or nine (LS9) pups. The 15N enrichment in liver, milk and caecal bacteria amino acids was determined by GC-combustion-isotope ratio MS. All amino acids in bacterial protein were enriched with the (15NH4)2SO4 treatment, with lysine 15N enrichment significantly greater in caecal bacteria (0·23 (se 0·0063) atom % excess (ape)) than in liver (0·04 (se 0·0004) ape) or milk protein (0·05 (se 0·0018) ape), confirming the double origin (bacterial and dietary) of tissue lysine. The contribution of microbes to tissue lysine was 0·23 (se 0·006) when milk protein was used as reference.

To study the absorption of microbial lysine in growing rabbits, a labelled diet (supplemented with 15NH4Cl) was administered to six animals (group ISOT); a control group (CTRL, four rabbits) received a similar, but unlabelled, diet. Diets were administered for 30d. An additional group of six animals were fed the unlabelled diet for 20d and then the labelled diet for 10d while wearing a neck collar to avoid caecotrophy (group COLL), in order to discriminate it from direct intestinal absorption. At day 30 animals were slaughtered and caecal bacteria and liver samples taken. The 15N enrichment in amino acids of caecal bacteria and liver were determined by GC–combustion/isotope ratio MS. Lysine showed a higher enrichment in caecal microflora (0·925 atom% excess, APE) than liver (0·215 APE) in group ISOT animals, confirming the double origin of body lysine: microbial and dietary. The COLL group showed a much lower enrichment in tissue lysine (0·007 (se 0·0029) APE for liver). Any enrichment in the latter animals was due to direct absorption of microbial lysine along the digestive tract, since recycling of microbial protein (caecotrophy) was avoided. In such conditions liver enrichment was low, indicating a small direct intestinal absorption. From the ratio of [15N]lysine enrichment between liver and bacteria the contribution of microbes to body lysine was estimated at 23%, with 97% of this arising through caecotrophy. Absorption of microbial lysine through caecotrophy was 119 (se 4·0) mg/d, compared with 406 (se 1·8) mg/d available from the diet. This study confirms the importance of caecotrophy in rabbit nutrition (15% of total protein intake).

Separation of low digestible fibres and fermentation of the digestible part of the food in the caecum is an adaptation of some small herbivores to cope with low-quality forage. The caecum content is later re-ingested as soft faeces so that the herbivore can benefit from this protein-rich material. This is known as caecotrophy and is a common phenomenon in species of leporids, although differences exist between hares and rabbits. Hares have amorphous soft faeces and the amount of soft faeces produced is smaller compared to that of rabbits. Both factors suggest that hares have smaller benefits from re-ingestion of the caecal contents compared with rabbits and, as a consequence, have a less efficient digestion (mainly of nitrogen) compared to rabbits. The assertion was tested whether digestive efficiency is different between the two herbivores and how this affects the choice of food plants in a natural situation. A feeding trial was conducted using hares and rabbits fed with diets with a range of fibre contents. Dry matter digestibility was not different, but nitrogen digestibility was lower in hares than in rabbits, indicating a less efficient digestion of protein. Both taxa showed a different response to increased fibre content in the diet. Rabbits maximized digestibility by increasing retention time of the food, hares maximized digestion rate by increasing the passage rate of the food through the digestive tract. The daily digestible nitrogen intake was higher in hares Lepus europaeus than that in rabbits Oryctolagus cuniculus, indicating that hares compensated for their lower nitrogen digestibility. Hares were predicted to select for higher quality plant species in a natural situation, but they had, on average, a lower nitrogen and higher total fibre content in their diet compared to sympatrically occurring rabbits. This indicated that hares did not compensate for their lower digestive efficiency by selecting higher quality food plants. The present experiment shows that hares and rabbits have different digestive strategies to cope with low quality forage. Rabbits had a higher N-digestibility by increasing the retention time, whereas hares appeared to compensate for their lower N-digestibility by increasing the processing rate, when food quality deteriorated.

The present study compares estimates of caecotrophes production from urinary purine derivatives (PD) excretion with that from preventing caecotrophy by using a neck collar. A total of 64 New Zealand growing male rabbits were used to study the effect of diet composition on caecotrophes production. Diets were formulated using two sources of structural carbohydrates (fibre): alfalfa hay (AH) and sugar-beet pulp (SBP), mixed at two constant proportions, (0·75: 0·25) AH diets and (0·25: 0·75) SBP diets. Both diets included either barley or maize grain at two fibre: grain ratios (F/G, 80: 20 and 45: 55). Diets were given ad libitum. Growth rate, dry matter intake and digestibility were not modified by the grain source, although high F/G diets resulted in a lower growth rate (19·8 v. 26·4 g/day; P < 0·001). Between fibre sources, dry-matter intake and growth were higher in AH than in SBP diets (122·5 and 25·6 v. 101·6 and 20·4 g/day, respectively, P < 0·001 and P < 0·01). Rabbits given high F/G ratio and AH diets excreted more caecotrophes than those given low F/G ratio and SBP diets (19·5 and 20·9 v. 16·3 and 14·85 g/day, respectively). Microbial-N recycling through the caecotrophy process was higher when considering data from PD excretion (1·33 g/d) than when estimated by preventing caecotrophy (0·72 g/day).

Twenty-five New Zealand White × Californian lactating doe rabbits were used to study the effect of different dietary fibre concentrations on the amino acid contribution of soft faeces to the total amino acid intake. Five isoenergetic diets containing 312, 334, 360, 384, and 412 g/kg neutral-detergent fibre (DM basis) were formulated. All diets were designed to maintain the same amino acid pattern. Soft faeces production showed a trend to increase (P = 0·07) as dietary fibre increased. The content of isoleucine (P < 0·01), glutamic acid, leucine and alanine (P < 0·05) in soft faeces decreased as the dietary fibre increased. A trend to decrease with the increase of dietary fibre was also observed for aspartic acid (P = 0·06), lysine (P = 0·07), methionine and valine (P = 0·1). As a consequence, the soft faeces contribution to the total intake of individual amino acids did not vary with the type of diet. The average contribution of soft faeces to the total amino acid intake for the most limiting amino acids of lactating doe rabbits under practical conditions, methionine, threonine and lysine, were 0·17 , 0·21 and 0·18 g/g, respectively.