New feeding and management strategies for the improvement of fattening rabbit intestinal health

Resumen

ABSTRACT The main objective of this thesis was to devise alternatives to the use of antibiotics in the feeding and management of young fattening rabbits to reduce the mortality rate due to digestive diseases. The first experiment aimed to quantify the concentration of Clostridium perfringens ¿- toxin in caecal samples of rabbits suffering from digestive diseases and relate these concentrations to C. perfringens counts. Additionally, this experiment characterised the symptoms of the diseased rabbits and toxinotyped the C. perfringens strains isolated from rabbits with clinical lesions of Epizootic Rabbit Enteropathy (ERE). To conduct this work, a total of 711 rabbits weaned at 35 days were housed in pairs and fattened until the age of 63 days. No experimental infection was done and no antibiotics were provided in the feed or drinking water. Naturally infected rabbits were slaughtered and necropsied at the detection of symptoms of digestive diseases. On balance, 88.5% of sick rabbits were affected by digestive diseases, from which 53.6% suffered from confirmed ERE. Apart from diarrhoea, the most constant ERE signs were abdominal bloating and borborygmi. Moreover, at necropsy, the anterior digestive tract was found filled with large amounts of gas and liquid. Seventy-three percent of ERE-affected rabbits had liquid caecal contents whereas caecal impaction only appeared in 27% of the animals. Live weight was lower by 49.2% (P<0.001) in diseased than in healthy rabbits of the same age. For C. perfringens counts lower than 6.0 log cfu/g, the ¿-toxin concentration remained below 2.6 ¿g/mg. However, for bacterial counts above 6.0 log cfu/g the concentration of ¿-toxin ranged from 0.12 to 60.9 ¿g/mg. Nevertheless, both caecal concentration of C. perfringens (7.65 vs. 3.09 log cfu/g, P<0.001) and that of its ¿-toxin (6.02 vs. 0.17 ¿g/mg, P<0.001) were higher in diseased rabbits than in healthy ones. C. perfringens toxinotype A was found in all ERE-affected rabbits. No other toxinotype was identified and no isolate contained the enterotoxin gene. Experiment 2 aimed to assess the effect of litter membership on fattening mortality due to ERE. For that purpose, 50 and 69 litters of rabbits from two consecutive batches were weaned at 35 days of age and housed in pairs in a fattening farm (471 and 655 weanling rabbits in the first and second fattening trial, respectively). In both trials, all rabbits were fed a standard fattening diet (33.0% NDF, 14.9% starch, 15.5% CP and 10.1 MJ DE/kg) until the age of 63 days. From 35 to 63 days, rabbits had always ad libitum access to feed and drinking water. Water was completely unmedicated and with the exception of 60 ppm of robenidine, no other medications were provided in the feed. On weaning day of the second trial, material from the nest (mainly straw, hair and hard faeces) of six rabbit does was sampled (three rabbit does were chosen because of the high mortality rate of their litters in the first trial whereas none of the rabbits of the other three rabbit does had died in the previous trial). On the following day, soft faeces were collected from these six females. Samples of dust were collected over the air extractors in the fattening farm at the beginning, on day 14 and at the end of each one of the two fattening trials. All samples of dust and those taken in the nests were tested for the count of C. perfringens spores. The count of C. perfringens vegetative colonies was determined in the soft faeces. Fattening mortality rate due to ERE amounted to 2.97 and 12.2% (P<0.001) in the first and second trial. Rabbits that died of ERE represented 76.6% of the total number of losses. Non-specific diarrhoeal syndromes caused 15.9% of the deaths and 7.5% of the rabbits died of pasteurellosis or respiratory diseases. Only 10 and 32% of the litters concentrated all the losses caused by ERE in the first and second trial, respectively. As main explanatory variable, litter membership accounted for 57.5 and 47.0% of the total variability registered in the mortality results, in the first and second trial respectively. However, it did not appear to exist any relationship between litter mortality rate in the fattening period and the concentration of C. perfringens spores in the nest on weaning day nor with the count of C. perfringens colonies in the soft faeces of the mother. Nonetheless, a parallelism was observed between the environmental count of C. perfringens spores and the mortality rate of growing rabbits at the fattening farm. The effect of a time-limited access to the feeders (8 hours per day from 9:30 to 17:30) on fattening performance and health status of growing rabbits was studied in the third experiment. For this purpose, a total of 192 rabbits (two consecutive trials; 96 rabbits per trial) were weaned at 35 days and assigned to two different feeding treatments (continuously fed ad libitum or restricted from 35 until 49 days). Rabbits were fed a standard non-medicated fattening diet and were fattened until the age of 63 or 70 days (first or second trial, respectively). In both trials, global feed intake and final live weight were lower (P<0.004) in rabbits that had been restricted than in rabbits always fed ad libitum. In none of the trials did feed restriction improve feed efficiency. In the first trial, no health problem was recorded (average mortality 1.04%) whereas in the second trial feed restriction permitted to decrease both mortality (25.6 vs. 6.34%, P=0.017) and morbidity (41.4 vs. 12.7%, P=0.004) rates. The interest of feed restriction in Spanish fattening rabbit farms is mitigated by the conventional market weight as restricted rabbits weighed less than 2 kg at 63 days of age. Nevertheless, in a context with high incidence of digestive disorders, a time-limited access to the feeder enhances health status at the farm and could justify the short delay necessary to reach the usual commercial slaughter weight. The objective of the fourth experiment was to evaluate the effect of two different NDF:starch ratios (425:67 vs. 330:149 g/kg) and two weaning ages (28 vs. 42 days) on intestinal enumerations of C. perfringens and fattening mortality of growing rabbits. Litters from a total of 80 hybrid rabbit does were used in this experiment in which neither feed nor drinking water was medicated with antibiotics. The diet containing 33% NDF and 14.9% starch reduced the caecal proliferation of C. perfringens by 9.2% (P=0.01) and did not increase fattening mortality rate while it enabled achievement of high productive performance. With impaired hygienic conditions, weaning at 42 days of age reduced fattening mortality rate with respect to rabbits weaned at 28 days (14.1 vs. 26.7%, P<0.05). Moreover, at 42 days of age, caecal counts of C. perfringens were lower in suckling than in weaned rabbits (P=0.03). Finally, in this experiment it was found that average fattening mortality rates were highly and positively correlated (P<0.001) to the average C. perfringens caecal counts. The purpose of the fifth experiment was to test the effect of barley and dehydrated lucerne grinding size (4.5- vs. 1.5-mm grinder screen size) on growth performance, fattening mortality, nutrient digestibility, jejunal histology and mucin ileal concentration of young non-medicated rabbits. In this experiment, a total of 1,056 mixed-sex rabbits weaned at 35 days of age were blocked by litter and assigned at random to the experimental diets (four dietary treatments in a 2 x 2 factorial structure with 2 main effects: grinding size of barley and grinding size of dehydrated lucerne). All these rabbits were fattened until the age of 63 days. Results of the third experiment seem to justify the interest of a different type of grinding for cereals and fibrous ingredients in rabbit feeding because the diet containing fine barley and coarse lucerne did not increase caecal content weight, resulted in a high feed intake (117 g/d in the 35¿63 d period) and led to a high ileal digestibility (0.947) and a low ileal flow of starch (1.08 g/d). On the contrary, grinding both cereals and fibrous ingredients with the same grinder screen size could have some disadvantages. Thus, grinding coarsely both barley and dehydrated lucerne impaired starch digestibility (-3.95%) and increased mucin ileal concentration (+38.7%), whereas grinding finely both ingredients led to increased caecal content weight (+11.2%) and reduced feed intake (-5.32%). Moreover, in the second, third, fourth and fifth experiments, samples of dust were collected on the air extractors of the fattening farm 14 days after weaning in order to assess the effect of different hygiene protocols on the environmental count of C. perfringens spores. All these enumerations have been jointly related to the contemporary values of fattening mortality recorded at the farm where the samples of dust were taken. From this regression analysis, a positive correlation (r = +0.93; P<0.001) was established between the fattening mortality rate due to digestive diseases and the environmental count of C. perfringens spores. In conclusion, the caecal count of C. perfringens colonies may be used as an indicator of the intestinal health status in growing rabbits while the environmental count of C. perfringens spores indirectly reflects the risk of onset of digestive disorders at the farm. Litter of origin accounts for a great part of total variability in mortality rates due to Epizootic Rabbit Enteropathy. Thus, blocking by litter at the beginning of a fattening experiment is of utmost importance to avoid having biased results. If not withdrawn, the use of antibiotics in the feed of growing rabbits can be strongly reduced in rabbits weaned at 35 days when fed a diet containing appropriate levels of insoluble fibre (around 34%, avoiding concentrations over 40%) and starch (around 15%), and housed in a farm thoroughly disinfected prior to the beginning of the fattening period. Once these premises are fulfilled, feed restriction for the first 14 days after weaning can further help prevent digestive diseases in young rabbits, although the fattening period needs to be extended some days to let rabbits reach an average liveweight of 2 kg. Finally, it could be interesting to combine a fine grinding of cereals with a coarse grinding of fibrous ingredients in growing rabbit feeding.