Strategies for rearing of rabbit does

This thesis describes the effects of different rearing strategies for young rabbit does on body development and reproduction performance. In current rearing, does are often fed to appetite from weaning to first insemination. First insemination is applied when 75 to 80% of mature body weight (BW) is reached, that occurs around 14 to 16 weeks of age. Under current rearing, young does loose significant part of their fat and energy reserves during first lactation. This seems related to the decreased reproductive performance and high replacement rate of young does, that are undesired from both economic and welfare point of view. Feed intake during lactation seems to be the limited factor. The objectives of this thesis were to optimize body development and feed intake capacity of young does to improve their reproductive performance and to prolong their lifespan.In Chapter 1, literature on body growth and body development of rabbits is summarized and management practices and factors that could be relevant during the rearing period to stimulate feed intake and body development are reviewed. Body development in rabbits is characterized by a high growth rate of organs and tissues at early age (before 12 weeks). After 12 weeks of age, mainly fat depots are being formed. Feeding level seemed an important factor to regulate body growth and development together with age of first insemination.Therefore, several experiments were performed, in which the level of feed intake and the age of first insemination on body development and composition, feed intake, reproductive performance, and culling rate of does were studied. Feeding level was manipulated in two successive periods during rearing; 1) the period before weaning (30 days of age) in which kits depend on their mother's milk for nutrient intake, and 2) the period after weaning in which kits eat solid food. Rearing period ended at first insemination, that was applied at 14.5 or 17.5 weeks of age.In Chapter 2, nutrient intake in the pre-weaning period was investigated. Milk intake was manipulated by varying the number of suckling kits in a litter. Kits were raised in litters of 6, 9 or 12 kits (LS6, 9, 12). Milk intake in the pre-weaning period affected body growth during rearing and BW at first mating (at 14.5 wk of age). Kits raised in small litters (LS6) were heavier at first mating than kits raised in large litters (LS12). Kits raised in litters of nine were able to compensate for the differences in BW at weaning and reached similar BW as LS6 at end of rearing. The amount of milk intake in the pre-weaning period affected body composition at first mating. Ash content (bone formation) was not improved in heavy does (LS6), but they had more fat tissue and energy than small does (LS12). Heavier does (LS6, and 9) produced more kits (+ 1.1 and + 2.2, respectively) than small does (LS12) in the first parity. Feed intake during first lactation was not affected. In the second parity, no differences in BW, feed intake or reproductive performance were found among treatments. Treatment did not affect culling rate. Does which had not become pregnant of the first insemination at 14.5 wk of age were re-inseminated at 17.5 wk of age. Reproductive performance of these does was significantly improved in the first parity.In Chapter 3, data of Chapter 2 and a set of unpublished data were used to investigate the effect of BW at 14.5 week of age on subsequent reproduction. Does were divided in three classes based on their BW at first insemination (14.5 week of age) (< 3.5, 3.5 - 4, > 4 kg, respectively). Differences in BW at 14.5 week of age were caused by difference in growth potential and feed intake during rearing. Heavy does (> 4 kg) were does with high voluntary feed intake, but according to their gain to feed ratio at end of rearing, no excessive fat deposition could have occurred. However, the fact that they were heavier implicates that they will have more body reserves in terms of protein and fat. Extra BW at start of reproduction improved litter size in the first parity. Heavy does at first insemination (BW > 4 kg) produced more kits (+ 2.5) compared to small does (BW < 3.5 kg). Extra BW at start did not contribute to an improved feed intake or increased BW development during reproduction and had no effect on culling. In Chapter 4, the effect of BW and age of first insemination on body development, body composition, and puberty characteristics at the end of rearing were studied. Does were fed ad libitum (AL) or restrictive ( R) and were inseminated at 14.5 or 17.5 week of age. Feed restriction was used to manipulate body development by preventing excessive fat deposition and stimulating sexual development. With the feed restriction we succeeded in "creating" does, which had similar body composition and body weight, but differed three weeks in physiological age (14.5 vs. 17.5), whereas puberty characteristics were comparable. Does fed to ad libitum and mated at 17.5 week of age were heaviest, contained more fat and had the best puberty characteristics. Does fed restrictively and mated at 14.5 weeks of age were physiologically too immature for reproduction. Only two of the 16 does (12.5%) fed restrictively were receptive at first insemination. In five of the 10 does that were killed to determine body composition corpera lutea were found, and only one doe had embryos.In Chapter 5, the effect of the treatments tested in Chapter 4 were investigated on performance in the reproductive period. Based on the results in Chapter 4, restrictive feeding and mating at 14.5 wk of age was not applied. Does fed ad libitum and mated at 17.5 week of age (AL-17.5) were heaviest at first insemination. Although these does were older and had more body reserves, reproductive performance was not improved. Heavier BW at the same age (AL-17.5 and R-17.5) resulted in a reduced feed intake during first gestation (-25%) and first lactation (-10%), probably due to the high amount of body fat. This resulted in weight loss during first gestation (-6%) and decreased litter weights (-19%) and litter growth (-14%) in the first parity. Although AL-14.5 and R-17.5 does had similar BW and body composition at first insemination, R-17.5 does produced more alive born kits (+ 1.4) and weaned more kits (+ 0.6) in the first parity. Feed intake in the first parity was similar. The ad libitum fed does inseminated at 14.5 week of age gained weight in the first gestation period as well as in the first lactation. Competition for nutrients between body growth and production must have occurred, and resulted in smaller litters and lower milk production. This could indicate that AL-14.5 does were physiological not mature enough for reproduction. Effect of rearing strategies was limited to the first parity. Culling rate of does during the first three parities was not affected by rearing strategy.In Chapter 6, an attempt was made to improve litter size after first insemination by restricting feed intake during the first 10 days of gestation. Feed intake in first 10 days of gestation did not affect kindling performance of young does. However, further analysis on ad libitum fed does revealed an effect of feed intake in the last week of gestation on kit survival and birth weight. In does with the highest feed intake during the last week of gestation, the lowest number of litters with stillborn kits with the highest birth weight was found. Feed intake in the last week of gestation was not influenced by feed restriction in early gestation.In the General Discussion, the consequences of rearing strategies for production were further analyzed and discussed based on BW at first insemination. Body weight at first insemination was divided into six BW-classes from very small (< 3.5 kg) to very heavy (> 4.5 kg). The effect of BW on reproductive parameters was analyzed for the following rearing strategies: AL-14.5, AL-17.5, and R-17.5, using the data of Chapter 2, a set of unpublished data also used in Chapter 3, and data of Chapter 5. Kindling rate was not affected by rearing strategy.In all rearing strategies, litter size increased when does were heavier, although this relationship was most profound in AL-14.5. Litter size was optimized between eight and nine kits in does weighing approximately 4 kg at first insemination. Feeding strategy during rearing influenced the uniformity in BW of does at first insemination. In AL-14.5 does, more than 70% of the does were smaller than 4 kg at first insemination. In does smaller than 4 kg litter size was reduced by approximately 1.5 kit. In AL-17.5 does, 75% of the does were heavier than 4 kg. Body composition determined in Chapter 4, revealed that heavy does have more fat. Based on results in Chapter 5, in does with excessive fat depots, the percentage of does with stillborn kits in their litter will be increased, because feed intake during first gestation is decreased.In R-17.5 does, the percentage of does that weighed around 4 kg at first insemination was 60 to 80%. In these does litter size was improved, because the percentage of small animals (with low litter size) as well as the percentage of very heavy animals (with high stillbirth) is reduced.There was not a clear relationship between milk production and BW at first insemination. However, in R-17.5 does milk production was higher than in AL-14.5 and 17.5 does, independent of BW-class. The difference in milk production between R-17.5 and Al-17.5 could be explained by the higher feed intake of R-17.5 in the first gestation and lactation period. The difference in milk production between R-17.5 and AL-14.5 could be explained by the fact that AL-14.5 gained weight during first gestation as well as during first lactation. Competition for nutrients between body growth and production must have occurred, and resulted in smaller litters and lower milk production. The higher milk production of R-17.5 does resulted in higher kit weight at weaning. The best reproductive performance in the first parity was found in R-17.5 does. Rearing strategies only affected body weight development, feed intake and reproductive performance in the first parity. Long-term effects over three parities were absent and rearing strategy did not influence culling rate of does.