Rabbit Production - James I McNitt

Скачать книгу

until marketing of the fryers. This increase in caging costs may be more than compensated for by increased production. Research studies on the “best” weaning age have given mixed results. Work at Oregon State University suggested that highest profits were obtained when all cages were filled with producing does rebred 42 days after kindling. Kits were left with the does until market age; therefore, no cages were used for finishing fryers. Other studies have shown that greatest profits were obtained when does were rebred 14 days after kindling.

      Various breed-back schedules, such as 7, 14, 21, 28, 35, and 42 days, are used. Generally, does are on a seven-day schedule basis, simply for ease of recordkeeping. A system that may produce greater profits and longevity of the does is to breed “depending on condition,” taking into account the number of offspring in a litter and the condition of the doe. If a doe has a small litter (less than four for medium breeds), she might be bred back in seven days if she is in good condition. However, if she has a big litter, is milking heavily, and is beginning to lose flesh, she should not be bred back until the young are weaned. It is generally agreed that breeding back at 21 or 28 days results in reduced conception.

      One of the best methods to judge the body condition of a doe is to use the sense of touch. It is difficult to tell the body condition of a doe by visual examination because of the fur. Put your hands on the doe’s back and feel the back-bone and the hipbones. If these bones are prominent, the doe is in poor condition. If she is bred in this condition, she might conceive, but the next litter may be born weak or small in number or weight, or she may abort or resorb the litter. The best thing to do if the doe is in poor condition is to delay breeding until her condition improves. In commercial herds it may be best to cull the doe.

      The opposite problem can occur with junior does, which can very easily become overweight, especially if they are on full feed. If they become fat (again, you can tell by feeling the body with your hands), they will not breed. They should be put on restricted feed until they reach normal condition. This can take several weeks to several months. It may be more economical to cull fat junior does and replace them with does in proper condition.

      The Mating Process

      The doe gives evidence of being receptive by behaving restlessly; by rubbing the chin (chinning) on the cage, water crock, feed troughs, etc.; and by making an effort to join other rabbits in nearby cages. The appearance of the vulva, whether it is pale or reddish in color, is generally indicative of the receptivity of the doe. A doe with a dry, pale vulva is less likely to be receptive than one with a pinkish-red, moist vulva.

      The doe may object to another rabbit being placed in her cage and quite often will attack and even injure the intruder, so she should always be taken to the buck’s cage for mating. If the doe is ready for service and the buck is active, mating should occur almost immediately (Fig. 5.3). When it is completed the buck usually falls over on his side (Fig. 5.4). The buck’s ejaculation occasionally misses the vagina. If there is evidence of this, the doe should be bred again before being returned to her cage. Studies have shown that allowing the buck to breed the doe two or three times when first placed in the buck’s cage will increase the conception rate and perhaps the litter size. This may be due to the additional stimulation causing a stronger ovulatory response.

      Fig. 5.3. An example of natural mating. (Courtesy of D.J. Harris)

      Fig. 5.4. When mating is completed, the buck usually falls over on his side. (Courtesy of J.I. McNitt)

      Sometimes a doe will squat in the corner of the cage and will not accept service; in this case, restraining her for mating may expedite and ensure service. This procedure makes many matings possible that would not occur otherwise. Restraining her, however, does not necessarily mean she will conceive. The conception rate from forced mating is much lower than from unrestrained mating.

      Figure 5.5 shows the proper method for restraining the doe. Either the right or left hand is used to hold a fold of skin over the doe’s shoulders; the other hand is placed under her body and between her hind legs. The thumb is placed on one side of the vulva, the index finger on the other, and the skin is pushed gently backward while avoiding any pinching of the vagina. This procedure throws her tail up and over her back. The weight of the doe’s body is supported by the hand, and the rear quarters are elevated, but only to the normal height for mating. Bucks accustomed to being handled will not object to this assistance by the attendant, and many does will respond and accept service naturally when the buck mounts. Forced mating of rabbits should only be used as a last resort.

      Fig. 5.5. Restraining a doe for mating. (Courtesy of D.J. Harris)

      The buck and the doe should not be left together unattended for more than a few minutes, as the buck may be injured. If mating doesn’t occur within a few minutes, it is advisable to put the doe in with another buck. Sometimes she will refuse service with one buck but will readily accept it with another. If she still won’t breed, she is probably non-receptive for that day and should be rescheduled for the next day or the day after.

      Gestation Period

      The gestation period, or the time from mating of the doe to kindling of the litter, averages 31 days. About 98 percent of normal litters will be kindled between 30 and 33 days after breeding but a small percentage may kindle as early as 29 days or as late as 35 days. Gestation lengths are generally longer with smaller litters (Fig. 5.6). In cases of prolonged gestation, the litter may contain only a few individuals with one or more abnormally large kits. Often the young may be born dead.

      Fig. 5.6. Relationship between number of kits born and gestation length. Figures within the bars show the number of litters of that size included. (Source: McNitt, J.I. and Moody, G.L., 1991. Gestation lengths of four medium breeds of rabbits in Louisiana. J. Appl. Rabbit Res . 14: 80–82)

      Factors Prevents Conception

      There are many factors that influence conception rate. Sterility, extreme age, poor physical condition, pseudopregnancy, poor genetic quality, sore hocks, injuries, retained fetuses, and disease are among the most important factors. Pay special attention to the physical condition of the doe. If she is too thin, you are probably underfeeding, or if she’s too fat, you’re overfeeding. Both conditions have negative effects on reproduction. Intensive breeding (on a 14-day or less remating schedule) can cause a doe to lose weight and condition, thus reducing her ability to conceive.

      Sterility

      In its natural environment the wild rabbit breeds during the spring and early summer and is barren during the fall and winter. During this barren period the doe’s ovaries become somewhat shriveled and inactive and fail to produce normal egg cells; bucks may fail to produce sperm, or the sperm may lack motility or be abnormally developed. In creating the domestic rabbit, humans have shortened the barren period somewhat. Its duration and intensity vary considerably. Some does and bucks are fertile throughout the year and for successive years. Others are not fertile for

Скачать книгу