LITMIR.BIZ

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periods. When does are out of production for an extended time, it is more difficult to get them to conceive; but if the herd has received proper care, a large percentage of both does and bucks should be over the barren period in a short time. In extreme cases, however, the period may last four to five months. Usually, such a prolonged period occurs in an area where excessive temperatures have prevailed, especially when unseasonably or excessively high temperatures occur and continue for some time or when the high temperatures are associated with drought conditions. Indications are that sperm production of the bucks is more likely to be impaired by these high temperatures than ovulation in the does, but both sexes can be affected.

      Another cause for prolonged sterile periods may be that the ration has not been properly balanced or that the proper amount of feed wasn’t used to keep the rabbits in desired breeding condition. While an animal is molting, the development of a new coat may tax its vitality, and conception may be delayed. Occasionally an animal is permanently infertile and should be culled.

      Because there is so much variation among does and bucks in regard to the regularity of breeding, the rabbit breeder may well give considerable attention to overcoming this factor through proper selection of breeding stock. Breeding stock should be selected from parents that produce regularly. Attempts to increase production during the sterile period by the use of vitamins and other feed supplements have not been effective.

      During the late autumn and early winter, when the percentage of does that conceive is below that of the spring and summer, the birth weight of the young may be below normal, litters may contain fewer individuals, and the does may neglect their young or the young may be unable to nurse. A decrease in conception in the winter may be caused by inadequate feeding. In the cold of winter in unheated rabbitries, the rabbits use more of their daily ration to produce heat to keep warm. Therefore, less energy is available for reproductive processes. To offset this increased body heat demand, the daily feed allotment to does should be increased by about 25 to 50 percent in the winter.

      One of the most important factors that influences seasonal fertility is the lighting program. In the wild, the rabbit is a seasonal breeder; this pattern is possibly regulated both by temperature and by light. During the fall, as day length decreases, a light-sensitive gland, the pineal gland, detects the decrease in light and decreases the secretion of certain reproductive hormones by the pituitary gland at the base of the rabbit’s brain. By maintaining a longer day length in the rabbitry through the use of artificial lights, you may be able to prevent the winter decline in reproduction. A lighting period of 16 hours per day seems adequate in most regions; the length of the lighting period should be as long as the longest day in your region. This is the principle used in the poultry industry to keep birds laying in the winter. Hand switching of the lights is too unreliable and irregular to give good results. An electric timer should be used to regulate the day length. Note that in the winter, extra hours of light will be required in the morning as well as in the evening. A great deal more research is needed to elucidate the role of environmental factors such as temperature and light in controlling rabbit production.

      Young does and bucks may be sexually immature, while other does and bucks may be too old, having passed their period of usefulness. The proper age for rabbits to be put into production was discussed earlier, and the animals may be retained in the breeding herd as long as they maintain good physical condition and produce satisfactory litters. This may be until they are 2 ½ to 3 years old. There is wide variation, and some individuals may reproduce satisfactorily for four years or more. However, in some commercial herds, up to 100 percent of the does may be replaced each year.

      A high culling rate may be desirable to continually upgrade the herd. If proper selection procedures are used, each doe that is culled can be replaced by a doe of higher genetic quality.

      Physical condition

      Rabbits that are abnormally fat or thin may have impaired fertility or they may become sterile. The ration should be adjusted and breeding delayed until they are in the proper physical condition. For those that are too fat, the pelleted diet should be restricted and a limited quantity of a good quality hay fed. For those that are too thin, the complete pellets or a grain and hay ration should be fed in increased amounts.

      Pseudopregnancy

      Pseudopregnancy can result from a sterile mating, or from stimulation caused by one doe riding another, or by a doe riding the young in her own litter. This condition lasts for about 17 days. During this time the doe may not conceive. If does have been kept in groups, they should be separated and put into individual cages at least 18 days before being mated. Pseudopregnancy is discussed further in Chapter 11.

      Sore hocks or injuries

      Sore hocks or injuries may affect the vitality of does and bucks. Rabbits should be examined carefully before being mated, and if any signs of these conditions are found, the affected animals should be isolated and held in quarantine until completely recovered or be culled from the herd. Because susceptibility to sore hocks is genetic and damage to the feet is permanent, affected animals in commercial herds should be culled. Bucks with sore hocks may refuse to breed, because the weight is shifted to the hind feet during mounting, which makes mating a painful experience.

      Retained fetus

      In some cases, a doe may fail to deliver the entire litter, and the fluids of the retained fetuses will be resorbed and mummified fetuses produced that remain in the uterus, usually causing permanent infertility. These cases can be diagnosed accurately by palpation. Because the doe will seldom conceive again, she should be culled from the herd. As is discussed in Chapter 11, development of mummified fetuses is one of the dangers of using test-mating as a means of pregnancy diagnosis.

      Determining Pregnancy

      Test-mating does by returning them to the buck’s cage to determine pregnancy is not a dependable practice. Some does will accept service again after they have conceived, and others will not accept service even though they have not conceived. Noting the development of the abdominal region and the gain in flesh by the doe as the period of gestation advances is not always accurate and will delay definite diagnosis until late in the period.

      Palpating the abdomen with the thumb and index finger to feel the development of the young in the uterus is an accurate and quick method for determining pregnancy. To make this test, restrain the doe by holding a fold of skin over the shoulders in either the right or left hand, with your free hand placed under the body slightly in front of the pelvis (Fig. 5.7). To make the test accurate, you must relax the doe so the abdominal muscles will not be tense. Generally, you can hold the doe in the air or place the doe on top of the cage for palpation. If it is necessary to use a table or cart, cover the top with a feed sack or carpeting to prevent the doe from slipping.

      Fig. 5.7. Palpating a doe for pregnancy. (Courtesy of J.I. McNitt)

      You must be acquainted with the relative size and location of the uterus and the embryos as pregnancy advances. Figure 5.8 shows the position of the uterus (the digestive tract has been removed) and illustrates the size of the uterus of a non-pregnant doe (left) and of a doe at the end of 10 days of pregnancy (right). It should be noted that the uterus has expanded to accommodate developing embryos; as the period of gestation advances, the embryos grow larger and the uterus is pushed forward as shown in Fig. 5.9.

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