Unit 3

Life Cycle Considerations -- Pregnancy

 Cattle, sheep and swine are the livestock species of interest in our discussion of pregnancy.  Once again, the avian species will be discussed separately.  Pregnancy is that period of time from the successful breeding of an animal with fertilization of the ovum, until offspring are born from that breeding.  This is often referred to as the "gestation" of the animal.  There are a number of requirements for the successful pregnancy.  In general, a sterile environment with warmth, moisture  and nutrients to support survival and growth of the offspring are essential.  However, in order to maintain the pregnancy, certain hormonal requirements must be met in the dam.  Among the important hormones are:
    • Estrogen
      • estrogen priming is necessary for proper development of the uterine endometrium and myometrium
  • Progesterone
  • high levels of progesterone are needed to maintain pregnancy
  • progesterone is produced by the CL and the placenta, once it is formed
  • progesterone normally effects target tissues only after estrogen priming
    • progesterone causes the endometrium to secrete "uterine milk"
    • progesterone prevents contraction of the myometrium
 
 Proper management of animal during pregnancy will assure the best possible outcome -- birth of live, vigorous offspring.  Regardless of the species or type of management system used, two key elements to proper management are:
      Stress  -- this is particularly important to minimize stress in the first phase of pregnancy (period of the ovum).  Heat stress or other forms of stress such as excessive, rough handling on the dam will result in death and resorption of the ovum.

      Nutrition -- nutrition should be well-balanced throughout the pregnancy.

      • energy requirements -- too much or too little energy (calories) in the diet can be detrimental to the offspring.  Usually there is no need to increase the energy content of the ration until the last half or last trimester of gestation and then only moderate increases may be needed.
      • weight gain -- a good rule of thumb is that the total weight gain of the dam during gestation should equal the weight of the offspring, the placenta and fluids and the mammary development.  The goal is to maintain the dam in average condition.  Excessive weight gain (overconditioning) during gestation can result in fewer offspring, stillborn offspring and greater difficulty in birthing (dystocia).  Excess energy converted to fat will be deposited in the birth canal, resulting in dystocia, and in the mammary gland, resulting in decreased milk production after birth.  Feeding too little to the dam in the last trimester of gestation will result in smaller, weaker offspring.
Pregnancy can be divided into three phases based on the development of the offspring, it's nutritional support and the susceptibility to loss of pregnancy.  These phases are not equal in length, and hence are not denoted as 1st, 2nd and 3rd trimester of pregnancy.
 

      Period of the ovum

      • Duration -- short time, usually 10-15 days, varies with gestation length
      • Ovulation and fertilization (Zygote) initiates the period of the ovum; fertilization occurs in infundibulum or oviduct
      • Division occurs, zygote moves to uterus; zygote is particularly sensitive to heat at time of oviduct transit
      • Uterus, primed by estrogen and progesterone, allows further division
      • Blastocyst forms - moves freely in uterus for several days, in the pig they will space along the uterine horns
      • Nutrition is derived from the uterine glands "uterine milk"
      • Death loss before day 12 will result in normal cycling of animal; if later will result in delayed return to estrus
        • estimates of 30-40% of deaths take place at this stage of gestation in the pig
        • many causes-genetic abnormalities, uterus not ready for blastocysts, hormonal imbalance, old egg or sperm, etc.
         

      Period of the Embryo

      • Duration -- 20-30 days, varies with gestation length
      • Organ development takes place during this stage of gestation
        • teratogens have their effect at causing malformations during this time
      • Implantation of the blastocyst into the uterine wall takes place during this time; this is also a critical time for death loss
      • Placental development takes place during this period also; the placenta supplies the nutrition to the developing embryo
       

    Period of the Fetus

    •  Duration -- this is the longest stage of gestation and comprises the last 2/3 of the gestation
    • Nutrition is supplied entirely by the placenta to the developing fetus
    • Death loss is the least during this stage of gestation, however it is often thought to be the greatest because dead fetuses are often expelled from the uterus (aborted) and are evidence of loss of the pregnancy.
    •  

Gestation Length

  • Cow = 283 days; approximately 9 months
  • Ewe = 148 days; approximately 5 months
  • Sow = 114 days; approximately 4 months
  • Mare = 336 days; approximately 11 months
  •  

Pregnancy testing

In animal management systems where reproduction is important to profitibility, it is important to shorten the interval between each pregnancy as much as possible.  As a consequence, estrus or heat detection is important to identify animals eligible for breeding and pregnancy testing is advisable to make sure that breeding resulted in pregnancy. If breeding did not result in pregnancy, it is important to identify the lack of pregnancy at the earliest possible time to decrease the amount of time the dam is "open".  The profitablity of a herd will decrease with an increase in the number of days open for the brood herd.  There are a number of ways in which to determine whether an animal is pregnant or not.  We will discuss the three most commonly used methods.
    • Rectal palpation -- Palpation of the ovaries and uterus by placing a hand into the rectum of the animal and feeling through the rectal wall for these structures can be performed with large animals such as cows and mares.  This is best performed by an experienced palpator, usually a veterinarian.  By identifying the corpus luteum, increased blood flow (fremitis) in the uterine artery and the amniotic vesicle within the uterus, the experienced palpator can identify an animal as pregnant or not within approximately 40 days of breeding in cattle.
    • Ultrasound -- Real-time ultrasound is being employed widely in the swine industry and it's use is increasing also in the sheep industry.  It can be used in place of or in conjunction with rectal palpation in cattle.  Pregnancy can be confirmed by ultrasound in swine at 17-20 days post-breeding.  Although it is not used as commonly as rectal palpation, pregnancy can be confirmed at 25-30 days post-breeding using ultrasound in cattle.
    • Hormone measurements -- Measurement of progesterone in serum or milk can be used to identify the presence of a functioning CL.  A functioning CL is necessary to maintain a pregnancy.  Sometimes, however, an animal that does not become pregnant will not experience regression of the CL during late diestrus and proestrus, resulting in a retained corpus luteum.  This can result in a false positive test for pregnancy is hormone measurements alone are used for pregnancy determination.
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