Goat producers face a problem every breeding season. Their does may exhibit signs of estrus, or standing heat, one cycle after they are bred. It is difficult to determine whether this behavior is due to lack of conception, without expensive tests available only through a veterinarian. Most breeders just re-breed the doe and mark two due dates on the calendar. It is unclear if there is a way to prevent this, but it is important to understand why it happens before a solution can be found.
To fully understand the phenomena of estrus while pregnant, it is important to understand two things: the estrus cycle of a goat and the process of pregnancy recognition.
The goat's estrus cycle is regulated by the season; the first heat usually occurs in late summer and the doe will cycle until mid-winter. Each cycle lasts approximately twenty-one days. On day zero, the doe begins to exhibit behavioral signs of estrus: tail flagging, mounting other does, urinating more often, and she will be more vocal than usual (Damero, 1993; Jarosz et al, 1971). Day two brings vaginal estrus, and the doe may still be exhibiting behavioral signs (Jarosz et al, 1971). In response to estrogen-induced discharge of the luteinizing hormone (LH), ovulation occurs on day three. Metestrus starts with ovulation and continues until day six. During this time, theca and granulosa cells differentiate into luteal cells under the influence of LH, thus forming the corpus luteum (CL). Starting on day four, the CL begins to secrete progesterone, which suppresses ovulation until after luteolysis (degradation of the CL). Between days seven and fourteen, the CL reaches its maximum size and functionality. On days thirteen and fourteen, the oxytocin receptor OTR is expressed. OTR appears to be responsible for the release of oxytocin (OT) and prostaglandin F2a (PGF). Proestrus begins about day fifteen, and the CL starts to regress. During this time, OT levels increase, which corresponds with the pulsatile release of PGF. The presence of PGF and OT begin to break down the CL. In order to completely disintegrate the CL, it must be exposed to five pulses of PGF over the course of at least twenty-five hours. The CL is almost completely reabsorbed by day seventeen. Estrogen present from days fifteen to seventeen regulates the pulses of OT and PGF to assure complete luteolysis. From days eighteen to twenty-one, a period known as diestrus, progesterone loses its ability to suppress the endometrial expression of the estrogen receptor and OTR, and thus the cycle begins again (Bazer et al, 1997).
Domestic ruminants are dependent on a coordinated interaction between the conceptus and the maternal system to maintain early pregnancy (Gnatek et al, 1989). During early pregnancy, PGF pulses are diminished in frequency, amplitude or both to prolong the life of the CL (Gnatek et al, 1989). Maternal recognition of pregnancy occurs between days fifteen and seventeen. It has been shown that a conceptus must be present at day fifteen to prolong the life of the CL, which secretes hormones needed to maintain early pregnancy (Weise et al, 1993). The conceptus will begin to maintain the pregnancy on day eighteen. At this time, pregnant does show an increase of 18 and 22 kDa proteins, as well as the presence of a 14 kDa protein that is not found in cyclic does. These proteins are proof that the pregnancy is no longer maintained by the CL, and is now dependent on the conceptus (Weise et al, 1993). The conceptus begins to change shape around day nineteen or twenty, elongating from a spherical blastocyst and fill the uterine horn ipsilateral to the CL (Gnatek et al, 1989).
The key to understanding why a pregnant doe may come into heat can be found in the processes of days fifteen, sixteen and seventeen. During this time of the estrus cycle, the CL begins to regress; estrogen and OT suppress progesterone which causes the doe to exhibit behavioral estrus and ovulate. In a pregnant doe, this is the period of pregnancy recognition. The time in which the CL begins to degradate and the doe starts to recognize the pregnancy overlap.
Due to this overlap, a doe may exhibit signs of heat and even go into heat while pregnant. Should CL regression begin before the maternal system recognizes the pregnancy, the heat cycle will continue. Degradation of the CL will begin and the conceptus will secrete the proteins to maintain pregnancy, thus allowing the continued regression of the CL. The doe will continue cycling and ovulate. Unfertilized eggs are resorbed and the pregnancy continues uninterrupted. If the maternal system recognizes the pregnancy before CL regression occurs, the presence of the conceptus prolongs the life of the CL. Once the conceptus begins to secrete the hormones necessary for pregnancy recognition, the CL will begin to disappear and is usually completely resorbed by day twenty-eight (Weise et al, 1993).
Understanding the heat cycle and pregnancy recognition, it is easy to see why a doe may go into heat when pregnant. To prevent this extra heat cycle, a way must be found for pregnancy recognition to occur before the degradation of the CL. Hormone treatments could be used to regulate the release of OT and PGF, but these treatments may prove expensive and tedious. The dairy goat industry is not profitable enough for the individual breeder to warrant the time and money it would take to prevent the extra heat cycle. Instead, most breeders are simply content to mark every breeding and due date on the calendar and tolerate this unusual quirk in their caprine friends.
[Editor's note: If a doe conceives on this "extra heat" when she is already pregnant, she can deliver two sets of kids three weeks apart. The scientific word for this is superfetation. This has been reported to occur in all farm animal species.]
Bazer, FW, TE Spencer, TL Ott. Interferon Tau: A Novel Pregnancy Recognition Signal. American Journal of Reproductive Immunology, June 1997: pp. 412-420
Damerow, Gail. Your Goats: A Kid's Guide to Raising and Showing. Storey Communications, Pownal, 1993
Gnatek, GG, LD Smith, RT Duby, JD Godkin. Maternal Recognition of Pregnancy in the Goat: Effect of Conceptus Removal on Interestrus Intervals and Characterization of Conceptus Protein Production during Early Pregnancy. Biology of Reproduction, November 1989: pp. 655-663
Jarosz, SJ, RJ Deans, WR Dukelow. The Reproductive Cycle of the African Pygmy and Toggenburg Goat. Journal of Reproductive Fertility, March 1971: PP. 119-123
Weise, DW, GR Newton, GC Emesih. Effects of Day of the Estrous Cycle or Pregnancy on Protein Secretion by Caprine Endometrial Tissues. Biology of Reproduction, August 1993: pp. 522-527
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