Early pregnancy: Concepts, challenges, and potential solutions

By Thomas Spencer. 2013

87 citations recorded by the source corpus.

Early pregnancy in domestic livestock encompasses the period of development of the embryo in the oviduct after fertilization of the oocyte(s) by sperm, formation of a blastocyst in the uterus, and elongation of the conceptus (embryo/fetus and associated extraembryonic membranes) after hatching of the blastocyst from the zona pellucida in the uterus. Conceptus elongation requires the uterus and is concomitant with pregnancy recognition signaling to maintain the corpus luteum (CL) and progesterone (P4) secretion as well as the onset of implantation for establishment of pregnancy. Uterine receptivity for conceptus elongation and implantation develops in response to P4 and is modified in response to products from the conceptus such as prostaglandins (PG) and interferons (IFN). Fertility can be defined as pregnancy rates to a single insemination. Based on that definition, fertility is high in sheep (85%) and pigs (90%), moderate in beef cattle (45%), and low in dairy cattle (35% or less in high-producing cattle) and humans (25%). Consequently, infertility and subfertility are major problems in domestic livestock as well as in humans. The majority of embryonic mortality occurs after fertilization and during the first two to three weeks of pregnancy in cattle and month of pregnancy in pigs. Direct effects of embryonic mortality are reflected in reduced conception rates in cattle and reduced litter size in pigs and sheep. Of particular note, pregnancy rates have declined in dairy cattle as a result of increased early embryonic mortality in association with selection for increased milk yield during the past 50 years (Diskin and Morris, 2008; Figure 1). New management tools are needed in livestock to decrease embryonic mortality and pregnancy loss, thereby increasing overall herd fertility, reproductive and productive longevity, and sustainability of livestock enterprises. Primary determinants of pregnancy success include nutrition, disease, environment, and genetics, and all of these are directly influenced by management strategies (Figure 2). There is sufficient genetic variability within major breeds for fertility traits, and genetic selection for dam fertility is likely a key to long-term improvements in livestock fertility, particularly in cattle. In general, fertility traits are complex and polygenic. The advent of high-throughput DNA sequencing and genomic technologies is useful to identify genes responsible for improved embryo survival in ruminants as well as uterine capacity for litter size in pigs. The incorporation of geFigure 1. Association of milk yield in dairy cattle with daughter pregnancy rate. Selection for greater milk production in United States dairy cattle has been very successful during the past 50 years; however, cow fertility has declined in spite of low heritabilities for reproductive traits. (Modified from the Dairy Cattle Reproduction Council.)

Subfields: Agronomy and Crop Science, Small Animals

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