Why Are My Cows Open?

As we approach the time of year when the pregnancy status of many spring-calving herds is determined, I am often asked to theorize why a particular group of cattle have less than the anticipated number pregnant. Generally, my options fall into one or more of four categories: 1) The bull’s fault 2) The cow’s fault 3) a disease’s fault or 4) nobody’s fault.

Bulls fault—In order for bulls to get cows pregnant, they must be able to produce adequate amounts of fertile semen, to find cows that are in heat, and to deliver fertile semen to the cow’s reproductive tract at least once during estrus (heat). Testicular damage from physical, toxic, genetic, or infectious causes can decrease both the number of sperm cells produced and the ability of sperm cells to reach and fertilize an ovulated egg within the cow’s reproductive tract. Poor eyesight can interfere with a bull’s ability to find cows in heat; and sore feet, legs or spine can cause a bull to refrain from walking after and mounting a cow he detects in heat. Damage to a bull’s penis or prepuce is likely to prevent it from successfully delivering semen to the cow’s reproductive tract.

Although very few bulls are completely sterile, surveys indicate that 10-20% of bulls given a Breeding Soundness Examination (BSE) by a veterinarian will fail to meet the criteria for a satisfactory breeding bull. Failing a BSE means that it is likely that those bulls will fail to cause a pregnancy in a high percentage of cows to which they are exposed.

Cow’s fault—Cows and heifers must be having regular estrous (heat) cycles during the time period they are exposed to bulls in order to become pregnant during the breeding season. By initiating cycling prior to or early in the breeding season, females have several opportunities to ovulate a fertile egg and become pregnant during a limited breeding season. If heifers have not reached puberty or cows have not resumed cycling following the birth of their last calf by the first few weeks of a 60 to 70 day breeding season, they will only have one or two opportunities to become pregnant before the bulls are removed from the breeding pasture. Age, weight, and genetics all influence whether a heifer has reached puberty by the start of the breeding season. How early in the previous calving season a cow calved (days postpartum) and what level of energy reserve (fat/body condition) she has during early lactation determine whether a cow has resumed fertile cycles by the start of the breeding season.

Previous years’ bull infertility problems or cowherd nutrition problems (low body condition score) during late gestation and early lactation can cause many cows to slip late into the calving season before giving birth. This late average calving date will increase the risk of cows not having enough days postpartum to resume cycling early in the next breeding season. In addition, limiting forage and feed quality or quantity late in gestation will cause body condition scores, as an indication of body fatness, to decrease to the point where the luxury of reproduction is delayed until energy reserves are adequate. Cows that don’t have enough days postpartum or energy reserve are not likely to be expressing estrus (heat) and ovulating fertile eggs, and there fore cannot become pregnant.

Disease’s Fault—A few infectious diseases can cause failure of fertile eggs and sperm to initiate pregnancy, or to cause death of a very early embryo. These include vibriosis and trichomoniasis. In addition, other diseases can cause the death and abortion of fetuses later in pregnancy. IBR and BVD viruses, bacterial diseases such as leptospirosis and protozoal diseases such as neosporosis can each cause abortion. Embryo loss or abortion early in the breeding season may allow enough time for the cow or heifer to resume cycling and become pregnant again before the end of the breeding season. Cows that abort after a 60-70 day breeding season, will be open when examined for pregnancy in the fall.

Nobody’s Fault—A certain percentage of fetuses have genetic flaws that are fatal. These flaws are not due to disease or heritable genetic problems, but rather are a testimony to the complexity of biology. These loses are probably unavoidable. Typical pregnancy loss due to genetic and other causes between the time of pregnancy diagnosis until calving is 1/2 to 2%.

A final reason for pregnancy percentages to be lower than desired that falls in the “nobody’s fault” category is that people tend to want above average results every year. Remember that if your average pregnancy percentage is 94%, year-to-year pregnancy percentages may range from 91-96% and occasionally from 89-98% due to normal biological variation.


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	Matt Herring - Agronomy/Natural Resources Ken Bolte - Farm Management/Livestock Franklin County University Extension Center Last revised: 11/28/05	Go to Franklin County University Extension Center page