Post-breeding management to minimize early pregnancy loss in heifers
By Shelby Rosasco
Heifers represent the future of the cowherd and significant emphasis is placed on the selection and management of replacement heifers prior to the start of their first breeding season. There are varying strategies and approaches regarding how heifers should be developed and what target body weight they should achieve.
Ultimately, the goal is for heifers to reach puberty and start cycling prior to the breeding season to allow heifers the best opportunity to become bred early in the breeding season. It is well established that heifers who become pregnant early in their first breeding season and calve in the first 21 days of their first calving season have increased herd longevity and pounds of calf weaned over their lifetime.
Therefore, getting heifers pregnant early in the breeding season can have a considerable impact on the profitability of a cow/calf operation. Significant amounts of time, effort and costs are necessary to develop heifers to the proper weight and body condition prior to the start of the breeding season. Post-breeding management of heifers, however, can be just as important for success during the first breeding season and influence the heifer’s ability to conceive early in the breeding season.
Embryonic development
Management decisions made during approximately the first 60 days after insemination or breeding can have a significant impact on reproductive success. Embryonic mortality can affect the number of heifers becoming pregnant early in the breeding season, the number of heifers carrying to term and ultimately the weaning percentage.
Generally, fertilization is established the majority of the time, approximately 90 percent, however, first service conception rates are often much lower, suggesting embryonic death and loss of pregnancy occurred between fertilization at day one, maternal recognition of pregnancy in days 15 to 17 and attachment of the embryo to the uterus in days 25 to 42.
While there are numerous causes of embryonic mortality that are out of the control of the producer, including poor oocyte quality, genetic abnormalities, disease, etc., management practices during the first two months after breeding can impact pregnancy success.
Post-breeding nutrition
In spring calving herds, heifers are often developed in a drylot over the winter and early spring before being turned out to pasture. This transition frequently occurs at the start of the breeding season either immediately following artificial insemination (AI) or when bulls are turned out.
This coincides with important time points during early embryonic development. Nutritional and metabolic stress, which occurs during this crucial period of embryonic development, maternal recognition and attachment, may affect embryonic mortality and first service conception rates.
Research from both University of Wyoming and South Dakota State University (SDSU) has established alterations in the plane of nutrition during the post-breeding period can negatively impact conception rates. Specifically, developing heifers in a drylot scenario and then immediately moving heifers to pasture following AI can result in reduced pregnancy rates to AI, if heifers lose weight once placed on pasture.
While spring pastures are generally high quality, the nutritive value of the diet consumed in the drylot should be evaluated. If heifers were consuming an energy dense diet in the drylot pre-breeding, transitioning to even high-quality spring pasture can result in a decline in energy density and a decrease in the plane of nutrition.
Adaptation of animals to a grazing environment can also result in increases in nutritional requirements due to an increase in activity level in grazing animals, creating a short-term energy deficit for heifers transitioning from drylot to pasture.
Maintaining heifers on the same plane of nutrition for the first month post-breeding can help alleviate alterations in the plane of nutrition during early embryo development. If heifers are developed on a high plane of nutrition in the drylot, keeping heifers in the drylot on the same diet for an additional 30 days may be the best option to minimize nutritional stress.
Research from SDSU also determined if heifers transitioned to pasture immediately following AI are supplemented to prevent weight loss post-AI, pregnancy rates are not negatively impacted. Supplementation may provide an option for operations that cannot maintain heifers in the drylot for an extended period of time.
An additional strategy could be to adapt heifers to a range-based grazing situation before breeding, roughly 30 days, allowing changes in the plane of nutrition to occur prior to breeding and more heifers to maintain their pregnancies early in the breeding season. Supplementation may be required to ensure nutrient requirements are being met, however, green-up of pastures in the spring may provide an increase in the plane of nutrition depending on when heifers are transitioned to pasture pre-breeding.
Shipping stress
Another source of stress during the breeding season can be when heifers or cows are being transitioned to spring or summer pasture. Hauling animals can increase stress hormones which can negatively impact the uterine environment.
During early gestation, blastocyst formation, migration to the uterus, maternal recognition and attachment of the early embryo is susceptible to alterations in the uterine environment. Stress related to shipping during critical time points of early gestation, migration to the uterus in days five to six, maternal recognition of pregnancy in days 15 to 17 and attachment of the embryo to the uterus at days 25 to 42 can result in an unfavorable uterine environment potentially resulting in embryonic mortality.
During the first five days after breeding, the embryo is in the oviduct and is not susceptible to changes in the uterine environment due to stress. The general recommendation for those producers who AI their heifers or cows is animals should be transported during the first five days after AI or approximately 45 days after insemination to minimize embryonic mortality and pregnancy loss.
Regardless of the development strategy utilized to develop heifers to the appropriate body weight pre-breeding, around 55 to 65 percent of mature body weight, replacement heifers represent a significant investment for producers. Providing heifers every opportunity to not only get pregnant early in the breeding season but maintain their pregnancy can have a significant impact on productivity and profitability of an operation.
Mitigating stress, nutritional and transportation related, during the critical time points discussed above, can be an important management strategy that can be utilized to help maximize the number of pregnancies maintained.
Shelby Rosasco is the University of Wyoming Extension Beef Specialist and an assistant professor at the University of Wyoming in the Animal Sciences Department. Rosasco can be reached at srosasco@uwyo.edu.
