Application of ultrasound technology in the reproductive management of small ruminants
Richard Ehrhardt, Almudena Veiga-Lopez and Barbara Makela
Department of Animal Science, Michigan State University
Introduction and history
Ultrasound technology has been used in the reproductive management of sheep and goats since the 1960s. The advent of portable A- and B-mode units in the 1970s expanded application at the farm level. B-mode units allowed visualization of the fetal compartment on a small screen, allowing more accurate detection of pregnancy and fetal counting. In contrast, A-mode units have been available for many years as well, but are limited by their ability to only detect large changes in sound wave transmission, as occur in primarily fluid-filled compartments in the abdomen such as the pregnant uterus. These units detect pregnancy by emission of a “beep” and/or light signal. These units have limitations in accuracy, as a full bladder may provide a false positive in the detection of pregnancy, and they are also limited in the scope of use since they cannot determine litter size or stage of pregnancy. Recent advances in B-mode units in terms of portability (battery powered with options including headset visualization goggles, allowing for enhanced portability), ease of contact lubrication application (some units allow for a stream of water to be pumped over the probe for fast and convenient skin contact), and more recently wireless streaming (some units allow for projection of ultrasound images on tablets and phones) have increased the speed and convenience of scanning. In addition, newer B-mode units have enhanced resolution even in relatively affordable units, making earlier and more accurate fetal scanning possible in the field. These advances in resolution and portability along with a decrease in cost have the potential to increase the application and adoption of this technology in the field.
Pregnancy detection. Transabdominal scanning of sheep and goats allows for a very efficient and accurate means of detecting pregnancy. The speed of scanning for experienced practitioners is largely a function of the efficiency of the animal handling system at a given farm. Speed and efficiency are also gained by use of an efficient means of applying lubricant (ultrasound gel) to the probe to allow for air-free skin contact, facilitating sound wave transmission. Experienced scanners with highly efficient handling systems can scan as many as 270 animals per hour at a 98% accuracy rate. The ideal window of pregnancy for transabdominal scanning is day 40-70. The most commonly used probes are variable frequency 3-5 mHz linear probes or 3 mHz sector scanning probes. Pregnancy can be detected as early via transabdominal scanning using higher resolution units. However, the fetal compartment is small and easier to miss at this stage, hence the recommendation of a minimum of 40 days. Producers interested in this application are advised to limit ram/buck exposure to no more than 2 estrus cycles (34 days in sheep and 42 days in goats) if a single scanning session is desired. Ideally, one would schedule scanning for 40 days following ram/buck removal. Pregnancy can also be detected with higher frequency probes (5-10 mHz) that are inserted in the rectum with a special, plastic extension tool. This allows placement of the probe closer to the gravid uterus, allowing visualization of pregnancy as early as day 18, with a conceptus size of less than 0.8 cm. The need for detection this early would likely be only in special cases when very early detection is desired and is not a general need.
Determination of stage of pregnancy. Stage of pregnancy can be quantified with accuracy (within ±4 days of gestational age for an experienced practitioner during mid-pregnancy) with transabdominal scanning. Highly experienced practitioners are able to look at the relative proportions of placentome size (component of the sheep/goat placenta) to fluid space volume to fetal size along with changes in density of fetal bones to predict stage of pregnancy with a high degree of accuracy. It is also possible to take measurements early in pregnancy of the size of the embryonic vesicle or, later, of various anatomical features of the fetus (biparietal diameter, crown to rump length, length of fused metacarpal bone) and apply these measures to a simple nomograph or table to predict fetal age with accuracy. More recently, these predictive values have also been made available through phone applications. We have found that each of these fetal measures has an optimal range for use, however, with these ranges overlapping as follows: embryonic vesicle day 28-40, crown to rump length day 40-70, biparietal diameter day 40-100. As pregnancy progresses beyond day 70, it becomes harder to take these measurements, however, as the distance to fetus may be too great due to fetal positioning within the gravid uterus.
Determination of fetal number. Fetal number can be quantified with reasonable accuracy in the day 40-70 stage of pregnancy as well. Skilled practitioners have error rates less than 5% in prediction of fetal number, although accuracy is hard to quantify from birth records due the natural occurrence of fetal death and reabsorption following the time of scanning in mid-pregnancy to term. Prediction of fetal number in higher order multiples (triplet or greater pregnancies) also has a higher degree of error compared to single or twin pregnancies. Accurate quantification of fetal number also takes more time than simple yes/no pregnancy detection, slowing animals scanned per hour to <50.
Use of ultrasound scanning information in small ruminant production
Better allocation of feed resources. Separation of ewes or does according to pregnancy status allows removal of non-pregnant animals, which then can either be culled or fed less due to their lower requirements. In some production systems, failure to remove the non-pregnant animals from the pregnant group will result in obesity. Not only is this a waste of feed resources, but a hidden cost in this may be the failure of these animals to breed when re-exposed or to be at greater risk of ketosis if they do conceive. In more prolific flocks/herds, grouping according to fetal number results in a better allocation of feed resources, particularly during late pregnancy when the nutrient demands will vary most markedly and to the greatest extent according to litter size. Those carrying singles can be fed less, with more and higher quality feed resources reallocated to those rearing multiples. This will result in a better pregnancy outcome for the entire flock/herd. The utility of fetal counting is greatest in more prolific flocks/herds as compared to those with a lambing/kidding rate of <120%.
Precision feeding to meet animal requirements during pregnancy. In more intensified production systems that utilize precision feeding management, animals can be fed precisely according to stage of pregnancy and litter size. Grouping animals using this information and feeding accordingly will reduce the incidence of metabolic disease and optimize maternal and fetal nutrition.
Optimizing birth management. Grouping according to fetal number and stage of pregnancy in both intensified and extensive production systems provides an opportunity to increase labor efficiency during the birth period. By separating ewes/does with potentially greater needs for labor at birth (those with multiples are more prone to dystocia and mis-mothering issues) from those with less (those carrying singles), one can allocate resources (labor, favorable birth paddocks with windbreaks, warmer housing facilities) appropriately to achieve the best birth outcomes.
Decision as to when to dry-off lactating animals in parlor milk production. The decision to dry-off lactating animals in parlor milk production can be made with knowledge of pregnancy status and stage. This allows animals to be managed in such a way as to allow optimal dry period length for better lactational performance and herd/flock health.
Detection of pregnancy in primiparous animals. Early detection of pregnancy allows sale of non-pregnant animals when they are still at a high market value (under 12 months of age). The ability to breed before one year of age can be used as a selection tool for increased lifetime productivity, as it has been demonstrated that ewes that lamb at 12-15 months of age have a higher lifetime productivity than those that do not.
Ultrasound technology can be applied as a tool to improve reproductive management of small ruminants in a diverse array of production scenarios, from extensive production systems emphasizing wool production to intensive production systems utilizing prolific genetics with an emphasize on meat production, to small ruminant dairies. Ultrasound scanning proves to be a low-cost, accurate method of determining pregnancy status, stage of pregnancy and litter size. By grouping animals according to pregnancy status, stage and/or litter size, managers can increase production efficiency, improve allocation of farm resources and improve the health and welfare of the flock/herd.