Researchers at the University of Illinois are searching for clues to low farrowing rates, small litter sizes, high returns to estrus, excessive non-productive days and even delayed puberty. Real-time ultrasound (RTU) technology is being used to assess the ovaries and uterus determining the reproductive status of females. Their goal is to arm pork producers with new information to make smart reproductive management decisions.

“Real-time visualization of the reproductive tract can be performed by insertion of a probe into the rectum of the female,” explains reproductive specialist Rob Knox. “This method is called the transrectal route and is optimal for visualizing ovaries and the reproductive tract.”

The rectal palpation method, commonly used in cattle and horses, is not practical in pigs. To overcome this limitation, researchers attached the ultrasound probe to a small rod, making the palpation easier. “Our experience with this technology indicates that visualization of the ovary is easier, faster and considerably more reliable than any other method to date,” states Knox.

Time of Mating

Knox and fellow researchers Gina Miller, Kilby Willenburg and Sarah Probst, DVM, from Carthage (IL) Veterinary Service have tested the RTU applications in commercial herds ranging from 1,000 to 6,000 sows. They identified factors that may be influencing the time of ovulation and subsequent fertility. Their work shows that time of ovulation after onset of estrus is most influenced by the interval from weaning to estrus.

“This information is critical for timing inseminations within a desired 24-hour window before ovulation, thereby improving pregnancy rates and average litter size,” explains Knox. “Although other factors such as parity, length of lactation and season play a role in wean-to-estrus interval, they appear to have little direct effect on time of ovulation after onset of estrus.”

The ability to characterize a herd for reproductive status after weaning and time of ovulation will ultimately allow producers to improve timing of insemination. This could lead to “fixed time” inseminations, lower semen dosage requirements and more use of frozen semen, perhaps reducing the number of inseminations to just one.

Knox speculates that if reproductive status (confirmed estrus, level of fertility) of replacement gilts could be accurately assessed, new efficiencies could be gained. These evaluations could also guide prudent use of estrus-inducing hormones to achieve optimal results, he adds.

Diagnostic Tool

The use of RTU may help reduce non-productive days further by helping diagnose the causes of infertility. “RTU is capable of diagnosing uterine tract infections, cystic ovaries and cases of limited ovarian activity,” says Knox. This information could help make decisions about treating or culling sub-par females.

By far the greatest application of RTU in commercial herds is for diagnosis of pregnancy and identification of open females after breeding. Its use has increased as machines have gotten smaller, more portable and more accurate in diagnosing pregnancy.

“Although reports of improved accuracy are very encouraging, results are often highly variable,” says Knox. “Part of the variability is linked to our inability to identify the day of pregnancy. This measure is inherently inaccurate because day of first service and first day of actual pregnancy may be off by as much two days.”

Also, false readings occur because RTU pregnancy diagnosis relies on identifying fluid in the uterus. Besides pregnancy, fluid may be prevalent in the uterus due to an infection, pseudopregnancy, cystic ovaries or recent insemination.

Despite the potential for errors, RTU can determine pregnancy in up to 90% of females when performed 3-4 weeks after mating. But the ability to diagnose non-pregnant females still lags far behind.

Pregnancy is confirmed when the operator can see multiple, fluid-filled pockets within the uterus. However, in the absence of clear, fluid-filled pockets, the technician may encounter cloudy fluid pockets, single clear fluid pockets, or no clear fluid pockets. It's these vague conditions that make it difficult to identify non-pregnant sows.

Equipment Sensitivity

The reliability and ease of pregnancy diagnosis is clearly related to the methodology and equipment. As the frequency in the probe increases from 3.5 to 7.5 MHz, more ultrasound energy allows for much finer image resolution. However, there's a tradeoff: since the higher-frequency waves do not also penetrate animal tissues, they can only scan at shorter distances. Lower-frequency probes can scan more deeply into the animal, but will produce a poorer image.

Examples of this high/low frequency variability are illustrated in a recent University of Illinois study. Using a 7.5 MHz transducer, transrectal visualization of both the ovaries and uterus at 20 days or less could potentially provide insight into 21-day returns to estrus. Although accurate, the methodology was time-consuming and identified less than 55% of pregnancies. At the same time, the transabdominal methodology (using a 3.5 MHz transducer) identified no returns and only 2% of pregnancies.

The current technology appears limited for pregnancy diagnosis or identification of 21-day returns at these early stages following mating. Still, diagnostic capability increases dramatically by Day 22, to 95% for transrectal and 51% for transabdominal. By Day 24, the full potential of the transrectal method is realized with 97% accurately diagnosed as pregnant, while transabdominal diagnosis reaches 90%.

Although transrectal ultrasound is more accurate, time and practicality may prohibit widespread use. It takes about 1.4 min. to diagnose pregnancy at Day 24; the transabdominal method takes 18 seconds or less for each sow.

Clearly, transducer frequency and technician experience influences the effectiveness of transabdominal ultrasound as early as 21 days after mating. Researchers at North Carolina State University (NCSU) showed that as probe frequency level increased from 3.5 to 5.0 MHz, the impact of an inexperienced technician was minimized.

Illinois research confirmed that as level of experience using transabdominal went from novice to expert, the time required to diagnose pregnancy was reduced by as much as 10 seconds per animal.

The NCSU findings also indicated that with experienced technicians and optimal frequency transducers, the ability to diagnose pregnancy could be very high (>95%) after Day 21. Unfortunately, diagnosing open females was still less than 60% at Day 21, improving only slightly, to 70%, by Day 27.

Again, the inability to accurately identify non-pregnant females is related to fluid accumulation and fluid diameter. Illinois research indicates that fluid diameter from ultrasound images of the uterus of pregnant females is minimal between 16-18 days after the first mating. This measure increases between 19 and 22 days, and then doubles between 23-26 days of pregnancy. Maximal fluid in the uterus occurs at Days 27-32 of pregnancy, marking the time when pregnancy diagnosis is quickest and easiest — requiring only about 6 seconds/sow. Fluid slowly declines around Day 40 before increasing once more around Day 55 of pregnancy.

The effect of this fluid change can significantly alter the ease and reliability of pregnancy diagnosis. “The difficulty in using RTU to confirm pregnancy may arise from a temporary decline in uterine fluid in conjunction with an increase in tissues of the growing fetus,” explains Knox. “Consequently, diagnosis before Day 21 and after Day 38 may lead to difficulty or inaccurate diagnosis of pregnancy.”