Determining whether sows and gilts that have not been detected in “heat” after periods of boar exposure are truly anestrus can be very useful. The most accurate way to accomplish this is to monitor progesterone concentrations. Progesterone is the hormone produced by corpora lutea. Corpora lutea are the structures that follicles become once they ovulate.
What happens in sows after weaning and gilts during their estrous cycles are follicles increase in size and produce estrogens. Once these estrogens reach high enough levels, they stimulate the standing reflex and the other behaviors associated with “heat” and also cause the release of gonadotropins. This rapid increase in gonadotropins, in turn, causes follicles to ovulate and turn into corpora lutea. Corpora lutea produces progesterone. If females are bred and conceive, then corpora lutea are maintained, and progesterone concentrations stay high for the duration of pregnancy. If females are in heat and not bred, then corpora lutea regress after 12 to 14 days and the process starts over. This results in progesterone being high for two weeks and then low for one week in females exhibiting normal ovarian activity that isn’t pregnant. Finally, if females are truly never in estrus, then ovulation never occurs; corpora lutea never form, and progesterone remains low.
Based on this information monitoring progesterone at two-week intervals should be an effective strategy for determining when females are truly anestrus versus when they are exhibiting normal ovarian activity but haven’t been observed in heat. If both samples are low, then females are anestrus and not exhibiting normal ovarian activity. If one (or both) samples is (are) high, then the ovarian activity is normal, and observation of estrus was missed in most cases.
While the interpretation of progesterone profiles are very straightforward, unfortunately, obtaining samples and then getting them analyzed is not. Blood is the best sample in which to measure progesterone but requires restraining females and bleeding them twice over a two week period which is time-consuming and can be stressful for both the pigs and the people. An alternative is to monitor progesterone in the feces. One of the ways that progesterone and many other hormones are cleared from the body is through urine, feces, saliva and other bodily excretions. Of these, the one that is available nearly all the time and the easiest to collect is feces. However, the time course over which changes occur in the blood compared with the feces is likely to be different depending on some factors including the type of feed; its digestibility; and how quickly it moves through the G.I. tract. These things don’t affect progesterone production much in the animal but do affect how quickly an animal would void feces. Since fecal matter is the source of progesterone being measure, they would affect its concentration — slow rate of passage would yield increased concentrations, and fast rate of passage would yield reduced concentrations given the same level of progesterone in the blood.
A field study in which blood and fecal samples were taken two weeks apart beginning seven days after weaning for a group of sows that had not been detected in estrus was conducted to determine how well fecal progesterone could be used to determine ovarian activity in presumably anestrus sows. The sows were sent to a processing facility after the last sample period (three weeks after weaning), and their reproductive tracts were examined. Ovarian activity was estimated based on fecal progesterone concentrations independently and without knowledge of the results of blood progesterone concentrations or ovarian morphology obtained when the reproductive tracts were recovered. The criteria used to classify sows as having normal ovarian activity based on their fecal progesterone values was that one sample had to be above 20 ng/g. This value was determined from previous studies in which blood and fecal samples were taken daily and compared in sows exhibiting normal estrous cycles.
This proved to be a very reliable way to determine whether weaned sows were anestrus or exhibiting normal ovarian activity. Of the 50 sows examined, 18 had corpora lutea, and at least one of the two blood samples had elevated progesterone. Sixteen of these were diagnosed correctly based on fecal progesterone concentrations while two were diagnosed incorrectly. Of the remaining 32 sows, 29 had small or medium follicles on their ovaries and low serum progesterone. Twenty-seven of these were diagnosed correctly as being truly anestrus based on their fecal progesterone levels, and two were diagnosed incorrectly. Finally, three sows had cystic follicles that had luteinized. In this situation, ovarian activity is abnormal since the follicles grow to preovulatory size but never ovulate. Instead, the follicular cells begin to produce progesterone. In essence, fecal progesterone did reflect accurately what was occurring regarding progesterone production by the ovaries. It is also interesting to note that the three sows with cystic, luteinized follicles were the only ones for which progesterone was high in both samples.
In this particular study, fecal samples were obtained from the rectum of gilts and placed in a small plastic bag. Once all the samples were collected and returned to the laboratory, 1 gram was removed; placed in a conical tube, and incubated with 10 mL of phosphate buffered saline at 25 degrees C for 18 hours. After centrifugation, the supernatant was removed; stored; and analyzed for progesterone. From a practical perspective, there are commercial laboratories that can analyze different types of samples for progesterone, so access to incubation and centrifugation capabilities would be a determining factor as to whether this is practical for specific production situations.