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Detection of estrus is an important component in A.I. breeding programs and is difficult to optimize. The keys to successful detection of estrus are to be observant, prevent over-exposure to boars and understand the physiological and behavioral signs that occur prior to the standing reflex. These signs should be used as a guide with regards to how much time should be spent trying to stimulate sows.
Identification of the physiological signs of estrus is not difficult. These include increased activity and vocalizations; swelling of the vulva and enlargement of the clitoris; the presence of a sticky discharge; elevation of the ears and immobilization in response to manual back pressure. However, estrus often is missed or misdiagnosed because many of these signs are overlooked. Accurate and efficient identification of estrus requires two things: (a) a thorough knowledge of the chronological order in which these signs occur as females enter their period of sexual receptivity ; and (b) establishment of heat detection protocols that enhance the expression of estrous behaviors.
The first sign that females are coming into heat is an increase in activity and vocalizations. When housed in crates, sows commonly move backward and forward or from side to side within the crate and often attempt to nibble or nose females in adjacent crates. In addition, they sometimes will paw at the front door of the crate and "chant" to animals in crates beside them. When housed in pens, characteristic activities include sniffing; nuzzling the rear and fore flanks; and attempting to mount or ride other females. It is important to remember that sows attempting to mount or those actually riding other females are not in heat, but rather either just coming in or going out of their period of sexual receptivity.
Reddening and swelling of the vulva usually either accompanies or occurs shortly after the increase in activity. The change in the size and color of the vulva is the result of an increase in blood flow and retention of fluids in the female external genitalia. As a result, the external lips of the vulva often are pushed outward exposing a portion of its inner lining which is highly vascular. Due to these changes, the opening to the vagina often appears to increase in size. In contrast, in females that are not in heat, the external lips of the vulva are curled inward and block the opening to the vagina giving it a small, puckered appearance. Reliance on only the appearance of the vulva as an indication of sexual receptivity and the cue for breeding is a common mistake associated with A.I. The size and color change of the vulva actually is greatest just prior to the period of estrus in which females will "stand" for breeding or exhibit the immobilization reflex. In addition, reddening and swelling of the vulva often do not occur in older parity sows during estrus. It is common for the skin around the vulva to become stretched or loose during farrowing. Thus, in older sows, even though the physiological changes which normally cause swelling and reddening still occur, the change in color and size are masked by the loose, flabby skin around with the vulva. The appearance of red, swollen vulvas should be recorded on breeding records and used as a reminder by breeding technicians to spend additional time over the next few days examining those sows or gilts for the immobilization response.
The presence of a "sticky" discharge and enlargement of the clitoris are changes that usually occur immediately before and during the standing reflex. The clitoris is a small, finger like structure which is located inside the base of the vulva in the crease formed by the two lips. When females are in heat, the clitoris is engorged with blood which causes it to protrude outward and have a bright red color. When females are not in heat, the clitoris is flat and has a pale, light pink color. To observe the clitoris, it is necessary to pull the external lips of the vulva apart and outward. This exposes the internal fold where the lips join and the clitoris is located.
Mucous obtained from the inside lining of the vulva from sows that are not in heat has a slimy or slick feel to it. Just prior to the initiation of the standing reflex, the consistency of the mucous changes and becomes tacky or sticky. This change in composition is caused by hormone changes that normally occur during sexual receptivity. Assessment of the consistency of the mucous commonly is referred to as the "thumb check". The thumb check is performed by wiping the inside lining of the vulva with the thumb to obtain a sample of mucous. Next, the thumb and first finger (pointer) are pressed together and slowly pulled apart. The sample is considered sticky or tacky is small strands of mucous remain connected to both the thumb and pointer as they are drawn apart. In most cases, an engorged clitoris and sticky mucous are good indications that the female is in standing heat or within several hours of exhibiting the immobilization reflex.
The best indication that sows are sexually receptive and ready to be inseminated is the occurrence of the standing reflex or immobilization response. When provided with the appropriate stimuli, receptive females will initiate isometric contractions of most of their skeletal muscles. This results in the female remaining rigid or "locked up" in anticipation of being mounted by a boar for breeding. Often the ears of sows and gilts will become erect during the standing reflex. This is commonly called the "ear popping" response.
Pheromones produced by boars are the most potent and effective inducer of the standing reflex in receptive females. For sows housed in crates, running a boar in front of sows while a breeding technician applies back pressure is a common and effective method of estrous detection. Sows in crates that are in heat will move forward and assume the standing reflex as the boar moves in front of the crate. In addition, when back pressure is applied, females that are truly in heat will actually push back when pressure is applied. This is a natural response as she is preparing herself to be mounted by the boar for breeding. Sows in crates that actually try and move away from back pressure, even though they may exhibit other positive signs, probably are not in estrus. In pens, sows will move to the front of the pen as the boar passes by. If back pressure is applied and the sow is in heat, then she will exhibit the standing reflex. If back pressure is applied and the sow is not in heat, then she usually will run away from the back pressure in an attempt to escape. Estrous females housed in pens will sometimes attempt to follow the movement of a boar as he passes in front of their pen. This is due to the fact that sexually receptive females seek out males more so than males finding sows that are in heat.
The manner in which boar exposure is presented to sows and gilts can influence the accurate detection of the standing reflex. Because it involves isometric muscle contractions, maintenance of the standing reflex requires a considerable amount of energy. As a result, estrous females exhibiting the standing reflex can become fatigued or tired. If a female becomes fatigued, then she usually can not resume a standing reflex for several hours. Consequently, boar exposure during estrous detection should be restricted to small groups of sows, either 5 to 10 crates or 1 to 2 pens. These small groups should be examined carefully and receptive females identified. Exposure of the boar to larger groups of females could result in detection errors due to estrous females becoming fatigued and not exhibiting a standing reflex when checked with the back pressure test.
1996 Pork Profitability Summit
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