Effect of Frequency of Boar Exposure on Estrus and Ovulation in Weaned Sows as Determined by Real-Time Ultrasound

Introduction Reproductive performance in swine is most related to inseminations occurring near the time of ovulation. This is due to the time-span for normal fertilization of eggs being limited by both the egg after the occurrence of ovulation (<12 h) and the sperm after insemination (<24 h). Therefore, the number of eggs that are fertilized, the percentage of normal embryos, the resulting pregnancy rates (Hunter and Dzuik, 1968, Soede et al., 1995), farrowing rates (Nissen et al., 1997), and litter size (Kemp and Soede, 1996; Rozeboom et al., 1996) are all related to time of insemination relative to ovulation. Insemination within 12 h before ovulation, appears to produce the most optimal pregnancy rates (>90%) and litter sizes (10.5-11, Kemp and Soede, 1996) but Nissen et al has observed that inseminations occurring from 28 h before to 4 h after ovulation result in high reproductive rates (Nissen et al., 1997). Unfortunately, the time of ovulation is difficult to predict since the only obvious marker for this event is the expression of standing estrus. Further, the time of ovulation varies between 24 to 60 h after onset of estrus (Soede et al., 1992). This high level of variation is important since insemination protocols are set from estrus, with inseminations typically occurring twice between 0 and 48 h. There appears to be a normal distribution for ovulation time after onset of estrus, with 20% of weaned sows ovulating by 24 h, 55% ovulating between 24 and 48 h, and 25% ovulating after 48 h (Knox et al., 1999a). It would appear then, that current insemination protocols may not fully account for the total variation in ovulation times within the breeding herd. This would help explain why average USA reproductive rates are less than optimal. This experiment was conducted to address whether estrous detection frequency could be used as a method to increase the frequency of fixed time inseminations that occur within 24 or 12 h prior to the time of ovulation in anticipation of early (≤ 24 h), expected (≥24 h and ≤48 h) and late times of ovulation (≥ 48 h). In commercial settings, a common industry practice employs a once daily estrous detection procedure with insemination targeted to occur at 0 h for the first service (estimated time relative to estrus is 0-24 h) and 24 h later for the second service (estimated time relative to estrus is 24-48 h). Although this procedure can generate 79% farrowing rates, it cannot account for the variation in time of ovulation, since onset of estrus can be off by as much as 24 h. Even twice daily estrous detection procedures (0800 h and 1600 h) that rely on an 8 h working day, cannot compensate for this variation, since sows detected in estrus at 0800 h, may have already been in estrus for up to 16 h. Additionally, since approximately 70% of the females will show the first standing response between 6 p.m. and 6 a.m., large estimation errors result (±16 h) for onset of estrus, leading to inseminations that fail to occur within 24 h prior to ovulation.