Factors Contributing to Variation of Duration of Estrus and Time of Ovulation in a Commercial Sow Herd

North Carolina State University Swine Reproduction Research from 2001. Despite considerable variation in duration of estrus (DE) and the onset of estrus-toovulation interval (OEOI), sows generally ovulate at about 70 to 80% of their individual DE. Unfortunately, DE and time of ovulation can only be established retrospectively and multiple inseminations are necessary to ensure that at least one insemination occurs near ovulation because the viability of gametes is brief. A sows weaning-toestrus interval (WEI) has been found to be a prospective indicator of time of ovulation since it tends to be inversely related to DE. The influence of other factors on DE and OEOI, such as the short lactation lengths (< 21 d) and different sow genotypes currently in use in the swine industry, has gone largely uninvestigated. The DE and time of ovulation of 86 weaned sows (parity 1 to 10; 13 to 19 d lactation) in a large commercial herd was monitored via the back pressure test and transabdominal real-time ultrasonography, respectively, at 0200, 0800, 1400 and 2000 h, beginning 2 d postweaning. Seventy-five of the 86 sows (87%) returned to estrus normally, within 2.5 to 7.0 d postweaning. Neither lactation length nor parity affected their WEI (range, 2.5 to 7.0 d; mean SEM, 4.4 0.1 d), DE (12 to 90 h; 59.5 1.6 h), OEOI (18 to 72 h; 45.0 1.6 h), or the percentage of DE at which ovulation occurred (DE%, 27 to 160%; 76.0 2.8%). There was only a weak negative correlation between WEI and DE (r = - 0.25, P < 0.03). One of the three sow genotypes studied tended to ovulate earlier than each of the other two genotypes (67.6 5.4 vs. 78.0 3.7 and 81.5 6.3%, P < 0.10). These data suggest that the short lactation lengths recently adopted by the US swine industry have not altered the temporal relationships between estrus and ovulation suggested in the literature and that sow genotype may be an important source of variation in DE%.