Introduction

Recent work conducted by Kemp and Soede (1996) has provided a comprehensive evaluation of the relationship among the duration of estrus, ovulation, timing of inseminations and fertility. In this study, ovulation was monitored via real-time ultrasonography allowing for a precise estimation of its occurrence. Data from this study demonstrate three important relationships. Firstly, the duration of estrus was inversely related to the weaning-to-estrous interval. In other words, sows that exhibit estrus within a short time period after weaning are sexually receptive for longer periods of times than their counterparts which have extended weaning-to-estrous intervals. Secondly, although the length of estrus varied among animals, ovulation consistently occurred at 71% of the duration of estrous. Sows exhibiting estrus for 48 and 72 hours ovulated at 34 and 51 hours, respectively. And finally, if matings occurred between 0 and 24 hours prior to ovulation, then fertilization rates were greater than 90%.

These results are exciting and have important implications for development of mating regimens. The inverse relationship between the length of estrus and the weaning- to-estrus interval may provide a way to further refine the optimal mating regimen for sows on an individual basis. Obviously, the length of estrus can only be obtained retrospectively after breeding decisions are made. However, if the weaning-to-estrous interval is known before breeding is initiated, then it may provide a means to assign sows to breeding regimens. The objective of this study was to determine the effectiveness of using the weaning-to-estrus interval as a means of adjusting both the timing and frequency of matings for sows.

Materials and Methods

A total of 1500 multiparous sows on two different commercial swine operations were used in the study. Sows on each farm were divided into 3 categories based on their weaning-to-estrus period: < 5 days; 6-7 days; and > 8 days. The control treatment consisted of breeding sows once every 24 hours beginning at first detected estrus. The adjusted mating treatments were based weaning-to-estrus intervals and were as follows: sows that returned to estrus in < 5 days were mated at 24 and 48 hours after first detected estrus; sows that returned to estrus in 6 or 7 days were mated at 8 and 24 hours after first detected estrus; and sows that returned to estrus > 8 days were mated at 0 and 8 hours after first detected estrus. Observation of estrous activity was conducted daily between 06:30 and 08:00 and was continued for all sows even after the completion of their assigned breeding regimen. All sows received artificial insemination doses from the same boars containing 4 billion spermatozoa within 36.4 + 2.1 hours of collection. Farrowing rate, number of pigs born alive and length of estrus were recorded and analyzed statistically with analysis of variance procedures. The study was conducted over a 6 month period between July and December.

Results and Discussion

A significant farm by return interval by breeding treatment interaction was present for both farrowing rate and number of pigs born alive. Therefore, comparisons between breeding treatments were made on each farm within each return interval (Table 1). On farm A, no differences between treatments for each weaning-to-estrous category were observed. In contrast, on farm B, reproductive performance was reduced for sows receiving the experimental breeding treatments in the 6-7 and > 8 day groups. Adjustment of breeding strategies based on the weaning-to-estrous interval was an effective practice on both farms for animals returning to estrus within 5 days after weaning because reproductive performance was similar with two versus three matings. However, on farm B, the changes in the timing and frequency of matings for sows with return intervals > 6 days appeared to be a detrimental practice resulting in reduced farrowing rates and litter size.

Reasons for the different responses to the same treatments between farms are not clear. However, one possibility could involve the variation in the normal pattern of estrous activity. In addition to reproductive performance, the relationship between the weaning-to-estrus interval and the duration of estrus was recorded during the study. This information is summarized in Table 2 and presented as a percentage of sows that expressed oestrus for a certain period of time (3 days, 2 days, etc.) as a function of the weaning-to-oestrus interval. For example, on farm A, 90% of the animals that exhibited estrus within 5 days of weaning remained in estrus for 3 days, while 9% and 1% exhibited a standing reflex for 2 and 1 days, respectively. It is interesting to note that on farm B, a large proportion of animals in the 6-7 and > 8 day return groups had 3 and 2 day estrous periods, respectively, compared with their contemporaries in herd A. Thus, the reduced reproductive performance in these groups of animals possibly was a consequence of suboptimal number and/or timing of matings relative to the duration of estrus (Flowers and Esbenshade, 1993; Kemp and Soede, 1996).

If results from both treatments are combined, then a reasonable breeding protocol for farm A relative to the onset of estrus would be as follows: < 5 days - 24 and 48 hours; 6-7 days - 0 and 24 hours; and > 8 days - 0 hours. In contrast, for farm B, a mating regimen of once each day of estrus seems more appropriate except, perhaps, for sows returning to estrus within 5 days of weaning. These females could be bred at 24 and 48 hours relative to the onset of estrus without compromising farrowing rates and litter size.

Summary

In summary, despite the variation between farms, establishment of breeding regimens based on weaning-to-estrous intervals appears to have potential. The key to its success, however, is dependent upon thorough documentation of the pattern of estrous activity within a herd.

References

Flowers, W.L. and Esbenshade, K.L. 1993 Optimizing management of natural and artificial matings in swine. J. Reprod. Fert. Suppl. 48, 217-228.

Kemp, B. and Soede, N.M. 1996.  Relationships of weaning-to-estrus interval to timing of ovulation and fertilization in sows. J. Anim. Sci. 74, 944-949.