Watch Weaning-to-Estrus Intervals

Management is especially important during this critical time.

By W.L. Flowers

One of the greatest influences on weaning-to-estrus interval is the management of sows during lactation. During this time, the reproductive organs of sows have a chance to recover from their previous pregnancy. It is well established that levels of reproductive hormones in the brain that stimulate estrus and ovulation are very low immediately after farrowing. 

Most research studies have shown that between12 and 16 days are required for the levels of these hormones to be replenished. Lactation plays a critical role in this recovery process because the suckling action of the piglets serves to keep the sow’s brain in a state of quiescence, and the secretion of these hormones at very low levels. Once weaning occurs, the suckling-induced inhibition of these hormones is gone; if they have been replenished sufficiently, then estrus and ovulation should occur within four to eight days. If they haven’t, then the rebreeding interval will be extended or, perhaps, a post-weaning estrus may not occur at all.

Opportunity for Evaluation

From a management perspective, weaning-to-estrus intervals present the first opportunity for producers to evaluate how well sows have recovered from their previous pregnancy. It also is a good opportunity to determine how well management during lactation has aided this process. The general assumption is that if sows return to estrus within eight days post weaning, then their recovery is complete. If the rebreeding interval is longer than this, then perhaps their recovery wasn’t quite finished when weaning occurred and their subsequent reproductive performance may be compromised. 

A recent analysis of adjusted farrowing rates and number of pigs born alive based on a farm’s average weaning-to-estrus interval seems to support this assertion (Table 1). Farms with weaning-to-estrus intervals of less than eight days averaged between 10.9 and 11.0 pigs born alive. In contrast, farms with weaning-to-estrus intervals of eight days or more averaged about 0.5 pigs less per litter. The relationship between weaningto- estrus intervals and farrowing rate was less clear. However, there was a general trend for farrowing rates to decrease as the weaning-to-estrus intervals increased.

Watch Weaning-to-Estrus Intervals

Work on Problem-Solving

If a herd has an extended rebreeding interval, then there are several areas associated with lactation management that should be examined. The most obvious is feed intake during lactation. It has been well documented that nutritional management during lactation has a significant impact on subsequent reproductive performance of sows. Lactation is a period in which the sow is under an enormous amount of metabolic stress. It has been estimated that about 75% of the nutrients that a sow consumes during peak lactation goes to support production of milk for her litter. Consequently, it is quite common and actually normal for sows to have to mobilize protein and fat to meet the metabolic demands of lactation. When this happens, the sow loses weight and body tissues. And if she loses too much body condition during lactation, her subsequent reproductive performance post-weaning can suffer. As a result, rebreeding intervals, subsequent farrowing rate and litter size can all be affected. Anything that can be done to increase feed intake during lactation should help improve weaning-to-estrus intervals.

Another area that can influence the weaning-to-estrus interval is lactation length. As mentioned earlier, the brain needs time to replenish reproductive hormones after farrowing. If sows are weaned before these levels are established, then suboptimal amounts are released. This creates a situation in which sows would probably show a delayed estrus and ovulate a lower-than-normal number of eggs. Recovery of the brain and replenishment of these hormones is also sensitive to the metabolic demands of lactation. Consequently, if excessive amounts of body tissue are lost during lactation, then recovery can take longer than the normal 12 to 16 days. Collectively, lactation lengths of less than 16 days often are not conducive for optimizing the subsequent reproductive performance of sows. Finally, split or partial weaning strategies can contribute to problems with extended rebreeding intervals. It is important to remember that whenever pigs are removed, the suckling stimulation is reduced. If enough pigs are removed, there could be a high enough reduction in the suckling intensity that the suppression of the endocrine system caused by suckling is removed and the sow may begin normal reproductive activity. What happens in many situations with split weaning is that the largest pigs in the litter are weaned two to three days before the rest of the litter. If enough piglets are removed from the sow at this time, then from a physiological perspective, she thinks the entire litter has been weaned. If this occurred on day 16, then the reproductive consequences are similar to those that occur with early weaning.

The Sow’s Perspective

As we’ve pointed out, management is the key to maximizing wean-to-estrus intervals. Keeping the needs of the sow herd as a top priority will help you make the necessary changes to improve this important production perameter. 

Editor’s Note: Dr. Flowers is a professor in the Animal Science Department at North Carolina State University with a 50% teaching and 50% research appointment in swine reproductive physiology. Dr. Flowers has published over 90 peer-reviewed journal articles and book chapters and more than 325 popular press / extension articles