February 2, 2010 – PZP’s Effects, part 3: Out of Season Foals

I apologize for my delay in getting this post up; I didn’t have time to get this done during this past busy week.

It is very common to hear discussions on PZP treatments resulting in out of season foals. This will be the topic of today’s discussion.

Though there has been a great amount of speculation and large-scale reporting on PZP’s effect on the seasonality of foaling, this is a topic that has been discussed in few scientific studies. Kirkpatrick and Turner (2003) reported on the seasonality of foaling on Assateague Island among treated and untreated horses. They defined out of season foals as those foals not born in April, May, or June. Their study involved analysis of the times when 168 foals were born, from 1986 to 2002, along with whether the foals’ mothers had been untreated, treated for one year before a foal was born, or born after at least two consecutive years of treatment. The survival rate among foals born during different seasons to treated and untreated mares was also analyzed.

It was found that there was no significant difference between the seasonality of foaling among treated and untreated mares. In breaking down mares treated for one year before birth and those treated for at least two years before birth, there were also no significant differences found between treated and untreated mares. Thus, this study concluded that PZP does not influence out of season births on Assateague Island. Further, the study concluded that there is no difference in the survival rates of foals born in and out of season on Assateague Island.

The Kirkpatrick and Turner study also has some interesting discussion of the broader picture of out of season foaling. They cite different studies that have statistics on the foaling patterns of other wild horse herds. These range from 4% of foals being born out of season, as was found among New Forest ponies, to 23% of the foals are born out of season, which was reported on Sable Island. They also describe how out of season foaling rates may change within populations. From 1984 to 2001, there was a 13% increase in out of season foals on Assateague Island. Because it was found that PZP was not influencing the seasonality of foaling, there must be other unknown factors at work. The authors propose that these changes may have come with an increasing population size; there were 80 horses in the Assateague herd in 1984 and 173 in 2001.

The above findings and statements from Kirkpatrick and Turner tell us that it is perfectly natural for some foals to be born out of season. Out of season foals came in the Pryors years before any mares were treated with PZP. Out of season foals come in herds whose mares have never been treated with PZP. Again, out of season foals are not unnatural though the number of foals born out of season seems to vary from herd to herd and from year to year.

How can this be? Let’s think about how wild horses reproduce. Earlier, we went over the way PZP works. In this discussion, we looked at the mechanism for reproduction: A series of hormonal interactions leads to the development and release of a mature egg that can be fertilized. This is a mare’s cycle. This doesn’t necessarily happen in one try, though. A mare’s reproductive system may initiate some of these steps but not complete the cycle by releasing a mature egg. For example, a mare may be showing behaviors associated with reproduction though she is not actually able to conceive. In horses, there can be a number of cycles each season. However, horses most commonly foal in the months of April, May, and June; and so there is obviously something going on that leads to mares conceiving during their spring cycles. If there wasn’t, it would be common to see equal proportions foals born at all times of the year. Obviously, this is not the case.

There are actually a few known factors that can influence a mare’s ability to release a mature egg. The amount of daylight is influential. As days get longer, there is a higher probability that a mare will be able to release a mature egg. Thus, from winter to spring, as the days get longer, the mare’s reproductive system will get closer and closer to being successful in releasing a mature egg. Eventually, it can be successful; and the mare can get pregnant. If she does get pregnant, a mare will foal about 11 months later. This is why it is most common to see foals born during the spring. About two weeks after foaling, a mare can go into what is often referred to as a “foal heat.” During this time, the mare releases another mature egg and can get pregnant again. This is why horses’ foaling seasons and breeding seasons are associated with each other. Interestingly, it is very rare among mammals for these two seasons to coincide. Though a mare can get pregnant after she foals, and thus have a good chance of having a foal around the same time the following year, we don’t always see this happen. This is because another factor can influence whether or not a mare is able to get pregnant; this has to do with the mare’s body conditions. If a mare has poor body conditions after she foals, which is not uncommon due to the stress that raising foal can cause, then it is unlikely that she will be able to get pregnant during her foal heat. Thus, the time when mares can get pregnant depends largely on the time of year (longer days) and the mare’s health.

Obviously, though, there will be some variation on a population. There will be some outliers that are able to get pregnant during the offseason. Genetics can actually be responsible for this. It is possible that a mare has descended from family lines that are able to reproduce at other times of the year. Though out of season foals have a high rate of survival on Assateague Island, this is not necessarily consistent with herds living in other areas. In these areas, out of season foals may not have a high survival rate; and so the genes responsible for allowing out of season foals may have a low chance of being passed on to future generations. It is also possible for a mare without such genes to get pregnant in the off season. Recall that the mare’s reproductive processes start to pick up in the spring time when the days get longer. They start to slow down when the days get shorter. However, it is still possible that a mature egg can develop and be released during this time; there is just a lower probability of this occurring at this time compared to it occurring during the spring. Having a foal out of season like this can possibly amplify the effects that the foal’s presence has on the mare’s health; and so it is unlikely that she will be able to get pregnant at the same time the following year.

Really, what this all comes down to is probability. There is a high probability for a mare to get pregnant in the spring time, and a low probability to get pregnant at other times of the year. Because there is a chance they can get pregnant at other times of the year, there will be a relatively small number of foals born out of season anywhere, whether or not their mothers were treated with PZP.

With that, we have established a biological explanation for the seasonality of foaling. Also, consider that the factors influencing when foals are born are going to vary from site to site. This is a probable reason why out of season foaling patterns can, and often do, vary from herd to herd. Still, though it may be acknowledged that PZP is not the exclusive source of out of season foals, it may be argued that out of season foaling rates can be amplified as an indirect side-effect of PZP. This was a question that the Kirkpatrick and Turner study discussed above answered for Assateague Island. Knowing what we know about differences herd to herd, it would likely be of use to perform similar studies on other herds. However, such studies must be well-designed and scientific because there is going to be a base number of foals born out of season no matter what; and this number of foals could very well differ from year to year. Such studies are rare, though, given that the data required for analysis is typically not gathered in wild horse herds. Thus, statements made regarding PZP’s effect on the seasonality of foaling must be taken lightly unless they reflect data from quality scientific studies.


(Please click on link to read full text article.)

Kirkpatrick, J.F., and Turner, A., 2003, Absence of Effects from Immunocontraception on Seasonal Birth Patterns and Foal Survival Among Barrier Island Wild Horses: Journal of Applied Animal Welfare Science, v. 6, p. 301-308

Published in: on February 2, 2010 at 11:30 am  Comments (5)  

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5 CommentsLeave a comment

  1. since PZP is a long acting form of birth control does the timing of its administration have anything to do with the timing of the mares next foaling?

    • Hello,
      Thanks for your question! First, PZP comes in one year doses (liquid injections) or the pellets. The pellets are longer acting while the liquid injections provide about one year of contraception. As you’ve probably noticed, most of the literature concerns the liquid injections; the pellets are still relatively new and so there isn’t as much research available on them. Since their contraceptive effect is resulting from PZP, it is probable that they act similarly though they will possibly have different efficacy over time.

      To answer your question, though, here are a couple points to think about:

      First, let’s assume that a mare was treated and that the vaccine had basically “worn off” in December. If this were to happen, then there would be the possibility that the mare could get pregnant. However, remember back to the post that I described how factors such as the length of day and the mare’s body conditions greatly influence when a mare can actually get pregnant. These two things are the controlling factors here – PZP prevents fertilization while light and weight control ovulation. (There actually is an immunocontraceptive that acts on ovulation instead of fertilization.) If a mare isn’t ovulating, she can’t get pregnant. Given the low light and possible low weight that a mare will be dealing with in December, it is unlikely that she would ovulate during the time when the PZP was starting to wear off. Because of this, PZP is probably not too influential.

      Second, let’s consider that a mare hasn’t foaled in a couple years, likely because she was treated with PZP. Let’s say that the PZP finally does wear off in December. It is very likely that this mare will have very good body conditions since she hasn’t had to support a foal for a couple years. Could this increase the probability that she will get pregnant at that time of year? It is possible that it could, but the amount of daylight is actually a more influential factor than body conditions. Thus, this is a harder question for me to answer.

      This is a tricky area to fully understand because of the numerous factors involved. Here is another of my personal observations as well: Also making it more complex is the fact that we are dealing with rather small sample sizes. I’ve tallied up the month that the foals born in the Pryors from 1996 to 2009 (that’s 467 total foals) were born in. During these years, about 90% of the foals were born in April, May, and June. Of the 10% not born in those three months, about 92% of them were born in July, August, and September. Thus, from 1996 to 2009, less than 1% of the foals born were born from October to March. Again, these are just my observations on the foals born; to make any conclusions on the difference in seasonality between treated and untreated mares would require detailed statistical analysis.

      Please let me know if I can answer any further questions! Thanks!

  2. Matt, I’m glad to see you putting the “facts” out here for people to read, and verify if they’d like from the sources you site, since I think a lot of people are under the impression that things are the same now as they were in the early part of the last century. Some things haven’t changed a lot, but some things have, and we need to be able to sort them out to make educated decisions about what to support and what not to. Thank you for your time and sharing your expertise and dedication.


    • Linda,
      Thanks for your comment. I agree with what you have to say about this stuff. Also, I was able to get a copy of the vet’s report on Phoenix’s injury. Basically, the vet didn’t seem to indicate that the injury was very deep. He didn’t think that there was any “foreign body in the leg.”

  3. Thanks Matt. The main thing is Phoenix is “fine as frogs’ hair”, now, or at least she was last summer. She sure seems to be a fine example of the toughness and many other characteristics of the mustangs that make them such survivors.

    I hope the herd is not experiencing too much difficulty foraging this winter. The ones who are left after the roundup deserve a break in that respect. Is the area where you guys removed the fence last fall somewhere they might do some grazing during the winter? My guess is no, but if it is, it sure would come in handy being “new ground” that isn’t already heavily grazed.

    Wish I could be out there now to see the “younguns” romping in the snow.

    Stay warm. Linda

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