February 12, 2010 – PZP and Mare Longevity

In past posts, I have discussed ways in which PZP could possibly affect the health of mares. These posts largely focused on possible negative side-effects. There has also been research done to show that mares treated with PZP may receive indirect positive benefits. This will be today’s topic.

A study (Turner and Kirkpatrick 2002) has been published regarding the health of treated and untreated mares on Assateague Island. In the study, comparisons were made between body conditions of mares in 1989 and in 1999. PZP started to be used on Assateague Island in 1994, and so 1989 and 1999 were good years for comparing the herd prior to and after treatments. Body conditions were assessed with a standard system with scores ranging from 1 to 5, with 1 describing the poorest body conditions and 5 describing the best body conditions. The average body condition score for mares in 1989 was 2.33 (plus or minus .35) while the average body conditions for all mares in 1999 was 2.96 (plus or minus .08). In looking at mares not producing milk, the average body condition score in 1989 was 2.42 (plus or minus .38) and in 1999 was 3.07 (plus or minus .05). Thus, it was determined that the body conditions of mares in 1999 were significantly higher than those in 1989 when looking at all mares and mares who weren’t producing milk. This isn’t the case with mares who were producing milk. The average body condition score for these mares was 1.96 (plus or minus .6) in 1989 and 2.0 (plus or minus .55) in 1999. All together, this makes sense: If mares aren’t having foals, they will have higher body conditions as they aren’t having to expend extra energy carrying a foal and then supporting it as it matures. Thus, when PZP was used to prevent mares from having foals, the body conditions of treated mares significantly improved.

In an earlier post, I described how wild horse populations take on a new age structure after PZP has been used on them. This is largely due to the fact that the life expectancy of mares increases in treated herds. This, as we now know, is the result of better body conditions that result from not having as many foals. The aforementioned study (Turner and Kirkpatrick 2002) describes the differences in age structure on Assateague Island in 1990 and 1999. It was found that from 1990 to 1994, when the contraception program started, the mortality rate for adult horses was over 10% while the mortality rate for foals was around 3%. In the years following treatment, the mortality rate for adult horses went down to less than 4%. Looking only at mares, it was also found that mortality rates dropped to less than 4%. Also, the mortality rate for foals went down to 1%. There are two main points here: First, there is a significant population effect that occurs due to the fact that treated mares are healthier; the mortality rate went down 6% following treatments. Second, the health of foals is also affected by PZP: Foals that are born to mares with good body conditions are more likely to survive than foals born to mares with poor body conditions.

This work was further expanded on in a later study on the Assateague horses (Kirkpatrick and Turner 2006). In this study, the mean age at death (MAD) was compared for stallions, untreated mares, mares treated for one to two years, and mares treated for at least three years. The study herd at the time consisted of 128 horses whose ages were known based on past records. There were 56 stallions involved, and their MAD was found to be 10.33 years. This wasn’t too different than 11 mares treated for one or two years; their MAD was 10.27 years. The MAD for 42 untreated mares was significantly less at 6.47 years. The MAD for 19 mares for at least three years was significantly higher at 19.94 years. Thus, mares treated for at least three years were living, on average, 13.47 more years than mares who were never treated. Even being treated less than three years had an effect as these mares were living, on average, 3.8 years longer than untreated mares.

These two aforementioned studies are quite clear in demonstrating that wild horse herds managed with PZP are going to likely have higher populations of older mares than herds where PZP is not used. Also, wild horse herds managed with PZP are likely going to have higher average body scores and lower foal mortality rates than herds where PZP is not used. These differences between treated and untreated wild horse herds are significant, and it is important to understand them.


Kirkpatrick, J.F. and Turner, A., 2006, Immunocontraception and Increased Longevity in Equids: Zoo Biology, v. 26, p. 237-244

Turner, A. and Kirkpatrick, J.F., 2002, Effects of immunocontraception on population, longevity, and body condition in wild mares (Equus caballus): Reproduction (Suppl. 60), p. 187-195

Published in: on February 12, 2010 at 5:49 pm  Comments (5)  

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5 CommentsLeave a comment

  1. Matt, once again thank you for all the valuable information contained in this series of posts. I am hoping that the use of in-field fertility drugs can become the rule instead of the exception in amnaging wild horse herds out west. It seems that this is the way to go, it would be great to see an end to the inhumanity of the roundups and the loss of life ensuant from them.

    I do have another question regarding the Pryor roundup last year. Were all the stallions gathered, with the exception of the Forest Service horses, castrated prior to the adoption event?? This question has come up as the Calico horses are about to go through “processing”, which in their case, seems to mean that ALL males are going to be gelded. And one more, do they use any sedation while gelding them?
    Thanks again for all your information and for always answering my questions,
    hope the weather has been good, it is cold and very snowy in the Midwest, perfect weather for me in February,
    take care,

    • Jan,
      Thank you for your comment. I tend to share your views on this matter!

      During “processing” here in the Pryors, the horses were tested and given vaccinations to transition them into the domestic world. The males were not gelded. I have heard from some adopters who have gelded their adopted males while others were considering keeping their male intact. However, during the 2006 gather, some of the unadopted stallions were processed at BLM facilities where they were gelded. I have had second hand information that all males brought to facilities are automatically gelded. As for whether or not they are sedated, I cannot answer this for sure. I will ask around to some people who might know more, though.

      Our weather’s been up and down here. It hasn’t been until fairly recently when we got more snow. I’ve been really bad about putting up a new post about the Pryor horses, but the ones I have planned will span weeks where it will be apparent how the snow has changed. It looked like the Pryors got more snow today too. I was in the McCullough Peaks, though; so I don’t know exactly how much snow was received.

      Thanks again!

  2. Is there a selection process to determine which mares are going to receive PZP, and if so, is it based on body condition alone or esthetic evaluation as well (such as color, type of horse, etc..) ?
    I am curious as well as far as the gelding process. I have read that it can be done with local anesthesia with the horse standing up or full anesthesia. Do you know by any chance which procedure out of the two is used for mustangs?
    Since it’s kind of a delicate operation, how long does it take? Is there a follow up?
    Also if by any chance you might know how many vets typically work at holding facilities and specifically at the one for the horses from Calico . Thanks.

    • Hello,
      Thanks for your comment. I wish I could answer more questions you have about gelding, but I can’t answer them with much confidence as it’s something I honestly haven’t had to deal with much. I will try to learn more about this, though.

      As for the selection for mares treated, this really depends on where you are. On Assateague Island, where you’ve probably noticed most of the studies and extensive use of PZP has taken place, treatments are based entirely on kinship. That is, they treat strategically so that every single horse has at least one offspring. This has obvious benefits for the herd’s genetic variability. Having this level of detail on kinship isn’t necessarily something that you find in wild horse herds. In the Pryors, I have been able to determine most of the herd’s kinship going back to 1976. In other areas, local observers are also working to start documenting this information. When females were treated in the Pryors last September, they were essentially treated if they were gathered. There were only a few females not treated; this had to do with their age and logistics. I think you’ll find this is about how it is in most places, they just give it to all the mares they can if they gather enough. In previous years in the Pryors, age also seemed to be the basis for treated females. This all partly seemed to be due to the body conditions of mares foaling early (and so there were young females treated) as well as old females foaling (and so there were old females treated). As far as the treated older mares, this is likely why our oldest horses on the range are now mares, such as Beauty and Tonopah. There is actually still a field darting EA active from back then; and it allows the treatment of females based on their age.

      This all is sort of related to the whole process of removal as well. How are horses chosen for removal? In recent years in the Pryors, selection was based on such things as conformation and kinship. Color seems to have been a factor in the past here as well. In other herds, it is typical to see conformation a major factor. Basically, they are looking to have a certain type of horse on their area; and so they release horses uniformly representing that type so that future generations will also look like they want them to look. This all has the potential to significantly affect the way a herd looks, of course. If you were to visit the Pryor Mountain Wild Horse Range in the late 1970’s, you would find that the proportions of colors were significantly different than they are today.

      Well, I hope this helped answer the questions I was able to answer for you! I’ll again look more into your other questions on gelding.

      • Matt — I have been studying your PZP articles again, hoping for a better understanding of the process and procedures. I think I have that now and really appreciate the time it must have taken to write these all up! Thanks.

        I still have some concerns about the PZP — my opinion is that it shouldn’t necessarily be given to mares at age 11 for the rest of their lives. And I would like to see it be given for no more than 4 years at a time. Although it is great to have the mares live longer, I’m not so sure it’s okay to then have to remove younger horses because the mares are still on the range.

        I am hoping you still check in here once in a while and see this comment. Although there are other blogs and websites out there, there is nothing like your insight and observations of the Pryor herd. We miss your input, although we realize you are busy. I hope you are able to continue posting occasionally.

        It’s great to see snow has finally come to your area — much needed moisture! 🙂 Merry Christmas and best wishes for a Happy New Year — 2013 will definitely be a “new beginning” 🙂

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