[Image: Mare]Joshua A. Cartmill, Laura R. Gentry and Donald L. Thompson Jr.
The large amount of media attention on human obesity in the past several years has made most of us keenly aware of the health hazards associated with carrying too much body fat. In the world of domestic farm animals, however, emphasis has typically been on how poor nutrition, and specifically poor body condition (little body fat), affect productivity and reproductive efficiency.
Considerable research with beef cattle has shown that a thin body condition is detrimental to postpartum rebreeding efficiency. This is understandable, given that the cow has a high nutrient demand on her for nursing her growing calf, and her body puts priority on the survival of the offspring. Getting pregnant again takes a back seat to the live calf at her side. In contrast, it has been reported that too much body fat in beef cattle is also detrimental to fertility. This may not be intuitive, but has been borne out by several research groups.
Back in 1983, it was reported that mares entering the foaling season in poor to moderate body condition took longer to get rebred, confirming that horses shared this common problem with cattle and other farm species. In contrast to cattle, no detrimental effect of very high body fat levels (scores of 8 and 9 on a scale of 1 to 9, where 1 is emaciated and 9 is extremely obese) were found. Since then, it has been assumed that very high body condition (high fat levels) in mares is not a factor in their reproductive efficiency.
Recently, research at the LSU AgCenter confirmed and expanded the effects of body condition on the reproductive capacity of mares. Scientists found that mares of high body condition tended to have significant follicular activity during the fall and winter, when mares generally tend to be reproductively inactive. In contrast, mares previously restricted in their grazing to produce thin body conditions (3 to 3.5) consistently did not exhibit estrous cycles or ovulate and had small, inactive ovaries during the same period. In fact, most of the thin mares did not begin to cycle until well into May.
One observation in the research was a dichotomy among mares in September, before they were divided into groups for either maintenance of high body condition or restriction to produce thin body condition. At that time, the mares had body condition scores between 6.5 and 8 (fleshy to fat). Of the 24 mares, about a third of them had very high concentrations of the hormone leptin in their blood; the remaining mares had much lower leptin concentrations. Leptin was being monitored in the research project because it was known to be a hormone produced by the fat cells and was correlated with total body fat in other species.
Why a third of these apparently healthy, normal mares had excessive leptin in their blood raised two questions:
1) What was special or different about these mares compared to the rest?
2) Was this difference associated with any pathology or did it have any implications regarding the mares’ health?
A follow-up research project attacked the first question. Mares of high body condition were assessed for their leptin concentrations, and equal numbers of mares with very high leptin and low leptin were chosen. Various metabolic and hormonal tests were then performed on the mares to determine which physiologic systems might be involved with or affected by the high leptin concentrations (hyperleptinemia, meaning excessive leptin in the blood).
Results of this experiment revealed that the mares with hyperleptinemia also had excessive insulin concentrations in their blood (hyperinsulinemia) as well as excessive glucose concentrations (hyperglycemia). Taken together, these results indicated that the hyperleptinemic mares had a syndrome similar to Type 2 diabetes in humans, in which the normal sensitivity to insulin is reduced, resulting in elevation of both glucose and insulin concentrations in the blood.
In humans, obesity and Type 2 diabetes are associated with several health risks, including heart disease, ovarian disease in women and eventual transgression to Type 1 diabetes (lack of insulin secretion by the pancreas). Because of this, current research at the LSU AgCenter Horse Farm is focusing on various aspects of health and reproduction to determine whether this syndrome has implications in these areas.
Two completed projects also attempted to devise methods to alleviate the syndrome through 1) chromium supplementation and 2) nutrient restriction and reduction in body condition. In the first experiment, six hyperleptinemic horses were fed a supplement containing chromium (in the form of chromium propionate); six other hyperleptinemic horses were fed only the carrier (limestone). After 30 days of supplementation, no benefit was realized in the supplemented horses either in leptin concentrations or in characteristics of glucose metabolism.
In the second experiment, six hyperleptinemic horses were restricted in their daily grazing to 6 hours per day (to reduce body condition), while six other hyperleptinemic horses were allowed to graze 23 hours per day. During the 45 days of restriction, restricted horses lost approximately 88 pounds of body weight and one full unit of body condition score. Moreover, their blood leptin concentrations dropped about 40 percent and their 24-hour insulin concentrations dropped more than 50 percent. However, their insulin sensitivity (which is a measure of how well their bodies can use glucose) was not improved. It was concluded that nutrient restriction in hyperleptinemic horses improved some, but not all, of the characteristics of the disturbed glucose metabolism. Further restriction may be needed to fully alleviate the symptoms. However, this is not guaranteed and needs to be determined in future research trials.
Two experiments further studying this hyperleptinemic syndrome in horses are in progress. The first involves monitoring the estrous cycles of hyperleptinemic mares versus normal mares of the same body condition during the winter and through the transition period (January through the first ovulation in the spring) to determine if there is any alteration in the date of first ovulation or in the hormonal concentrations during the cycle. The second is a survey of Thoroughbred and quarter horse mares from Louisiana breeding farms in an effort to associate mares with hyperleptinemia (determined via blood sampling) with any breeding inefficiencies after foaling. Results from these trials will be used to assess the practical implications of the syndrome in mares and whether further research is needed on methods of correcting the problem.
Joshua A. Cartmill, Graduate Student; Laura R. Gentry, Instructor; and Donald L. Thompson Jr., Professor, Department of Animal Sciences, LSU AgCenter, Baton Rouge, La.