Le ers

Dear Dr. Jones, I am writing with respect to the

article "The Transition From Cyclicity To Anestrum in Mares" by John New- combe which appeared in your May issue; I would like to add some informa- tion. A full review of the literature reveals that we do know a good deal about mode of transition from cyclicity to anestrus in the mare. Our laboratory has been working on defining this aspect of equine reproduction for over a decade and has made some very interesting discoveries.

Over the past 14 years, we have followed over one hundred mares through the autumnal transitions. We have investigated and defined the ovarian dynamics leading into anestrus - both follicular and luteal - and have reported that mares enter into anestrus, in roughly equal proportions, from one of three different ovarian states: follow- ing normal tuteolysis with subsequent lack of follicular growth, following a spontaneously prolonged corpus luteum, or following a period of follicular growth and atresia during which ovulation does not occur. ~ We investigated and reported on a number of endocrine factors related to this transitional period, including ovarian steroids, 2,3,4,5 (estradiot-17g and progesterone), gonadotropins 6 (FSH and LH), prostaglandin F2.7 and prolactin. We found that, as a rule, ovarian endocrine function declines prior to a significant change in pituitary function.

Interestingly, progesterone secre- tion is the first endocrine factor to make a seasonal decline. Average progesterone concentrations show a linear decline during the luteal phases following the summer solstice. 1,26 This has been a consistent finding in over five fall transitional studies undertaken by our research group. Follicular function in the fall begins to decline around the time of the autumnal equinox. 6 Estradiol is not produced from fall transitional follicles with the same efficiency as

summer folliciles. 3,4,5,6 The incidence of spontaneously prolonged luteal activity significantly increases during this period, 1,6,8,9 a situation whose cause is still unclear. Although PGF2, is not secreted at the time of luteolysis in the fall, precipitating SPCL cycles, it does not appear to be for lack of the uterus' ability to produce or release it. 7 Luteinizing hormone concentra- tions decline during mid-transition, and are correlated with the increased incidence of anovulatory follicular waves; however, we did not discover a significant change in FSH secretion in fall transitional mares until transition into anestrus had been completed. 6

The information that we have accumulated on the mare's seasonal transition into anestrus indicates that the mechanisms driving the transition are different from those behind the spring transition. Although much is still left to be understood about this area of equine reproduction, there does exist a wealth of information which was not brought to light in the article. Since this area of research holds no immediate financial impact to the horse breeding industry, it has been one that has not been investigated by many equine reproduc- tion laboratories. As a reproductive scientist, I find this area of investigation fascinating in its own right, and hope that the information which I have presented here will stimulate the interest of other scientists as well. Below I have listed some additional readings in this area which should be of interest to researchers and clinicians.

Sheryl King, PhD, Professor Director of Equine Studies

Southern Illinois University Carbondale, IL

Suggested Reading

1. King SS, Neumann KR, Nequin LG, Weedman B J: Time of onset and ovarian state prior to entry into winter anestrus. J Equine Vet Sci 1993; 13:512-515.

2. King SS, Nequin LG, Drake S, Hebner TS, Roser JF, Evans JW: Progesterone levels correlate with impending anestrus in the mare. J Equine Vet Sci 1988;8:109-111.

3.Weedman B J, Neumann KR, King SS, Nequin LG: Relationship between follicular development and circulating estradiol during the autumn-winter transition in the mare. J Anim Sci 1992;($1 ):270.

4. Weedman B J, King SS, Neumann KR, Nequin LG: Comparison of circulating estradiol-1713 and folliculogenesis during the breeding season, autumn transition and anestrus in the mare. J Equine Vet Sci 1993; 13:502-505.

5. Weedman B J, King SS, Nequin LG: Anomalous cycle events precede onset of anestrus in the mare. Biol Reprod 1993;48;($1 ):87.

6. Nequin LG, King SS, Roser F, Soderstrom BL, Carnevale EM, Neumann KR: Uncoupling of the reproductive axes during transition into anestrus. Proc 7 h Intern Symp Equine Reprod, 1997.

7. Soderstrom BL, King SS, Nequin LG, Mallar ML, Hoffmann L, Crawford J J, Silvia W J, Burns P: The temporal sequence of progesterone and 13,14-Dihydro, 15-keto PGF a in summer and fall transitional mares. Proc 16 th Equine Nutr & Physiol Symp 1999;P202-203.

8. King SS, Nequin LG: Spontaneously prolonged corpus luteum activity is a common route into anestrus in the mare. Proc. 12 th Equine Nutr & Physiol Symp 1991 ;P207-208.

9. Neumann KR, Weedman B J, King SS, Nequin LG: An analysis of the patterns of cycliclty and acyclicity during the winter months in northern hemispheric mares. J Anim Sci 1991 ;(S)1:270.

Dear Dr. Miller, I have read many of your articles

over the years with appreciation for your horsemanship and common sense. I particularly wish to say well done for "Reflections of a Formerly Opinionated Horseman" in the latest JEVS.

You are correct about the concern for alfalfa being expressed before you (or I - - we are of an age) were born. Under the name lucerne French and German veterinarians were concerned with deleterious effects on the gastrointestinal tract in the 1800s.

Thanks again for your interesting pieces.

James R. Rooney

Volume 20, Number 6, 2000 363