Some Horse Breeding Problems John M. Kays Brood mares and foals on pasture at University of Missouri Farm. UNIVERSITY OF MISSOURI COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION J. H. Longwell, Director BULLETIN 590 JANUARY 1953 THIS BULLETIN AT A GLANCE Breeding Season, Estrus and Estrual Cycle ____________ Page 3 Ovarian Changes and Ovulation _________ _____ __ __________ _____________ ________ 5 Physiological Changes During Early and Late Pregnancy __ 6 Relation of Psychological and Physiological Phenomena of the Estrual Cycle to Breeding Results ___________________________________________ 7 Breedi ng Practi ces __________ __ ________ ____________________ ___ __ . __ _________________ ________ 9 The Foaling Mare __ _______________________________________ __ ___________________ 12 Diseases and Infections of the Newborn Foal __ 14 Handling and Management of the Stallion 16 Feeding grai n to brood mares and foals. Acknowledgments Grateful acknowledgment is made to Dr. Joseph E. Weinman, School of Veterinary Medicine, and Dr. Dennis T. Mayer, Department of Agricultural Chemistry, for their advice and criticisms given on the manuscript. This bulletin is a partial report on Department of Animal Husban­ dry Project Number 38 entitled "Animal Reproduction" . Some Horse Breeding Problems JOHN M. KAys* Horse breeding is an art practiced for 5000 years. Scientific knowl­ edge of recent times has helped to make the results of breeding prac­ tices more certain and predictable. However, problems associated with reproduction still remain to perplex and plague the practical horseman. These problems are important. Compared to other livestock, horses have a lower reproductive rate and a longer gestation period. A longer time is required to grow a usable or salable animal. With racing and show stock the potential value of the horse is great. Hence, anything that will improve the results usually obtained in horse breeding will help the horseman. The aim of this bulletin is to discuss in popular terms what has been learned by a number of scientific research inves­ tigators about some of the problems involved in horse breeding. THE BREEDING SEASON, ESTRUS, AND THE ESTRUAL CYCLE Horse breeding has become a seasonal activity in some regions, not essentially because of physiological causes but for economic and geo­ graphic reasons. Mares of all types kept under ordinary farm conditions in the United States and on a reasonably high plane of nutrition ex­ hibit reproductive phenomena throughout the year. Foals are born in all months. But most breeders of Thoroughbreds and Standardbreds, as well as those of other light breeds, prefer early spring foals so that the young horses will be well developed when they are put into train­ ing for racing, showing, or other uses at 11/2 to 2 years of age. Hence, the breeding season for horses in some areas is limited to the late win­ ter and spring months. Investigations of the problems of estrus, ovulation, and related phenomena in the mare were made by Frederick N. Andrews and Fred F. McKenzie' of the University of Missouri at the United States Range Livestock Experiment Station, Miles City, Montana, between April 20 and July 10, 1937 and April 20 and July 15, 1938. Observations were made on 45 draft and 35 light mares most of which were in the herd for each of the two seasons. The mares were kept under range condi­ tions. *Assistant Professor of Animal Husbandry, University of Missouri. 'Mo. Ag. Exp. Sta. Res. Bul. 329, "Estrus, Ovulation, and Related Phenomena in the Mare," by Frederick N. Andrews and Fred F. McKenzie. 4 MISSOURI AGRICULTURAL EXPERIMENT STATION Regular, systematic teasing of the mares at a teasing pole was prac­ ticed through the use of one of six active, vigorous, healthy stallions. The degrees of receptivity and resistance shown by the mares were subject to great individual variation. Most mares became less antagon­ istic to the stallion 1 to 2 days prior to the onset of heat and were most receptive at the time of and 1 to 2 days preceding ovulation. Ovu­ lation usually occurred 1 to 2 days before the end of the heat period. Twenty-eight cases of "split" estrus were observed. This phenome­ non was characterized by an initial heat period of one or more days, followed by a non-receptive period usually of 1 to 2 days duration, and a subsequent return of heat for one or more days. "Split" estrus ap­ peared among both the draft and also the grade Thoroughbred mares of all age groups. "Physiological" estrus, or "silent" heat, was manifested by the ovar­ ian, uterine, and vaginal changes characteristic of psychological estrus, but receptivity, or sex-desire, was absent. Eleven cases were observed, all in mares from 2 to 6 years old. The length of 196 heat periods ranged from 1. to 37 days with a mean of 5.3 days. The mean length of 90 estrual cycles was 20.7 days with a range of,10 to 37 days. The length of 106 interestrual periods ranged from 5 to 33 days with a mean of 15.3 days. The interval from foaling to the first heat period was observed in 33 instances. The average interval was 11.4 days with a range of 2 to 30 days. TABLE 1* -- DURATION OF ESTRUS IN DRAFT AND LIGHT MARES Range of Type Duration of Duration of No. of No. of Estrus of Estrus Mare Mares Periods (Mean days) (days) Draft 40 127 5.2 1 - 14 Light 35 69 5.5 1 - 37 All 75 196 5.3 1 - 37 *From Table 7, p. 28, Missouri Agr. Exp. sta. Res. Bul. 329. TABLE 2* -- LENGTH OF THE ESTRUAL CYCLE IN DRAFT AND LIGHT MARES Range of Type Length of Cycle of No. of No. of Cycle Length Mare Mares Cycles (mean days) (days) Draft 37 63 20.1 10 - 29 Light 18 27 22.0 13 - 37 All 55 90 20.7 10 - 37 *From Table 10, p. 30, Missouri Agr. Exp. Sta. Res. Bul. 329. BULLETIN 590 5 TABLE 3* -- LENGTH OF THE INTERESTRUAL PERIOD IN DRAFT AND LIGHT MARES Type Length of Range of of No. of No. of Period Period Mare Mares Periods (mean days) (days) Draft 37 71 14.9 7 - 27 Light 18 35 16.0 5 - 33 All 55 106 15.3 5 - 33 *From Table 13, p. 32, Missouri Agr. Exp. Res. Bu!. 329. To the practical horseman, the variations noted above point up the importance of regular, systematic teasing of all mares during the breeding season if a large percentage of the mares are to be bred at the optimum time for conception to occur. OVARIAN CHANGES AND OVULATION Ovulation refers in a broad sense to the development of the egg cell and ovarian follicle and the discharge of the mature egg cell by the rupture of the follicle in the ovary. Follicular growth is accom­ panied by the secretion of the hormone estrogen which gradually in~ duces heat. For most successful breeding of a species in which the heat period is relatively long, as it is in the mare, a knowledge of the time of ovulation is of major importance. It is highly desirable for the mating to occur near the time of ovulation because neither the egg cell nor the spermatozoa live a long time in the genital tract oj the mare. A horseman who has an intimate knowledge of equine ana­ tomy can determine the condition of the ovaries by palpation per rectum. The ovaries of the mare are kidney- or bean-shaped and vary great­ ly in size and shape. Age, the stage of the estrual cycle, and pregnancy affect their conformation but they are usally from 4 to 9 cm. in length and 2.5 to 5.0 cm. in thickness. One ovary may be larger than the other. The position of the ovaries is not constant but in most mares they are located in the sub-lumbar region of the abdominal cavity ventral to and 8 to 15 cm. posterior to the kidneys and 5 to 12 cm. from the median plane. During pregnancy the ovaries are drawn down­ ward, and by 10 to 12 weeks are on a level with the pubis. With age and succeeding pregnancies the ovaries tend to move ventrally. Andrews and McKenzie, working at the United States Range Live­ stock Experiment Station, Miles City, Montana, observed ovarian changes during two breeding seasons by palpation daily during heat in 45 draft and 32 grade Thoroughbred mares. Examinations were 6 MISSOURI AGRICULTURAL EXPERIMENT STATION also made at intervals of 5 to 7 days during the interestrual period and during late and early pregnancy. Most follicles were taut on the first day of estrus and had an aver­ age diameter in draft mares of 3.1 cm. on the left and 2.7 cm. on the right ovaries and in light mares 2.6 cm. and 2.5 cm. on the left and right ovaries respectively. Prior to ovulation 58.4% of the follicles of draft mares were 4 cm. or more in diameter in contrast to 33.3% of a corresponding diameter in light mares. Ovulation occurred in the left ovary in 60% of the cases in draft mares and in 61.6% of the cases in light mares. No definite pattern for the order of ovulation between the two ovaries was observed. Al­ though more than one follicle was often palpated on one or both ova­ ries during heat, double ovulation occurred in only 9 instances (3.8% of the cases). Ovulation was not dependent upon estrus, nor estrus upon ovula­ lation, but in 76.7% of the cases ovulation occurred during the interval from 1 day before to 1 day after the end of heat.