Influence of Temporary Absence from

Career Profile of the Canadian Standardbred III. Influence of Temporary Absence from Racing and Season P.W. Physick-Sheard and M. Russell* ABSTRACT The objective of the study was to examine the career profile of the average racehorse in order to establish normal values for performance. Records of race performance for a population of 762 horses randomly selected from the 1972 registrations of the Canadian Standardbred Horse Society were summarized and the pattern of racing by year and month from 1974 to 1983 inclusive recorded. Results were analyzed to determine the influence of sex, gait, age, and year of first race (starting year). The transformation log (1 + x) was used to achieve normality where necessary. Of the 762 horses, 507 raced, 56 (11.0%) of which took one or more complete seasons (years) off. No age predisposition was detected, and no significant difference in mean career performance between the group taking a season off and the remaining horses were found, with the exception of career length, which was longer for those temporarily absent from racing (p < 0.001). No influence upon career pattern for horses off early in their career was observed but those off late showed indications of a subsequent fall in number of races per year and in times for the mile. Mean career duration was 25.66 to 28.38 months (95% confidence limits for the mean, n = 507). Absence from racing of at least one months duration was taken by 437 horses, mean percentage of career off 42.36 ± 0.94% (mean ± SEM), mean duration of periods off 4.43 to 4.71 months. Career duration in months and percentage off were only weakly related (r = -0.16). Trotters took significantly more time off than pacers (p < 0.01), and females more than geldings (p < 0.05). Females had significantly shorter periods of continuous racing (p < 0.05). Otherwise there were minimal gait or sex effects. Delay in the starting year resulted in an increased frequency of time off and a fall in duration of periods on and off. Marked seasonal influences upon the numbers of horses racing, and upon the numbers of horses leaving racing both temporarily and permanently were noted. These observations, coupled with the strong relationship between frequency of absence from racing and career length in months, suggest that breaks in the continuity of racing may frequently result from scheduled lay-off as opposed to acute injury. The results suggest that some prediction of the chances of absentees returning to active competition may be possible. Key words: racehorse, dardbred. Career, demography, performance, Stan- RESUME Cette etude consistait a examiner le profil de carriere du cheval de course moyen, afin d'etablir des normes de performance. Les auteurs resumerent a cette fin les records de performance en piste de 762 chevaux Standardbred choisis au hasard dans le registre de l'association Standardbred du Canada, parmi les inspections de 1972, et ils enregistrerent les profils de course annuels et mensuels, pour la periode de 1974 a 1983 inclusivement. Ils analyserent les resultats, pour determiner l'influence du sexe, de l'allure et de l'age, au moment de la premiere course. Ils utiliserent la transformation log (1 + x) pour normaliser, lorsque le besoin s'en fit sentir. Parmi les 762 chevaux, 507 courserent et 56, ou 11% de ces derniers, ne participerent pas aux courses, pendant une annee ou plus. Les auteurs ne detecterent pas de predisposition d'age, ni de difference significative dans la performance moyenne de la carriere, entre ces derniers et les autres, si ce n'est que la carriere des chevaux temporairement absents de la competition s'avera plus longue (p < 0,001). Cette absence n'influenca pas le profil de carriere, lorsqu'elle survint a ses debuts; elle entraina cependant une diminution du nombre annuel de courses et une perte de vitesse, lorsqu'elle survint chez des chevaux plus ages. La duree moyenne de la carriere des 507 chevaux varia de 25,66 a 28,38 mois, d'apres l'intervalle de confiance 95%. On enregistra une absence de la competition d'une dur&e d'au moins un mois, chez 437 chevaux, pour un pourcentage moyen d'absenteisme de 42,36 ± 0,94%, au cours de la carriere, et une duree moyenne de 4,43 a 4,71 mois, pour ces periodes d'absenteisme. La duree de la carriere, en mois, et le pourcentage d'absenteisme n'afficherent qu'une faible relation (r = 0,16). Les trotteurs manquerent plus de courses que les ambleurs (p < 0,01), tout comme les juments, par rapport aux hongres (p < 0,05). Celles-ci connurent des periodes de course constantes, significativement plus * Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, Ontario NIG 2W1. This study was funded by the Canadian Trotting Association. Submitted August 23, 1985. Can J Vet Res 1986; 50: 471478. 471

breves. Les autres influences dues 'a l'allure ou au sexe se revelerent minimes. Lorsqu'un cheval commengait a courser a un age plus avance, il s'absentait plus souvent de la competition et il affichait des periodes plus breves d'activite et d'inactivite. On constata des influences saisonnieres marquees, sur le nombre de chevaux en competition et sur le nombre de ceux qui s'en retirerent, de fason temporaire ou permanente. Ces constatations, jointes a la relation etroite entre la frequence de l'absence de la competition et le nombre de mois de carriere, suggerent que les arrets dans la continuite de la competition resulteraient souvent des periodes planifiees d'inactivite plutot que des blessures. Les resultats suggerent la possibilite de predire, jusqu'a un certain point, les chances des chevaux inactifs de retourner 'a la competition. Mots cles: carriere, demographie, cheval de course, performance, Standardbred. INTRODUCTION The majority of studies which have analyzed race records in the horse have involved the assessment of heritability and repeatability for various measures of race performance for application in breeding. Investigation of performance parameters has been incidental. The object of the present study was to use the available information to develop a career profile of the average racehorse in order to establish relative norms for production and performance. The intention was that the results should be used in undergraduate teaching and in clinical decision-making. Previous papers in the present study have evaluated the basic career profile of the Canadian Standardbred horse and have investigated the effect of age, sex, and gait upon chances of racing, and the effect of these parameters and of year of first race upon career length, number of career races, career money won, and race times (1,2). The present paper evaluates the pattern of temporary absence from racing and its relationship to various performance parameters. The relationships 472 between career structure, season, and some aspects of population dynamics are also examined. MATERIALS AND METHODS These were as previously described (1). Briefly, a random sample of 762 horses (the total study population) from the population of Canadian-bred Standardbred racehorses registered with the Canadian Standardbred Horse Society (CSHS) as foals of 1972 was taken, and records of race performance for the period 1974 to 1983 inclusive summarized. The records were studied to determine the pattern of career performance for the population. In order to achieve normality, the transformation log (I + x) was used to treat money won, number of races, months raced, and variables derived from these measurements where necessary. RESULTS The term "race" refers to an officially sanctioned (by the Canadian Trotting Association [CTA] or United States Trotting Association [USTA]) competitive event. Records of race performance are the official records maintained by the above associations. Races do not include qualifying races (races upon which there was no betting and which were staged solely to establish minimal performance standards in order to make a horse eligible for entry for races at a particular track/ s). Males (intact males, colts, stallions), are treated separately from geldings. Horses registered as males but gelded by the time of their first race are treated as geldings for purposes of analysis. Males castrated after their first race are treated as males throughout, since on average they raced the greater part of their careers as stallions (1). Unless specifically stated otherwise the results discuss the subset of 507 horses for which one or more races were recorded, as opposed to the total study population of 762 horses, 255 of which never raced. The term "mark" refers to a horse's best (fastest) winning time for the mile in an official race. A horse will only have a mark if it won at least once. The mark may continually change if the animal wins in a faster time. A horse can thus have a mark for each year, and a career best (career mark), representing the fastest winning time for its career. Most horses also have a best, nonwinning recorded time which may or may not be different from the mark. A horse can have a best, nonwinning time for each year and a career best, nonwinning time (best recorded time). For the purposes of analysis, a layoff by year was defined as a temporary absence from active competition for one or more complete seasons. Lay-off by month was defined as any absence of one month or longer, and thus includes horses considered under layoff by year. Means are expressed as mean ± SEM, or as mean range (95% confidence limits for the mean) where transformation was used in the analysis of the data. All results are presented in the original units. LAY-OFF BY YEAR Active racing career was defined as the inclusive period from the first to the last year in which races were recorded. Intervening years in which no races were recorded were identified as temporary absences from racing (lay-offs). In this discussion of lay-off by year, the restricted population referred to may be regarded as a subset of all horses which either experienced problems of sufficient severity to interrupt their racing careers, or whose performance did not warrant their being kept in training. Whereas the racing career of most of these horses was at an end, the subset represents animals which subsequently returned to racing. The number of horses taking a year off in each career year is shown in Table I. In this table numbers racing refers to the number of horses whose career was current in that year, and includes those which were temporarily absent from racing as well as those actively competing. Of the 507 horses which raced, 43 took one year off racing, ten took two years, and three took three years, for a total of 56 or 1 1.0% of horses which raced. For three and nine year old horses numbers taking time off represent a low 0.62%

TABLE I. Temporary Absence by Career Year Career year 2 3 4 5 6 7 8 9 10 Number racing 84 322 397 387 325 250 176 116 72 Number temporarily absent 2 17 18 12 11 8 3 7ta -- 0.62 4.28 4.65 3.69 4.40 4.55 2.59 - 'Percentage of horses racing in that year and 2.59% respectively of horses whose career was current in that year. For all other ages the percentage of horses racing taking year-long lay-off is fairly consistent, indicating no age predisposition. The relationship between starting age (age at the time of the first race) and number of horses taking a year off racing is shown in Table II. There is an overall trend towards smaller numbers taking a year off as starting age is delayed. Comparison of the population of horses taking a full season off with those not doing so revealed no significant difference with regard to mean career values for number of races, money won, mark, or best recorded time (p > 0.05). However, there was a very significant (p < 0.001) difference in mean number of years raced. Horses laid off raced a mean of 1. 15 years longer, despite subtraction of the period of lay-off from active racing career prior to determination of mean. Despite the lack of statistically significant differences between group means for performance parameters other than number of years raced, the group taking time off did show a narrower range than those not doing so for career money won and career mark. Thus only 3.5% of horses taking time off had career earnings less than $100.00, compared with 12% for the TABLE II. Starting Age vs Temporary Absence from Racinga remaining horses, while only 1.8% exceeded $100,000.00 compared with 33.9% for horses not taking time off. The fastest and slowest career marks for the first group were 1.993 min and 2.267 min, as opposed to 1.940 min and 2.350 min for the second. The distribution of sexes for the subset taking time off was the same as that for the remainder of the population, suggesting that all three sex groups were equally likely to be given a season off. There was a mild gait effect. Trotters represented 14.3% of the group taking a break from racing but they only comprised 6.2% of the group which raced without taking a year off. Twenty-two percent of trotters took one or more seasons off compared with 10% of nontrotters. In order to evaluate whether age at the time a year was taken off had any detectable effect upon career pattern, the subset was subdivided according to the age at which each horse was absent from racing. For each age group, means for six different performance parameters for the periods before and after time off were determined separately (Fig. 1). Group sizes for horses taking the three and nine year old years off were too small to allow meaningful comparison and thus only the intervening years are plotted. In interpreting the trends presented in this figure caution should be used, since the abscissa does not Starting age 2 3 4 5 6 Number startingb 84 243 110 50 15 Number off' 14 28 11 2 1 % 16.5 11.5 10.0 4.0 6.0 atemporary absence of one complete season (year) bnumber first racing at each age cnumber which took a year off represent a continuum. Each pair of plotted points (before and after time off) represent means derived from a separate population of horses. Years raced and total number of races showed consistent patterns. Performance after lay-off (dotted line) approximated a mirror image of performance before lay-off, implying that mean career expectation was achieved and that there was no effect created by taking a year off. Number of races per year before time off showed a predictable, gradual increase as age at year off increased. For groups off early in their career, the mean number of races per year after time off was very stable, but the mean number of races per year achieved by horses which took a year off after their sixth career year fell, implying some deterioration in performance upon return to racing. Up to the group taking time off at six years, the best recorded time improved after return to racing. For 4 Lu 2-1 0 m 40 U 2.201 I- 2.1 z 2 2.10-7 5 50- LO 245 -K 3000] O- YEARS RACED -; RACES / YEAR ----- _TOTA L RACES - ~ BEST,-< TI ME EARNIN GS / RACE TOTAL - '---- - EARNINGS 4 S 6 7 6 AGE AT YEAR OFF Fig. 1. Influence of age at year off upon career performance. Each pair of points plotted represent means for performance parameters for the group of horses taking a year off at that age. Solid line-before, and dotted line-after, a year of absence. 473

TABLE 111. Career Duration in Months' Male Female Gelding Total Pace 34.98-41.46 18.36-21.44 29.97-35.90 26.35-29.27 (118) (200) (142) (460) Trot 19.80-59.81 9.89-19.00 38.36-78.62 17.62-28.79 (7) (21) (8) (36) Mixed ND ND ND 10.27-22.01 (II) Total 33.51-39.93 18.36-21.44 29.97-35.90 25.67-28.38 (128) (226) (153) (507) '955% confidence limits for the mean Total numbers in each group are shown in parer subsequent ages best time was slower than before time off. Of the 56 horses taking one or more years off, 19 achieved their best time before the break, 32 afterwards. Career money won before and after lay-off showed no obvious influence of the break in racing. Before time off earnings per race differed little for horses off at four to six years, but were high for horses off at seven and eight. Earnings per race after time off increased slightly for horses taking time off up to and including age six, but for horses off at seven and eight there was a marked fall in this parameter, once again indicating some deterioration in performance. The two horses taking their three year old year off were exceptional animals, having above-average years raced, number of career races, and career earnings. LAY-OFF BY MONTH To allow analysis of performance by month, records of race performance were examined, and for each month of the ten year period under study a horse was considered as actively involved in racing if at least one race was recorded TABLE IV. Mean Percentage of Career Spent Off during that period. The data was then summarized to reveal number, and length of periods on and off racing by month. With the exception of career duration and duration of periods on, results refer to- a restricted population of horses which took one or more months off. Sex and Gait Effects Of the 507 horses which raced, 437 showed an absence from racing of at least one months duration on at least one occasion. The 70 horses which did not take time off had generally short racing careers and were inferior horses in regard to all performance parameters. Most only raced for two to three months (see below). Career duration in months and the effect of sex and gait are shown in Table III, which includes both months on and months off. Mean range for duration of career was 25.67 to 28.38 months (n = 507). Females exhibited a significantly shorter racing career than males or geldings (p < 0.01). Longest career duration was for gelding trotters (mean range 38.36 to 78.62 months). No significant gait effect was Male Female Gelding Total Pace 41.55 ± 1.84 (109) 43.31 ± 1.58 (168) 39.14 ± 1.58 (123) 41.56 ± 0.97 (400) Trot 48.62 ± 9.61 55.33 + 5.59 49.70 + 7.47 (6) (16) (8) 52.48 ± 3.97 (30) Mixed ND ND ND 45.62 + 5.55 (7) Total 41.87 ± 1.78 44.51 + 1.52 39.78 ± 1.54 (117) (187) (133) 42.36 + 0.94 (437) 'Months, mean + SEM Total numbers in each group are shown in parenthesis detected. The longest career was for a trotter which raced for 100 out of a possible 120 months. This animal started as a two year old and made 13 starts in four months, then took six months off. It started again as a three year old and was still racing at the end of 1983 with only eight months off, a total of 281 career races, career earnings of $143,257.00, and a career mark of 2.073 min for the mile as a six year old. Percentage of the active career spent off in months is shown in Table IV. Mean percentage was 42.36 ± 0.94 (mean + SEM, n = 437). Trotters took significantly more time off than pacers (p < 0.01), and females more than geldings (p < 0.05). There were no significant gait/ sex subgroup effects. The relationships between percentage of career taken off and transformed money won and number of races were significant (r = -0.5345 and -0.5876 respectively, p < 0.001), and are described by the regressions y = 4.669-0.024x and y = 2.315-0.0137x for transformed money won and transformed races respectively. Career mark and transformed money won per race were less strongly correlated (r = 0.3969, and -0.3656 respectively). Career duration in months and percentage career off were only weakly related (r = -0.1628). Duration of periods of lay-off is shown in Table V. Mean range for duration off for the total population (based upon the calculated mean for each horse), was 4.43 to 4.71 months, (n = 437,95% confidence limits for the population 1.01 to 14.38, longest period off 51 months, shortest 1). No sex or gait effects were noted. The number of periods taken off during a racing career was strongly influenced by sex, females showing significantly fewer absences than males or geldings (p < 0.001). This effect resulted from differences in career duration however, and was lost when this factor was controlled for. Mean range for number of periods off was 3.27-3.49. Career duration and number of periods off were strongly correlated (r = 0.8933, p < 0.001). The relationship was log-log (exponential), and is described by the regression y = 0.5685x-0.2698 of transformed career duration on transformed periods off. As career 474

. TABLE V. Mean Duration of Periods Off' Male Female Gelding Total Pace 4.18-4.70 4.51-5.11 4.01-4.40 4.38-4.66 (109) (168) (123) (400) Trot 4.28-6.33 4.37-6.18 3.97-5.61 4.56-5.65 (6) (16) (8) (30) Mixed ND ND ND 3.48-6.85 (7) Total 4.19-4.69 4.65-5.17 4.07-4.43 4.43-4.71 (117) (187) (133) (437) 'Months, 95%, confidence limits for the mean Total numbers in each group are shown in parenthesis 55 7 45. (Ạ - z 35 Career 0 25 Duration 15s 4S Perc ent. LLB Li 40 Career I= 35 Off 0.150 0.125- number -0.100- IF Periods 0.075. Of f 5.5 - L, Sq ~~~~~~~Duration I 2:512 Off 4.5 _ Duration 3.5< --; _ Periods 2.5 2 3 A IN 5-6 AGE AT STARTINJG Fig. 2. Influence of age at starting racing upon temporary absence from racing by month. Depicted are the 95% confidence limits for the mean in standard units. Mean and SEM were derived by log (1 + x) transformation. Number of periods off has been corrected for career duration and represents the number of periods of absence per month of career. Numbers at each starting age were 79,211,99, 33 and 14. duration increases total number of periods off increases, but the actual increase in number of periods off becomes progressively smaller. A sex influence was determined for the mean duration of periods of continuous racing, the periods for females being significantly shorter (p < 0.05, Table VI). Population mean range for the duration of periods of continuous racing was 4.25 to 4.52 months (n = 507). The longest period of continuous racing for any horse was 82 months. There was a highly significant (p < 0.001) difference in transformed mean total months of continuous racing between those horses which did not take time off (mean range 2.22 to 2.74 months), and those which did (mean range 20.82 to 21.57 months). Effects of Starting Age The effects of age at first race (starting age) upon the variables examined above are shown in Figure 2. Tests of significance were only applied between consecutive starting groups. Significant (p < 0.05) differences were revealed between three, four, and five year old starters for career duration (in months) and the number of periods off (uncorrected for career duration). Between four and five year old starters significant (p < 0.05) differences were found for percentage of career off and mean duration of periods off. Mean duration of periods off was significantly different between two and three year old starters. As starting age increased, career duration shortened, as did the mean duration of both periods on and off. Percentage of career taken off decreased by an insignificant amount. Numbers of periods off (frequency of absence from racing) increased when corrected for the effect of career duration (Fig. 2). SEASONAL PATTERN OF TIME OFF The seasonal pattern of racing for the study population is presented in Figure 3, which shows the number of horses racing in each month from January 1974 to December 1983 (open bars, outer scale). The greatest numbers of horses racing occurred for four and five year olds, with a continual decline thereafter. There was a consistent pattern for the first eight seasons, the least number of 475

TABLE VI. Mean Duration of Periods of Continuous Racing' Male Female Gelding Total Pace 4.60-5.17 (118) 3.73-4.12 (200) 4.60-5.08 (142) 4.38-4.66 (460) Trot 2.84-5.19 (7) 2.00-2.90 (21) 3.14-5.21 (8) 2.61-3.44 (36) Mixed ND ND ND 3.49-4.75 Total 4.46-5.01 3.73-4.12 4.60-5.08 4.25-4.52 (128) (226) (153) (507) 'Months, 95/f confidence limits for the mean Total numbers in each group are shown in parenthesis horses racing in February and the greatest in the summer and early fall. With successive seasons there was a tendency for the peak racing month to occur progressively earlier in the season. The number of horses leaving racing each month is also shown in Figure 3 (black bars, inner scale). There was a consistent tendency for horses to be withdrawn from racing in the second half of the season, approximately one month after the time when the greatest 250 200 i150 u 10 1974 1975 YEAR -1979 1980 1981 1982 Fig. 3. The open bars show the number of horses in the study population racing in each month for the ten year period under study (outer scale). For each year except 1974 the bar for January is shaded. The superimposed solid bars show the number of horses leaving racing permanently in each month. Note the different scale for this variable (inner scale)..100 801.60: j~~~~~~~9. 77il 1971. 1975 1976-1977 1978-1979 1980 1981 198 983 Fig. 4. Number of horses taking a month off in each month as a percentage of the total number of horses still actively involved in racing in that month. For each year except 1974 the bar for January is shaded. YEAR 476

number of horses were racing. For the career that year, plus those which were the previous 12 months, but which population as a w/hole the rate of loss laid off temporarily. The second row were not racing in the current year, of horses followed a sigmoid curve, the shows the actual number of horses approximately 14% could be expected greatest rate of loss occurring in newly absent from racing in each year, to return to racing, most of them in the middle aged horseis. and also includes both categories. The following year. The balance of the The number c)f horses taking a number of horses temporarily absent absent horses would be retired from designated month off racing expressed from racing in each year is shown in racing. as a percentage c)f the total actively the third row. The fourth and fifth involved in racirig at that time, is rows show numbers temporarily shown in Figure 44. Only those horses absent in each year as a percentage of DISCUSSION which were tempo rarily (as opposed to the first two rows. permanently) abs ent from racing are Over the age range of three to ten In the present study two different included. Of the horses which were years from 33.3% of three year old, to approaches were taken to the analysis racing at two years of age, 90% were 0.2% of ten year old horses which had of time off racing, pattern of absence off in December of their first racing raced in any previous year but which by race season (year), and pattern of year. For each ye-ar after the second were not currently racing could be absence by month. Horses taking a race year and up to the eighth racing expected to race again in subsequent complete season off racing were year (nine year )lds), peak absence years, i.e. from 66.7% of three year old analyzed separately because they were from racing occiurred in February, horses to 99.8% of ten year olds regarded as a special case of temporof horses off being previously started but not currently ary absence. They most likely reflect with the number lowest in mid to 1late summer. This is racing could be assumed to have either a planned absence (e.g. a year to an approximate reflection of the retired permanently from racing. The mature, or for a mare with mild number of horsees racing (Fig. 3). chance of returning to racing fell problems to carry a foal), or a serious During this perio4d (1976 to 1981) the progressively as age at which time was physical problem such as a fracture. percentage of horsses actively involved taken off rose. The last row in Table Because of the retrospective nature of in racing which x vere off during each VII shows the percentage of horses the study, information regarding the month did not vary greatly. The newly absent from racing in each year reason for temporary absence from pattern was lost for the last two race which subsequently returned. racing was not available. years surveyed. Extending these findings to the total Limited numbers precluded a population over the same age range, critical statistical analysis, but it is POPULATION DYN AMICS and assuming the particular cohort valid to examine the pattern of the There are som4e observations with studied to be representative of other results. If absence from racing for a regard to population dynamics which cohorts racing at the same time, it may year has no effect upon the career can be derived fro] m the distribution of be predicted that approximately 4% of performance, then performance after horses either temjporarily or perman- all racehorses absent from the starting absence should simply reflect the ently withdrawn firom racing. The first line in any particular year, having anticipated career pattern (1,2). This row in Table VII slhows the cumulative raced in any previous year, could be appears to be the case for most of the total of horses wlhich had previously expected to return to racing. Most of variables studied. raced which were absent from racing these horses would return to racing The gradual decline in number of at each age. The n[umber for each year after an absence of one year. Similarly, races per year after a year off for later includes horses which ended their of the horses which had raced within groups could reflect both deterioration in performance and the population trend towards a decline in the number of annual races in later career TABLE VIl. Distribuition of Horses Temporarily and Permanently Withdrawn from Racing by seasons (2). The gradual rise in mean Age annual races before time off reflects Cumulative horses not racing (A) Horses newiy absent from racing (B) Horses temporarily absent (C) (C) as %6 of (A) (C) as % of (B) (years) population patterns (2). The best Age 6 7(sears) recorded time before a year off reflects 3 4 5 6 7 8 9 10 the population trend for progressive improvement. The best recorded time 6 57 119 190 267 336 391 432 after year off shows improvement for horses off at four to six years. This reflects the finding that the average horse achieves its best recorded time at 6 53 79 89 89 80 63 44 5.00 years of age, 2.68 years into its racing career (2). Slower times after year off could reflect deterioration in 2 17 18 12 11 8 3 1 performance, and could also partly be explained by the natural trend 33.3 29.8 15.1 6.3 4.1 2.4 0.8 0.2 towards slower times in later seasons. 33.3 32.1 26.1 13.5 12.4 10.0 5.0 2.3 However, for both number of races 477

per year and best recorded time, the magnitude of the changes seen is greater than can be accounted for by population trends alone (2). The overall picture is thus one of minimal detectable interference in anticipated career pattern for groups off up to and including six years of age. After this age the indications are that horses taking a season off were better than average animals to start with and that they did not regain their previous level of performance after returning to racing. The percentages of two to six year old starters taking time off indicates there is a greater chance that early starters which have been withdrawn will subsequently return to racing (Table VII). This finding may be taken to indicate an increased potential for recovery among younger-starting horses. However, it has been demonstrated that as starting age is delayed there is a significantly greater attrition rate for each starting group (1). The falling percentage of horses which are temporarily as opposed to permanently withdrawn may thus reflect a higher incidence of problems. Additionally, since animals starting in later years are generally inferior animals (1,2), it may not be financially sound to give such animals a year off. Since horses took a year off at all possible ages for the first three starting years, the falling percentage of temporary absences cannot be ascribed to any direct effect of year of starting upon the age at which horses were absent from racing. The explanation for the significantly longer career (in months) for horses taking a year off may also relate to the quality and potential of different animals. Trainers/ owners are more likely to persevere with horses with a good performance history and potential for further race success. The tendency for the peak racing month to move back in the year may reflect both a progressively earlier start in successive seasons for the majority of horses racing, and a tendency for those horses still racing in later seasons to be raced continuously rather than be laid off over winter. These tendencies may reflect agerelated factors, and the manner in which older horses are campaigned. The latter in turn reflects both racing opportunities and management considerations. Trainers and owners must consider the* inadvisability of laying off chronically lame older horses for long periods, and the reduced time needed to prepare the older horse to race. The tendency for horses to be withdrawn from racing in the second half of the season, approximately one month after the time when the greatest number of horses are running, suggests that the majority of horses return from winter lay-off successfully but succumb to the effects of competition. The reason for withdrawal probably relates more often to acute injury or exacerbation of an existing problem during the most intense part of the season than to planned withdrawal. The results indicate a greater tendency to lay off two and three year olds during the winter. While the same pattern remains clear in subsequent years, there is an increase in the tendency to rest horses at other times of the year, and the percentage of horses resting over winter falls. The number of horses taking short-term lay-off as a percentage of the total currently actively involved in racing approximately mirrors the numbers of horses racing. The pattern is lost for the last two race years surveyed, probably reflecting a tendency to race continuously. The management factors underlying these findings are complex. Horses may take time off as part of a natural sequence of events during the racing year as well as for injury-related reasons. While serious injury may lead to significant periods of absence from racing, its effect on population dynamics is obscured by the tendency for such animals to be withdrawn from racing permanently. A prospective study is required to further evaluate the factors involved in temporary absence from racing. ACKNOWLEDGMENTS The authors wish to thank Amanda Crowe and Ted Reitsma for their invaluable work in the reviewing of records of race performance. The study was greatly aided by the cooperation of the Canadian Trotting Association and the Canadian Standardbred Horse Society. Their support is gratefully acknowledged. REFERENCES 1. PHYSICK-SHEARD PW. Career profile of the Canadian Standardbred I. Influence of age, gait and sex upon chances of racing. Can J Vet Res 1986; 50: 449-456. 2. PHYSICK-SHEARD PW. Career profile of the Canadian Standardbred 11. Influence of age, gait and sex upon number of races, money won and race times. Can J Vet Res 1986; 50: 457-470. 478