Paternal Age - the effects of a sire's age on progeny performance

ATERNAL age has been found to have a variety of adverse effects on offspring characteristics, including the prevalence of mental disorders and decreased intelligence, in multiple other species such as humans and mice. The probable cause is the large increase in the rate of genetic mutations that occur in the sperm cells of older males. What is the situation in the thoroughbred horse? This study sought to determine if the proportion of successful progeny produced changed as the stallions increased in age. The study analysed the racing performances of 26,650 progeny produced by 20 stallions Naturally, the sire and dam contribute equally to the genetic constitution of a racehorse. Previous research has found advanced mare age to have negative infl uences on racehorse performance after nine years of age and the fi fth foal parity. However, to date there has been no single study where the role that stallion age plays in progeny performance has been examined. WORDS: ALISON BRASSIL – UNIVERSITY OF LIMERICK PHOTOS: SHUTTERSTOCK, CAROLINE NORRIS over a 25-year period from 1985-2010. In order to be eligible for the study, the stallions had to be retired from stud duties so that all progeny performance could be analysed. The stallions had to have had an active breeding career of over 15 years in order to be considered of advanced age. The last crop of foals had to be produced at least fi ve years prior to the study to allow all progeny to have raced from the ages of 2-5 years. As a sizeable number of foals were needed for each stallion in order to make the study a viable one, it was necessary to use popular stallions. Success in Flat racing is greatly measured in black-type and by the number of Group and Listed race winners a stallion produces. It was therefore important to use premier stallions that had been proven to produce successful progeny and also to help prevent poor quality mares having an infl uence on the results, as a higher stud fee is associated with a higher quality mare. In order to differentiate between stallions of different quality, the stallions were divided into three stud fee categories based on what the peak stud fee was: 30,000-85,000, 85,000-125,000, and 125,000-plus. The progeny performance data was also examined in four separate groups depending on the age of the stallion at the time of conception: 4-9 years, 10-15 years, 16-21 years, and 22-27 years. Foal crop size Overall, mean foal crop size produced by stallions at the age of 4-9 (62 foals) was the smallest, as it increased at age 10-15 (83) and was maintained thereafter. Only stallions in the 85,000-125,000 stud fee category produced a smaller foal crop (33) after the age of 22 than they did at ages 4-9. The foal crop size of both the lower 30,000-85,000) and higher (125,000+) stud fee groups never fell below the number of foals produced at ages 4-9. Starters The proportion of starters produced was high in all stud fee categories and was signifi cantly maintained throughout all stallion age groups (80%). Progeny gender Stallion age did not have an effect on the proportions of male and female progeny that was produced at any age. Stud fees The stud fees of the 30,000-85,000 stallion category increased after age 4-9 and were maintained at a signifi cantly similar level from the age of 10 through to 27 years of age. For the middle stud fee group of 85,000-125,000, the stud fee increased STALLIONS AND THE CONCURRENT STUD FEE GROUPS, NUMBER OF FOALS PRODUCED AND LENGTH OF BREEDING CAREER after age 4-9, peaked at age 16-21, and subsequently fell at age 22-plus; however, it never fell below what the initial stud fee was at age 4-9. The high-priced stallions in the 125,000-plus category increased steadily in mean stud fee from the age of 4-9 to 10-15. After age 16, the stud fee reached a plateau and was maintained through 27 years of age. Progeny success in Listed races The stallions were overall 2.5 times more likely to produce a horse with black-type by being placed in a Listed race at age 4-9 (10.8%). This reduced to 7.8% after age 16 and 4.5% after age 22. Overall, the stallions produced more Listed race winners at age 4-9 (7.5%) than at any other age. The proportion of listed winners decreased to 5.6% after age 10, 4.8% after age 16, and 1.8% after age 22. The stallions were 4 times more likely to produce a Listed race winner at age 4-9 than at age 22+ (1.8%). Progeny success in Group races Younger stallions produced the highest proportion of Group race-placed progeny, with 9.9% at age 4-9. This decreased by half (5.8%) after age 16 and again by half (2.4%) at age 22-plus. The overall proportion of Group race winners produced decreased progressively as the stallions increased in age. The stallions produced the highest mean “ The overall proportion of Group race winners produced decreased progressively as the stallions increased in age ” percentage of Group race winners at the ages of 4-9 (6.4%). This decreased to 4.9% after age 10 and 3.6% after age 16. The stallions were overall fi ve times more likely to produce a Group race winner at ages 4-9 than at 22+ (1.4%). When the progeny from each stud fee category was analysed individually, it was found that all three stallion bands produced signifi cantly less Group racewinning progeny after age 16 and again at age 22-plus. Even the high-priced, 125,000- plus stallions were fi ve times more likely to produce a Group winner at age 4-9 (8.3%) than at 22-plus (1.9%). This decrease in the 125,000-plus category occurred after age 16 (4.9%) and again after age 22-plus despite the fact that a high stud fee, foal crop size, and proportion of starters was maintained. As a high stud fee is associated with a higher quality of broodmare, it is possible to suggest that a good quality mare was also being bred. Group 2- and 3-winning progeny The levels of Group racing were examined in closer detail and it was found that the lowest stud fee category of 30,000-85,000 was 6.5 times more likely to produce a Group 3 winner at age 4-9 (3.9%) than at 22-plus (0.6%). The 85,000-125,000 stallions followed the same pattern and were four times more likely to produce a Group 3 winner at age 4-9 (3.7%) than at 22-plus (0.9%). Even in the most expensive stud fee category, the proportion of Group 3 winners produced decreased signifi cantly after age 10 (4%) and were 5.5 times more likely to produce a Group 3 winner at age 4-9 (5.5%) than at 22-plus (1%). While the proportion of Group 2 winners produced by the two lower stud fee categories was consistently low throughout, stallions priced 125,000-plus produced the most Group 2 winners at the ages of 4-9 (3.7%) and 10-15 (3.1%). The high-priced stallions were 2 times less likely to produce a Group 2 winner after age 16 (1.8%) and 5 times less likely to produce a Group 2 winner at age 22-plus (0.8%) despite the high stud fee being maintained. Group 1-winning progeny Group 1 races are the pinnacle of racing and the ultimate goal for breeders. When the Group 1-winning progeny were examined among the different stud fee categories, it was found that the lower stud fee category of 30,000-85,000 did not show a signifi cant decrease due to the low proportions of Group 1 winners produced throughout their entire careers. In the 85,000-125,000 category, the stallions were three times more likely to produce a Group 1 winner at age 4-9 (2%) than after age 16 (0.7%), and they were 20 times more likely to produce a Group 1 winner at age 4-9 than at 22-plus (0.1%). Even in the highest stud fee category of 125,000-plus, the stallions produced a signifi cantly smaller proportion of Group 1 winners after age 16 (1.9%) and they were 25 times more likely to produce a Group 1 winner at age 4-9 (3.2%) that at 22-plus (0.1%). This decrease occurred despite the stud fee, foal crop size, and proportion of starters being maintained after the age of 16 and 22-plus. As a higher stud fee is “Even in the highest stud fee category of €125,000- plus, the stallions produced a signifi cantly smaller proportion of Group 1 winners after age 16 (1.9%) ” Stallion selection The stallions were selected without bias by screening through ‘new to stud’ lists from 1985-1997 until 20 stallions who met the criteria were found. The selection process resulted in 11 USAbased stallions and nine stallions based in Europe (GB and IRE). Stud fees The stud fees for each stallion used in the study were recorded for every year the stallion was active. This data was collected from the Racing Post database and by contacting the studs at which the stallions stood for the duration of their careers. All stud fees were converted to euro based on what the currency conversion rate was for each year using an online historic currency conversion calculator, confi rmed by fi gures from the Central Bank of Ireland. Progeny performance data All progeny racing performance data was recorded from the Equineline database using ‘Sire Crop Analysis Reports,’ which includes every foal born and all progeny raced in every country. Due to the fact that Equineline is an American database, all earnings were automatically converted to US dollars, regardless of the country in which the earnings were won. Statistical analysis All data was coded and transferred to the IBM SPSS Statistics 22.0 software for statistical analysis. Various tests were carried out using this software to determine if the changes in the progeny racing performance were scientifi cally signifi cant. D associated with a higher quality mare, the progeny performance decreased after age 16 and worsened after age 22 despite a higher quality of broodmare. Champions, or horses that have won multiple Group 1 races, were usually sired by stallions in the top price range. When the proportion of multiple Group 1-winning progeny was examined, it was found that the highest mean proportion of multiple Group 1 winners were produced by the stallions at age 4-9 (1.3%). This fell to 0% produced by stallions at age 22-plus. Earnings Overall, the mean annual earnings won by annual progeny earnings still decreased from $113,623 at age 4-9 to $75,672 at age 16, and fell to just $34,860 when produced at age 22-plus. Conclusions This study is the fi rst reported fi nding of an effect of stallion age on progeny racing performance in thoroughbreds. The relevance of stallion age as a factor affecting progeny performance is supported by the decreases in Listed raceplaced progeny, Listed race winners, Group race-placed progeny, Group race winners, and progeny earnings. The most notable decreases in progeny performance occurred at the age of 16- 21 and again at age 22-plus. Both of these age groups produced signifi cantly less successful progeny in all variables of racing performance when compared with progeny produced at age 4-9. The study found that even in the highest stud fee category of 125,000-plus, where mare quality, stud fee, foal crop size, and proportion of starters were maintained, progeny performance decreased signifi cantly after age 16 and worsened after age 22-plus. These stallions were 25 times more likely to produce a Group 1 winner at age 4-9 (3.2%) than at 22 (0.1%). While the mechanism behind the effects of advanced paternal age on progeny performance is not clear, this may be an interesting area for future research. The fi ndings of this study may suggest that breeders may benefi t from breeding to younger stallions due to a simultaneous decrease across all levels of progeny racing performance and rise in stud fees. ■ progeny were the highest when produced at the age of 4-9 ($87,251) and 10-15 ($74,567). When produced after the age of 16, the mean annual progeny earnings decreased to $58,549. The progeny produced by stallions at age 22-plus ($32,166) earned 3 times less than progeny produced at age 4-15. When the stud fee categories were analysed separately, it was found that in each category the progeny produced by stallions at age 22-plus earned three times less than progeny produced at age 4-9. Even in the highest stud fee group of 125,000- plus--where the foal crop size, proportion of starters, high stud fee, and presumably mare quality was maintained--the mean

CLICK ON THE IMAGE ABOVE TO READ ONLINE

First published in European Trainer issue 56 - January '17 - March '17

BREEDINGWeb Master31 December 2016EUT56, Stallion Age, Fertility, Stallion Fertility, Progeny performance